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Seeing into the Earth: Noninvasive Characterization of the Shallow Subsurface for Environmental and Engineering Applications (2000)
Commission on Geosciences, Environment and Resources (CGER)

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Why Characterize the Subsurface? A large number of applications require the ability to "see into the earth," to determine physical, chemical, and biological properties and to detect, monitor, and predict natural and induced processes. The characterization of the earth's subsurface is motivated by a mix of economic, scientific, environmental, regula- tory, and health and safety concerns. This chapter discusses a sample of the various applications. The foremost question is: What is there and what is its extent or boundaries? Information can be obtained from noninvasive observa- tions from the earth's surface or from direct sampling. Depending on the applica- tion, noninvasive observations often have to be supplemented by direct sampling of the volume of ground under investigation. Characterization needs or objectives are dependent on the specific applica- tion. These characterization objectives are described in greater detail in Chapter 3. In some applications, the required information involves determining the physi- cal, chemical, and biological properties of the solids and fluids below the surface. Characterization also commonly involves obtaining information about the pro- cesses that occur in the subsurface. These processes include the natural processes that form and modify the geological materials and the structure of the subsurface, and induced processes such as fluid pumping or injection. The level of accuracy and spatial resolution required in the characterization of properties and processes is determined both by the application and by the motivation. NATURAL RESOURCES As the human population increases and developing countries become more industrialized, the demand for natural resources will continue to grow. Offshore 18

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WHY CHARACTERIZE THE SUBSURFACE? 19 exploration for oil is expanding into deeper water. Mineral and water resources are in ever-increasing demand. To find and extract these resources economically with minimal environmental impact requires technologies that can efficiently determine their location, extent, and quality. The common characterization objective in resource exploration is the direct detection of the resource (e.g., ore body, groundwater aquifer, hydrocarbon reser- voirs). The methods used depend on the physical and chemical contrast with the background or host geology. In some cases it is not the ore body, the aquifer, or the petroleum reservoir that is the target but a geological setting or structure likely to host the resource. An example is the location of a fault zone in the search for gold mineralization or the location of an anticline in the search for a gas reservoir. For many resources, noninvasive techniques, including remote sensing, represent a standard approach to initial characterization. Geophysical surveys, either airborne or ground-based, can be used to obtain reconnaissance data over large areas to determine regions with high resource potential. Such geophysical surveys have had many successes including the location of the multibillion-dollar Olympic Dam ore deposit in Australia (Roberts and Hudson, 1983~. More detailed assessment of an area requires higher-resolution geophysical and geological mapping along with drilling and direct sampling. Characterization at this stage is designed to obtain a more accurate determination of location and lateral distribution. A shortcoming of noninvasive techniques for these purposes in mineral exploration has been the limited depth of investigation and poor reso- lution at greater depths. Although not as widely recognized as a limitation, reso- lution, even at shallow depths, is often insufficient to produce the type of ore body discrimination that can be useful in modern exploration surveys and mine development activities. In addition to locating a resource, a common goal of characterization is to determine the extent and quality of the resource. Although drilling and sampling can provide this information, noninvasive techniques may be useful for some determinations of resource quality. For example, induced polarization methods can discriminate certain minerals in an ore deposit, and electrical conductivity can indicate the salinity of a groundwater aquifer. In summary, noninvasive techniques are widely used in some resource exploration and well-integrated with the often more expensive drilling and direct sampling programs. GROUNDWATER CONTAMINATION AND REMEDIATION There is a growing concern about contaminated soil and groundwater, both from surface spills and from underground sources such as leaking storage tanks or landfill sites. Such contamination affects our natural environment and can have a direct impact on public health and safety and the utility of groundwater. Remediation of contaminated sites is often required, whereas at other sites, con- tainment of the contaminants may be an option (National Research Council,

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20 SEEING INTO THE EARTH 1994~. Regardless of the action to be taken at a site, there is a need for accurate, high-resolution information about the physical and chemical state of the subsur- face. (For a recent treatment of containment technology and site characterization needs for containment, see Rumer and Mitchell, 1996.) Specific characterization objectives depend on the nature of the contamina- tion (if known) and the type of remediative actions being considered at a site. A common objective is to determine the source or present location of a contami- nant. Although drilling and direct sampling can be used to help locate a subsur- face contaminant plume, such invasive methods may be undesirable because they can spread contamination to surrounding areas and may pose a safety risk. Noninvasive techniques can be used for initial characterization and as a guide for determining the location and density of required direct sampling. For this objec- tive, the appropriate noninvasive technique is one that can directly detect the presence of the contaminant. Alternatively, the technique may detect the subsur- face structures or pathways in which a contaminant may preferentially collect or flow. As discussed later, electromagnetic methods in some cases can directly detect inorganic contaminants, depending on the concentration of the contami- nant and the geological setting. Direct detection of organic contaminants by noninvasive methods is considerably more challenging, with some reported suc- cess using resistivity sounding, ground penetrating radar (GPR), electromagnetic methods, and soil-gas measurements (Figure 2.1~. For contaminant plumes, the sensitivity of the noninvasive technique can improve dramatically if the move- ment of the fluid can be monitored over time (see Plate 2~. A common problem at a contaminated site is the detection of a buried con- tainer, which may be a source of contaminant. The chemical and physical proper- ties of the container material, as well as its size, shape, and depth of burial, can be highly variable. The location of underground storage tanks is a common charac- terization objective for the use of noninvasive technologies because invasive techniques risk damaging or puncturing the container. In other cases, the characterization objective might be to determine a site's geological framework. Knowing the geological setting, including the heterogene- ity (e.g., lithological and structural) and its associated anisotropy, is critical in predicting contaminant transport and planning future remediation efforts. Inva- sive techniques such as drilling and direct sampling can provide very accurate information about the subsurface, but only for the sampling location and for a limited volume of the subsurface. To obtain the resolution and coverage required for adequate site characterization, direct sampling can become time-consuming and expensive. Noninvasive techniques, on the other hand, can provide high- resolution information about the subsurface over a large area without any direct contact with the contaminated region. Monitoring the remediation process can significantly improve effectiveness and reduce costs. A monitoring system makes possible ongoing, real-time modi- fication of the remediation process. For example, some remediation methods

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WHY CHARACTERIZE THE SUBSURFACE? S ITE A GROUNDWATER ZONE SOURCE .SOURCE 0.0~ loololl 0 20 40 60 m I i I I N 21 S ITE B VADOSE ZONE SOURCE .SOURCE PILOT VAPOUR EXTRAC~ONSnE:4 ;~\' SOIL GAS GROUNDWATER FIGURE 2.1 Areal extent of soil gas and groundwater contamination derived from ICE (trichloroethelene) emplaced below the water table (site A) and in the vadose zone (site B). (all units in grams per liter.) Figure from Rivett and Cherry (1991), which should be referred to for additional details. usually involve displacing one fluid with another, which can produce spatial and temporal variations in the physical and chemical properties of the subsurface. Such temporal changes can be ideal targets for detection with noninvasive meth- ods and provide an accurate monitoring tool. Noninvasive methods have had some success in this area, as discussed in detail in Chapter 3. LAND MINES AND UNEXPLODED ORDNANCE Noninvasive techniques are frequently used to locate buried land mines and unexploded ordnance (UXO). Safety concerns are paramount in such applica- tions. There are an estimated 110 million unexploded land mines, and these "have maimed at least 250,000 people in the world and kill more than 10,000 people each year, more than 90 percent of whom are civilians" (Inter-Parliamentary Union, 1996~. In the United States an estimated 15 million acres may be contami- nated with UXO (Defense Science Board, 1998~. Noninvasive technologies have the potential to reduce the danger by locating and identifying land mines and

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22 SEEING INTO THE EARTH UXO, and obtaining sufficient information about the surrounding material to allow safe removal or detonation in place. Methods of detecting of land mines and UXO differ considerably (U. S. Army Environmental Center, 1994; Joint Unexploded Ordnance Clearance Steer- ing Group, 1997~; see Box 2.1. Land mines are usually buried at shallow depths (tens of centimeters) and are designed to detonate if disturbed. The sensitive nature of land mines necessitates consideration of remote or standoff detection methods. The shallow depths make detection possible using standoff GPR, high- resolution induction electromagnetic (EM) systems, and infrared and multispec- tral scanners. Buried UXO, however, is encountered at depths up to 10 m and varies in size (from 20-mm projectiles to 2000-pound bombs). UXO detection requires not only high resolution but also a large depth of investigation. Low- metal-content land mines make detection with magnetometers difficult if not impossible, whereas magnetometers are the common "tool of choice" for UXO detection (see Figure 2.2~. Locating land mines and UXO involves both invasive and noninvasive tech- nologies. Noninvasive technologies are usually the first step in determining the location and the identity of land mines and UXO. For certain types of land mines and UXO, it is reasonable to proceed with invasive methods to detonate in place or to excavate and neutralize a region. In other cases, the only safe decision may be to isolate and mark the region as a danger zone and prevent entry. Land mine and UXO detection is an application area in which the technolo- gies selected must, in some situations, be truly noninvasive because even placing a sensor on or in the ground could detonate a land mine or UXO. The great challenge in the use of noninvasive technologies for UXO detection is the enor- mous variety in the size, shape, and burial depth of the ordnance and the geologi- cal background and cultural clutter. These factors, combined with the demands for high measurement and positional accuracy and sophisticated data integration and interpretation for target discrimination and identification, make UXO detec- tion an extremely challenging and critical area for the application of geophysical methods and help underscore the need for additional research and development.

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WHY CHARACTERIZE THE SUBSURFACE? Total Magnetic Intensity - - sooo - 400.0 T ~ too ~ at ~3 FIGURE 2.2 High-resolution magnetic survey for UXO detection Unexploded ordnance (UXO) contamination exists on active and former military training and testing ranges. Environmental restoration of these sites to support fu- ture training and testing and return them to public use is a high priority. High- accuracy magnetic determinations can be particularly effective in the detection of ferrous UXO. If the type of ordnance is known, such surveys permit the areal location, depth, and approximate size of sub-surface UXO to be determined. The top figure is from a high-resolution magnetic survey over a contaminated World War II artillery range. Data were collected with an automated survey system consisting of an array of optically pumped magnetometer sensors combined with a differential Global Positioning System, operated from an all-terrain vehicle. A simi- lar survey (bottom) after the area had been cleared of ordnance produced a uni- formly flat figure with the exception of residual rust flakes from other metallic debris. (Example courtesy of Geophysical Technology Limited, Armidale, Austra- lia.)

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24 SEEING INTO THE EARTH CIVIL INFRASTRUCTURE Many civil engineering projects require characterization of the shallow sub- surface. Such projects include the design and construction of roads, airfields, bridges, dams, water supply and wastewater treatment facilities, housing, indus- trial and office buildings, tunnels, power plants, and safe storage facilities for wastes of all types. In addition to design and construction, subsurface information is needed for the rehabilitation of existing underground infrastructure. A common application of noninvasive techniques is for locating existing underground utilities (e.g., telephone, gas, water, electric) and structures (see Plates 3 and 4~. The National Transportation Safety Board (1997) cites the needs for proper use of geophysics in locating underground utilities before digging, excavating, or drilling, and for statistics on inadequate implementation of geophysical sensing. It also states that "a single pipeline accident has the potential to cause a catastrophic disaster that can injure hundreds of persons, affect thousands more, and cost millions of dollars in terms of property damage, loss of work opportunity, ecological dam- age, and insurance liability." Typically about 70 such events occur in the United States every year (National Transportation Safety Board, 1997~. Many geotechnical projects have traditionally relied on field penetration tests, in situ tests of various types, and laboratory tests on samples of varying quality and representation. However, noninvasive tests have been used increas- ingly in recent years because they often cost less, are relatively easy to conduct, and provide information not readily obtained by other means. In addition, noninvasive methods can test a much larger volume of the subsurface than tradi- tional sampling or in situ testing approaches. These methods provide an excellent supplement than can limit the number of invasive methods used in most projects. A coordinated approach that combines invasive and noninvasive methods is likely to yield the most reliable site characterization. Most infrastructure projects have several characterization objectives in com- mon. At a minimum, geologists and engineers seek to know and understand the types of soil and rock materials and their stratigraphy, as well as the engineering properties of the different materials and the depth to groundwater. Construction in urban areas also requires information about existing underground works such as utilities, tunnels, and preexisting foundations. The engineering property re- quirements consist of five types: (1) volume change characteristics, so that settle- ments or heaves may be estimated; (2) strength, so that the stability of slopes, embankments, and excavations can be analyzed and the supporting capacities of foundations determined; (3) deformation characteristics, so that ground move- ments may be anticipated, dynamic response to earthquakes analyzed, and soil- structure interactions studied; (4) hydraulic conductivity properties (and in cer- tain situations, thermal, electrical, and chemical conductivity) so that flow

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WHY CHARACTERIZE THE SUBSURFACE? 25 quantities can be estimated; and (5) the likelihood that these properties may change with time. Current limitations of noninvasive methods for geotechnical applications include the inability to define boundaries and identify material types with suffi- cient accuracy, the inability to analyze small volumes or zones that may have a critical importance (e.g., failure to detect small-scale heterogeneity), and a lack of noninvasive methods for determining strength, volume change, and hydraulic conductivity properties (except as they might be deduced through correlations with material type). In addition, there is often a lack of unique interpretation from a given set of geophysical measurements. HAZARDS Noninvasive methods can play a critical role in characterizing certain natural hazards. Ground failure risks from natural hazards (e.g., surface manifestation of earthquakes, floods, landslides, and expansive or collapsing soils) require identi- fication and mitigation to ensure public safety, as well as for reasons of economy. Knowledge of stratigraphy and engineering properties is essential for analysis of ground responses to forces of nature, such as gravity, earthquake ground motion, wind, and waves. Seismic methods are particularly well suited for evaluating the mechanical properties and interpreting ground behavior under dynamic loading. Subsurface cavities are another type of hazard commonly associated with sudden ground failure. These cavities, which include natural sinkholes and cav- erns as well as human-made tunnels or subterranean chambers, must be properly located (see Figure 2.3~. Sinkholes might have no surface expression until they breach the surface and cause considerable damage to engineering infrastructure. By knowing where cavities are, one can avoid building on them. In addition, knowledge of underground cavity distribution often gives information on the water flow network that such cavities can provide. Conduits and caves can act as pipes, allowing contaminated groundwater to migrate rapidly over great distances. Some of the more troublesome groundwater contamination disasters have occurred in karst (limestone) aquifers where the existence or location of conduits was initially unrecognized. Structural engineer- ing projects can also be severely impacted if there are large openings in underly- ing bedrock. On a much more localized scale, noninvasive techniques (particularly GPR) can be used in road maintenance, particularly in monitoring asphalt pavement thickness and detecting air-filled voids or bridge deck delamination (NRC, 1998~. ARCHAEOLOGY Recent federal legislation such as the Native American Graves Protection and Repatriation Act, the Archaeological Resources Protection Act, and the Na

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26 SEEING INTO THE EARTH ..! 'A _ is. ~ '; 10 'O -10 a: -20 _ _ at: In ~_ I I ~ I I I I -'I I I 1 - 0 20 40 60 80 100 120 140 160 180 204 220 244 260 - N~S DISTANCE, FT GRAVITY PROFILE \ \ 170 106 1110 1" 'im Z 1"' 1" 13S 130 1 1 1 ~ TOP OF KNOWN CAVITY ~3 in) OTTO 1 VERTICAL EXAGGERATION 1 1- 1 1 _. 1 __1 1 ~1 1 l 125 0 20 40 60 80 100 120 140 t60 180 200 220 240 260 NIPS DIST - CE, FT GEOLOGY PROFILE FIGURE 2.3 Microgravity profile (top) and corresponding geological section (bot- tom) determined by closely spaced drilling. Known air-filled cavities passing under the profile line are shown in the geological section. The gravity anomaly profile indicates a negative anomaly over the cavities and positive and negative anomalies correlating to limestone pinnacles and clay-filled pockets, respectively. The nega- tive anomaly over the cavities is a superposition of the effects of the cavities plus solution-enlarged porosity and fractures above and around the cavities. Adapted from Butler (1984~.

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WHY CHARACTERIZE THE SUBSURFACE? 27 tional Historic Preservation Act of 1966 mandate cultural resource assessments prior to construction or other activity that could endanger either known or undis- covered cultural artifacts. At an archaeological site, the overall characterization objective is to find the artifacts and features. Specific characterization objectives include the direct de- tection of these objects and the detection of disturbed ground that indicates past human activity. Traditional archaeological field research involves invasive meth- ods such as the careful digging of pits and trenches to find, extract, and document artifacts. The time and cost involved in these methods have increased interest in the use of noninvasive techniques, particularly geophysical methods, to map archaeological sites and plan the locations of invasive sampling. In addition, geophysical methods often allow archaeologists to detect and map patterns at sites that are often extremely difficult to detect and visualize from standard procedures. For example, Katsonopoulou and Soter (1996) used GPR in the exploration of ancient Helike. Some geophysical investigations of archaeological sites have received considerable public and popular-scientific media attention- for example, Lakshmanan and Montlucon's (1987) discovery of hidden chambers in the Great Pyramid at Giza, Egypt. Because geophysical anomalies caused by archaeological artifacts and features are often small and subtle, the geophysical methods and survey procedures used often must be high resolution and precise. They generally must be multimethod, integrated investigations. Archaeological application requirements often stimulate innovative, cutting-edge developments in geophysical equipment, field procedures, and interpretation methods. The work by Butler et al. (1994) to locate the exact site of the Wright brother's 1910 hanger demonstrated this multimethod, integrated approach through its use of scanned period aerial photographs georeferenced to the geophysical survey maps and site facility maps (see Figure 2.4~. Their work also showed that archaeological investigations do not always involve ancient or prehis- toric objectives and that anomalies due to even relatively recent cultural site fea- tures can be very small in magnitude and subtle in expression. BASIC SCIENCE The upper tens of meters of the earth hold information critical to understand- ing many of the natural processes occurring within and on the earth. Studies of outcrops and of cores obtained through drilling have traditionally provided earth scientists with the samples used to investigate the properties and distributions of geological materials. These outcrops and samples are used to measure physical and chemical properties, to gain insights into the spatial distribution of these properties, and to develop models of the geological processes that formed the materials. For example, in studies of sedimentary deltaic environments, observed lithologic variation is used to develop models of the processes involved in the

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28 7GH Flying held J fog Am_ . tOE.~JNI SON 50N N CON 3QI\I 20N 1051 ON O 5 10 SEEING INTO THE EARTH magnet red ._-...- . 10E 20E 306 406 60E POE 70E 80E gOE lOOE ~Interested Location of Rectangular Anornalo~Js - - -a lnterPreted Storm Drain Location _ Ara ftom GPR l:,ate Duct, mea" _ _ Ares of Rich Concontrasion of EM Anomalies ~ - Approximoto Loeedon of Monger front 1924 Aerial Photograph ............. or A, - agneti C. (;PR nor_ _. . . __~_~.a. MA ::;ir GPR. E Ink of Slm ._` . I ~ ....~..~. mu_ ~ i ! -~- := - .. . ~ I, An, "D ~.- An, ~ = ~ - 1= ~ -. . _ - CPR/ EM, G R ~ auk to flay 19~ 60N 50n 40N ' 30N ~ 2or. ION ON · Conerete and Bronze Monument to 1910 Hangar ,^ ADproxlrnate Location of Original Bronze Marker @a 191ultiple Geophysical Anomaly Ares FIGURE 2.4 Wright Patterson Air Force Base plans to construct a replica of the 1910 Wright Brothers' hangar in the exact location of the original, especially for the centenary of powered flight in 2003. The original hanger was razed in the late 1930s or early 1940s and no surface indication of its exact location exists. An archaeological geophysics investigation was conducted to locate any remaining sig- nature or evidence of the old hangar foundation. This integrated geophysical anomaly map was constructed using surveys of GPR, magnetics, and frequency-domain elec- tromagnetic induction, which were georeferenced to a digitized 1924 aerial photo- graph that showed the hangar. Subsequent archaeological excavations confirmed the geophysical results by finding concentrations of period artifacts. Figure from Butler and simms, 1994. transport and deposition of sediments. In areas of recent volcanism, outcrops provide information about the stages of a volcanic eruption. In areas adjacent to a fault zone, sampling provides information about the style of mechanical defor- mation within the earth. For all of these examples and many others, noninvasive characterization of a three-dimensional volume of the subsurface can provide valuable information about geological properties and processes. Rather than being restricted to a two- dimensional exposed section of material at the surface or a one-dimensional sample of the earth obtained from drilling, noninvasive characterization provides a unique opportunity to study an undisturbed region of the earth. In addition, noninvasive technologies can provide continuous sampling of a region at a sam

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WHY CHARACTERIZE THE SUBSURFACE? 29 pling density unlikely to be obtained through more expensive invasive technolo- gies. One recent example of the use of noninvasive technologies for the advance- ment of basic science is the use of GPR to image the volcanic deposits of Santonn~ (Russell and Stasiak, 1997~. The clear delineation of the basement rocks and the venous layers (corresponding to stages of volcanic activity) either provide infor- mation about the volcanic process that could not be obtained from other sampling methods or help interpret one-dimensional sampling methods. Even in situations where extensive information can be extracted from sur- face outcrops, noninvasive techniques can provide continuous, high-resolution coverage in the third dimension. Studies of sedimentary environments require a quantitative description of the spatial variability in hydraulic properties for mod- eling fluid flow in groundwater aquifers. Although detailed analyses of outcrops can provide direct measurements of variation in properties such as porosity and permeability, noninvasive techniques can characterize the three-dimensional spa- tial variability of a region. The use of noninvasive technologies to characterize the heterogeneity inherent in geological systems will contribute directly to char- actenzation needs for many applications, and may provide basic information required to understand geological processes. Noninvasive technologies provide a way of imaging the earth and quantifying many earth processes. As noninvasive measurement techniques become more accurate, a new level of complexity will probably be revealed in physical processes of rocks and soils. For example, observations of nonlinearity and anisotropy of physical properties might result from improved techniques and sources. Such observations would provide new basic scientific information on subsurface matenals. REFERENCES Butler, D. K., 1984. Microgravimetric and gravity gradient techniques for detection of subsurface cavities, Geophysics 49(7), 1084-1096. Butler, D. K., and J. E. Simms, 1994. Archaeological geophysics investigation of the Wright Broth- ers 1910 Hangar site, Geoarchaeology 9(6), 437-466. Defense Science Board, 1998. Task Force Report on Unexploded Ordnance (UXO) Clearance, Active Range UXO Clearance, and Explosive Ordnance Disposal (EOD) Programs, Task Force Report to the Office of the Under Secretary of Defense (Acquisition and Technology), April 1998. www.acq.osd.mil/ens/esb/dsbfnlrpt.pdf. Inter-Parliamentary Union, 1996. Resolution (20 September 1996) by the 96th Inter-Parliamentary Conference of the Inter-Parliamentary Union, held in Beijing, United Nations. The Joint Unexploded Ordnance Clearance Steering Group, 1997. Unexploded Ordnance Clear- ance: A Coordinated Approach to Requirements and Technology Development, Report to Con- gress, Office of the Under Secretary of Defense (Acquisition and Technology), March 1997. Katsonopoulou, D., and S. Soter, 1996. Ancient Helike in the light of recent discoveries, Archaeo- logical Institute of America Annual Meeting, New York. Lakshmanan, J., and J. Montlucon, 1987. Microgravity probes the Great Pyramid, The Leading Edge 6(1), 10-17. National Research Council (NRC), 1994. Alternatives for Ground Water Cleanup, Water Science and Technology Board, National Academy Press, Washington, D.C.

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30 SEEING INTO THE EARTH NRC, 1998. Ground Penetrating Radar for Evaluating Subsurface Conditions for Transportation Facilities, Transportation Research Board (NCHRP Synthesis of Highway Practice, Report 255), Topic 26-08), Washington, D.C., 37 pp. National Transportation Safety Board (NTSB), 1997. Protecting Public Safety Through Excavation Damage Prevention, Safety Study NTSB/SS-97/01, Washington, D.C., 106 pp. Rivett, M. O. and J. A. Cherry, 1991. The effectiveness of soil gas surveys in the delineation of groundwater contamination: Controlled experiments at the Borden field site. In Proceedings of the Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater, Hous- ton, Texas, November 20-22, pp. 107-124. Roberts, D. E., and G. R. T. Hudson, 1983. The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia, Economic Geology 78(5), 799-822. Rumer, R. R., and J. K. Mitchell, eds., 1996. Assessment of Barrier Containment Technologies: A Comprehensive Treatment for Environmental Remediation Applications, NTIS Publication No. PB96- 180583, 437 pp. Russell, J. K., and M. V. Stasiak, 1997. Characterization of volcanic deposits with ground-penetrat- ing radar, Bulletin Volcanology 58, 515-527. Sternberg, B. K., 1993. Construction of a lined basin for tests of the high resolution subsurface imaging ellipticity system, EPRI (Electrical Power Research Institute) Final report on Research Project 2485-11. U. S. Army Environmental Center (USAEC), 1994. Unexploded ordnance advanced technology demonstration program at Jefferson Proving Ground (Phase I), Report No. SFIM-AEC-ET-CR- 94120, U.S. Army Environmental Center, Aberdeen Proving Ground, Maryland.

Representative terms from entire chapter:

land mines