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Airborne Geophysics: A Powerful Tool for Studying the Earth

Introduction

Since the early days of balloon photography and military reconnaissance, people have been struck by the broad view of the Earth that the airborne perspective provides. From the first photographs of Earth taken from aircraft with hand-held cameras to the highly refined swath terrain profiling systems now being developed, aircraft have provided a unique approach to studying earth science. Compared with ground-or space-based methods, airborne techniques offer the advantages of improved access, rapid sampling at scales that are optimal for many geophysical problems, and a tremendous potential for interdisciplinary studies at intermediate scales.

Access

Aircraft provide the capability of traversing regions that are otherwise difficult or impossible to cover. Examples include remote areas of the Rocky Mountains, the treacherous waters of the Drake Passage in South America, and the thickly-vegetated Amazon basin. The map of the Trinidad Quadrangle, Colorado, demonstrates the advantages of an airborne program: the land-based gravity survey is restricted to the passable roads and stream valleys in the region (Figure 1.1), whereas an airborne survey could systematically sample the entire region.



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--> 1 Airborne Geophysics: A Powerful Tool for Studying the Earth Introduction Since the early days of balloon photography and military reconnaissance, people have been struck by the broad view of the Earth that the airborne perspective provides. From the first photographs of Earth taken from aircraft with hand-held cameras to the highly refined swath terrain profiling systems now being developed, aircraft have provided a unique approach to studying earth science. Compared with ground-or space-based methods, airborne techniques offer the advantages of improved access, rapid sampling at scales that are optimal for many geophysical problems, and a tremendous potential for interdisciplinary studies at intermediate scales. Access Aircraft provide the capability of traversing regions that are otherwise difficult or impossible to cover. Examples include remote areas of the Rocky Mountains, the treacherous waters of the Drake Passage in South America, and the thickly-vegetated Amazon basin. The map of the Trinidad Quadrangle, Colorado, demonstrates the advantages of an airborne program: the land-based gravity survey is restricted to the passable roads and stream valleys in the region (Figure 1.1), whereas an airborne survey could systematically sample the entire region.

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--> Figure 1.1 Gravity map of a portion of the Trinidad Quadrangle, Colorado.  Note that the locations of gravity measurements are clustered near the roads  and in small stream valleys. Contour interval is 5 mGal. (Figure modified from  Peterson et al., 1968. Courtesy of P. Hill, U.S. Geological Survey).

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--> Areas that are physically accessible but that have social, economic, or political barriers are also potential candidates for an airborne survey. Where land access is restricted in environmentally hazardous areas because of health risks or industrial or military interests, the geochemical signature of these regions can be mapped remotely. A recent example is an airborne survey over a strip mining region in Czechoslovakia that mapped the extent of uranium contamination in the local river system (Figure 1.2). Sampling Proper sample spacing is crucial for obtaining meaningful scientific results. Undersampling of a region may not allow resolution of the major features that control the system. Oversampling is inefficient and may restrict the extent of the area to be studied, but may be necessary to isolate the signal from the noise. The sampling strategy chosen depends on the objectives. For example, magnetic anomalies that constrain rates of seafloor spreading can be adequately resolved with 20-kilometer (km) line spacing, aligned orthogonal to the ridge crest. In regions where plate motion is complex, a much closer line spacing (on the order of 4 km) with a complementary set of crossing lines is required to identify small-scale features and rotated crustal blocks. With airborne techniques, the scientific objectives of a study, rather than restrictions of access, allow the optimal sampling strategy to be selected. The improved sampling that is possible with an airborne program has particular advantages in the study of the potential fields of magnetics and gravity. Many of the errors and uncertainties that arise in the interpretation of land-based studies result from removing the effects of noise. For example, humanly-generated magnetic fields introduce ''social'' noise in magnetic studies, and surrounding, but poorly known, topography can introduce noise in gravity studies. Both of these measurement types are more easily interpreted if they are taken a systematic distance from the source. To achieve optimal results, airborne gravity is typically collected at a single altitude, whereas airborne magnetic surveys are generally flown at a constant elevation above the terrain.

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--> Figure 1.2 Environmental Monitoring System airborne gamma ray radiometrics survey of Czechoslovakia's Mimon Uranium Mine (northeastern area) showing uranium anomalies along the local river system. The brighter areas indicate high (≥9 parts per million) uranium concentrations. The survey was flown at a height of 80 m and a line spacing of 250 m. The data are overlain on a SPOT satellite scene of the area so that ground features can be identified easily. (Figure courtesy of B. Larson, World Geoscience Corporation).

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--> Airborne platforms also offer the advantage of rapid sampling rates. A recent gravity survey of Gabon in western Africa illustrates the speed in which an airborne survey can be conducted. The area surveyed was a combination of mangrove swamp, shallow water, and jungle. Using airborne gravity measurements, an area of 150,000 square kilometers (km2), about the size of Florida, with lines spaced 6 km apart, was surveyed in 50 flights, conducted over 55 days (see Figures 1.3(a) and 1.3(b)). The final product, a detailed gravity map and geologic interpretation, was delivered to the contracting petroleum company less than one month later. A similar land-based survey would have required many more months of effort, as well as a large team of surface parties with diverse instruments and equipment to conduct the survey in areas with a variety of vegetational cover. Interdisciplinary Studies An important advantage of airborne survey systems is their potential for a flexible interdisciplinary platform. In the 1960s and 1970s, research groups collaborated in a systematic survey of the ocean floor using deep-water research vessels. The strategy was not simply to collect topographic measurements, to sample the ocean floor with cores, or to probe the structure of the ocean water column. Rather, these vessels provided a center for integrated earth science research. This approach has been the standard for oceanographic vessels since the Challenger expedition in the mid-1870s, which circumnavigated the globe and gathered a broad range of samples and observations. Alone, any one of the sample collections from this expedition would have been interesting, but together they provided a powerful stimulus for rethinking the structure and habitats of the globe. Aircraft can provide a similar centerpiece for interdisciplinary research. The capabilities aircraft offer for such research can be exemplified as follows. In many regions, the seepage of saline groundwater into agricultural areas adversely affects crop production. In order to determine groundwater flow patterns, aircraft could carry instrumentation for both airborne magnetic capabilities and electromagnetic mapping. The airborne magnetic data would define the geologic structure, and the electromagnetic data would delineate the extent of saline groundwater. Together these data

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--> Figure 1.3(a) Land gravity measurements (dots) and marine gravity surveys (lines) of coastal  Gabon. The land measurements were made principally on roads  close to major cities. (Data from Watts et al., 1985).

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--> Figure 1.3(b) Airborne gravity survey (lines) flown over Gabon in a 6 × 6 km grid.  The sampling strategy is not limited by access to roads.  (Figure courtesy of B. Gumert, Carson Geophysical).

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--> may indicate, for example, that the flow patterns are controlled by the presence of a series of igneous dikes that cut across the area. It is this capability to carry and utilize multiple instruments on an aircraft that provides new insights into the linked processes that control the earth's systems. The major drawback of using multi-instrumented aircraft, however, is the need to compromise individual measurement types. As noted above, the optimal altitude for collecting airborne gravity and magnetic measurements may differ. Nevertheless, in many instances such interdisciplinary efforts, coupled with new high-resolution techniques, offer increasing opportunities for airborne geophysics. Precise positioning, based on GPS technology, will be increasingly necessary to advance these new applications. Precise Position Capabilities and Requirements Airborne geophysical experiments require that the location of the measurements be accurately known and, in some cases, that the orientation of the platform from which the measurements are taken be well constrained. Accurately locating the experiment is the real-time, or navigational, problem. In large-scale regional studies, real-time navigation is of minimal concern, but as the applications of airborne geophysical techniques become increasingly detailed and as the instrumentation becomes increasingly sensitive, navigation issues become more significant. Navigation issues are critical, for example, in monitoring changes in topography after an earthquake. The scientific objective of understanding the deformation of the Earth requires that the same track be flown both before and after the seismic event. Recent applications of pseudosatellites have produced encouraging results in very precise real-time applications and could have application in post-processing. Most geophysical applications require post-processing of the data because of our inability to hold the aircraft to within a meter. In general, postmission positioning requirements are more stringent than are real-time navigation requirements, and thus, more sophisticated algorithms are used for aerotriangulation applications, for example, or in computing aircraft acceleration for gravity applications.

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--> Recovering the aircraft orientation is important to many applications, as this information may be required to correct for aircraft motion during data acquisition. Examples are the correction of laser ranges to the vertical and also the rectification of imagery. The accuracy required for navigation, positioning, and attitude varies among operations, but, in general, the exploitation of data obtained from airborne sensors increases with improved positioning and navigation capabilities. GPS is a powerful tool for determining aircraft positioning and navigation and is potentially useful for recovering orientation. The system has the advantages of being globally accessible, little affected by local weather conditions, and having relatively low user costs. Table 1.1 summarizes the achievable accuracies for position and orientation using GPS. The GPS system is based on a constellation of 24 satellites that transmit biphase encoded signals at two frequencies, denoted by L1 (1.575 gigahertz [GHz]) and L2 (1.227 GHz). Two basic codes are written on the GPS carrier signals at two frequencies: the coarse/acquisition code (C/A code) and the precise code (P-code). The system was designed to provide navigation to an accuracy of approximately 10 m with the P-code, and approximately 100 m with the C/A code (U.S. Department of Transportation/U.S. Department of Defense, 1992). Researchers accessing the full range of signals transmitted by the GPS satellites have expanded the capabilities for positioning moving vehicles (such as aircraft) to the decimeter level using differential techniques. The three main types of observables that are used in the analysis of GPS data to position objects precisely are (1) the pseudorange, (2) the carrier phase, and (3) the Doppler shift. Pseudorange measurements are made by determining the difference between the arrival time of a GPS signal (as measured on the receiver clock) and its transmission time (as determined by the satellite clock). By simultaneously observing four or more satellites, it is possible to determine the position of the receiver and to correct for differences in time between the receiver's clock and the GPS satellite time system. When Selective Availability (SA) is turned on, there is high-frequency "dithering" of the satellite clock by up to 0.2 microseconds (µs). This dithering cannot be recovered from the ephemeris message without access to a classified decryption key (see APPENDIX A). This noise introduces 60-m errors into the pseudorange solutions with periods of several minutes.

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--> TABLE 1.1 Summary of GPS Positioning and Attitude Accuracies Model Distance from Reference Receiver Achievable Accuracy Pseudorange point positioning*   100 m horizontal 156 m vertical Carrier-smoothed pseudorange differential positioning 10 km 0.5–3 m horizontal 0.8–4 m vertical 500 km 3–7 m horizontal 4–8 m vertical Carrier phase differential positioning 10 km 0.03–0.2 m horizontal 0.05–0.3 vertical 50 km 0.15–0.3 m horizontal 0.2–0.4 m vertical Attitude determination 1 m separation 18–30 arcminutes 5 m separation 4–6 arcminutes 10 m separation 2–3 arcminutes * Selective Availability on, position dilution of precision (PDOP) &x156; 3, 2 times distance-root-mean-square (DRMS) to 95 percent probability (U.S. Department of Transportation/U.S. Department of Defense, 1992). Carrier phase measurements are made by reconstructing the carrier signal, the fundamental frequencies of L1 (1.575 GHz) and L2 (1.227 GHz). This process includes removing the biphase encoding and measuring the phase difference between the reconstructed carrier phase and a local oscillator within the receiver for both the L1 and L2 frequencies. As this carrier-beat phase rotates through cycles, the number of cycles is accumulated. Thus, the phase measurement is the accumulated phase change from the time the satellite is acquired by a receiver (locked on) until the receiver loses the signal (loss of lock). The carrier phase noise is a few millimeters; however, it also contains an ambiguity that must be determined to exploit this accuracy.

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--> The Doppler observable is most often determined from the time derivative of the carrier phase. The Doppler observable is rarely used for geodetic positioning, but it can be useful for connecting phase measurements across small gaps (<5 s) when the signal from a satellite is lost (e.g., during banked turns or rapid accelerations). Together these three observables provide a powerful suite of tools for determining the time and position of a vehicle anywhere around the globe. The principal errors in a GPS position arise from errors in timing and errors introduced as a result of the propagation of the signal from the satellite to the receiver. The SA signal is a clear example of an artificially induced error that affects the pseudorange measurements and thus position determinations. The SA error size is much larger than the timing errors that are intrinsic to the GPS clocks. The propagation errors are associated with dispersive delay in the ionosphere, atmospheric delays, and the signal reflecting off objects surrounding the antennas before it is recorded by the receiver (multipath). Differential techniques based on multiple satellites and receivers are used to minimize many of these errors. Further information on GPS measurements, errors, and processing techniques is given by Wells et al. (1986), Hofmann-Wellenhof et al. (1992), Seeber (1993), Cohen et al. (1994), and Enge et al. (1994). Differential techniques are based on the concept that many of the errors in timing and propagation can be eliminated by using a reference receiver in a fixed location and positioning the moving receiver relative to this reference. This configuration assumes that systematic errors seen by both receivers can be removed by differencing the signal. The accuracy of differential positioning decreases with increasing baseline lengths. This approach can be applied to both pseudorange and carrier phase measurements. Differential pseudorange positions are accurate to 0.5 to 3.0 m, depending on the receiver type used, and are being used experimentally for commercial aircraft systems. Differential carrier phase measurements are as accurate as 2 to 20 centimeters (cm) and generally require extensive postmission analysis, although real-time systems are under development (e.g., Frodge et al., 1994). Several efforts have been made to demonstrate the robustness of the differential GPS approach for accurately locating aircraft. As the most difficult component to constrain is the vertical position, the recovery of this component is generally used as a benchmark. A ranging system that provides an independent constraint on the aircraft height is used in

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--> satellite altimetry (less than 10 cm for the current TOPEX/POSEIDON satellite) characterize the instantaneous sea-surface height, which can deviate from the geoid by as much as 1 to 2 m. Therefore, satellite altimetry is not the final answer to better marine gravity models. Gravity measurements made from a moving vehicle, such as an aircraft or ship, are contaminated by motion-induced and inertial accelerations due to the rotating coordinate frame, even when mounted on a stabilized platform. These accelerations must be accurately determined and removed from the measurements to recover meaningful gravity anomalies. The amplitude of the vertical acceleration can be much greater than that of the geologically and geophysically significant anomalies. For example, a sedimentary basin may have a gravity anomaly of 125 milligal (mGal = 10-5ms-2), whereas the typical vertical accelerations of an aircraft are approximately 20,000 mGal. Also, the eötvös correction (Coriolis and centrifugal effects), being a function of flight direction and velocity, can reach amplitudes of over 1,000 mGal and must be calculated for gravity measurements. On ships, the requirements on position and velocity accuracy are less demanding because of the lower speeds and the somewhat predictable average vertical position. These less stringent positioning requirements have permitted widespread use of marine gravimetry through the use of satellite positioning and radio navigation systems, such as Loran-C. Early airborne gravimeter measurements also used Loran-C, as well as radar, laser, and barometric altimetry. These techniques, however, were largely imprecise and regionally limited. It was not until the late 1980s that precise kinematic positioning with GPS was shown to meet the rigorous positioning requirements for airborne gravity measurements (Brozena et al., 1989). GPS positioning, which is easily made on global scales, has led to the broader application of airborne gravity surveys, both in scientific research and in commercial exploration. Numerous basic and applied research groups in the United States, Canada, Switzerland, and Germany are actively pursuing both proof of concept and operational airborne gravity capabilities that are now possible with GPS. A recent gravimetric survey of the subcontinent of Greenland illustrates the powerful new capabilities of GPS-navigated and-positioned airborne gravity surveys (Brozena et al., 1992). Because thick ice covers more than 96 percent of the surface of Greenland, the geology of the subcontinent is poorly known. Scientists flew a detailed gravity survey of

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--> the land and coastal waters of Greenland using a P-3 Orion aircraft. The aircraft was equipped with a LaCoste-Romberg air-sea gravimeter mounted on a three-axis stabilized platform, a GPS receiver, and pressure and radar altimeters. Over the course of two years, the scientists were able to map the entire subcontinent at a line spacing of 50 to 100 km with an accuracy of 4 to 6 mGal. The resulting data suggest the location of major sutures between the northern and southern regions (see Figures 1.4(a) and 1.4(b)). This survey would have been impossible without the precise positioning capabilities of differential GPS. Measuring the Earth's Surface Topography Measurements of surface elevations are fundamental to many applications of geology, geophysics, hydrology, terrestrial ecology, geomorphology, glaciology, and atmospheric physics (Table 1.3). For example, the topography of the polar ice caps and mountain glaciers is important because it is a direct measure of ice flow dynamics and is closely linked to global climate and sea level change. Existing topographic data in ''well mapped'' North America, western Europe, and Australia, however, are inadequate in terms of accuracy and consistency to support most types of scientific research (Topographic Science Working Group, 1988). For example, in high-relief terrain, the vertical accuracy of available data may be worse than 30 m, too poor by at least an order of magnitude to support many applications. Less accessible regions of the Earth, including large areas of Africa, Asia, South America, and Antarctica, do not have even this level of topographic coverage (Figure 1.5). Topography has traditionally been measured by triangulation and levelling methods. This land-based effort required skilled crews to measure accurately distances and elevation changes. Over the past several decades, ground determinations of topography have been supplemented by airborne photogrammetry, which provides the potential to map the Earth's surface to a decimeter. Photogrammetric techniques are adequate to meet topographic mapping standards for scales of 1:10,000 or less, but they are not sufficiently precise to monitor dynamic changes in the Earth's surface. The signals of interest to earth scientists are typically on the order of

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--> Figure 1.4(a) Terrestrial gravity data distribution over Greenland as of 1992,  reflecting more than 30 years of ground survey efforts.  The contour interval shown is 10 mGal. Contours are  masked if no data point exists within 40 km. (Figure  courtesy of J. Brozena, Naval Research Laboratory).

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--> Figure 1.4(b) Airborne gravity data distribution from the Greenland Aerogeophysics Project, flown during the summers of 1991 and 1992 by the Naval Research Laboratory and the Naval Oceanographic Office in collaboration with NOAA and the Danish National Survey and Cadastre. Approximately 200,000 line-km of tracks were flown over a period of 4 months, covering the entire subcontinent of Greenland (>2,100,000 km2). Interferometric-mode GPS provided aircraft positioning, and magnetics and surface topography were mapped simultaneously with gravity. Contour interval is 10 mGal. The airborne data were combined with historical terrestrial and shipboard data, except the traverse near 77°N, which proved to be in error by more than 30 mGal over the ice cap. (Figure courtesy of J. Brozena, Naval Research Laboratory).

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--> TABLE 1.3 Topographic Accuracy Needed for Earth Science Applications (Compiled from Burke and Dixon, 1988; Coolfont, 1991; Dixon et al., 1989; Fletcher and Hallet, 1983; Garvin and Williams, 1990; Qidong et al., 1984; Simkin et al., 1981; Telford et al., 1990). Feature Vertical Accuracy (m) Horizontal Accuracy (m) Repeat Interval (yr) Geology/Geophysics Plate Boundaries and Intraplate Deformation Large-scale structures 10 1,000 - Rifts 20 2,000 - Diffuse extension 10 1,000 - Mountains 10 500 - Land Geology/Fault Zone Tectonics Mapping 4 30 - Surface structure 1 10 - Neotectonics 2 100 5–20 Volcanology Flow and ash volumes 0.5–3 30–100 3 Volcano morphology 2–10 30–500 - Volcano dynamics 0.15–1 30–100 1 Marine Geology/Geophysics Topography prediction from geoid 0.1 3,000 - Gravity/Magnetics Admittances—gravity 10 10,000 - Terrain correction—gravity 1 200 - Satellite gravity 3 1,000 - Magnetics 10 200 - Polar Science Basic Inventory Large-scale features (ice domes, divides, streams, ice shelves, drainage basins) Medium-scale features (flow lines, undulations, crevasses, rifts) 10 500 5–10 1 100–500 5–10 Mass Balance and Dynamics Accumulation 0.1 500 5 Ice dynamics (gradients, flow features) 0.1–0.5 100–500 1–5 Ablation (grounding lines, ice shelf margins, rifts, crevasse fields, icebergs) 0.1–0.5 100–500 1–5

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--> Figure 1.5 Global availability of topographic data at a variety of scales derived from contour  and digital maps. (Figure from United National Development Project).

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--> millimeters to tens of centimeters for postseismic rebound, for example; millimeters for stream erosion rates; and tens of meters for mass wasting processes. Precise positioning is required for these applications. With the implementation of geodetic networks supported by land-based GPS crews, points can now be located on the order of millimeters. Recent studies have documented plate motions and have raised important questions about the nonrigid nature of the Earth's deformation along plate boundaries. A drawback of this technique, however, is that the geodetic networks provide only point measurements, and knowledge of the entire three-dimensional surface is required if earth processes are to be understood. The recent integration of airborne mapping techniques with precise positioning has demonstrated the ability of this approach to detect dynamic processes. A striking example of the success of this approach was the detection of a subtle but significant depression in the West Antarctic ice sheet (Blankenship et al., 1993). Although the region had been photographed, surveyed for topography, and imaged by satellites, these earlier efforts failed to identify a depression in the ice surface that was 50 m deep, 6 km wide, and 12 km long (Figure 1.6). An aircraft survey with 5-km spacing equipped with a laser profiling system integrated with GPS positioning revealed that the depression was associated with active volcanism beneath the ice sheet. This discovery has important ramifications for the dynamics of ice sheet collapse, and yet the depression never would have been resolved without precise positioning. Very High Resolution Studies with Airborne Techniques Land-based gravity measurements have long been used to identify the salt domes and other structural traps for oil along the Gulf Coast and the large mineral deposits in the western United States. As the easily identified reservoirs and mineral deposits become depleted, however, exploration industries will require increasingly detailed information on the subsurface of the Earth. High-resolution mapping of the subsurface is also rapidly developing as a requirement for the characterization and cleanup

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--> Figure 1.6 Evidence for active volcanism beneath the West Antarctic Ice Sheet from precise surface altimetry measurements and other airborne geophysical observations. The measurements were made as part of a major study of the stability mechanisms of the West Antarctic ice sheet and were collected along a north-south profile. (a) Surface elevations from a GPS-positioned laser altimeter reveals an anomalous depression in the ice surface located at 28 km. (b) Depth to bedrock (ice thickness) from ice penetrating radar observations. The prominent feature at 26.5 km is centered in a shallow rimmed caldera. (c) Total magnetic field observations reveal a large anomaly between 0 and 40 km that is strongly correlated with the caldera and central edifice. (d) Free air gravity anomaly of 7 mGal is associated with the central edifice at 24 km. (Figure modified from Blankenship et al., 1993).

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--> Figure 1.7(a) Map showing waste sites and buildings in the survey area, Oak  Ridge National Laboratory. (Figure modified from Doll et al., 1993).

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--> Figure 1.7(b) Short-wavelength magnetic anomalies (vertical component) measured from a  GPS-navigated helicopter are associated with high- and low-activity silos, surface pipes  (labelled "high range wells"), high- and low-activity materials disposal trenches  (capped areas 1, 2, 4, 7, and Control Trenches), biological disposal trenches  (capped areas 5 and 8), and asbestos disposal trenches (capped area 6).  (Figure modified from Doll et al., 1993).

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--> of Superfund and other waste sites (e.g., Labson et al., 1993; Phillips, 1993). Measurements of these waste sites often pose health hazards to ground crews, so there is great interest in the development of remote tools. These high-resolution applications can be addressed by airborne geophysical technology and precise positioning. The ability of airborne techniques to characterize waste sites safely and rapidly was recently demonstrated at Oak Ridge National Laboratory (Doll et al., 1993). An innovative approach was developed utilizing a helicopter equipped with GPS and airborne electromagnetic, magnetic, and radiation sensors. A reconnaissance survey was flown at an altitude of 30 m with a line spacing of 45 m to detect large environmental targets and to identify the faults and fractures that could influence contaminant migration. Preliminary results showed that electromagnetic and magnetic anomalies corresponded to radioactive, biological, or asbestos waste sites, and ferrous objects, such as drums, silos, trenches, and well casings (see Figures 1.7(a) and 1.7(b)). Simultaneously, these measurements provided constraints on the regional hydrologic systems and geologic framework that partly control the extent of the hazardous material.