The Adaptation of Tactical Imaging Spectrometry to Applications
in Earth Science
Long-wave infrared imagery is being applied to Earth observation activities in anticipation of future needs to assess climate change impacts around the globe and assist the national security community in defining future observational requirements.
There is increasing awareness within the U.S. government that climate change has the potential for considerable and unpredictable disruption around the globe. In 2007, Congress called for a national intelligence estimate on how climate change might affect national security and consequent U.S. force projections. In 2008, the intelligence community completed this analysis; the findings pointed to important and extensive implications for U.S. national security interests within the next 20 years. The impacts are expected to exacerbate existing problems abroad in many countries struggling with poverty, social tensions, environmental degradation, ineffective leadership, and weak political institutions.
The convergence of Department of Defense climate concerns with related activities that have traditionally been pursued within the civil sector prompted The Aerospace Corporation to initiate a pilot program of Earth remote sensing observations. Aerospace has been developing the theory and practice of high-resolution spectral imaging at long-wave infrared (LWIR) wavelengths (7–14 microns) for more than two decades. The instruments developed have been applied almost exclusively to tactical applications; however, since 2008, these technologies have demonstrated their applicability to a variety of diverse topics in Earth science, such as the detection and tracking of trace atmospheric gas plumes, the mapping of surface composition and geological structure, and the characterization of geothermal activity.
Although LWIR imagery has a long history in Earth observation, the number of spectral bands employed is typically small: ten channels for MODIS (Moderate Resolution Imaging Spectroradiometer) and five for ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), two instruments currently flying on satellites as part of NASA’s Earth Observing System constellation, and only one channel for the Enhanced Thematic Mapper Plus aboard Landsat. Even the most advanced plan for a next-generation LWIR imager (the Hyperspectral Infrared Imager, or HyspIRI) calls for just seven channels, whereas considerably more knowledge could be gained by even a moderately higher resolution imager. Although remote LWIR thermometry has a long heritage and can be carried out to reasonable accuracy with very few channels, more fully resolving the thermal radiation distribution enables researchers to discern residual spectral structure, which can then be correlated against known spectral signatures to test for a variety of chemical constituents in a scene.
Natural and Anthropogenic Methane Emission
The climate science community has predominantly focused on understanding the global distribution of atmospheric carbon dioxide, which is the most prominent marker of industrial activity; however, methane is receiving growing attention because the current warming trend threatens to dramatically increase methane fluxes into the atmosphere from solid reservoirs (in the form of methane hydrates) in Earth’s oceans and permafrost regions. While carbon dioxide is significantly more abundant and remains in the atmosphere much longer, the relative potency of methane as a greenhouse agent is approximately 100 times that of carbon dioxide on decadal timescales. With methane being this much more effective at retaining heat in the atmosphere, the potential for major climate change from rising atmospheric methane levels is therefore a serious long-term issue.
The marine oil seeps in the Santa Barbara Channel off Goleta, California, are among the most productive natural methane emission sites in the world (~100 cubic meters per day). The methane, along with other liquid and gaseous hydrocarbons, is contained in geological reservoirs formed by anticlinal folding in the strata that make up the ocean floor. Breaching of the anticline crests by erosion releases these trapped hydrocarbons—a trend that has persisted for at least the last few centuries, as attested by early European explorers and the indigenous cultures of the region. As more of the submarine hydrocarbon load has been captured by petroleum production operations throughout the twentieth century, the observed levels of seepage have declined considerably. Nevertheless, atmospheric hydrocarbons originating from the Santa Barbara Channel seeps far exceeds that from all highway traffic in Santa Barbara County on an annually averaged basis.
Aerospace’s SEBASS (Spatially Enhanced Broadband Array Spectrograph System) instrument was flown over this location, focusing on an area that encompasses the most vigorously emitting seeps, the so-called Trilogy Seep. The primary objective was to assess the suitability of LWIR imaging spectroscopy for detecting methane in the atmosphere.
A number of methane haze features were detected in the data. Another interesting finding was the rich structure evident in the retrieved ocean surface temperature field. This complexity results from a combination of current interactions, cold-water upwelling, and surfactant-driven emissivity modulation arising from capillary wave suppression by surface oil films.
A pronounced methane emission was also observed near the La Brea Tar Pits in Los Angeles, though not from the tar pits themselves. The emission was localized and initially thought to be a remnant of tar eruption reported a few days earlier. However, further investigation uncovered the fact that in the late 1990s a vent pipe was erected at this location to allow gas to escape from an especially prolific subterranean methane pocket, which is consistent with the observation of a highly localized source of gas with a distinctive plume morphology suggestive of a point source emission.
Another significant source of methane is the decomposition of organic matter in fetid water bodies, marshes, and landfills. According to the Environmental Protection Agency, landfills accounted for roughly 23 percent of total U.S. methane emissions in 2007. The SEBASS instrument was also flown over an active landfill in Los Angeles, and was successful in detecting fugitive methane emissions.
Geothermal Observations in an Active Seismic Zone
The Salton Trough, located in Southern California’s Imperial Valley, represents a major structural feature in Earth’s crust because it contains the northern portion of the transition zone between the San Andreas and Imperial Faults, which themselves demarcate the interface between the North American and Pacific tectonic plates. The transition zone is marked by a latticework of minor faults that in places provides conduits to the surface for hydrothermal fluids generated by an underlying magma chamber. In prehistoric times (15,000–20,000 years ago), this resulted in significant surface volcanism (forming the Salton Buttes, a line of quiescent vents along the southeast edge of the Salton Sea), but the only remnant of this activity is a proliferation of hydrothermal vents that are variously described as fumaroles, mud pots, or mud volcanoes. The area is currently being tapped by a number of geothermal power plants.
The Salton Sea occupies the lowest reaches of the Salton Trough. Although a freshwater lake has existed here at various points in history, the present lake was created inadvertently in the early twentieth century when an irrigation project went awry and the Salton Trough, at that time dry, filled with water from the Colorado River. This created a peculiar environment, because the lake had no outlets. The subsequent cycle of evaporation and continued inflow resulted in hypersaline conditions, with the water being further polluted by high levels of agricultural runoff from adjacent terrain.
This area was selected to evaluate the suitability of the Aerospace airborne LWIR imagers for making high-resolution observations of surface geothermal activity. A flight line was established along the presumed surface track of the Calipatria Fault, one of the most consistently geothermally active areas in the Salton Sea geothermal complex. The 1-meter-resolution LWIR imagery revealed numerous prominent thermal sources associated with an active fumarole group that has been exposed in recent years by the declining water level of the Salton Sea. In addition, a pronounced thermal hot spot is apparent in the shallows a short distance to the northwest of the landward cluster of fumaroles. The center of this particular fumarole was measured at almost 80°C. This was one of many fumaroles that emit vapor plumes, most of which were shown by spectral analysis to contain ammonia. It is presumed that this ammonia arises from geothermal pyrolytic decomposition of the nitrogen-rich agricultural runoff that permeates the lake water and sediments. These measurements are the first indication that geothermal activity contributes to the atmospheric ammonia content in this area.
With sufficient knowledge of the prevailing meteorological conditions, it is possible to estimate quantified emission fluxes. When this was done for six of the most clearly defined plumes, their combined ammonia output was calculated to be 30–90 kilograms per hour. The range in values expresses the total aggregate uncertainty attaching to the LWIR-derived quantities and local meteorological parameters for this particular data collection. (see sidebar, Fundamentals of Spectral Imaging)
Ammonia is one of many atmospheric pollutants that environmental agencies monitor because it creates airborne particulates that not only influence climate through their effects on the terrestrial radiation balance, but also present health risks to humans. Large-scale maps of ammonia distribution across the United States are currently compiled from data gathered by a sparse network of surface-based in situ sensors that are then processed by mesoscale atmospheric transport models to estimate ammonia concentrations over broader areas. These operational products are subject to inaccuracies from incomplete sensor coverage and performance uncertainties associated with the transport models. The measurements described here, in combination with recent advances in satellite retrieval approaches, point toward a means for more thorough characterization of atmospheric ammonia burdens on regional to global scales. (see sidebar, Aerospace LWIR Instrumentation).
The research findings described in this article have borne out the original premise that hyperspectral LWIR equipment and techniques developed for the tactical realm are readily adaptable to a diverse range of Earth and environmental science applications. The principal limitation of such systems concerns the modest areal coverage rate, and this issue is being addressed through the development of two new LWIR imaging instruments being built with support from Aerospace and NASA. The Aerospace research plan was conceived at the outset to be synergistic with the latter program, while the investigations themselves were chosen based on the likelihood of hyperspectral LWIR imagery to uncover new or unique knowledge relating to natural phenomena. The greatly expanded areal coverage rate of these new sensors will enable considerably more comprehensive studies to be undertaken over much larger areas than has been possible with the previous generation of instrumentation.
The work described in this article was supported by The Aerospace Corporation’s IR&D and Civil and Commercial Operations program offices and involved staff from several different departments within the corporation, including Imaging Spectroscopy, Remote Sensing, Advanced Sensor Engineering, Sensor Exploitation, and the Commercial, International and Homeland Security Programs office. Data processing and visualization for this article were provided by Kerry Buckland, Stephen Young, and Patrick Johnson. Mission planning was carried out by Brian Kasper, David Lynch, Jeffry Padin, and Mark Polak, while instrument preparation and support to field operations involved David Gutierrez, Jeffrey Hall, Eric Keim, Michael Martino, Luis Ortega, Jun Qian, Shaun Stoller, Adam Vore, and Karl Westberg.
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