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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.
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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. 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. 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. 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. 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-
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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. 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. 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. 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-
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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.
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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?
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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? Are earthquakes predictable? How do fluids such as groundwater, hydrocarbons, and CO2 trigger earthquakes? Volcanic-eruption prediction Can a worldwide volcanic eruption forecasting system be established using remote sensing data? What pre-eruption surface manifestations are amenable to remote measurement from orbit? What surface temperature change patterns are relevant? What can the measurement of emissions such as SO2 and silicate ash indicate, and what patterns of change are relevant? How can multiple change patterns and measurements (topography, gas, temperature, vegetation) at craters be better interpreted for eruption forecasting? How often must a volcano be observed to provide a meaningful prediction? Landslide prediction Which places show slowly moving landslides, and how likely are they to fail catastrophically? Where are oversteepened slopes and susceptible rock types located? 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? How does this affect future storage capabilities of the aquifers? 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? 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? What changes in surface chemistry and thermal properties are diagnostic of hydrocarbon and mineral resources? 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? 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? What key surface geochemical indicators detectable by remote sensing are relevant to describing mining waste or landslide hazards? 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? Agricultural soil degradation What is the true extent of the loss of topsoil due to poor management practices? Can remote sensing be used to measure carbon sequestration in agricultural soils? How well can the leaching of nutrients and increasing salinization be measured by remote sensing? Can remote sensing provide the kind of information needed for policy decisions by government entities worldwide? 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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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
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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.
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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.
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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.
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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
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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.
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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. BIBLIOGRAPHY Amelung, F., D.L.Galloway, J.W.Bell, H.A.Zebker, and R.J.Laczniak. 1999. Sensing the ups and downs of Las Vegas: InSAR reveals structural control of land subsidence and aquifer-system deformation. Geology 27:483–486. Amelung, F., S.Jónsson, H.Zebker and P.Segall. 2000. Widespread uplift and “trapdoor” faulting on Galápagos volcanoes observed with radar interferometry. Nature 407:993–996. Bawden, G.W., W.Thatcher, R.S. Stein, K.W.Hudnut, and G.Peltzer. 2001. Tectonic contraction across Los Angeles after removal of groundwater pumping effects. Nature 412:812–815. Ben-Dor, E. 2002. Quantitative remote sensing of soil properties. Adv. Agron. 75:173–243. Biegert, E.K., J.L.Berry, and S.D.Oakley. 1997. Oil filed subsidence monitoring using spaceborne interferometric SAR: A Belridge 4-D case history. Proceedings of the Annual Meeting of the American Association of Petroleum Geologists, Dallas, April 1997. American Association of Petroleum Geologists, Tulsa, Okla. Bilham, R. 1988. Earthquakes and urban development. Nature 336:625–626. Bilham, R. 2004. Urban earthquake fatalities: A safer world or worse to come? Seismol. Res. Lett. 75(6):706–712. Blair, J.B., M.Hofton, and S.B.Luthcke. 2002. Wide-swath imaging lidar development for airborne and spaceborne applications. Pp. 17–19 in International Archives of Photogrammetry and Remote Sensing, Volume XXXIV-3/W4. Available at http://www.isprs.org/commission3/annapolis/pdf/Blair.pdf. Bourne, S., K.Maron, S.Oates, and G.Mueller, 2006. Monitoring deformation of a carbonate field in Oman: Evidence for largescale fault re-activation from microseismic, InSAR, and GPS. Proceedings of 68th EAGE Annual Conference and Exhibition/SPE Europec, June 12–15, 2006. EAGE Publications BV, Austria, Vienna. Chabrillat, S., A.F.H.Goetz, L.Krosley, and H.W.Olson. 2002. Use of hyperspectral images in the identification and mapping of expansive clay soils and the role of spatial resolution. Remote Sens. Environ. 82:431–445. Cozzolino, D., and A.Moron. 2006. Potential of near-infrared reflectance spectroscopy and chemometrics to predict organic carbon fractions. Soil Till. Res. 85:78–85. Crowley, J.K., B.E.Hubbard, and J.C.Mars. 2003. Analysis of potential debris flow source areas on Mount Shasta, California, by using airborne and satellite remote sensing data. Remote Sens. Environ. 87:345–358. De Rouffignac, E.P., P.L.Bondor, J.M.Karinakas, and S.K.Hara. 1995. Subsidence and well failure in the South Belridge diatomite field. Pp. 153–167 in Proceedings SPE Western Regional Meeting, Bakersfield, Calif., March 8–10, 1995. Society of Petroleum Engineers, Inc., Richardson, Tex. Degnan, J.J. 2002. A conceptual design for a spaceborne 3-D imaging LIDAR. Elektrotech. Informat. 4:99–106.
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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: 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. 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. 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. 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. 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.
Representative terms from entire chapter: