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4 Methods of Characterization The principal methods for determining subsurface properties are reviewed in this chapter. The methods are reviewed briefly (scientific and technical details can be found in the referenced literature), followed by their range of application and limitations, and the prospects for their improvement. The major noninvasive characterization tools involve geophysical sensing of potential and propagating fields. In addition, a limited number of noninvasive geochemical and geobio- logical measurements can be made. Measurements for characterizing the subsurface may be performed from laboratory to planetary scales; from instrument platforms in boreholes, on the surface, and in vehicles (trucks, boats and airplanes); and from satellites in orbit. Some methods work only from certain platforms (e.g., seismic measurements cannot be made from satellites or aircraft), and a few can be done from all (e.g., electromagnetic observations). In general, the closer the instrument is to the material being measured, the higher is the resolution. Measurement techniques "at a distance" (usually from aircraft or satellites) are remote-sensing methods with meter to tens of meter resolution. Measurement techniques requiring bore- holes (single-hole well logging; geophysical sensing from hole to hole, hole to surface, surface to hole, hole to tunnel, etc.) are invasive, requiring the drilling of a hole. However, they can often provide greatly improved resolution compared to surface measurements. Many of these invasive well-logging techniques are thor- oughly reviewed by Ellis (1987~. In noninvasive characterization, the depth of investigation is highly dependent on technique, logistical constraints, and other factors discussed below, ranging from no surface penetration (surface photoimaging) to hundreds of kilometers in depth (seismic and electromagnetic induction). 52
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METHODS OF CHARACTERIZATION 53 Independent of sensor type, all of these methods of characterization also can produce anomaly maps. Such maps yield information about the location of places or regions that are somehow different (or anomalous) from other places. Even if only anomaly information is available, it at least guides later invasive investiga- tion (drilling) to sample these differences. To go beyond simple anomaly maps requires knowledge of the sensor func- tion, logistics of deployment, sensor location and orientation, sources of noise and interference, and so forth. Such information allows computer processing to correct for biases introduced in the measurement process, for example, because of limitations of the instrument (the instrument transfer function), logistical con- straints, or sources of noise. Further, such detailed information can allow the modeling of the measurements and prediction of success in problem application as well as interpretation of derived quantities. For example, if fluid flow is of interest, because the techniques only directly measure changes in some physical field (such as electric or elastic fields), the fluid flow parameters have to be derived through modeling. In all of the methods of characterization, there are certain common problems. Historically, the single largest error often has been precise knowledge of the location and orientation of the measurement sensor. It does not help to have a good measurement but be unable to relocate the measurement site to guide a drill rig to penetrate a contaminant plume. This location sensitivity is especially true of moving sensors in vehicles and satellites, but also of fixed sensors (such as seismic geophones) where later processing and modeling brings out features in the data that must be located. Inadequate locational information has been ru- mored to be the reason for the failure of more than one site characterization or exploration survey. Adequate location surveying may also take longer and cost more than the geophysical survey, although the growing use of GPS (Global Positioning System) technology is ameliorating this problem. Another common problem is lack of property contrast. In comparison to lack of optical contrast, which makes it difficult to find a black cat in a dark coal bin, it is easy to find a furry cat against hard coal by touch. Thus, it is important to consider the available contrast in properties between the target and the host background materials. In a practical sense, many environmental and many engineering geophysical surveys are conducted under less than ideal conditions; for example, often a site is disturbed by human activities including prior excavation of soil or delivery of fill material. Other problems include the presence of either buried or surface utilities such as tunnels, gas lines, sewer drains, and water mains. The mere presence of these more or less passive anthropogenic features disturbs the signals that would otherwise be obtained. Other noise sources include active field disturbances caused by human ac- tivities such as interferences (electromagnetic methods pick up all nearby good conductors, e.g., metallic pipes, wires, and fences), and sources of noise (seismic
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54 SEEING INTO THE EARTH noise from wind or nearby traffic; electromagnetic noise from radio stations, cellular phones, and so forth). Seismometers can be susceptible to 60 Hz noise from power lines, as well as higher modes of 60 Hz (such as 120 Hz, 240 Hz, etc.~. In addition to noise problems, there are often logistical constraints (e.g., denial of access to secure or hazardous areas) and physical requirements (e.g., seismic methods require ground contact and are not often effective through con- crete) that are difficult to meet. Each of these is discussed in further detail for the individual method. There are two major types of geophysical measurements. One is measure- ment of potential fields that result from forces decaying away from a source of stored energy. The most common potential-field techniques measure gravita- tional and magnetic fields; less commonly used are thermal and stress fields, which exhibit a quasi-static, nearly time-invariant (or slowly varying) depen- dence on a force generated by a gradient in a field. For all of these methods, the depth of investigation and the resolution are controlled by the measurement sampling interval. Closely spaced measurements give higher resolution for nearby changes in properties, but the resolution decays exponentially with increasing depth. In general, a discrete object with a high contrast against its background is detectable at a depth ten times the size of the object. Measurements of small perturbations in the large source field are made with part-per-million precision and accuracy. The other major type of geophysical measurements, uses propagating fields. Propagating fields result from a disturbance in a field within a material medium that has the capability to store energy. Principal techniques include various adap- tations of seismology and ground penetrating radar (GPR). Resolution is con- trolled by the frequency and the velocity of the propagating wave and is generally comparable to a wavelength. Resolution is also related to the geometry of the sensors and may be much better than one wavelength for arrays of sensors. The depth of penetration is linear with the inverse of frequency (period) and con- trolled by the losses that cause the eventual decay of the propagating wave. Measurements are made of scattered waves in the absence of the source field. POTENTIAL-FIELD METHODS Gravity Measurements Gravity (a potential field) methods measure changes in the earth's natural gravitational field caused by internal variations in bulk density. Density is a basic property of all materials describing the volumetric packing of mass in space. Gravity describes not only the density of minerals but also the packaging of minerals, including fluids and voids in the interparticle spaces (porosity) between mineral grains. The gravity field is a vector quantity pointed toward the center of the earth, with a minor horizontal component near extremes of topography (moun
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METHODS OF CHARACTERIZATION 55 tains and canyons). Commercially available sensors are quite simple in prin- ciple measuring the vertical field strength, but they are delicate, expensive, and sophisticated in practice owing to the required precision of measurement (parts per billion) and necessary corrections for location on the earth (altitude and latitude) and for environment (temperature, barometric pressure, tides). Funda- mental principles are described in Blakely (1996) and Hinze (1994~. The subsurface condition that leads to a surface anomaly in the gravitational field is a density variation that changes with horizontal location (lateral density contrast) or depth. A variety of geological conditions cause lateral density con- trasts (e.g., lithologic changes, cavities, faults, folds) as do buried human-made features (e.g., trenches, tunnels, disposal containers). For example, Roberts et al. (199Oa) detected density differences within landfill material in a glaciated area in the U.S. midcontinent. Applications and Limitations Applications of gravity address engineering, environmental, groundwater, and archaeological requirements, such as detection of cavities and tunnels, map- ping of density variations in landfills or aquifer materials, location of under- ground storage tanks, location of buried river channels, detection of faults and fracture zones, and infrastructure assessments. Butler (1984) discusses the use of gravity gradients for near-surface investigations. Because gravity measurements can be taken virtually anywhere, surveys are possible on, inside, and immediately adjacent to structures; on pavement and concrete slabs; and under conditions where other noninvasive methods are not always applicable (Yule et al., 1998~. However, certain frequencies of mechanical vibrations can make attaining preci- sion measurements difficult. Future Prospects Because most technical and theoretical aspects of gravity measurements are quite mature, future improvements will probably be evolutionary in nature. New possibilities are starting to be realized by the application of airborne gravity surveys, which combine gravity determinations with accurate land and sensor positioning using the GPS in a differential mode (NRC, 1994~. At present, the resolution of airborne gravity systems are on the order of a few milligals (1 gal = 1 cm/s2), which is about a thousand times less accurate than microgravity surveys on land. However, the resolution will probably improve with increased attention to this relatively new approach, particularly since it can cover large areas at a smaller cost than land surveys. For certain types of applications, gravity measure- ments from satellites will be possible (NRC, 1997~. A Department of Defense program in the 1980s helped develop a viable gravity "radiometer system (Jakeli, 1993~. The system allows determination of
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56 SEEING INTO THE EARTH all independent components of the gradient tensor from moving platforms. Di- verse applications of the gravity "radiometer measurements are a rapidly evolv- ing area of research (Bell, 1997~. Magnetic Measurements Magnetic methods measure changes in the earth's natural magnetic (poten- tial) field caused by variations in magnetic susceptibility and remanence. Mag- netic susceptibility is the property of some minerals (mostly iron bearing) that describes their ability to be magnetized by an external magnetic field. Magnetic remanence is the property that describes the ability of a material to retain mag- netic field strength and direction in the absence of an external magnetic field. Magnetic fields are static vector fields with three-dimensional variation in direc- tion over the surface of the earth with a small superimposed time-varying compo- nent. It is sometimes important to measure both field strength and direction. Modern commercial sensors are simple in principle, measuring either the total field strength (a scalar) or the three-components of the directional field (a vector). Gradient measurements (derivatives of the field) are less often measured. Mea- surements are performed easily and routinely at the part-per-million level. The techniques are quite mature. Fundamental principles are described in Blakely (1996) and Hinze (1994~. Magnetic interpretation is similar to gravity interpretation because both are based on potential-field theory, except that magnetic anomalies are almost al- ways asymmetrical. It is important to realize that anomalies express the net effect of two bipolar vector magnetic fields (induced and remanent) that usually have different intensities and directions of magnetization. Wavelength filtering can be used to better separate the effects of shallow versus deep-seated sources. Using a high-pass filter brings out anomalies at greater depths. Derivative methods accen- tuate the boundaries of anomalies, both shallow and deep. As with all potential-field techniques, it is impossible to calculate the anomaly's depth unambiguously without knowing the shape and magnetic prop- erties of the source of a magnetic anomaly. However, with a prior knowledge about the source, it is often possible to estimate depths to within a factor of 10 to 20 percent depending on the complexity of the anomaly and site noise conditions. Magnetic "radiometry involves simultaneous measurement by two magnetom- eters close to each other (about 0.5 m). The interval gradient is the difference in magnetic intensity readings divided by the distance between sensors. Commonly, two total field instruments are placed on a vertical staff and the vertical gradient is determined. Two key advantages of gradient surveys are (1) they tend to resolve complex anomalies into their component parts (higher resolution than the magnetic field alone), and (2) because the readings are taken simultaneously, it is not neces- sary to correct for diurnal variations and magnetic storms. The orientation of the line between the two sensors must be kept constant, or at least monitored, because
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METHODS OF CHARACTERIZATION 57 the gradient will vary with the orientation of this line. In addition, the magnetic cleanliness of the operator (belt buckles, watches, etc.) and magnetic cleanliness of the surface of the area surveyed become even more important than for simple total field measurements. The "radiometer technique is extremely sensitive to surface debris such as nails, cans, wire, and other metallic objects. Applications, Limitations, and Prospects In site characterization, magnetic methods commonly are used for finding buried objects such as drums and abandoned underground fuel storage tanks. Often, the analysis can be very simple. A survey is carried out on a grid or profile line, the results are contoured or plotted, and anomaly locations are noted. Buried metallic objects usually show up as dipolar anomalies (magnetic highs with an adjacent low on their north sides in the Northern Hemisphere). However, sophis- ticated filtering and analysis techniques for separation of superimposed anoma- lies and depth determinations can make processing and interpreting magnetic surveys more complicated (e.g., Telford et al., 1990; Burger, 1992~. Where there are localized changes or contrasts in magnetic properties, the earth's field will induce a secondary or anomalous magnetic field. For buried ferrous metal objects, the magnetic permeability is large relative to surrounding soil and rock and results in a large induced magnetic field. Many magnetic objects, particularly ferromagnetic objects, also have a large remanent or perma- nent component. Accurate interpretation of magnetic data depends on being able to distinguish between the induced and remanent components. Gravity and magnetic methods can be used in a complementary fashion to more tightly constrain geological interpretations. Roberts et al. (199Ob) give an example in which magnetic data recorded over a landfill was enhanced by digital processing. Hinze et al. (1990) show how the gravity and magnetic data from the landfill can be combined to assist in the interpretation of its extent and of the material within it. Future prospects include several innovations related to increased use of GPS, high-temperature superconductivity, and cheaper electronics. Increased use of GPS could lead to robotic control of magnetometers, including unmanned air- craft. High-temperature superconductors may lead to additional sensitivity for portable magnetometers. Cheaper electronics and computing could lead to real- time contouring of data in the field and to increased use of magnetic "radiometry in which two or more magnetometers are read simultaneously at slightly different locations. ELECTRICAL AND ELECTROMAGNETIC METHODS "Electrical methods" refer to measurements of natural or impressed electri- cal fields (potential fields) at low-frequency alternating current (ac) or direct
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58 SEEING INTO THE EARTH Electncai Resist~vity Method C, P1 ~, ~ . P2 ; C '~..~.,~ , ^., ~ .r, at, .~ . ~ ~ ,\ ,., ,~. ,~, _' b ': g '. r2 ~ X 2~' ~ i'; =dii~ dX ~ } ~ ~ ~ ~ r -; ;~ ~ ~ ~ ~5 ·~ ~ I, ., 2 ,~ ,5 5 Electromagnetic Induction | Methods I ( ~ Ti HE ~. ~i ~ ;~ Hs:RX ` ... . . I' ~ ~ ~ at,, . ., -. ~ ~. ~ >~N FIGURE 4.1 Simple comparison of electrical resistivity methods and electromag- netic induction (EM) methods. EM methods are generally noncontact, whereas resis- tivity methods require driving metal electrodes into the ground. current (do) using electrodes attached to the ground. By contrast, electromagnetic (EM) methods measure magnetic fields associated with time-varying subsurface currents induced by a natural or artificial electromagnetic source (propagating fields). A schematic comparison of the two is shown in Figure 4.1. GPR is based on high-frequency electromagnetic wave propagation.
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METHODS OF CHARACTERIZATION 59 Field Electrical Measurements Electrical field methods measure changes in the earth's natural and induced electrical fields caused by changes in the source origins of the fields and in the electrical properties of the earth. Electrical field methods include do resistivity, complex resistivity, and self potential. Electrical properties of interest are (1) the electrical conductivity, which describes the ability of a material to transport electrical charge, and (2) certain electrochemical and coupled processes. Sources of the electrical fields are the natural fields in the earth caused by the natural magnetic field, solar-wind interaction with the earth, lightning from storms, elec- trochemistry (e.g., the battery-like corrosion of naturally occurring sulfide miner- als in water), and coupled processes (e.g., a voltage called the streaming potential is generated by fluids flowing through pores). Human-made sources also exist from grounding of power grids, corrosion of buried metallic objects, and inten- tional artificial sources connected to the ground (do resistivity sounding). Electrical fields are time-varying vector fields with three-dimensional varia- tion in direction over the surface of the earth. Commercial electric field sensors are simple in principle, consisting of a porous container filled with nonpolarizing electrolyte and electrode. Measurements are made by connecting voltmeter ter- minals to electrodes in the ground at two locations. Measurements are easily and routinely performed at the microvolt or millivolt level. Fundamental principles are described in Keller and Frischknecht (1970), for example. dc Resistivity The do resistivity method is a widely used, inexpensive technique for near- surface investigations. Electrical resistivity methods measure the bulk electrical resistivity of the subsurface directly by measuring the voltage generated by trans- mission of current between electrodes implanted at the ground surface (Figure 4.2~. Resistivity data are collected using single or multiple pairs of current and voltage electrodes (dipoles) with known relative positions. They are interpreted by matching them to theoretical models having a subsurface structure of varying conductivity. In the past, resistivity measurements were usually taken in a straight line on the surface, and the interpretation was done in terms of one-dimensional or two- dimensional models. In a sounding, the measurement array can be expanded about a central position and the data interpreted with a vertical one-dimensional model. In profiling, the relative array geometry and electrode spacing are fixed, but the entire array is moved laterally. Variations indicate lateral or two-dimen- sional changes in subsurface resistivity. This work can be reviewed in texts such as Keller and Frischknecht (1970) and Koefoed (1979~. Sounding provides a resistivity map as a function of depth, comparable to drilling a well and logging it for this information. Resistivity measurements are
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60 SEEING INTO THE EARTH current sounce ~ measured 22~ potential . ~ . ~ ~ . ~< FIGURE 4.2 Schematic of a resistivity survey. made at a variety of electrode separations, the depth of the investigation increas- ing with larger separations. Electrode geometry can differ among various appli- cations of this method. Profiling is performed using a constant spacing between electrodes (two outside current electrodes and two inside voltage probes), in which case the arrangement is known as a Wenner configuration (Figure 4.2), and sounding using the Schlumberger configuration where the potential elec- trodes are located in the center of two widely spaced current electrodes. Modern resistivity systems use a multicore cable, multiple electrodes, and computer- controlled switches in a noise-reducing and field-efficient procedure that speeds the data collection process. Depth and resistivity estimates are made with one- or two-dimensional inversion programs. Complex resistivity or induced polarization measurements refer to nonlinear or frequency-dependent resistivity measurements and are treated later in this chapter. It was clear even from early studies (Schlichter, 1933; Pekeris, 1940) that results of the resistivity method must account carefully for both nonuniqueness and resolution issues. More recently, there have been systematic studies of the uniqueness of one-dimensional resistivity sounding results (Parker, 1984; Zohdy, 1989; Simms and Morgan, 1992~. A number of easy-to-use one-dimensional inversion programs are commercially available. Recently, a number of approximate imaging schemes have been developed (e.g., Niwas and Israil, 1987; Zohdy, 1989) that give a better representation of a spatially varying geoelectric section than simple layered-earth models. Two-dimensional resistivity models of the subsurface avoid some of the limitations in one-dimensional models. Clearly, they also entail the collection of much more data. The first step in data processing is usually to display the data graphically in a pseudosection, which is constructed by assigning a resistivity
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METHODS OF CHARACTERIZATION 61 value, measured with a specific geometry, to an approximate position at depth. The pseudosection is a data display, not a geoelectric section. Unfortunately, some practitioners mistakenly contour this pseudosection and use it as the end product for interpretation. Resistivity data may be inverted on a computer using algorithms (e.g., Tripp et al., 1984~. Today, the state of the science is to collect extensive current and electrical potential data not in one direction but rather in two surface directions. These data are then interpreted by computer inversion in terms of two-dimensional or three- dimensional subsurface models. Hundreds to thousands of data points must be collected. The generation and inversion of a full three-dimensional subsurface model requires complex computer codes (Ellis and Oldenburg, 1994; Li and Oldenburg, 1994; Zhang et al., 1995~. However, the general uniqueness and resolution of three-dimensional resistivity inversion have not been investigated sufficiently thus far. Self Potential Self potentials (SP) (sometimes called spontaneous potential) are natural do voltages that exist in the earth. They are measured with a high-input impedance voltmeter using nonpolarizing electrodes, often as a by-product of a do resistivity measurement. Natural voltages rarely exceed 100 mV over several hundred meters, and they usually average to zero over distances that are a few times larger than whatever size anomalies may be present. These electrical fields are caused by fluid flow, subsurface chemical reactions, and temperature differences. Depth of placement of the electrodes can have an effect on the reliability of the readings, as can roots and nearby vegetation. Through the fluid flow streaming-potential mechanism, the SP method rep- resents the only known noninvasive passive method directly related to subsurface fluid flow (the seismoelectric method also can measure fluid flow). Small fluid flows associated with cracks in contaminant containment barriers are probably too small to be observed. However, significant fluid movement associated with remediation, such as pump-and-treat and sparging, should produce measurable anomalies. Furthermore, significant fluid flow from leaking dams can be moni- tored and modeled (e.g., Wurmstich et al., 1991; Wurmstich and Morgan, 1994~. Underground chemical pollution, by definition, produces chemical concen- tration or diffusion potentials. However, a number of factors must be favorable for surface anomalies to be detectable. Large chemical concentration differences, shallow depth, and a high electrical resistivity background all contribute to en- hancing the effect. Furthermore, the specific chemistry involved in setting up the diffusion potentials will determine the level of sustainable electric current avail- able from such an electrochemical battery. The SP method is one of the oldest geophysical methods and shows signifi- cant correlation with subsurface processes. Field data are also relatively easy and
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62 SEEING INTO THE EARTH inexpensive to obtain. However, detailed interpretation is relatively difficult. Because the voltages are low, they are subject to noise from power lines, pipe- lines, electrical storms, and other environmental sources. Care must be taken with the data acquisition field procedures to ensure that the data are repeatable. Induced Polarization (IP) Using an electrode setup identical to that of the resistivity method, the re- sponse of the ground to the removal of an induced electrical signal can be inves- tigated. The IP method involves measurement of the decay of voltage in the ground following the cessation of an excitation current pulse (time-domain method) or low-frequency (less than 100 Hz) variations of earth impedance (fre- quency-domain method). Most of the stored energy involved is chemical, involv- ing variations in the mobility of ions and variations due to the change from ionic to electronic conduction where metallic minerals are present, and can be likened to a capacitive discharge. Various electrode configurations can be used, com- monly dipole-dipole arrays. In the resistivity method, the passage of electric current through the pores of rocks and soils is dominated by the movement of ions in the pore solution. The earth behaves capacitively at low frequency. Induced polarization measures the low-frequency or capacitive behavior. As ions progress through the pore fluid of rocks they also accumulate along and across surface boundaries. It is this induced accumulation of charge that produces the capacitive effect. Induced polarization is present in varying degrees in all earth materials. However, it manifests itself strongly in two situations. When electrically con- ducting metallic minerals are present, charges accumulate at surface boundaries as charge flow changes from ionic in solution to electronic through the mineral. IP effects are also significant in earth materials such as clays with high internal surface areas. Here the charge accumulation or capacitance is associated with the ubiquitous electrochemical boundary layer. Application, Limitations, and Improvement of IP Methods Traditionally, IP was assessed in the field by measuring the resistivity at two frequencies or by monitoring the change in decay in response to a current pulse. Modern instruments can also measure the phase difference between the real and imaginary parts (complex conductivity) over a wide range of frequency. Such instrumentation opens a new domain because it allows a broad frequency or spectral response to be recorded in the field. The idea is that the spectral response will have behaviors characteristic of the specific chemical reactions taking place. Historically, IP has been used mainly to locate metallic minerals in the near surface (Wait, 1959; Madden and Cantwell, 1967; Bertin and Loeb, 1976; Sumner, 1976~. There were a few early attempts to use the method in groundwa
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86 SEEING INTO THE EARTH use is effective coupling to the ground to obtain the broader bandwidth and higher frequencies necessary for high-resolution near-surface applications. · For many years, the seismic receivers of choice in reflection seismology have been velocity geophones. However, manufacturers' specifications some- times do not reach to the high frequencies used in shallow reflection surveys. Consequently, an unbiased and independent research evaluation of receiver at- tributes of high-frequency geophones, following the work of Duff and Lepper (1980), could be useful. Tests should include amplitude, phase, spurious response analysis over a broad bandwidth, at least from 10 to 2 kHz. For shallow high- resolution purposes, accelerometers have become a possible alternative. Other motion-sensitive technologies may be applicable in the future. REMOTE SENSING Remote sensing offers unique observations of the earth's surface and shal- low subsurface that complement conventional mapping and exploration methods. When employed in timely conjunction with field observations, remote sensing can be used to extrapolate local observations over extensive regional areas. A report summarizing remote sensing from satellite and aircraft (Watson and Knepper, 1994) provides a comprehensive evaluation of the state of the art for geological mapping, mineral and energy resources, and environmental studies. It recognizes the evolution from aerial photography to multispectral systems that record solar reflected, thermal emitted, and radar illuminated radiation, and the emergence of imaging spectrometers, which acquire data with spectral resolution comparable to laboratory instruments. There are also several texts on remote sensing. A good source for explaining the physical basis is Elachi (1987~; a report that summarizes many of the opportunities for remote sensing was issued by the National Research Council (NRC, 1995~. An annual conference with published proceedings is sponsored by the Environmental Research Institute of Michigan and is a good source for current application focus. Technical instrument work- shops on instruments are sponsored by the Jet Propulsion Laboratory, and sub- stantial information and illustrative material are available on the Internet. Aerial Photography An ideal environmental remote-sensing system requires high spatial resolu- tion, high sensitivity to changes in baseline characteristics, proven and accessible technologies, and low cost. Airborne photography, which is familiar and rela- tively inexpensive, is still ubiquitous in environmental studies despite obvious limitations awkward archiving, lack of spectral resolution and sensitivity, and difficult integration with correlative geospatial data and digital technologies. Historical photos may provide evidence of waste sites and facilities that are now abandoned. Recent photography facilitates the analysis of current waste disposal
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METHODS OF CHARACTERIZATION 87 practices and locations, drainage patterns, geological conditions, signs of vegeta- tion stress, and other factors relevant to contamination site assessment. Addition- ally, aerial photograph fracture trace analysis is used at sites where bedrock contamination is a concern. Overlapping photo pairs can be used to model topog- raphy. Photography is gradually being replaced by digital image data, a trend that will be hastened as commercial satellites with spatial resolution in the 1- to 5-m range are launched in the next few years. Multispectral Scanners Multispectral scanners digitally record several images simultaneously at dif- ferent wavelength bands. The bands are selected to exploit the greatest sensitivity to features of interest and allow significantly more definitive characterization of surface composition and state than does photography. The data are processed using computer image analysis algorithms based on physical or statistical models and knowledge of laboratory-measured physical properties. The most familiar system is the 30-m-resolution TM (Thematic Mapper) satellite instrument that has six reflectance channels (and a 120-m-resolution thermal channel). A number of aircraft systems are available to acquire additional spectral channels with comparable spectral resolution and somewhat higher ground resolution. Imaging radar, acquired as part of a national program is archived (along with photography from a similar program) at the U.S. Geological Survey's EROS Data Center. These data and their derivative images provide uniform spatial coverage, avail- ability at different resolutions, and the digital format that are important for geo- graphic information systems (GIS) analysis. Reflectance data have been success- fully used to distinguish among geological units, to find hydrothermally altered rocks, to infer tectonic setting and local fold and fault structures, to map linear features that may indicate fracture controls, and to indirectly infer lithologic and structural information in heavily vegetated areas based on empirical correlations between vegetation type, density, distribution, and local geological conditions. Thermal infrared data can be used to map silicification and igneous lithologies, fractures, heat (due to near-surface exothermic reactions or underground coal fires), and changes in near-surface thermal properties and to examine surface water changes and groundwater discharge and seepage. Airborne and satellite radar provides all-weather weather capability to define terrain units, to map topo- graphically expressed features that reflect local and regional geological struc- tures, and in hyperarid terrain, to penetrate the upper meter or two. Imaging Spectroscopy Imaging spectrometry can be used to map minerals at the surface for a wide variety of environmental studies. An excellent example (see Plate 7) is the map- ping by aircraft of acid-generating minerals at the Superfund site in Leadville,
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88 SEEING INTO THE EARTH Colorado (Swayze et al., 1996~. Mine waste material is dispersed over a 30-km2 area in which oxidation of sulfides releases heavy metals that are carried into the Arkansas River, a major source of water for urban centers and agricultural com- munities along the Rocky Mountain Front Range. The spectroscopy identified areas with higher acid-generating capacity based on the identification and map- ping of distinctive zones of iron-bearing minerals. Research Instruments There are also a number of remote sensing instruments that have consider- able promise for surface characterization but are not yet well established. Passive Microwave Radiometry Natural surfaces radiate mainly in the thermal infrared region; however, radiation at lower intensities extends throughout the electromagnetic spectrum into the submillimeter and microwave region. The radiant power emitted is a function of the surface temperature and its emissivity, which in turn are functions of surface composition and roughness. The large emissivity difference between ice and open water makes mapping polar ice cover and its change one of the most useful applications of microwave radiometry. The high dielectric constant (low emissivity) of water relative to most natural surfaces leads to applications involv- ing mapping of soil moisture variation. However, because large variations can also result from differences in surface roughness or composition, repeat measure- ments are required to resolve ambiguities in interpretation. Microwave data can also be used to infer snow extent, onset of snowmelt, and water equivalent of snow. Limitations are the availability of data and the low spatial resolution. (For passive electromagnetic radiation, resolution is proportional to the ratio of re- ceiver diameter to wavelength; thus, to preserve spatial resolution, very large receivers are required at longer wavelengths.) Radar Interferometry Radar interferometry from satellites can be used to detect minute changes in land surface geometry by comparing the phase difference between observations at two different times. Because the method is sensitive to differences as small as a few centimeters, it is sensitive to active faulting, subsidence caused by fluid withdrawal, pre-eruption volcanic swell, erosion, or tectonic creep. Atmospheric differences between the two observation times can cause substantial errors, and it is necessary that the surface has not been too greatly disrupted. This technique appears to have substantial potential for worldwide study of geological hazards once a good database and case history experience have been established.
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METHODS OF CHARACTERIZATION Lasers 89 A number of experimental laser systems have been used from aircraft to illuminate the ground in order to measure surface conditions including surface texture, composition, elevation (decimeter accuracy), and water quality and depth (using fluorescence). GEOCHEMICAL METHODS Assessment of the subsurface geochemistry involves describing the chemi- cal composition of solids, liquids, or gases. That is, the geochemistry of the subsurface may be defined as the chemical composition of bedrock and soil, groundwater and its dissolved or suspended load, and the atmosphere in the unsaturated zone. It is unlikely that all aspects of subsurface geochemistry can be determined remotely. In fact, relatively few chemical parameters can be readily detected without direct sampling and analysis. However, remote methods of chemical sensing for some constituents of interest in contaminated aquifer sys- tems show promise. Volatile Gas Emission Wide use of organic solvents in the industrial and commercial sectors and of refined petroleum as fuels in numerous applications has led to nearly ubiquitous contamination of the environment with volatile organic compounds (VOCs) of a variety of compositions. Volatilization at the surface of the water table and diffusion through the air- filled pore spaces in the vadose zone cause VOCs to be present at the surface overlying a contaminated site (see, for example, Figure 2.1~. Soil-gas analysis became a popular screening tool for detecting VOCs during the 1980s. Soil-gas surveys can generate extensive chemical distribution data quickly at a fraction of the cost of conventional invasive methods and offer the benefits of real-time data. There are two types of soil-gas sampling. Grab sampling typically involves the insertion of a hand-held probe to depths of only tens of centimeters, with the volatiles pumped directly into a portable gas chromatograph. Passive sampling provides a measure of VOCs over time. It uses a sorbent material, such as acti- vated carbon, that is placed below ground and later retrieved for analysis. VOCs and gases can also be important as indicators of biological degrada- tion reactions proceeding at depth. Isotopic information on these gases, obtained through mass spectroscopic methods in the laboratory, may yield even more information about the nature and extent of biodegradation reactions occurring within an aquifer.
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9o SEEING INTO THE EARTH Water Composition Ground water moves into, through, and out of a given portion of the shallow subsurface. In doing so, reactions occur among the components of the aquifer system (water, minerals, atmosphere, and associated biota) that can lead to a change in the composition of the groundwater. Sampling the groundwater in wells or springs downgradient of the site may allow inferences to be made about a portion of the subsurface that we cannot sample directly. That is, the composi- tion of the dissolved or suspended load in the groundwater may be used as an indicator of the composition of the solids, liquids, and gases in the study area, as well as of the reactions they are undergoing. (Many of the principles are similar to the geochemical water sampling developed in ore deposit exploration.) Tracers may be passive or natural products of the environment, or they may be introduced purposefully for the purposes of sampling. Natural or artificial tracers may be introduced and sampled without disturbing the physical integrity of the study site. Analysis of the outcome is by traditional chemical methods in the field or the laboratory. Most solutes in natural and contaminated groundwater are ionic; that is, they are present as charged cations and anions in solution. Dissolved ions can carry an applied electrical current; if they are present in high enough concentrations in groundwater, noninvasive electrical geophysical methods can detect their pres- ence and location. An example of a useful and successful application is in the mapping of saltwater intrusion fronts in coastal water supply aquifers. Fresh groundwater has highly contrasting electrical properties to the intruding seawa- ter, and because of density differences and poor mixing in a porous medium, the contact between the two types of water can be fairly sharp and, in these cases, relatively easily detected and mapped. Another widespread problem is the presence of plumes of landfill leachate within an otherwise clean groundwater system. Electrical methods can map such plumes as well as their migration because the leachates are typically high in dissolved salts and metals and contain a variety of organic compounds. Similarly, acid mine drainage can also be mapped because of high concentrations of dis- solved solids and metals. Significant challenges remain in detecting nonionic contaminants, including many dissolved organic compounds such as pesticides. Composition of the Solid Phase Remote assessment of the chemical composition of the subsurface's solid portions (soil and bedrock) is problematic. Relatively few material properties that can be remotely measured yield information about the chemical composition of solid materials, although the presence of some minerals can be modeled. One approach to the composition would be to use a combination of the knowledge of site geology with geophysical determinations of density or porosity contrasts to support an interpretation of rock type, but this does not go much beyond what a
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METHODS OF CHARACTERIZATION 91 geologist can do without noninvasive technologies. A metallic object can be detected from the surface through the contrast of its electrical or magnetic prop- erties with the enclosing silicate, carbonate, or oxide rock, but little specific knowledge can be gained about chemical composition. Use of self potentials and induced polarization methods potentially could be applied to such chemical de- terminations. Radioactive Methods Detection of natural radioactivity (or that resulting from disposal of radioac- tive materials) can be of use in characterizing the shallow subsurface. Applica- tions include, for example, regional mapping, prospecting for some minerals, and detection of leaking storage facilities containing radionuclides. The same proper- ties that make radionuclides dangerous also make them easy to track in the environment. "The Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) provides a nationally consistent consensus approach to conducting radiation surveys and investigations at potentially contaminated sites" (Environ- mental Protection Agency, 1997~. The manual describes well-tested methods and details the specific methodology and analysis that should be used. Several other aspects of radioactivity that can be valuable in site characterization involve inva- sive (e.g., borehole logging [Ellis, 19871) or direct sampling (e.g., tritium or bomb-pulsed chlorine tracers in subsurface water). GEOBIOLOGICAL METHODS Properties of the biota of most interest to site characterization biologists may be the most difficult to determine noninvasively. The identity, abundance, diver- sity, and ecology of the resident organisms, as well as their overall physiological status, are the most important general properties to assess. Biological processes in the near surface ultimately depend on the genetic makeup of the near-surface biota, which in turn depends on physical and chemi- cal environmental factors that select the biota at a given site. Generic properties of the biota (identity, abundance, diversity, and ecology and their overall physi- ological status and activity) will be important in most site characterizations. However, given the spatial variability and heterogeneity of geological settings, large variations in metabolic activities may occur across a given site. For ex- ample, a process such as aerobic respiration of a pollutant chemical (or biomineralization) depends upon the availability of oxygen, which itself can be controlled by water content, inorganic oxidation-reduction reactions, and content of exchangeable organic compounds. The pollutant chemical itself may be more or less available for respiration depending on its solubility in water, its octanol- water partition coefficient, its organic matter content, or competition with soil particle surface absorbers. Finally, the total abundance of aerobic heterotrophic
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92 SEEING INTO THE EARTH organisms controlled by oxidizable organ~cs will greatly influence the oxygen available for respiration of the pollutant. Presently there are no noninvasive methods for direct measurement of bio- logical presence or metabolic activity in the near surface. However, in some geological settings, subsurface biological activity can be inferred indirectly from near-surface biogeochem~cal activity, which might be measurable using noninvasive methods. For example, near-surface biogeochem~cal activity in the vicinity of oil reservoirs has been mapped by electrical resistivity methods (Sternberg, 1991), and airborne imaging spectroscopy has been used to detect and map biogenic minerals in acid and neutral drainage areas of acidified watersheds (see Plate 7~. Minimally invasive methods such as soil-gas analysis by gas chro- matography, chemical assays of bioaccumulating plants, and bacterial indicator culturing of surface soil have been used to a limited extent for petroleum and mineral exploration as well as environmental pollution studies. Noninvasive tech- nologies show some promise in biological assessments, but until more research is done to develop other methods, the characterization of site biology will still depend to a large degree on analysis of samples obtained by invasive methods. Development of coordinated noninvasive and minimally invasive methods for geobiological site characterization remains a challenge (e.g., Ghiorse, 19971. REFERENCES Annan, A. P., M. L. Brewster, J. P. Greenhouse, J. D. Redman, G. W. Schneider, G. R. Olhoeft, and K. A. Sander, 1992. Geophysical monitoring of DNAPL migration in a sandy aquifer, 62nd Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts 62, 344-347. Bachrach, R., and A. Nur, 1998. High-resolution shallow-seismic experiments in sand, Part I: Water table, fluid flow, and saturation, Geophysics 63, 1225 1233. Bell, R. E., 1997. Gravity "radiometry resurfaces, The Leading Edge 16(1), 55-59. Bertin, J., and J. Loeb, 1976. Experimental and Theoretical Aspects of Induced Polarization, Vols. I and II, Gebruder Borntraeger, Berlin-Stuttgart. Birkelo, B. A., D. W. Steeples, R. D. Miller, and M. A. Sophocleous, 1987. Seismic-reflection study of a shallow aquifer during a pumping test, Ground Water 25, 703-709. Blakely, R. J., 1996. Potential Theory in Gravity and Magnetic Applications, Cambridge University Press, Cambridge, 441 pp. Bradford, J., M. Ramaswamy, and C. Peddy, 1996. Imaging PVC gas pipes using 3-D GPR, Pro- ceedings of the Symposium on the Application of Geophysics to Engineering and Environmen- tal Problems, Environmental and Engineering Geophysical Society, Wheat Ridge, Colorado, pp. 519-524. Branham, K. L., and D. W. Steeples, 1988. Cavity detection using high-resolution seismic reflection methods, Mining Engineering 40, 115-119. Brewster, M. L., A. P. Annan, J. P. Greenhouse, B. H. Kueper, G. R. Olhoeft, J. D. Redman, and K. A. Sander, 1995. Observed migration of a controlled DNAPL release by geophysical methods, Groundwater 33, 977-987. Buker, F. A., G. Green, and H. Horstmeyer, 1998. Shallow 3-D seismic reflection surveying: Data acquisition and preliminary processing strategies, Geophysics 63, 1434-1450.
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Representative terms from entire chapter: