8
Solid-Earth Hazards, Natural Resources, and Dynamics
OVERVIEW
The solid Earth is the repository of the raw materials that support life. Further, it is the continual discovery of new Earth resources, or new approaches for exploitation of known resources, that sustains societal functioning. Our resources and habitat are ultimately the result of dynamic processes within our planet, processes that are also a source of danger. Hundreds of thousands of lives will be lost in the next century from catastrophic earthquakes, explosive volcanic activity, floods, and landslides. Investment of billions of dollars will be needed to mitigate losses from these disasters as well as from slower ongoing processes such as land subsidence, soil and water contamination, and erosion. Fundamental scientific advances are needed to inform these investments to protect human life and property. These scientific advances require new global observations to quantify rates of accumulation of crustal stress and strain and the evolution of land-surface chemistry and topography. Detailed remote sensing of the evolution of the topography and composition of Earth on regional and global scales and decadal timescales will lead to fundamentally new understandings of Earth essential to informing decision makers and citizens alike.
The continual change of the solid Earth on a wide range of timescales necessitates the use of global observations to develop the knowledge necessary for mitigation of natural hazards. For example, the earthquake cycle in seismically active regions typically has characteristic timescales of centuries to millennia. Thus, observations at any one place over intervals of days to decades, or even over a century (the length of the instrumental record), often capture only a tiny fraction of the cycle. However, when studied over the whole globe, the frequency of events is high, and the study of events at one location can provide the knowledge needed to save lives in other locations. For example, observations of tsunamis generated by earthquakes in Indonesia and South America help improve assessment of the earthquake and tsunami risk in the Pacific Northwest. Observations of seismically induced landslides in Pakistan improve understanding of similar risks in California. Observations of volcanic eruptions and their precursors in Kamchatka and the Philippines help to improve forecasting of volcanic hazards in the United States. More precise knowledge of the timing and likely impact of these sudden catastrophes, as well as constraints on the processes driving slower changes in Earth’s surface chemistry and topography, will increasingly have geopolitical implications.
Society cannot afford to miss opportunities to make space-based observations in areas where land-based observations are unavailable or are impractical because of physical or political restrictions on access.
In this chapter, the Panel on Solid-Earth Hazards, Natural Resources, and Dynamics identifies the three highest-priority satellite missions essential for advancing the knowledge base that society needs to manage, understand, forecast, and mitigate natural hazards; to improve discovery and management of energy, mineral, and soil resources; and to address fundamental questions in solid-Earth dynamics. The first mission addresses when, where, why, and how much the surface of Earth is deforming. Surface deformation can be a measure of the accumulation and release of stress and strain through the earthquake cycle, and it can be the harbinger of catastrophes such as volcanic eruptions or landslides. The second mission addresses how and why Earth’s surface composition and thermal properties vary with location and time and has implications for resources, susceptibility to natural hazards, and ecosystem health. The third proposed mission seeks to determine much more accurately the topography of all seven continents; this would allow improved prediction of flood inundation and landslide likelihood and would provide an understanding of how topography evolves over time.
To put these missions into perspective, it is important to realize that we now have the capability to monitor the events and processes responsible for natural hazards in real time, allowing the possibility for short-term forecasting of their occurrence. Tremendous advances in computational power provide the platform to model complex systems over a variety of timescales. What is lacking is sufficient quantitative observation of the relevant physical processes. If such observations are combined with realistic parameterizations of Earth material properties over the spatial scales needed to understand events that trigger catastrophic hazards, as well as the processes that unfold after initiation, it will be possible to improve forecasting for protection of property and human lives. The three missions required to implement this vision are summarized below and discussed in more detail in the remainder of the chapter.
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Mission to monitor deformation of Earth’s surface. The first priority for solid-Earth science is a mission to observe and characterize subcentimeter-level vector displacements of Earth’s surface. Surface deformation is a visible response to processes at depth that drive seismic activity, volcanism and landslides (Figure 8.1). Local subsidence and uplift from groundwater extraction and recharge and hydrocarbon production are also visible in maps of deformation.
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FIGURE 8.1 (Top) Deformation resulting from fault slip that occurred in the 1999 Hector Mine earthquake in the Mojave Desert, California, is revealed in this synthetic aperture radar interferogram. An interferogram is generated by taking the difference in phase of two radar images taken from the same location in orbit, but at two different times (here, September 15, 1999, and October 20, 1999). Just as the interference fringes seen on an oil slick reveal small changes in thickness of the oil film,the interference fringes shown represent small changes in distance from the satellite to the ground. (Bottom) The centimeter-level sensitivity to the surface deformation pattern permits a determination of the distribution of slip many kilometers below the surface,yielding unprecedented insight into earthquake physics. The C-band satellite used to make these observations performs adequately in desert regions; longer-wavelength L-band InSAR satellites are needed to obtain similar information in vegetated areas. In addition, because of the 5 weeks that elapsed between observations,the image of coseismic deformation is corrupted by the postseismic deformation that occurred after the earthquake (see Box 8.2). Finally, even though this is a desert region,the image is degraded by noise due to atmospheric effects that could be removed if many more observations could be stacked. SOURCE: Zebker et al., 1999. Courtesy of H.Zebker, Stanford University.
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Requirements: An L-band (1.2-GHz) InSAR mission that will meet the science measurement objectives requires a satellite in a 700- to 800-km orbit that is maintained to a repeat track within 250 m. The mission should have a 5-year lifetime to capture time-variable processes and to achieve improved measurement accuracy by stacking of interferograms to remove atmospheric noise. Measurements at the L band minimize temporal decorrelation in regions of appreciable ground cover. Two subbands separated by 70 MHz allow correction of ionospheric effects. Left- and right-pointing images on both ascending and descending orbits are needed to obtain vector displacements. An 8-day revisit interval balances complete global coverage with frequent repeats. The baseline mission, providing fundamental constraints for questions related to volcanos, earthquakes, landslides, resource production, and ice sheet dynamics, requires a single polarization antenna; a multiple polarization capability would add determination of variations in ecosystem structure (see Chapter 7) to the science return.
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Mission to observe surface composition and thermal properties. Changes in mineralogical composition affect the optical reflectance spectrum of the surface, providing information on the distribution of geologic materials (Figure 8.2; Swayze et al., 2007) and also the condition and types of vegetation on the surface. Gases from within Earth, such as CO2 or SO2, are sensitive indicators of impending volcanic hazards, and plume ejecta themselves pose risks to aircraft and to those downwind. These gases also have distinctive spectra in the optical and near-infrared (IR) regions. Thus, the panel’s second priority is a sensor that can resolve both in finely detailed spectra.
Requirements: For this mission, two pointable sensors on the same platform are needed: an optical hyperspectral imaging sensor operating in the 400- to 2,500-nm region and a multispectral sensor operating in the 8- to 12-µm thermal-IR region. The hyperspectral sensor, with spectral discrimination greatly enhanced beyond Landsat and MODIS-class sensors, would make key observations for resource exploration, soil assessment, and landslide-hazard forecasts. The combined, pointable hyperspectral and infrared sensors would greatly improve volcano monitoring and eruption prediction, aid in prospecting for resources and mapping long-term changes in physical and chemical properties of soil, and contribute data essential for characterizing ecosystem changes.
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Mission to measure high-resolution (5-m) topography of the land surface. Many hydrologic and geomorphic processes are revealed in detailed topographic data. The panel’s third priority is a mission to determine Earth’s elevation at every point on land to the sub-decimeter level, approaching the quality of information already available for Mars. This recommended first-epoch mapping of Earth’s surface would set the stage for repeat imagery, which would allow the quantification of rates of many natural and anthropogenic processes such as loss of topsoil and disruption and degradation of wetlands.
Requirements: A promising technology for high-resolution spatial topographic mapping is imaging lidar (Figure 8.3). Two-dimensional surface coverage can be accomplished with multiple-beam laser systems,
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FIGURE 8.2 Significant differences in surface chemistry in a mining region caused by natural and anthropogenic processes can be monitored by satellite. For example, these hyperspectral images of Cuprite, Nevada, acquired by the AVIRIS satellite and overlaid on a digital terrain model were processed to identify iron mineralogy (top) and hydroxide- and carbonate-bearing minerals (bottom). The dominant mineral in each pixel is identified and color coded. Both topography and mineralogy control the formation of alteration minerals that contain hydroxides and carbonates, largely because such alteration can create the most acidic waters on Earth. Such drainage flows downhill and creates surface alteration zones. As humans alter larger and larger regions of Earth’s surface, documenting such impacts through satellite imaging will be of use in assessing the impacts. SOURCE: Courtesy of U.S. Geological Survey.
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FIGURE 8.3 Mapping natural hazards and understanding the processes that shape Earth’s surface both require high-resolution topographic data. The two images show shaded-relief maps of California’s Salinas River and surrounding hill-slopes. The left-hand image shows the finest resolution (30 m) that is currently available over much of Earth’s surface. The right-hand image shows the same scene at the resolution achievable with lidar mapping from space (5 m). Mapping landslide and flood hazards in this landscape is achievable with 5-m topographic data, but impossible with 30-m data. SOURCE: Courtesy of J.Taylor Perron, University of California, Berkeley.
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scanning platforms, and/or pixilated detectors in which each pixel has an associated time-of-flight chip that provides a measurement of elevation. Providing 5-m resolution topography at sub-decimeter accuracy would facilitate forecasting of landslides and floods and allow fundamental advances in geomorphology.
Although these three space-based missions are the primary recommendations and focus of this chapter, the panel also notes several other high priorities for solid-Earth science. These include the measurement and determination of the terrestrial reference frame and the use of suborbital technology for measurements that must be made either locally or at shorter distance and time intervals than is allowed by space observa-
tion. In addition, two space-based missions addressing spatial and temporal variations in the gravity field of primary interest to other panels are also of interest to solid-Earth science.
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Requirement for precise measurement and maintenance of the terrestrial reference frame. The geodetic infrastructure needed to enhance or even to maintain the terrestrial reference frame is in danger of collapse (see Chapter 1). Improvements in accuracy and economic efficiency are needed. Investing resources to ensure the improvement and continued operation of the geodetic infrastructure is a requirement of virtually all the missions proposed by every panel in this study. The terrestrial reference frame is realized through integration of the high-precision networks of the Global Positioning System (GPS), Very Long Baseline Interferometry (VLBI), and satellite laser ranging (SLR). It provides the foundation for virtually all space-based and ground-based observations in Earth science and studies of global change, including remote monitoring of sea level, sea-surface topography, plate motions, crustal deformation, the geoid, and time-varying gravity from space. It is through this reference frame that all measurements can be interrelated for robust, long-term monitoring of global change. A precise reference frame is also essential for interplanetary navigation and diverse national strategic needs.
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Important suborbital missions. Two kinds of suborbital missions would provide important information about Earth’s interior, gravity, and magnetic properties. (1) Development of an unmanned aerial vehicle (UAV) capability will allow temporally dense InSAR coverage of deformation associated with earthquakes and volcanos and also provide high-resolution measurement of spatial variations in Earth’s gravity field with better accuracy than from space. Such observations would enhance knowledge of geologic structures where higher-order gravity field expansion terms are too weak to be reliably observed. (2) Magnetic studies from balloons (“stratospheric satellites”) could lead to new understandings of Earth’s crust.
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Other important space missions. Two missions given high priority by other panels would greatly enhance understanding of processes acting within the solid Earth. (1) Measurement of temporal variations in Earth’s gravity field at improved resolution via an improved version of the GRACE mission would provide important constraints on the rheology of Earth’s interior. This would lead to improved models of the convective processes driving plate tectonics and hence nearly all active deformation, and would provide fundamental constraints on processes related to movement of water masses for hydrology and oceanography. (2) Measurements of sea-surface topography via radar altimetry would allow an order-of-magnitude improvement in the size of seamounts on the ocean floor that could be discovered and analyzed. This would both reduce navigation hazards and increase knowledge of volcanic processes. Although these missions are not this panel’s highest priority, they are of substantial value. Important observations of temporal and spatial variations in Earth’s magnetic field will be provided by international missions.
In summary, the challenges posed by resource discovery and production; by forecasting, assessment, and mitigation of natural hazards; and by advancing the science of solid-Earth dynamics call for ongoing investment in satellite capabilities. The panel has identified above the set of three highest-priority satellite missions that, in combination with a robust global geodetic network and a continuation of the long-term instrumental record and other supporting observations from missions recommended by other panels and flown by other countries, will enable scientific progress and improved strategies for management of solid-Earth hazards, resources, and dynamics.
THE STRATEGIC ROLE OF SOLID-EARTH SCIENCE
The events of the past few years—for example, the volcanic unrest of Mt. Saint Helens in 2004, the devastation of the December 26, 2004, Sumatra earthquake and resulting tsunami, the loss of life and destruc-
tion from the great Pakistan earthquake and associated landslides of 2005, and the chaos following Hurricane Katrina (Figure 8.4)—demonstrate humankind’s vulnerability to naturally occurring disasters. These events highlighted the costs associated with inadequate information and the consequences of inadequate planning for the dissemination of available or obtainable information. Slower ongoing changes—depletion of resources, degradation of soils, sea-level rise, and depletion and contamination of groundwater—will also continue to have serious consequences. It is possible to mitigate the impacts of these events with carefully planned actions that have their foundation in science. Sustainable management of resources and hazards requires information that is costly, but less costly than inaction. Post-facto remediation can be prohibitively expensive.
Scientists, resource providers, policy makers and other stakeholders need an array of information to anticipate and mitigate natural hazards, ensure a steady supply of natural resources and energy, and develop appropriate international policies capable of sustaining life on Earth. Risks posed by hazards such as earthquakes, volcanos, and other natural disasters have to be quantified and documented, and precursors or other early warning signals have to be detected. Long-term changes in Earth’s surface chemistry and topography must be quantified to predict soil degradation and flooding. Demand for energy supplies drawn from Earth will become an even more critical policy issue as worldwide competition for already-scarce resources increases.
The necessity of developing a forward-looking U.S. energy policy will be one of the major political drivers for reorganizing priorities in the Earth sciences. The energy consumption per capita in Asia will grow in the next decade to at least European levels. This will require increased access to resources both for energy and for mineral consumption. Easy access to hydrocarbons based on rudimentary scientific understanding of upper crustal processes is coming to an end. Hence, energy producers must find new hydrocarbon resources and produce more efficiently from existing reservoirs. In addition, the need to exploit resources in hostile environments will continually increase. Future energy supplies must be more diverse to meet global demand, and the assessment of total resources will have to be much more accurate globally to maintain political stability. In the next 30 to 50 years, a transition to less dependence on hydrocarbons to fuel society will be technically possible, but any energy-producing resource will have climate and environmental impacts. In addition, the demand for hydrocarbon resources will grow substantially in the next two to three decades in absolute terms, regardless of alternative-energy policy priorities. A scientific basis will be required to estimate the impacts on the biosphere of any given energy plan. (Note that much of the ecology panel’s proposed program of missions outlined in Chapter 7 will contribute substantially to assessing the environmental impacts of energy choices.) Currently, most studies of energy consumption and resource recovery are qualitative, and the debate about exploitation of resources takes place without much scientific basis. These political and global realities will drive innovation in Earth science over the next decades.
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FIGURE 8.4 (Top) Breach in the New Orleans 17th Street canal levee that allowed flooding in the city following Hurricane Katrina. SOURCE: Marty Bahamonde/FEMA. (Bottom) Map derived from InSAR observations by the Canadian C-band RADARSAT satellite showing the rate of subsidence in millimeters per year for New Orleans and its vicinity in the 3-year interval preceding the hurricane (2002–2005). Insets show the location (white frame) and magnified view (red frame) of the region west of Lake Borgne, including eastern St. Bernard Parish. Note the high rates of subsidence (>20 mm/yr) on the levee bounding the MRGO canal, where large sections were breached when Hurricane Katrina struck. (Scale bar, 10 km). Note also that much of the map has no data because of a lack of coherence in phase caused by vegetation; an L-band InSAR satellite such as that recommended in this chapter should provide better coherence. SOURCE: Dixon et al., 2006. Reproduced by permission of Macmillan Publishers Ltd. Copyright 2006.
Development and implementation of appropriate policies at the national and international levels will require a complete understanding of the fundamental nature of Earth and how it evolves over time. In the past decade, remote sensing methods have greatly improved understanding of the localization of strain in Earth’s crust and how it drives catastrophic processes. It is critical to be able to anticipate and understand these forces and their associated risks. Space-based data play a similarly critical role in mapping and monitoring resources such as oil, gas, and water, which are exploited now on a truly international scale—only global views from space will continue to enable sustainable policies.
These needs are encapsulated in the considerations of strategic roles that guide the observational requirements over the next decade (Box 8.1).
BACKGROUND ON OBSERVATIONAL NEEDS AND REQUIREMENTS
To provide the information and tools essential to policy makers and other stakeholders, an Earth observation strategy is required to address the strategic needs described in the previous section. The panel’s approach, informed by previously stated needs and goals from the scientific community (SESWG, 2002), is focused on two primary themes (Table 8.1): (1) forecasting, assessment, and mitigation of natural hazards and (2) resource discovery and production.
Forecasting, Assessment, and Mitigation of Natural Hazards
Natural hazards pose an enormous threat to many parts of the United States and the rest of the world (Figure 8.5). In 2000, annual losses from earthquakes were estimated at $4.4 billion per year for the United
BOX 8.1 STRATEGIC ROLES AND QUESTIONS FOR SOLID-EARTH SCIENCE AND OBSERVATIONS Forecasting and Mitigating the Effects of Natural Hazards What observations can improve the reliability of hazard forecasts? What are the opportunities for early detection, ongoing observation, and management of extreme events? What are the policy options for managing events that threaten human life and property? Can systems be managed to reduce their vulnerability before such events occur? How can useful information, including uncertainties, be communicated to decision makers for the benefit of society? Discovering and Managing Resources How can the ability be improved to locate resources that can be profitably produced? How can the ability to produce known resources more safely and effectively be improved? How can potential environmental damage from exploitation of resources be limited? How can long-term changes in soil characteristics, land use, and Earth surface topography be monitored to understand soil degradation and erosion in the context of climate change? How can information about surface chemistry be coupled to topographic information to yield predictive models for landslide activity? Enabling Science What new observations, coupled with improved modeling capability, are most likely to advance fundamental understanding of nature? How can this fundamental understanding be used to decrease hazards arising from natural disasters and to protect and improve the economy? |
TABLE 8.1 Solid-Earth Panel Key Questions for Identifying Satellite Observation Priorities
Science Themes |
Subthemes |
Key Questions |
Forecasting, assessment, and mitigation of natural hazards |
Earthquake forecasting |
Where and how fast is seismogenic strain accumulating? |
How fast does crust stress change, and how does this trigger earthquakes? |
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Are earthquakes predictable? |
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How do fluids such as groundwater, hydrocarbons, and CO2 trigger earthquakes? |
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Volcanic-eruption prediction |
Can a worldwide volcanic eruption forecasting system be established using remote sensing data? |
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What pre-eruption surface manifestations are amenable to remote measurement from orbit? |
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What surface temperature change patterns are relevant? |
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What can the measurement of emissions such as SO2 and silicate ash indicate, and what patterns of change are relevant? |
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How can multiple change patterns and measurements (topography, gas, temperature, vegetation) at craters be better interpreted for eruption forecasting? |
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How often must a volcano be observed to provide a meaningful prediction? |
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Landslide prediction |
Which places show slowly moving landslides, and how likely are they to fail catastrophically? |
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Where are oversteepened slopes and susceptible rock types located? |
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Resource discovery and production |
Water resources |
Where and when are groundwater reservoirs being depleted or recharged? |
Which critical aquifers are being driven into irreversible inelastic compaction? |
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How does this affect future storage capabilities of the aquifers? |
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Can surface hyperspectral and thermal measurements be coupled with measurements of surface deformation from InSAR to enable new concepts for detecting and understanding slow deformation processes related to fluid seepage phenomena? |
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Petroleum and mineral resources |
What fundamentally new concepts in surface geochemistry will allow for more comprehensive and precise surface geology characterization relevant for the hydrocarbon- and mineral-extraction industry? |
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What changes in surface chemistry and thermal properties are diagnostic of hydrocarbon and mineral resources? |
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How can the efficiency of hydrocarbon and mineral production be improved? Using three-dimensional dynamic stress modeling at reservoir scales, is it possible to more accurately model stress dynamics and in particular to predict failure processes on a basin scale? |
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Terrains creating chemical risk |
Can the risk of surface-water and groundwater pollution from mineral and hydrocarbon waste sites be quantified from surface geochemical measurements? |
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What key surface geochemical indicators detectable by remote sensing are relevant to describing mining waste or landslide hazards? |
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What are the detection limits at which soils containing natural health hazards such as asbestos, or swelling clays unsuitable for building construction, can be detected by hyperspectral imaging? |
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Agricultural soil degradation |
What is the true extent of the loss of topsoil due to poor management practices? |
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Can remote sensing be used to measure carbon sequestration in agricultural soils? |
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How well can the leaching of nutrients and increasing salinization be measured by remote sensing? |
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Can remote sensing provide the kind of information needed for policy decisions by government entities worldwide? |
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Will documenting the loss of prime agricultural soils force land-use planners to assist in preserving soil resources? |
States alone (FEMA, 2001 a). Volcanic eruptions destroy cities and towns, affect regional agriculture, and disrupt air transport. (In 1989, a KLM jet encountered a volcanic ash cloud from Redoubt Volcano near Anchorage Alaska and sustained more than $80 million in damage.) Flood hazards threaten civilian safety and commerce. The 1993 Mississippi River flooding caused $15 billion to $20 billion worth of damage and displaced 70,000 people; damage from the recent earthquake-spawned tsunamis in southeast Asia, which killed an estimated 270,000 people, will not be fully appreciated for some time. Additional long-
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FIGURE 8.5 Earthquakes produce substantial economic and human loss that could be mitigated with better warnings. In the Northridge, California, earthquake of January 17, 1994, buildings, cars and personal property were all destroyed when the earthquake struck. Approximately 114,000 residential and commercial structures were damaged and 72 deaths were attributed to the earthquake. The cost of damage was estimated at $44 billion (NRC, 1999). SOURCE: FEMA News Photo.
and short-term hazards result from sea-level change and landslides. Moreover, world population is rising most rapidly in areas of high risk from earthquakes, volcanos, flooding, and landslides (e.g., Bilham, 1988, 2004).
Assessing risk and developing successful policies to minimize loss of life and destruction of property require precise measurements and powerful geophysical models. A recent FEMA report describing the effects of a hypothetical earthquake on the Hayward fault estimated economic losses associated with
damage to buildings at nearly $37 billion (year 2000 dollars) (FEMA, 2001 b). According to the FEMA scenario, under a targeted rehabilitation program, major injuries and deaths could be decreased by nearly 60 percent. If a comprehensive rehabilitation program were fully implemented, economic losses would be reduced by over 35 percent ($37 billion to $24 billion) and 1,300 lives could be saved. Better understanding of earthquake physics and the expected shaking from a given event will enable strategies such as building rehabilitation that can substantially reduce the risks to human life.
Natural disasters take their heaviest tolls in developing countries that do not have the resources for mitigation strategies. For example, the 1994 Northridge earthquake, although an economic disaster (Chapter 1), claimed only 72 victims, in large part because much of the rupture plane lay beneath a sparsely populated mountainous area. Conversely, the comparably sized 1995 Kobe earthquake, which occurred beneath a densely populated area, claimed nearly 6,500 lives. The moderately larger 2001 Gujarat earthquake claimed 15,000 lives, more than 200 times the number of lives lost in the 1994 Northridge earthquake. These disparities reflect both the relative preparedness of earthquake-prone zones and their population density. More accurate warnings of impending hazard can be used to avoid substantial suffering by focusing resources on preparedness and disaster response.
Forecasting Earthquakes
Stress transfer processes are important in triggering seismic activity. Current research is elucidating the nature of earthquake-to-earthquake interactions, rigorously quantifying the statistical likelihood of linkages, and elucidating time-dependent processes (e.g., postseismic relaxation, state and rate of fault friction) that influence triggered activity (Box 8.2). Emerging clues suggest longer-range interactions that are not fully understood. Such interactions are notoriously hard to identify and quantify, but they should have detectable deformation signatures. Synoptic space-based imaging offers a new and promising means to identify deformation causes and effects linking regional earthquake events.
Identification of deformation events that are seismic precursors is the “Holy Grail” for earthquake research. Current earthquake-hazard maps provide only coarse resolution of time and geography. Such maps depict the probability of exceeding a specified magnitude of shaking over the next 30 to 100 years. The spatial resolution is typically on the order of tens to hundreds of kilometers. These maps are based on information about past earthquakes observed in the geological or historical record.
Future scientific studies of crustal deformation will yield insights into earthquake behavior, including answers to questions such as whether high strain rates indicate the initiation of failure on a fault or a quiet release of stress, and how stress is transferred to other faults (Box 8.3). These studies will drive the science that places more useful earthquake hazard maps into the hands of decision makers.
Forecasting Volcanic Eruptions
Volcanos represent growing hazards to large local populations (Ewert and Harpel, 2004) and also present hazards to aviation passengers worldwide through engine dust ingestion (Salinas and Watt, 2004). At Pinatubo in 1991, a sustained increase in seismic energy and changes in the nature of seismic events were critical for successful prediction of eruption (Harlow et al., 1996). However, only a small percentage of the world’s volcanos are instrumented sufficiently to facilitate predictions of eruptions from seismic data, and use of satellite-based remote sensing could provide crucial assistance in identifying regions where volcanic unrest is likely (Pritchard and Simons, 2002).
Significant progress in unraveling the mechanics of magma transport from source regions to shallow crustal reservoirs in volcanos has been made through field studies of ancient eroded volcanic systems and
BOX 8.2 RELATIONSHIP OF SURFACE DEFORMATION TO EARTHQUAKE PROBABILITY Recent developments in investigations of the mechanics of earthquakes have demonstrated that the frictional properties of faults, and hence their probability of rupturing, depend strongly on the rates at which the faults are stressed. The fundamental concepts are illustrated in Figure 8.2.1. The stressing rate in a fault system such as that in southern California (Figure 8.2.2.) is the result of both loading from motions of the tectonic plates and loading from stresses generated by earthquakes within the fault system. ![]() FIGURE 8.2.1 The top row shows example histories of shear stress in a region as a function of time; the bottom row shows the resulting rate of seismicity, which is directly proportional to the probability of an earthquake on a given fault segment. A change in stressing rate (left column) leads to an offset in the rate of seismicity. A sudden change in stress (right column) leads to an abrupt change in seismicity rate, followed by a relaxation back to the original rate. SOURCE: Toda et al., 2002. Reprinted by permission of Macmillan Publishers Ltd. Copyright 2002. |
through theoretical models. However, direct observational constraints on the style and dynamics of magma ascent are still lacking. Such constraints are crucial for forecasting the replenishment and pressurization of shallow magma chambers that may potentially feed volcanic eruptions. Volcanic unrest episodes for any given magmatic system may be quite infrequent, and only a few volcanic systems around the world are closely monitored (Figure 8.6). Therefore, a global observation system capable of detecting ongoing magmatic unrest will result in dramatic improvements in the understanding of volcanic activity and associated societal hazards.
To improve hazard prediction for populated active volcanos, the size and shape of magmatic reservoirs must be determined from geodetic, seismic, gravity, and other geophysical observations. Researchers must also identify the type of magmatic unrest associated with eruptions, characterize detectable deformation prior to volcanic eruptions, and predict the volume and size of impending eruptive events. High-quality geodetic observations are needed to constrain timescales and mechanisms of these processes.
Volcanos are advantageous targets of remote sensing because, unlike many natural hazards, their positions are well known: several hundred potentially active subaerial vents or craters are known today. Eruptions can therefore often be forecast on the basis of observations from either the ground or space. Crater regions are affected by heat from magma and associated fluids and show detectable thermal changes (Harris et al., 2002). Gas emissions, especially SO2, and volcanic ash are well-known crater features linked to activity (Watson et al., 2004). Topographic changes (uplifts, slumps, and landslides) are frequent. Vegetation in crater regions provides a sensitive barometer of all the other changes. Many, but not all, eruptions may be forecast if changes in these observables are measured frequently at fine spatial and spectral resolutions. Half of all eruptions may be preceded by detectable surficial changes with lead times of 30 days or more.
Currently, eruptions are monitored from orbit at coarse spatial resolution using MODIS on the EOS missions Terra and Aqua and at moderate resolution (90-m pixels) using ASTER on Terra (Patrick et al.,
BOX 8.3 “SLOW" OR ASEISMIC EARTHQUAKES Faulting at subduction zones (Figure 8.3.1) produces the world’s largest earthquakes, characterized by the rapid release of strain over very large areas. Over the past two decades, improvements in geodesy, or the precision with which researchers can measure crustal deformation, have made visible similar motions that occur over long periods rather than the almost instantaneous shock to Earth that is associated with earthquakes. These “slow” events, expressing themselves in waves with periods far too long to be easily measured by seismometers, redistribute strain throughout the crust and are important in determining the overall strain balance, and hence directly affect earthquake probabilities. ![]() FIGURE 8.3.1 The Cascadia subduction zone in the U.S. Northwest is a potential source of truly great earthquakes, perhaps as large as magnitude 9. Current GPS deformation measurements (left) show interseismic deformations from ongoing tectonic motions. These motions reverse themselves for periods of 2–6 weeks every 14 or 15 months, as repeated slow earthquakes propagate across the area (see GPS measurements, right). The vertical bars drawn on top of the GPS measurements represent known occurrences of slow earthquakes and correspond to “abrupt” (week-long) changes in GPS station position. These events are most easily seen in deformation maps and can greatly increase the ability to assess strain accumulation information and can lead to a better forecasting model. Slow earthquakes are not quite silent, and a unique non-earthquake seismic tremor signal has been detected accompanying them (Rogers and Dragert, 2003). SOURCE: (Left) Melbourne and Webb, 2003; reprinted with permission from AAAS. (Right) Courtesy of H. Dragert, Geological Survey of Canada, Natural Resources Canada. |
2005). Improving the spatial resolution and swath width of an ASTER-like sensor would make it possible to detect changes earlier and could provide the foundation of a global eruption-prediction system. A major initiative to improve volcano monitoring and eruption forecasting using surficial methods has been proposed by USGS for U.S. volcanos (Ewert et al., 2005). If implemented, this initiative would greatly expand ground truth data for a small subset of Earth’s volcanos. This is important because it would greatly facilitate validation of the panel’s proposed satellite techniques and could lead to an effective global volcanic-eruption mitigation effort.
Forecasting Landslides
Landslides threaten property and life in many parts of the world. Steep slopes, soil conditions, and rainfall patterns are among the underlying causes of landslides. Thus, improved knowledge of surface composition (see Figure 8.2) and topography (see Figure 8.3) are important for characterizing landslide risk.
Prediction is aided significantly by detailed observation of down-slope movements at the milimeter to centimeter level (Figure 8.7). Such observations can identify unstable patches of soil and have been correlated with landslide events (Hilley et al., 2004). Because these areas are relatively small and often heavily vegetated, conventional InSAR has not been an effective tool for mapping these small deformations. A new InSAR analysis technique, utilizing so-called persistent scatterers, has been shown to yield high-spatial-resolution information, including reliable milimeter-scale down-slope motions in terrains that challenge existing measurement systems. In this method individual points on the surface that do not suffer from radar “speckle” are isolated, and displacements at these points form a network that resolves the tiny motions over time. This method appears to be a reliable approach to finding areas prone to landslide, before any catastrophic collapse occurs.
![](/https/nap.nationalacademies.org/openbook/0309103878/xhtml/images/p200131deg234001.jpg)
FIGURE 8.7 InSAR image acquired over the Berkeley Hills, California, showing coherent down-slope motions that may be precursors of more rapid landslides. Increased down-slope movement in years with higher rainfall shows that potential hazard areas may be pinpointed in these high-resolution data and that hazard level may be assessed yearly. More rapid acquisition of images from the mission recommended in this chapter would allow assessment of the threat almost weekly. SOURCE: Hilley et al., 2004. Reprinted with permission from AAAS.
Resource Discovery and Production
As world population increases, the demand for nonrenewable resources grows. In particular, the need for hydrocarbon resources and other mineral resources will rise for the next few decades. Rising demand will result in more vigorous exploration for new hydrocarbon- and mineral-bearing resources, as well as a higher level of production from known resources. At the same time, the need to reduce environmental impact during resource exploitation will increase. The petroleum industry is now developing methods to both detect and monitor hydrocarbon reservoirs remotely through a combination of airborne and space-based data.
Experiments exploiting hyperspectral data have allowed accurate and high-resolution interpretations of subtle surface geological effects related, albeit indirectly, to mineral deposits and hydrocarbon reservoirs (van der Meer and de Jong, 2003). These activities require better data acquisition over larger bandwidths to understand fundamental geophysical and geochemical processes active in the upper layers of the crust. Indeed, the availability of high-resolution hyperspectral data will lead to comprehensive and precise surface geology characterization relevant for both resource exploitation and amelioration of environmental impact in the hydrocarbon- and mineral-extraction industry.
Management of hydrocarbon resources is facilitated by measurements of surface deformation and surface composition (Box 8.4). Extraction of oil or gas from reservoirs leads to subsidence (e.g., Fielding et al., 1998) and occasionally triggers earthquakes (e.g., Segall et al., 1994). Quantitative interpretation of the deformation pattern can assist in assessment of reservoir storage properties, as well as help guide an extraction strategy. Monitoring of deformation is also important in areas where ongoing subsidence from years of production results in significant subsidence in inhabited areas. In the United States such settling is problematic for communities around Houston, Texas, and Long Beach, California.
Recent academic and industry research has shown that accurate monitoring of surface deformation caused by fluid extraction can be directly related to the onset and evolution of microseismic events (magnitude <2) occurring on natural faults and fractures in reservoir rock (e.g., Bourne et al., 2006). These observations are now being used to build subsurface models that may help to predict reservoir fluid flow dynamics accurately and to quantify the risk of well-bore failure due to localized increased strain accumulations and fault reactivation. Well-bore failure is one of the most important hazards in the hydrocarbon fluid extraction process; it can lead to decreased production efficiency and to serious safety or environmental hazards (Mayuga and Alen, 1970; De Rouffignac et al., 1995; Biegert et al., 1997; Patzek and Silin, 2000).
Space-based monitoring techniques provide more comprehensive and more accurate surface-deformation data than conventional geodetic techniques are able to achieve. The need for such monitoring is increasing because the industry is targeting large but ultralow-permeability reservoirs, which require application of major production-enhancement techniques that often involve the injection of water and steam to produce artificial fractures. In principle, that leads to increased productivity; however, unless the injection process and the resulting localized strain increases can be monitored accurately, such operations can be highly ineffective and lead to significant damage to infrastructure costing on the order of $100 million. In particular, the monitoring of sudden local compaction events is crucial to avoid costly well damage.
The capability to measure surface deformation from space would have two profound consequences. High-resolution InSAR monitoring may allow efficient extraction at acceptable environmental risks in remote (and often environmentally sensitive) areas not possible with surface-based techniques, except at sometimes prohibitively great cost and an unacceptably large footprint. And surface-deformation and monitoring data for accurate three-dimensional geomechanical models of subsurface strain accumulations may provide an efficient way to study such dynamics at a larger but less controlled basin scale. Through applica-
BOX 8.4 HYDROCARBON PRODUCTION, SURFACE DEFORMATION, AND FAULT REACTIVATION IN THE YIBAL FIELD IN OMAN Obtaining observations to better manage hydrocarbon production can have substantial benefits for extracting precious resources. Typically only a fraction of the hydrocarbons stored in reservoir rock are extracted, in effect wasting what is left behind. In addition, deformation from compaction-induced internal deformation of reservoirs risks failure of the wells. The example shown in Figure 8.4.1 is from an oil field overlain by a gas reservoir, both producing from carbonate layers. Three types of data have been acquired to monitor the reservoirs: (1) microseismic, (2) InSAR, and (3) GPS. These data have the potential to image changes in reservoir fluid pressure, structure, and the resulting fault reactivation. As a result, geomechanical models can be built that enable accurate prediction of the risk for well-bore failure due to fault reactivation. Microseismic events located using a down-hole geophone array are shown in Figure 8.4.2. ![]() FIGURE 8.4.1 The schematic cross section illustrates how a gas reservoir with rapid variations in thickness could cause fault reactivation as a result of depletion. Pressures decline uniformly throughout the reservoir, but compaction varies by up to 20 percent because of abrupt changes in reservoir thickness across major faults. These differences lead to stresses that could cause failure of the faults and the accumulation of fault slip to accommodate the different rates of reservoir compaction. ![]() FIGURE 8.4.2 Cross section (a) and map view (b) with thick black lines denoting fault traces interpreted at the depth of an oil reservoir. Most of the seismic activity is sandwiched between the gas and oil reservoirs. The field of surface displacement measured by InSAR over a 22-month period shows primarily subsidence due to reservoir compaction (c; color range is ±90 mm). In addition, a discontinuity in surface displacement is observed across the fault segment outlined by the ellipse. This suggests shallow aseismic motion on the fault coincident with the increased microseismic activity at depth. SOURCE: Bourne et al., 2006. Courtesy of Shell International. |
tion and up-scaling of new insights obtained recently in the hydrocarbon industry with respect to accurate three-dimensional dynamic stress modeling at reservoir scales (Bourne et al., 2006; Maron et al., 2005), it will be possible to model stress dynamics more accurately and in particular to predict failure processes on a basin scale. This enhanced ability could also lead to important new insights in earthquake prediction because typically there is much more information on the state of stress in reservoirs than elsewhere.
Injection of CO2 into the crust is expected to become an increasingly important means for sequestering this greenhouse gas from the atmosphere. Monitoring the surface deformation caused by fluid injection will likely become an important technique for understanding reservoir behavior and monitoring its integrity.
In addition, remediation of mine wastes is a costly undertaking and it has been shown that finely resolved remote sensing can provide valuable guidance in cleanup (Montero et al., 2005; Swayze et al., 2000). Current and historical mine-waste dumps are sources of heavy metals, which under appropriate conditions can leach into surface and groundwater supplies and harm people, as well as wildlife and vegetation.
Another important natural resource that can be remotely monitored is agricultural soil. Agricultural soils around the globe are being degraded rapidly by a variety of different mechanisms (Figure 8.8). Poor management practices and removal of crop residues for livestock feed and bedding cause loss of topsoil worldwide. Flooding results in the deposit of sediments and leaching of nutrients. Increased salinity due to poor irrigation practices or sea/ocean surge (caused by tsunami or hurricanes and urbanization) is permanently removing prime farm land from production (Lal et al., 2003). All of these changes can be detected and monitored globally by remote sensing of surface properties. Farmers and the public have not always worried about soil loss, because crop yields have increased despite these problems. The detrimental effects of soil erosion have been masked by increased applications of fertilizers, use of better crop varieties, denser plantings, more intense pest control, and more effective tillage and water management, as well as favorable weather. Nevertheless, in many countries crop yields lag growing populations. Soil productivity losses are often a main cause of some nation’s inability to provide adequate food supplies. In addition, the emerging emphasis on producing corn and soybeans for use as biofuels will cause farmers to bring more lands into production. Most of these new production areas now have hay or pasture on highly erodible land where tilling will result in increased soil erosion (since the soil is bare during critical high-rain events in the spring, when corn and soybean crops are planted).
The existence of nonmarine life on Earth depends not only on soil fertility but also on the availability of freshwater, human dependence on which is amply demonstrated during droughts around the world. Ground-water, surface water, soil moisture, and snow pack all contribute to the global freshwater budget, and it is necessary to understand how natural and anthropogenic processes redistribute water in space and time.
Groundwater currently provides 24 percent of the daily freshwater supply in the United States but remains a poorly characterized component of the terrestrial water budget. As drought conditions persist in the western United States and populations continue to grow, new groundwater development will exacerbate national subsidence problems that cost $168 million annually and have already led to coastal inundation and infrastructure damage; there are also unquantifiable hidden costs (NRC, 1999).
Characterization of how the land surface above aquifers responds to groundwater pumping is very important, providing insights into subsurface controls of the aquifer system, the location of groundwater barriers and conduits, and the extent of the aquifer. When combined with records on groundwater level and pumping, it also provides knowledge of hydrodynamic properties of the aquifer systems critical to measuring changes in the groundwater supply, modeling the aquifer system, and constraining the terrestrial water budget. Deformation measurements with national coverage and routine imaging would significantly advance the ability to characterize both regional- and continental-scale aquifer systems. Moreover, measurements of deformation could uniformly quantify the nation’s aquifer system for the first time.
Natural and human-induced land-surface subsidence across the United States has affected more than 44,000 square kilometers in 45 states (Figure 8.9). More than 80 percent of the identified subsidence in the United States is a consequence of the increasing development of land and water resources, which threatens to exacerbate existing land subsidence problems and initiate new ones (Galloway et al., 1999).
Surface deformation associated with natural processes and human activity is observed but is difficult to separate in geodetic network data. For example, sediment compaction, tectonic extension, sinkhole collapse, groundwater pumping, geothermal and hydrocarbon production, CO2 injection, and mineral extraction all produce both vertical and horizontal surface motion. By combining geodetic and hydrologic time-series data with spatially dense deformation observations, it is now possible to recognize and in some cases separate multiple sources of land-surface deformation at a given location (e.g., Bawden et al., 2001). National coverage and routine imaging from space with high spatial resolution unachievable with a network of discrete surface stations would significantly advance understanding of the contributions of both human-induced and tectonic surface motions.
![](/https/nap.nationalacademies.org/openbook/0309103878/xhtml/images/p200131deg239001.jpg)
FIGURE 8.9 Many regions of Earth are in motion,affecting the lives of millions of people; for example, this subsidence near Las Vegas is due to the withdrawal of groundwater (Amelung et al., 1999). InSAR provides the only tool capable of mapping these changes globally. SOURCE: Image courtesy of F.Amelung, University of Miami.
TABLE 8.2 Solid-Earth Panel Priorities and Associated Mission Concepts
Summary of Mission Focus |
Variables |
Type of Sensor(s) |
Coverage |
Spatial Resolution |
Frequency |
Synergies with Other Panels |
Related Planned or Integrated Missions |
Surface deformation |
Strain accumulation in seismogenic zones; volcano monitoring; stress changes and earthquake triggering; hydrocarbon reservoir monitoring; landslides; solid-Earth dynamics |
InSAR |
Global |
50–75 m |
~weekly |
Climate Ecosystems Water |
DESDynl |
Surface composition and thermal properties |
Volcano monitoring; hydrocarbon, mineral exploration; assessment of soil resources; landslides; solid-Earth dynamics |
Hyperspectral visible and near IR, thermal IR |
Global; pointable |
50–75 m |
30 day, pointable to daily |
Ecosystems Water |
HyspIRI |
High-resolution topography |
Landslides; floods; solid-Earth dynamics |
Imaging lidar |
Global |
5 m |
Monthly to occasional |
Ecosystems Water |
LIST |
Temporal variations in Earth’s gravity field |
Groundwater storage; glacier mass balance; ocean mass distribution; signals from post-glacial rebound, great earthquakes |
Microwave or laser ranging |
Global |
|
~Monthly |
Climate Water |
GRACE-II |
Oceanic bathymetry |
Seafloor topography |
Altimeter |
Global |
~6 km |
|
Climate Ecosystems Health Water |
SWOT |
Summary of Data Needs
For each of the science themes listed in Table 8.1, ground-based measurements are already in use. However, the coverage offered by satellite measurements would extend ground measurements to global scales and provide documented time histories of change. Although intense land-based monitoring at a given location might yield insights for a given system, the recurrence interval for individual hazards is long, and such monitoring will not yield the multiplicity of data needed for true advances in prediction. Global monitoring that allows observations of many hazards over a short period will drive the improvements in prediction that decision makers need. Table 8.2 summarizes the measurements that will contribute most markedly to the subthemes listed in Table 8.1.
PRIORITY MISSIONS
The mission concepts recommended by the panel are based on a long and thoughtful planning process. The community engaged in research on solid-Earth hazards, natural resources, and dynamics has traditionally focused on NASA-sponsored observations of Earth using space-based techniques, such as the Crustal Dynamics project (NASA, 1991). That community has carried out a series of planning exercises, starting with the Williamstown report (Kaula, 1969). The most recent formal assessment of current capabilities and future needs culminated in the release of the SESWG report (SESWG, 2002).
In preparation for writing its report, the panel considered the SESWG report, the NRC review of the SESWG Report (NRC, 2004), inputs from the RFI process (see Appendixes D and E), and presentations made by leaders in the community and in relevant federal agencies. The suggestions were evaluated on the basis of their potential to transform science and to promote societal applications and their associated risk, including degree of readiness and cost. The panel also considered whether the proposed measurements addressed international or national needs. On that basis and in keeping with the requirement for maintenance of a robust geodetic network infrastructure, six measurement needs and conceptual missions were evaluated in some detail: (1) measuring and monitoring surface deformation via InSAR; (2) remote sensing of chemical and thermal properties of Earth’s surface via hyperspectral and thermal imaging; (3) high-resolution (5-m) land topography via lidar; (4) improved resolution of seafloor bathymetry via satellite altimetry; (5) measuring and monitoring variations in Earth’s gravity field via a GRACE follow-on and gradiometry; and (6) measuring and monitoring variations in Earth’s magnetic field via satellite, balloon, and UAV observations.
The panel deliberations enabled a prioritized list of mission concepts. The panel agrees with the SESWG report’s conclusion that a dedicated L-band InSAR mission is the highest-priority mission for solid-Earth hazards, resources and dynamics. The panel goes beyond the SESWG report in setting priorities for additional missions. The panel also notes, as did the SESWG report, that a robust geodetic infrastructure is a prerequisite for a wide array of missions that depend on precise tracking: this infrastructure is needed by many communities both within and outside the solid-Earth sciences.
Mission to Monitor Deformation of Earth’s Surface
Mission Summary—Surface Deformation
Variables: |
Strain accumulation in seismogenic zones; volcano monitoring; stress changes and earthquake triggering; hydrocarbon reservoir monitoring; landslides; solid-Earth dynamics |
Sensor: |
InSAR |
Orbit/coverage: |
LEO/global |
Panel synergies: |
Climate, Ecosystems, Water |
The science challenges related to observing surface deformation can be met through the use of repeat-pass interferometric synthetic aperture radar (InSAR) from an orbital platform. This mission would yield spatially continuous maps of ground displacements over wide areas at fine resolution and with subcentimeter accuracy. The technical requirements for a radar mission capable of meeting these goals are (1) L-band wavelength; (2) approximate weekly repeat cycle; (3) sensitivity at the millimeter scale; (4) tightly controlled orbit to maximize usable InSAR pairs; and (5) both left- and right-looking capability for three-dimensional vector displacement capability, rapid access, and more comprehensive coverage.
Some added objectives would be possible with the following technology enhancements: (1) ScanSAR operation for wide swaths; (2) increased power and storage to operate 20 percent of the orbit on average; (3) fully calibrated amplitude and phase data for polarimetry; (4) multi-wavelength capabilities; C- and L-band imagery to provide the necessary control to map ice sheet dynamics; and (5) along-track interferometry for ocean surfaces and other fast-moving objects.
The InSAR mission recommended by the panel would be a major technological advance over existing systems (Table 8.3), which were not designed to measure centimeter-level Earth deformations. InSAR would offer short repeat intervals for two important reasons: they would resolve fine space-time details of deformation events and would also allow multiple averaged acquisitions to lessen single-acquisition noise caused by atmospheric propagation variations. Such noise limits the precision of current systems to centimeter or poorer accuracy in regions of even moderate humidity (Massonnet and Feigl, 1995; Goldstein, 1995; Zebker et al., 1997).
Use of the L-band avoids much of the temporal decorrelation that plagues shorter-wavelength systems (Zebker and Villasenor, 1992). Dual-frequency observations allow correction for ionospheric propagation variations. A ScanSAR mode, designed to allow interferometric comparison of 330-km instead of 110-km swaths, could triple coverage on selected data acquisitions, so that either more frequent observations or more coverage could be obtained.
Data availability is another problem that limits the usefulness of the current generation of radar satellites. The principal bottlenecks are reliance on centralized receiving stations and processor facilities, and data policy. A new mission should address data availability by using a radically different, distributed ground system approach. Technological advances in communications, computers, and interferometric
TABLE 8.3 Comparative Interferometric SAR Characteristics
Sensor Characteristic |
ALOS |
ERS/Envisat |
RADARSAT |
Desired InSAR |
Signal-to-noise ratio |
Moderate |
Moderate |
Moderate |
High |
Coverage |
Good within station masks |
Good within station masks |
Few repeat-pass areas |
Global |
Orbit control |
Good |
Moderate |
Moderate |
Excellent |
Orbit knowledge |
Excellent |
Good |
Moderate |
Excellent |
Atmospheric propagation effects |
Poor |
Poor |
Poor |
Good (can average many passes) |
Ionospheric propagation effects |
Poor |
Good |
Good |
Very good (differential band correction) |
Temporal correlation |
Good (L band) |
Poor (C band) |
Poor (C band) |
Good (L band) |
Data availability |
Moderate |
Moderate |
Costly |
Excellent |
Wide swath for greater coverage |
ScanSAR but no interferometry |
Interferometric ScanSAR |
ScanSAR, no interferometry |
Triple-width swath (experimental ScanSAR interferometry) |
signal processing now allow the development of very inexpensive receiving, processing, and distribution nodes that can then be networked together to form a worldwide data system.
To begin to understand earthquakes requires spatially distributed vector measurements of surface displacement with absolute single-component errors of 5 mm measured over 5-year intervals. This accuracy can be achieved by averaging multiple observations. Such vector measurements require observations from at least three directions, which can be achieved by observing from both the left and the right sides during ascending and descending passes (four directions). For objectives related to ice deformation, a deformation rate of 1 m/yr implies a displacement accuracy of 11 mm, which is easily achieved by averaging only a few interferograms. Volcanic studies require less accuracy but better temporal resolution. The single-observation accuracy of 3–14 mm over length scales of 25–100 km is sufficient to meet volcano-related objectives.
In summary, an InSAR mission can meet Earth deformation science objectives with an SAR system aboard a single dedicated spacecraft. The wavelength of operation should be the L band with at least 80-MHz separation, providing ionospheric corrections similar to the L1/L2 GPS approach. In addition, the orbit should be measured to an accuracy of better than a few centimeters with on-board GPS systems and should be maintained within a 250-m tube, which would guarantee that every image will be interferometrically valuable. In other words, with such instrumentation, interferograms documenting centimeter-scale changes will document regional and ongoing deformation. The side-looking antenna should point to either side of the orbit plane, ensuring the displacement measurements needed for such maps. The spacecraft should fly on a tightly controlled, exact-repeat Sun-synchronous polar orbit at an altitude of approximately 800 km to accommodate 3- and 8-day repeat periods. In that orbit the ground separation between orbit tracks is roughly 330 km at the equator. In the 8-day repeat phase, with an average radar swath of 110 km that is steerable over a 330-km range, every point on the Earth will be imaged from one of three repeated orbits every 8 days. Coverage of any specific area from an exactly repeated orbit will be provided every 24 days.
Earth Surface Deformation Mission Contributions
New science: |
Global, fine-resolution map of strain accumulation, subsidence from water and hydrocarbon extraction, and characterization of earthquake, volcano, and landslide natural hazards |
Applications: |
Earthquake risk assessment, volcanic hazard prediction, monitoring of changes in groundwater and hydrocarbon reserves |
Mission to Observe Surface Composition and Thermal Properties
Mission Summary—Surface Composition and Thermal Properties
Variables: |
Volcano monitoring; hydrocarbon exploration; mineral exploration; assessment of soil resources; landslides; solid-Earth dynamics |
Sensors: |
Hyperspectral visible and near IR, thermal IR |
Orbit/coverage: |
LEO/global access |
Panel synergies: |
Ecosystems, Water |
Many solid-Earth problems that can be addressed by remote sensing from Earth orbit are in the category of environmental geology. The effects to be studied manifest themselves in change at Earth’s surface, and in particular over the upper micrometers that can be observed by remote sensing. The changes in surface geochemistry or surface temperature patterns provide clues to processes in the subsurface.
This mission would use two sensors that represent a considered compromise between requirements for measurement and feasibility of implementation at reasonable cost. Researchers sometimes express desires
for spatial and spectral resolution as well as swath coverage and revisit times that are not compatible with technical feasibility, space-to-ground communication bandwidth or budgeted costs. Fortunately, the decadal survey’s RFI produced many proposed missions that were well conceived and had realistic costs. With these realities in mind, the panel established the following set of requirements to provide data and information to answer many of the questions stated above.
The core requirement is for a hyperspectral imaging sensor operating at 400–2500 nm. Many applications have been developed with airborne sensors such as the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) (Green et al., 1998). Efforts in mapping of alteration minerals to show potential debris-flow source areas on volcanos (Crowley et al., 2003), asbestos in soils in California (Swayze et al., 2007), acidic mine waste (Swayze et al., 2000), swelling soils along the Front Range in Colorado (Chabrillat et al., 2002), agricultural soil properties (Ben-Dor, 2002), carbon in soils (Cozzolino and Moron, 2006), and mineral exploration (Gingerich et al., 2002) all have similar requirements for spectral and spatial resolution and signal-to-noise ratio. These studies could not have been accomplished with multispectral sensors.
One of the major challenges of imaging spectrometry is the high data rate resulting from the acquisition of images in hundreds of contiguous spectral bands. In the past, spatial resolution at 30-m pixels and swath widths of 30 km or less have been proposed. The only hyperspectral imager in Earth orbit is Hyperion, and its swath width is 7 km (Ungar et al., 2003). Given recent advances in detectors, optics and electronics, however, it is now feasible to acquire pushbroom images with 620 pixels cross-track and 210 spectral bands. Mouroulis et al. (2000) describe such an instrument design that allows 45-m pixels at nadir resulting in a 28-km swath. By using three spectrometers with the same telescope, a 90-km swath results when Earth’s curvature is taken into account.
The hyperspectral system described above is exactly that being proposed by the ecology panel for ecological studies. Both require a pointable imager—for solid Earth, to accommodate high-temporal-resolution measurements of volcanos for monitoring purposes. Given the high likelihood of short-time-frame predictability for volcanic eruptions through crater-based monitoring, pointability of a sensor has great potential for saving lives and mitigating destruction in areas that are volcanically active.
For volcano monitoring and eruption prediction, experience has been gained using the ASTER instrument on the Terra spacecraft. ASTER is a multispectral sensor with, among others, five bands in the 8- to 12-µm thermal infrared region. Pieri and Abrams (2005) showed that it is possible to detect subtle changes in heat flow causing snowmelt in the otherwise snow-covered slopes of the Chikurachki volcano on Paramushir Island, Russia, prior to an eruption. The pixel size in the thermal channels is 90 m. The requirements for volcano-eruption prediction are high thermal sensitivity, on the order of 0.1 K, and a pixel size of less than 90 m. An opto-mechanical scanner, as opposed to a pushbroom scanner, would provide a wide swath of as much as 400 km at the required sensitivity and pixel size. Placement of the thermal multispectral scanner on the same platform with the hyperspectral imager described above would provide a new level of understanding of the problems discussed above and at the same time provide data for ecology and other disciplines.
Surface Composition and Thermal Properties Mission Contributions
New science: |
Surface composition from maps of fine-resolution hyperspectral observations in optical and near-infrared, thermal emissivity and thermal inertia, mapping of gas release from processes at depth |
Applications: |
Volcanic hazards, resource exploitation and extraction, ecological drivers |
Mission to Measure High-Resolution (5-m) Topography of the Land Surface
Mission Summary—High-Resolution Topography
Variables: |
Landslides; floods; solid-Earth dynamics |
Sensor: |
Imaging lidar |
Orbit/coverage: |
LEO/global |
Panel synergies: |
Ecosystems, Water |
The topography of Earth’s surface is a fundamental property is relevant to all manner of physical processes. Meeting the science goals of high-resolution topographic studies requires mapping the global land surface on a 5-m grid with decimeter vertical accuracy. The preferred technology to achieve these objectives is imaging lidar. (InSAR could also provide global mapping capabilities at somewhat lower precision, which would still represent a major improvement over what is currently available.)
Global topographic data are available at 30- to 90-m resolution, with vertical accuracy of several meters. As Figure 8.3 illustrates, the proposed high-resolution topography mission would give Earth scientists literally a new view of Earth’s surface. At 30- to 90-m resolution, many important topographic features are obscured, including many stream channels, floodplains, hillslopes, and landslide deposits. However, these same features are clearly visible at 5-m resolution, making it possible not only to map natural hazards, but also to detect changes in surface topography through time, and to better understand the processes that shape Earth’s surface.
Lidar systems permit very precise (<10 cm height error) mapping from space. Lidar has already been used to globally map the surface of Mars (Smith et al., 1999, 2001) and will be used to map the Moon at even higher resolution than Mars in 2009. Interestingly, lidar has not yet been used to map Earth’s continents, and the topography of Mars is now known at far finer resolution than Earth’s topography.
While earlier-generation space-based laser systems such as the shuttle laser altimeters (Garvin et al., 1998) were generally single-beam systems that collected profiles of the surface along the spacecraft ground track, emerging technology will enable spatial-elevation mapping. Three approaches could enable spatial mapping of Earth’s surface from an orbital platform. The first uses a single laser beam and a scanning mechanism to spatially map the surface, as demonstrated by the GSFC airborne laser vegetation imaging sensor (LVIS; Blair et al., 2001). Analysis indicates that kilohertz-ranging rates could be achieved from an orbital scanning system (Degnan, 2002). The second approach splits a single beam into numerous parts via a diffractive optical element, and separate detectors are used to measure elevation in each backscattered beam; this approach is being implemented in the design of the lunar obiter laser altimeter (Smith et al., 2006), to be flown on the Lunar Reconnaissance Orbiter to be launched in 2008.
The third approach uses a single laser beam to illuminate a broad swatch of surface and a pixilated detector in which each pixel makes a time-of-flight measurement. An example that uses this approach is the Lincoln Laboratory JIGSAW airborne system (Heinrichs et al., 2001), and analysis has shown that 5-m mapping of the Moon could be achieved in 2 years with an adaptation of this system (Zuber et al., 2004). Study will be required to determine the optimal technological approach for the high-resolution topographic mapping mission. In any case, megabit to gigabit data rates will need to be managed during mapping operations.
Cloud cover will limit the coverage available from each individual pass, so multiple passes will be required for complete coverage. A relatively long mission lifetime may be needed to achieve the desired spatial density and coverage, and repeated measurements over several years would facilitate detecting surface changes such as topsoil losses to erosion.
Lidar measurements can be corrected for many vegetation effects, in that the full-range profile at each post can be recorded. Thus the structure of the vegetation canopy could also be mapped at high spatial resolution, along with the underlying topography, which would provide an improvement over the sparser sampling that would be obtained earlier in the decade by the DESDynl mission. Because the height precision of lidar is unsurpassed, it is the preferred method for a topographic mission. However, the mission is not intended to be flown until late in the decade, allowing time to invest in technology development before a final selection must be made.
InSAR has been used for both local (TOPSAR, GEOSAR) and global (SRTM) topographic mapping and is capable of retrieving elevation data from precise parallax measurements by using two radar antennas. Although the highest precision results from systems with two antennas on one platform, repeat-pass orbit geometries have realized 5-m height accuracy (Zebker et al., 1994). The Shuttle Radar Topography Mapping (SRTM) mission used two antennas simultaneously to minimize atmospheric propagation effects and mapped Earth at arcsecond (30-m) posting. The German space agency DLR plans to acquire global topography with 12-m posting and 2-m vertical precision via the tandem X-band InSAR (TanDEM-X), scheduled for launch in 2009. For the high-precision topographic mission, the posting and vertical precision could be improved to 5 m and 1 m, respectively, by using a dual-antenna system or two satellites flying in tandem. Multiple passes of a single-antenna system could provide areal coverage in regions not subject to limiting temporal decorrelation. The panel recommends pursuing the lidar mission because of its greater accuracy and complementary use for improving measurements of ecosystem structure, but data from TanDEM-X would allow important progress to be made before the lidar mission is flown later in the decade.
High-resolution Topography Mission Contributions
New science: |
High-resolution, high-precision topographic data, in most cases with vegetation effects quantified and removed |
Applications: |
Geomorphology, landslide hazards, flooding, hydrology, ecology |
Mission to Monitor Temporal Variations in Earth’s Gravity Field
Mission Summary—Temporal Variations in Earth’s Gravity Field
Variables: |
Ground water storage; glacier mass balance; ocean mass distribution; signals from post-glacial rebound, great earthquakes |
Sensors: |
Microwave or laser ranging |
Orbit/coverage: |
LEO/global |
Panel synergies: |
Climate, Water |
The problem of temporal variations in Earth’s gravity field is inherently interdisciplinary. The largest variations on timescales of months to decades are associated with the water cycle (see Chapter 11). Changes in ocean circulation also result in mass variations that are associated with changes in the gravity field. Observations of temporal changes in Earth’s gravity can provide important information about solid-Earth dynamics.
The largest signal from solid-Earth processes is the variation associated with postglacial rebound, which leads to substantial secular increases in the gravity field over formerly glaciated regions, including the region surrounding Hudson Bay in Canada; Scandinavia; Antarctica; and Greenland. The pattern and amplitude of predicted secular changes in gravity are sensitive both to Earth’s radial and lateral variations in viscosity and to the details of the ice load history. Combining observations of changes in gravity with observations of deformation of Earth’s surface improves the ability to constrain models and to separate
changes in gravity caused by postglacial rebound from changes caused by ongoing redistribution of water and ice mass.
Great earthquakes cause large redistributions in mass that lead to observable changes in Earth’s gravity field. Monitoring the postseismic relaxation of these mass changes would provide unique information about Earth’s viscosity structure in subduction zone regions and make valuable contributions to understanding of the variation of stress with time.
Even time-varying processes associated with the generation of the geodynamo in the fluid core result in variations in the gravity field. These include elastic deformation of the overlying mantle and crust associated with dynamic pressure variations at the core-mantle boundary and rotation of the aspherical inner core caused by torques from the geodynamo. Although these signals are weaker than those from redistribution of water mass at Earth’s surface, they can be recognized because they have distinct spatial patterns.
The Gravity Recovery and Climate Experiment (GRACE), a collaboration between NASA and the German space agency to monitor temporal variations in Earth’s gravity field, was launched in 2002 with a mission life now estimated as 9 nine years. Already signals from postglacial rebound beneath Hudson Bay are visible. However, regions of ongoing postglacial rebound are typically regions where variations in ice mass and water storage are also substantial, so the solid-Earth and hydrologic signals are mixed together. In order to separate these two signals, a multidecade period of observation is required, which requires a follow-on mission to GRACE. Any gap in coverage between GRACE and GRACE-II will disrupt the time series of observations, complicating its interpretation.
The change in gravity from the great 2004 Sumatra earthquake has also been observed by GRACE. Monitoring the temporal variation of this feature is crucial. It is also important to improve the spatial resolution of the measurements of the time-varying gravity field. For each improvement in spatial resolution by a factor of three, an order of magnitude more earthquakes will be observable.
Temporal Variations in Earth’s Gravity Field Mission Contributions
New science: |
Separation of time-varying gravity signal from postglacial rebound from changes caused by ongoing redistribution of water and ice mass; monitoring of postseismic relaxation |
Applications: |
Geodynamic studies, improved estimates of tide gauge motions |
Mission to Measure Oceanic Bathymetry
Mission Summary—Oceanic Bathymetry
Variables: |
Seafloor topography |
Sensors: |
Altimeter (nadir or swath) |
Orbit/coverage: |
LEO/global |
Panel synergies: |
Climate, Ecosystems, Health, Weather |
Variations in the pull of gravity caused by seafloor topography cause slight tilts in ocean surface height, measurable by satellite altimeters. Estimates of seafloor topography from previous altimetric missions have led to spectacular global bathymetric maps with spatial resolution down to ~12 km (e.g., Smith and Sandwell, 1997). These altimetric missions have had nadir-pointing radars flown in repeat orbits, and the spatial scale has been limited by the distance between orbits. Higher-resolution measurements could be obtained by flying a nadir-pointing altimeter in a nonrepeating orbit or by swath altimetry, as in the SWOT mission discussed in Chapter 11.
Doubling the spatial resolution of data on seafloor topography would improve understanding of the geologic processes responsible for ocean-floor features, including abyssal hills, seamounts, microplates, and propagating rifts. It would improve tsunami hazard forecast accuracy by mapping the near-field ocean topography that steers tsunami wave energy. Determining the distribution of seafloor roughness would improve models of ocean circulation and mixing. Bathymetric maps have numerous other practical applications, including navigation (on January 8, 2005, a billion-dollar U.S. nuclear submarine ran at full speed into an uncharted seamount), reconnaissance for submarine cable and pipeline routes, improvement of tide models, and assessment of potential territorial claims to the seabed under the United Nations Convention on the Law of the Sea.
Ocean Bathymetry Mission Contributions
New science: |
Geologic processes responsible for ocean floor features, distribution of seafloor roughness |
Applications: |
Tsunami hazard forecasts, ocean circulation, navigation |
Monitoring the Geomagnetic Field
Understanding the origin of Earth’s magnetic field was ranked by Albert Einstein as among the three most important unsolved problems in physics. Although it is now known that the magnetic field is generated in the convecting metallic outer core, where self-generating dynamo action maintains the field against decay, the detailed physics by which the dynamo operates is not well understood. Researchers do not know how much longer the current rate of decay of the dipole field, sufficient to eliminate the dipole field in 2000 years, will go on. This is of more than academic interest since it is the magnetic dipole field that shelters Earth from bombardment by charged particles from space. On shorter time scales, the ongoing dipole decay is connected to the South Atlantic magnetic anomaly, where the field at Earth’s surface is now about 35 percent weaker than average. This “hole” in the field affects the radiation dosage experienced by satellites in low-Earth orbit.
Advances in understanding the geodynamo rely on global observations of the geomagnetic field and its temporal changes to constrain ever-more-sophisticated numerical models of magnetohydrodynamics. In recognition of the importance of this rapidly advancing scientific discipline, the SESWG report (2002) recommended improved access to and analysis of existing observations, as well as flying constellations of satellites in varying orbits in order to better determine future changes in the global magnetic field. The NRC review of the SESWG report (NRC, 2004) strongly supported these recommendations and noted that the SWARM mission (http://www.esa.int/esaLP/LPswarm.html) planned for launch by the European Space Agency in 2009 would largely satisfy the SESWG goals.
The solid-Earth panel concurs that this field is in the strong position of having the acquisition of important satellite data already committed to by international collaborators. It is important for NASA to make significant contributions to these missions, as well as to ensure that U.S. scientists have access to the data. Later in the decade it will be important to reassess the situation and plan future missions.
OTHER SPECIAL ISSUES
This section describes additional observing priorities that NASA should consider. They would support the main science objectives discussed above, but with non-space-based technology. The maintenance of the terrestrial reference frame is discussed above. This section concentrates on suborbital platforms, international collaborations, and policy issues.
The Role of Suborbital Remote Sensing
Many problems in Earth science require global data; however, some important problems require higher spatial or temporal resolution in specific regions. Many applications could benefit from tactical deployment of manned or unmanned aerial platforms and instruments. Examples are rapid-repeat observations of deformation of active volcanos using InSAR, IR, and hyperspectral measurements; observation of post-seismic deformation from recent earthquakes; and observation of transient events related to localized floods, landslides, and other disasters. All of these augment and localize the synoptic work described above, extending the science objectives listed previously.
The solid-Earth panel believes that it would be valuable for NASA to develop technologies implemented on operational airborne platforms to augment the space-based program. In particular, repeat-pass InSAR on a UAV with real-time interferogram generation would be invaluable for directed study of rapidly changing surfaces. Rapid deformation before or after earthquakes or during volcanic eruptions could be analyzed suborbitally on time scales not easily sampled with spacecraft.
The use of stratospheric platforms for in situ and remote Earth science measurements warrants revolutionary concepts. NASA contracted with Global Aerospace Corporation to lead a small study to evaluate the capabilities of the candidate platforms to meet NASA’s Earth science objectives. The fields in which the platforms are expected to have substantial effects include atmospheric chemistry, Earth radiation balance, and geomagnetism. Potential platforms include ultra-long-duration balloons (ULDBs), other balloons, airships, UAVs, and crewed aircraft. Of those, ULDBs are by far the most affordable.
Individual stratospheric balloon platforms, built in quantity, are estimated to cost less than 1 percent of the cost of a satellite. A constellation of 100 could give synoptic coverage for the cost of a single space satellite. Instrumentation could be recovered to allow postflight verification. As technology advances, balloon platforms offer ease of upgrade through recovery and relaunch of payloads. The cost of a single guided-balloon mission configured for the crustal magnetic-field measurement mission is estimated at about $3 million, not including advanced technology development, for a 100-day flight after the appropriate technology is developed. Because the mission cannot be accomplished with current space satellites or other current stratospheric platform technologies, its cost-to-benefit ratio is very high.
Suborbital magnetic studies hold particular promise for answering a number of interesting questions including, (1) What are the natures of the upper, middle, and lower crust? (2) How is the South Atlantic magnetic anomaly changing? (3) What is the sub-ice circulation in polar regions? (4) What are the stratospheric/atmospheric processes with magnetic signatures? (See, for example, http://core2.gsfc.nasa.gov/research/mag_field/purucker/huang/RASC_WorkshopReport_final.pdf.) Obviously, these questions overlap with questions in climate science as well as environmental sciences related to space weather phenomena.
There are two reasons why suborbital observations are relevant for studying those questions. First, data recorded at stratospheric altitudes would fill an important gap in bandwidth that cannot be filled with compiling measurements from satellite platforms or airborne platforms; at stratospheric altitudes, processes in the crust can be measured directly. Second, stratospheric missions, such as ULDB missions, are of low cost relative to satellite missions and could provide efficient and wide-ranging observations over a relatively short period of time.
The advantages of using “stratospheric satellites” are that observations at stratospheric altitudes allow the separation of various components of Earth’s magnetic field. In addition such observations allow for the inclusion of intermediate spatial wavelength information to existing surface and satellite surveys. Stratospheric platforms can enable long-term coverage over hard to access sites and provide space weather event warnings for polar satellites.
International Collaborations
Radar Observations
A number of international colleagues have developed space-borne radar sensors over the past decade, including the European Space Agency (ESA) with the ERS and Envisat satellites, Canada with its RADARSAT satellite, and Japan with the ALOS system (see addendum to this chapter, “International Cooperation: The Case for a U.S. InSAR”). These are mainly short-wavelength radars emphasizing radar imaging rather than the deformation-measuring capability of InSAR. Although these satellites provide important information on an “as available” basis, there are three serious problems: (1) short-wavelength radar rapidly loses phase coherence over areas of vegetation, and so its applicability is limited mainly to arid regions; (2) short-wavelength sensors do not provide useful constraints on ecosystem structure; and (3) many conflicting demands for scheduling observations severely limit acquisitions of images for the science described in this chapter. Because partner agencies have invested in short-wavelength radar, it remains for the United States to develop the technically more challenging long-wavelength sensors that better maintain phase coherence and are also useful for obtaining information on ecosystem structure. If U.S. sensors are flown coincident with the international platforms, the microwave spectrum will be covered and the maximum science return can be obtained.
Magnetic Field Observations
Observations of spatial and temporal variations in Earth’s magnetic field will be dominated in the next decade by international missions such as SWARM. It is crucial for NASA to facilitate participation and access to the data for U.S. scientists.
End-to-End Systems for Integrating Observations to Decision Making
It is also imperative that technological advances be tightly integrated with the policy infrastructure so that the science return can be adequately incorporated into important decisions, whether for hazard mitigation, national security, or the sustenance of life on Earth; the science proposed here is critical to informed policy making. However, simply making the observations and measurements is not enough to answer essential questions. To fully reap the rewards and benefits of an integrated and focused system of Earth observations requires that comparable investments be made in an integrated analysis of the data—across disciplines, across missions, and across other space programs.
In Situ Observations
Space-derived data provide a global synoptic view of the processes studied, but many projects require input from field observations. A strong field component of any of the science presented here can provide information that is unavailable or difficult to obtain from space. Seismic networks, continuous GPS networks to provide sampling of higher-frequency deformation, and ground-based measurements of soil erosion are notable examples.
As emphasized in the overview at the beginning of this chapter, high-precision global networks of GPS, very-long-baseline interferometry, and satellite laser ranging provide the foundation for virtually all space-based and ground-based observations of Earth. The terrestrial reference frame is realized through integration of those observing systems, and it is through this reference frame that all measurements can be
interrelated for robust, long-term monitoring of global change. A precise reference frame is also essential for interplanetary navigation and diverse national strategic needs.
Synergistic Observations from Other Panels
Spatially dense crustal-deformation measurements are the primary data need recognized by the solid-Earth panel. Acquisition of the data is also a high priority of the climate and ecosystem panels, specifically for the observation of ice flow in the polar ice sheets and characterization of vegetation canopy structure and biomass. All three panels have endorsed a conceptual baseline mission that operates at wavelengths of 6–24 cm, with 24 cm the preferred wavelength for natural-hazards applications in the solid-Earth field. In addition, augmentations to the baseline mission and refinement of the parameters would add appreciably to the utility of InSAR in the climate and ecosystem fields.
The climate panel requires InSAR data for observation of ice-sheet flow and dynamics, specifically to address the role of glaciers and ice sheets in sea-level rise and possible changes in Earth’s climate. The data are used to map ice velocity and discharge by ice streams and glaciers worldwide and to quantify their contributions to sea-level rise. InSAR data will help to characterize the temporal variability in ice flow well enough to separate short-term fluctuations from long-term change. InSAR will also identify fundamental forcings and feedbacks on ice-stream and glacier flow to improve the predictive capability of ice-sheet models.
Most research to date has been carried out at a shorter, 6-cm wavelength (C band), but theoretical models show that the ice objectives can be met with the 24-cm wavelength preferred by the solid-Earth community. The longer wavelength will penetrate 100 m or more into dry snow so that the measured signal is from a deeper region than the 20 m usually seen with the 6-cm wavelength. Multiple frequencies would allow profiling of the ice motion and structure with depth. Hence one possible improvement is the inclusion of a second frequency on the radar platform; this would result in a more capable and versatile instrument, albeit at a cost in complexity and budget.
For the ecosystem dynamics panel, one major uncertainty is the three-dimensional structure of vegetation on Earth’s terrestrial surface and how it influences habitat, agricultural and timber resources, fire behavior, and economic value. InSAR is one valuable tool for characterizing structure, inasmuch as the waves that penetrate the canopy have a different phase in the radar echo from those reflected off the top of the canopy. Those differences are even more apparent if the polarization of the reflected signal is recorded.
In the case of vegetation studies, the longer wavelength of 24 cm is preferred because it penetrates deeper into the canopy and the return does not saturate at low biomass values. However, the desire to separate scattering mechanisms with polarization makes a polarimetric addition to the instrument desirable for ecosystem research. Although many objectives can be met with the single polarization instrument proposed by the solid-Earth panel, a polarimetric instrument would return more scientific benefit.
All the above are advantages of the multiple use of InSAR measurements. Potential scheduling conflicts could arise, however, from multiple requests for the instrument at the same time. The objectives of three panels mentioned can be satisfied with the return orbit of 1–2 weeks, so that is not likely to be a planning problem. No substantial conflict in operation is foreseen, because the geographic regions of most interest to the communities are largely disjoint. The ice community requires data acquisitions over Greenland and Antarctica. The solid-Earth scientists need data acquired over active tectonic areas, mainly the Pacific rim, and the Alpine-Himalaya belt. Major forests are in tropical Asia, Africa, and South America—some overlap occurs in the southwestern Pacific region with seismic and volcanic activity. Volcanos often are in areas of ecological interest, and so coordination in radar modes and frequency of coverage will have to be addressed for these sites.
SUMMARY
Sustaining quality of life necessitates a thorough understanding of the physical and chemical processes that shape Earth. Cooperating with natural processes and planning for hazards and other catastrophes prudently will minimize loss of life and property. Successful exploitation and discovery of energy and mineral resources will pose an increasing challenge. Thus, there is a critical need to understand, assess, and predict catastrophic events—such as earthquakes, volcanos, and floods—and continue to mine energy and other natural resources from Earth. Detailed and accurate measurements of the surface are needed to analyze and manage Earth and the fragile water and soil resources that sustain life. Because hazardous events happen only infrequently at any one location, there is a need for global observation capacity.
The panel has identified three space missions as crucial: an InSAR mission to accomplish global characterization of the deformation of Earth’s crust, a hyperspectral optical and near-infrared mission to observe and record surface composition and thermal properties, a mission to measure land-surface topography precisely. Missions to determine long-term variations in Earth’s gravity field, to determine ocean bathymetry with improved spatial resolution, and to observe the spatial and temporal variations in the geomagnetic field are also important. Improvements in and continued operation of the global tracking network are crucial for the success of all satellite missions. Suborbital and field programs would also continue to play a vital role in managing Earth. Those supporting measurements and analyses are needed for the development of national and international policies and for informed public decision making. The missions proposed here will be valuable not only to solid-Earth science but also to several other communities. The ecology, hydrology, and climate panels in particular will find substantial benefit in all three of the highest-priority missions.
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ATTACHMENT
International Cooperation: The Case for a U.S. InSAR
Many nations are pursuing space-based radar programs. However, for a variety of reasons, it is at best uncertain if these programs can return the quantity and kind of data required to meet the science objectives discussed in this report. Furthermore, many of these systems exist only as concept studies. Given below is a brief assessment of the usefulness of several of these systems for the crustal deformation-, climate-, and ecology-related monitoring and commercial applications important for the nation to undertake:
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ALOS. This L-band satellite, listed in Table 8.3, was launched by Japan in early 2006 and is currently operating. The data quality appears high, and, after some trouble with controlling the orbit, the satellite is now delivering test data to the calibration/validation team. ALOS, in a 41-day repeat cycle, will image much of east Asia several times per year. However, it will not image the U.S. swaths more than once or twice per year over its 5-year lifetime due to data rate constraints.
A U.S. interagency working group is trying to offer NASA data-relay capabilities to JAXA to increase coverage over the United States, but it has not yet succeeded. Thus, while these data can yield some engineering studies for L-band SAR, the temporal density is an order of magnitude too sparse to eliminate atmospheric interference or to give insights into transient phenomena. In any case, ALOS will be at the end of its functional lifetime before a new satellite can be launched by the United States, and so it is at best a stop-gap engineering mission.
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HJ-1 satellites. China has an ambitious plan to orbit up to 10 radar satellites (4 of which form the HJ-1 series) over the next 10–15 years. Reports by word of mouth that the first satellite was launched last April have not been substantiated in existing Web-reachable documents. It is reputed to have been an L-band system, and the orbit, repeat cycle, and capabilities of the sensor are not widely known. Published reports state that the next two satellites to be launched will be a pair of S-band radars in 2007; these are possibly nearly as effective as L-band radars in reducing decorrelation. However, the panel considers it unlikely that enough data will be made available to the U.S. science community to address its science objectives, and in any case does not see how there will be sufficient participation by U.S. scientists to define the proper orbits and coverage to begin to meet U.S. needs. If U.S.-Chinese relations change drastically, and NASA agrees to support the Chinese space program significantly, then of course these satellites could be useful.
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Arkon-2. Arkon-2 is a military system with three radar frequencies. No U.S. scientists are known to have been asked to join a Russian team to plan for scientific use of the sensor. It is possible that the Russian team could decide to place the radar in an orbit useful for scientific radar remote sensing investigations, rather than in a militarily useful orbit, and then sell the data commercially. If that is the case then the United States could consider a make/buy decision on data. Past experience has been that Russian radar data products do not satisfy the science community’s needs with respect to data volume, satellite tasking, orbit geometries, and, most importantly, data quality.
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MAPSAR. MAPSAR is a Brazilian radar designed for equatorial coverage of the Amazon region. Even if the capacity of the sensor could be increased and U.S. scientists could acquire satellite data for their use, the conflicts regarding orbit configuration and data allocations are formidable if the same satellite is to be used for the polar regions as well as the Amazon. This is Brazil’s first imaging radar satellite system, and it is difficult to assess whether it will be capable of delivering the amount, type, and quality of data needed to monitor and characterize hazards and to address environmental, climatic, and commercial needs.
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Sentinel-1. This ESA system, based on a C-band radar, cannot address science needs that require a long-wavelength system. Despite Europe’s long history with SAR, the United States should have access to a longer-wavelength system to enable the important next steps described above in this chapter. The nation has benefited from using ESA radars over the last 15 years and will continue to benefit from them in the future; however, a change in technology is needed to achieve real breakthroughs. Although operating future Sentinel radars at the L band or the even longer P band has been discussed, these are concept studies (equivalent to NASA Phase A studies) and not real systems. A commitment by NASA to a real ESA partnership has the potential for substantive cost savings.
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Other systems. RADARSAT-2, a Canadian system that will replace RADARSAT-1, is a C-band radar optimized for observations of sea ice. It cannot meet the objectives described in this chapter. TerraSAR-X and TanDEM-X are German radars operating at the even shorter X-band wavelength, and while they are very similar to existing U.S. high-resolution military technology, they likely will suffer from too much decorrelation to provide reliable InSAR over vegetated terrains. TerraSAR-X by itself cannot supply the needed data volume. TanDEM-X, operated in concert with TerraSAR-X, will probably obtain the highest-quality digital elevation model of Earth that will then exist. But it still cannot do repeat-pass interferometry, the cornerstone of all the planned major science objectives.
In summary, the panel notes that the U.S. science community continues to propose L-band InSAR because it appears to be the only known technology for meeting identified Earth observation needs. Repeatpass InSAR methods will make the fine-scale and dense measurements needed to characterize Earth for the several disciplines that have proposed it as the first priority for a new mission.