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1 Introduction Just beneath our feet is an environment that supports our built infrastructure, yields much of our water, energy, and mineral resources, supports agriculture, and serves as the repository for most of our municipal, industrial, and radioactive wastes. Processes within this environment can lead to a variety of soil instabili- ties that constitute natural hazards, such as landslides and sinkholes; the subsur- face is also susceptible to contamination and modification by human activity. During the next century, there will be increasing pressures to use and understand the shallow subsurface for a myriad of applications (see Box 1.1~. Safe, effective use of the near-surface environment of the earth will be a major challenge facing our global society in the twenty-first century. An accurate description or charac- terization of the shallow subsurface environment is critical to the solution of many resource, environmental, and engineering problems, but our ability to do so is often limited. Applications of subsurface information abound. Figure 1.1 is a conceptual diagram that attempts to show four major types of applications, many with differ- ent motivations. For instance, basic science is driven largely by understanding and knowledge acquisition, whereas infrastructure applications tend to be driven by engineering needs and the use of subsurface knowledge. Subsurface applica- tions of public health and safety are dominated by regulatory concerns, whereas resource extraction is driven by economic returns. These four general applica- tions form a continuum of sometimes interrelated and sometimes competing objectives. Because those applications driven by economics tend to progress more rapidly, this report emphasizes the need for public sector involvement in developing applications relating to regulatory, health and safety, and scientific concerns. 6
INTRODUCTION 7 In the United States, the total cost of cleaning up an estimated 300,000 to 400,000 contaminated groundwater sites over the next 30 years may range as high as $1 trillion (NRC, 1994~. Accurate subsurface characterization of a site (see Box 1.2) prior to cleanup is essential in designing and implementing effec- tive remediation systems (NRC, 1997~. Subsurface characterization is likewise critical to infrastructure development, repair, and replacement, the cost of which is estimated to be more than $1 trillion (American Society of Civil Engineers, 1998~. Rapid, inexpensive, reliable characterization could save an enormous amount of money through improved performance in environmental and engineer- ing applications. Techniques for describing the subsurface environment involve many disci- plines and have myriad potential applications. In this report, characterization of the subsurface refers to the determination of physical, chemical, and in some cases, biological information about subsurface properties and processes that we can neither see nor easily sample from the surface. At present, most subsurface characterization involves invasive methods drilling, trenching, excavation- and the methodologies are well established. Direct or invasive methods of characterization (such as drilling and excava- tion) can be expensive, and they often may disrupt human activities and cause unnecessary environmental damage. Indirect or noninvasive methods hold the promise for rapid, low-impact, and relatively inexpensive characterization of the earth's subsurface just as X-rays, sonograms, and other medical imaging tech- nologies have reduced the necessity for invasive diagnostic surgery and have revolutionized medical practice. As in medicine, there are many situations in which invasive methods are required to characterize the shallow subsurface.
8 SEEING INTO THE EARTH PUBLIC HEALTH AND SAFETY ... 1 it' Mines ~ BUILT INFRASTRUCTURE ) Contaminants Waste Disposal_ ~ Utilities ~ ( Transportation ) Ground ~ Petroleum Cow BASIC SCIENCE Fuels / RESOURCE EXTRACTION FIGURE 1.1 Four general agendas (rectangles) met by noninvasive subsurface characterization, and a spectrum of some representative applications (ovals) in rela- tion to their corresponding agendas. In general, there is a trend from regulatory- driven applications in the upper left corner to more economically driven applica- tions in the lower right corner; there is also a trend from knowledge acquisition in the lower left corner to knowledge application in the upper right corner. NOTE: UXO = unexploded ordnance. Advances in instrumentation, computation, transportation (in terms of robot- ics and aerial and space instrument platforms), and communication have signifi- cantly expanded the practice and the research opportunities for seeing into the earth noninvasively. These new advances and the resulting wealth of data they produce not only have intensified the task of data management, but also have presented problems for practitioners and site managers, who must decide what techniques are appropriate at a given site. Nonetheless, noninvasive characteriza- tion methods hold great potential for defining subsurface details with a high level of accuracy, precision, economy, and safety, if they are used consistently and effectively. Realizing this potential will require concerted interdisciplinary ef- forts by earth scientists, geotechnologists, government agencies and regulators, and the user community. PURPOSE OF THIS REPORT A variety of noninvasive techniques for subsurface characterization may offer distinct advantages over traditional invasive methods. This report focuses on techniques that hold the potential to reduce the need for invasive site charac
INTRODUCTION 9 terization by providing more continuous coverage of subsurface features and properties. The report (1) assesses current capabilities for characterizing the near- surface environment using noninvasive technologies, (2) identifies weak links in current capabilities and the potential for new and improved methods, and (3) rec- ommends research and development to fill these gaps. Following this introductory chapter, several illustrative applications are given in Chapter 2. Chapter 3 follows with a discussion of what is measured or charac- terized. The bases of the various noninvasive techniques are developed in Chap- ter 4, with an emphasis on the strengths and limitations of the techniques and the nature of the research and development needed to improve the capability of a specific technique. This is not an exhaustive treatment some methods are only briefly mentioned, whereas most of the "standard" methods are treated more fully. The committee purposely avoided discussion of the pros or cons of specific commercial instruments and chose to treat the general method. Chapter 5 deals with issues of data interpretation integration of data from multiple methods, modeling, and visualization. A discussion of some of the nontechnical issues that the committee found hampered the broader application of noninvasive techniques is given in Chapter 6. Finally, Chapter 7 looks at some steps that could further develop noninvasive characterization and their practice. WHAT IS NONINVASIVE? Noninvasive methods, involving little or no disruption of surface materials, are able to (1) sense and record the location of buried objects; (2) determine geological, geochemical, and geobiological properties; (3) detect and map con- taminants and monitor their movement; and (4) assess structural, lithologic, strati- graphic, and hydrogeologic conditions. Many are geophysical techniques that measure responses to acoustic, electromagnetic, or electrical stimuli or detect changes in natural physical or chemical properties of the earth (e.g., gravitation,
10 TABLE 1.1 Relative Invasiveness of Measurements SEEING INTO THE EARTH Least Invasive Satellites, aircraft Helicopter-borne Walk on ground Disturbance, <1 m Disturbance, <3 m Example Technique Remote sensing, aerial photographs Remote sensing, electromagnetics, magnetics Magnetics, gravity, conductivity, ground penetrating radar Surface seismic, resistivity, geochemical sampling, biological sampling, soil-gas sampling Shallow trenches, penetrometers, direct-push technologies Drill holes adjacent to volume being investigated Drill holes into volume, Direct sampling Borehole methods (including tomography) using seismic, radar, electromagnetics, resistivity) trenching Most Invasive magnetic field, composition). Geophysical methods applied on the surface and from airborne and space platforms (including multispectral remote sensing tech- niques) are relatively mature, whereas noninvasive geochemical and geobiological techniques are less advanced. Any physical measurement can potentially disturb the material being inves- tigated. The significance of the disturbance, of course, depends on the applica- tion. A continuum exists from least invasive (remote sensing from satellites or aircraft) to highly invasive (drilling into and making direct measurements in the volume to be investigated). Several techniques may be minimally invasive, such as small-diameter penetrometers and soil probes that penetrate from a few tenths to several tens of meters into the subsurface. More invasive methods include borehole logging techniques and geophysical methods that use powerful energy sources and require larger-diameter boreholes. Actions considered invasive and disruptive in one instance (e.g., driving a four-wheel-drive vehicle on tundra) may be benign in other circumstances (driv- ing along a paved road). Drilling into the volume of earth being investigated would certainly be regarded as invasive, but drilling boreholes adjacent to the volume being investigated to perform cross-borehole measurements or measure- ment between the drill hole and the surface might not be. Table 1.1 lists some measurement techniques in relative order of increasing invasiveness. The concept of noninvasive must necessarily be flexible. NEAR-SURFACE APPLICATIONS OF NONINVASIVE TECHNIQUES Noninvasive characterization of the shallow subsurface can serve many ends (see Box 1.3~. Many of the techniques have been developed from the decades-old geophysical methods used to explore for petroleum and other mineral resources.
INTRODUCTION 11 The near-surface application of these well-established techniques, however, usu- ally demands higher resolution than that commonly found in the petroleum in- dustry. Sheriff (1991) defined resolution as "the ability to separate two features which are very close together; the minimum separation of two bodies before their identities are lost on the resultant map or cross section." The limits of both horizontal and vertical resolution for noninvasive methods can be determined from the laws of physics (e.g., Widess, 1973~. There is a difference, however, between resolving a body and detecting a body, because detection does not imply determination of size. The physical requirements for detection are less stringent than those for resolving a body. The extension of the established techniques to near-surface applications is relatively new, circa the 1970s, and less mature. For instance, the practical use of ground penetrating radar (GPR) and seismic reflection techniques for environ- mental purposes dates only from the mid-1980s, though many of the techniques themselves date from the 1920s. A survey of the evolution of geophysical meth- ods applied to engineering and environmental problems is available through a series of manuals produced by the U.S. Army Corps of Engineers (Department of the Army, 1948, 1979, 1995~. The roots of many of the methods applied to noninvasive subsurface charac- terization go back decades. For example, measurements of natural voltages asso- ciated with weathering of sulfides in Cornwall, England, date to the 1830s. The
2 SEEING INTO THE EARTH first practical use of explosion seismology occurred in World War I, when the French used seismic refraction arrivals to locate German artillery emplacements. Seismic reflection was first used successfully in 1921, and the petroleum industry has routinely used reflection methods in its quest for hydrocarbons ever since. Except for magnetotelluric methods, almost all the other classical electrical meth- ods of geophysics had been investigated to some degree by 1930. In a sense, then, some of the noninvasive methods have come full circle from initial development for surficial research, to use for resource exploration at depths of a few kilometers, to renewed use in near-surface applications. Geophysical surveys, soil-gas analyses, and interpretation of aerial and space imagery can help to characterize contaminant sources (e.g., location of buried tanks or abandoned wells), identify geological influences on fluid and contami- nant movement (e.g., stratigraphy and faults), or determine and monitor the ex- tent of subsurface contamination at environmental remediation sites. Such char- acterization can help plan the location of drill holes and physical sampling (and their representative nature). Noninvasive techniques can also be used as part of engineering design to prevent future engineering and environmental problems. Increasingly, geophysical methods are being used prior to construction to help assess the subsurface integrity of proposed locations for industrial and govern- ment facilities such as chemical plants and facilities for waste storage and dis- posal. Geophysical and other noninvasive methods continue to be developed for near-surface resource exploration, particularly mineral resources and groundwa- ter. Finally, noninvasive methods are important for purposes of research and enhancement of basic geological and hydrologic knowledge. Measurements made at or just below the ground surface using noninvasive methods can cost less than invasive methods that involve trenching or drilling, installation of monitoring wells, sampling, or chemical analysis. Digging holes into the soil and drilling wells into deeper layers are necessary to directly sample constituents and determine the exact composition of shallow and deeper layers underground. By itself, however, drilling provides a narrow, one-dimensional sample of the ground. Noninvasive methods can provide continuous coverage of features and properties and reveal trends and patterns that might easily be missed by drilling. Because drilling can be expensive, noninvasive techniques can save money by optimizing well placement and reducing the number of wells required. Further, there are many situations in which drilling or disturbing the earth is impractical, unsafe, or prohibited. In polluted areas, drilling may pose a risk to workers and the environment because wells could promote the spread of contami- nants. Drilling on busy urban streets can be disruptive to traffic as well as being risky to buried utilities. Table 1.2 lists the main classes of noninvasive techniques. Many of the relevant noninvasive geophysical techniques for characterizing the near surface were developed for the oil, water, and mineral exploration industries or for geotechnical applications in civil engineering. The detection of underground
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4 SEEING INTO THE EARTH structures, caves, land mines, and unexploded ordnance has been a research priority of the U.S. Department of Defense, requiring very high resolution, noninvasive techniques. USING THE TOOLS AND TECHNIQUES Several geological, geochemical, and geophysical techniques are used in the field for characterization of the shallow subsurface with varying degrees of suc- cess and cost-effectiveness. Reviews of current applications of shallow explora- tion techniques, their methods, and a variety of case histories can be found, for example, in the three-volume book Geotechnical and Environmental Geophysics (Ward, 1990), in the annual Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems (e.g., SAGEEP, 1998), and in the Journal of Environmental and Engineering Geophysics as well as many other technical journals and texts (e.g., Sharma, 1997~. Recently, the Ameri- can Society for Testing and Materials (1999) issued a guide for selecting surface geophysical methods. All near-surface techniques sense some physical or chemi- cal parameter at the surface of the earth. The resulting measurements are then used to infer permeability, porosity, chemical constituents, stratigraphy, geologi- cal structure, and other properties beneath the survey area (see Table 1.3~. Where there is the need, the noninvasive characterization can be checked by invasive "ground truth" measurements, which may allow further calibration of the noninvasive methods and help in modeling the specific site's subsurface condi- tions. Resulting characterizations provide critical input to the development of a conceptual model for a site, which is the initial step in an "expedited site charac- terization process" (American Society for Testing and Materials, 1997~. Figure 1.2 schematically indicates representative steps that can be used for noninvasive characterization, if the objectives of the characterization effort are defined. As this figure illustrates, a series of steps links the technique used to the actual property that is to be estimated. A desired property, such as aquifer poros- ity, is not measured directly but rather must be determined from the measured parameters using models and interpretation. As implied by this figure, the process is inherently iterative. For example, the desired parameter to be measured is determined before beginning the survey design. However, modeling done during the survey design phase might show that the interpretation of this parameter will not produce the desired result; thus, it would be necessary to reconsider the charac- terization strategy and select a different parameter. Alternatively, real-time inter- pretation in the field might show that the geological basis for the survey design was wrong, making it necessary to interrupt data collection and redesign the survey. In assessing the capabilities and limitations of a particular noninvasive tool (selected using steps 1 and 2 in Figure 1.2), the critical factors include the mea- surement process (step 3), the interpretive model (step 4), and the derivation of the desired parameters from the interpretation (step 5~. Table 1.3 indicates gen
INTRODUCTION TABLE 1.3 General Applicability of Selected Geophysical Methods to Typical Site Assessment and Monitoring Objectives. 15 a ~£ ° O ~R Example Objectives v, v, ~¢ Geologic mapping Hydrogeology characteristics Water table depth Top of bedrock Cavity detection Disposal trench mapping Nature of trench fill Inorganic contaminant plume Organic contaminant plume Disposal container (metal drum) Underground storage tanks UXO detection o o o o na na na na na na na na O O o O o o o o o o O ? O O O O O O O O O O O O O O O O ? o o o (~) (~) na O o na ? O na ? na na na na na na O na na na O na O o na KEY: 0 = primary applicability; (~) = secondary supporting applicability; (~) = limited applicability; na = no general applicability or not widely used; and ? = area of active research and rapidly evolving technology or questionable application. NOTE: This table indicates the relative applicability of the various methods; however, there are many exception, and this should not be used as a basis for definitive planning and contracts. Similar tables have been constructed as general guides by others (e.g., ASTM, 1997). eral applicability of individual noninvasive geophysical methods to various char- acterization goals. How the measurement process is designed and performed in the field and how the resulting data are processed, modeled, and interpreted will largely determine how accurately the desired parameters can be determined (e.g., Plate 1~. Logistical decisions, such as the spacing of survey lines, and the inherent limitations owing to the physics of the measurement process will affect measure- ment accuracy and resolution. Modeling capabilities and our understanding of the relationship between the measured parameters and the estimated properties will control the accuracy and uniqueness of the final result. Furthermore, some prop- erties simply cannot yet be deduced unambiguously using noninvasive methods. For example, groundwater flow calculations depend on estimates of hydraulic conductivity for which direct noninvasive measurement techniques currently do not exist. Other examples of parameters that cannot be unambiguously deter- mined include porosity, grain size and orientation, and clay content and mineral
6 SEEING INTO THE EARTH Step I: Determine Too] and/or Technique r Step 2: Identify Measured Parameters __ 1 Step 3: Determine Measurement Process 1 t (Step 4: Determine Models ~ 1~ and/or Interpretation J 1 g Identify ~Parameter - J FIGURE 1.2 Steps in noninvasive parameter identification ogical variations. More work is needed to develop new tools, techniques, and interpretive methods for determining such parameters. In general, there is also a difficulty in linking physical parameters to geological and hydrological features and processes. REFERENCES American Society of Civil Engineers, 1998. 1998 Report Card for America's Infrastructure, Wash- ington, D.C. American Society for Testing and Materials (ASTM), 1997. Provisional Standard Guide for Expe- dited Site Characterization of Hazardous Waste Contaminated Sites, ASTM PS 85-96. American Society for Testing and Materials (ASTM), 1999. Provisional Guide for Selecting Surface Geophysical Methods, ASTM PS 78-97, 10 pp. Department of the Army, 1948. Subsurface Investigations: Geophysical Explorations, Engineer Manual EM 1110-2-1802, U.S. Army Corps of Engineers, Washington, D.C.
INTRODUCTION 17 Department of the Army, 1979. Engineering and Design: Geophysical Exploration, Engineer Manual EM 1110-1-1802 (31 May 1979), U.S. Army Corps of Engineers, Washington, D.C. Department of the Army, 1995. Engineering and Design: Geophysical Exploration for Engineering and Environmental Applications, Engineer Manual EM 1110-1-1802 (31 August 1995), U.S. Army Corps of Engineers, Washington, D.C. Internet: www.usace.army.mil~inet/usace-docs/ eng-manuals/eml 110-1-1802/toc.htm Environmental Protection Agency (EPA), 1997. Expedited Site Assessment Tools for Underground Storage Tank Sites: A Guide for Regulators, EPA Office of Underground Storage Tanks, EPA- 510-B-97-001, Washington, D.C. National Research Council (NRC), 1994. Alternatives for Ground Water Cleanup, National Acad- emy Press, Washington, D.C. NRC, 1997. Innovations in Ground Water and Soil Cleanup: From Concept to Commercialization, National Academy Press, Washington, D.C. SAGEEP, 1998. Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP-98), Environmental and Engineering Geophysical So- ciety, Wheat Ridge, Colorado, 1305 pp. Sharma, P. V., 1997. Environmental and Engineering Geophysics, Cambridge University Press, Cambridge, U.K., 475 pp. Sheriff, R. E., 1991. Encyclopedic Dictionary of Exploration Geophysics, 3rd Edition, Geophysical Reference Series #1, Society of Exploration Geophysicists, Tulsa, Oklahoma, 376 pp. Ward, S. H., ea., 1990. Geotechnical and Environmental Geophysics, Investigations in Geophysics No. 5 (3 volumes), Society of Exploration Geophysicists, Tulsa, Oklahoma. Widess, M. B., 1973. How thin is a thin bed? Geophysics 38, 1176-1180.
