Seeing into the Earth Noninvasive Characterization of the Shallow Subsurface for Environmental and Engineering Applications (2000) / Chapter Skim
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3 What is Characterized?
3 What is Characterized?
Pages 31-51
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Passive investigations measure variations in naturally occurring fields (the earth's gravity, magnetic, electric, thermal, radiometric, stress, solar irradiation, and hydraulic fields)
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Heat flow, induced electrical current flow, and hydraulic fluid flow are all processes described by the diffusion equation, with the diffusion coefficient describing the property of conductivity. Mechanical particle movement and the coupled electromagnetic field behavior are described by the equations of wave propagation.
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All aspects of site characterization and remedial investigations are influenced by the geological setting. Lithology The different physical properties of different lithologies make it possible to obtain information about them from geophysical measurements.
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Limestone and other soluble rock types may be extensively dissolved at depth, creating secondary porosity and permeability. Being heterogeneous in their distribution, such subsurface conduits in limestone are difficult to map, though certain noninvasive techniques can detect large openings in shallow bedrock (see Figure 2.3 using a microgravity method)
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have significant porosity and permeability, but most rock types, whether sedimentary, igneous, or metamorphic, do not. Ground water occurrence and movement in such rocks is almost entirely controlled by structural features.
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that prevents the penetration of radar signals. The groundwater table can have a similar limiting effect if the conductivity of the water is high.
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provides a comprehensive review of research on techniques and approaches to fracture characterization and fluid flow in rock fractures. Fracture detection depends on detecting physical property change across the fracture or within the fracture itself (see Figure 3.2~.
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Detecting fractures beneath the surface often depends on observing contrasts in physical properties such as dielectric constant, electrical conductivity, P-wave seismic velocity and attenuation, magnetic susceptibility, and density all of which can be related to interconnected void space or moisture content of the fracture zone. High spatial resolution is required for both location and detection of fractures.
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In almost all cases, surface geophysical methods cannot characterize completely a fractured rock site because the fractures that have flow cannot be separated from fractures without flow. To characterize such sites, hydraulic testing and borehole geophysical methods usually are required.
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Soil gas is primarily air, but also contains on the order of 1 weight percent water vapor and may contain trace amounts of organic chemical vapors as well as noncondensable gases such as CO2 and radon. Beneath the water table, the gaseous phase is usually unimportant, and there is a single aque
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Demands on spatial resolution and identification of minor fluid components tend to be greatest in the area of contaminant hydrology. Noninvasive techniques are usually incapable of unambiguously resolving site characterization needs relating to fluids, but they can contribute valuable information, especially when used in conjunction with a minimum amount of invasive methods for providing "ground truth." Common site characterization tasks include two that are identified as part of geological characterization: the location of permeable features and the location of features with low permeability such as clay layers.
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42 C cn IL O Q Cd In ~ ~ O Z ~ D C]
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In addition to geophysical detection, increased biological activity at the water table may cause oxygen depletion, changes in pH and eH, production of biomass, specific mineral accumulations, and gas production (methane, CO2, dissolved hydrogen)
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Dissolved and immiscible organic contaminants remain virtually impossible to detect noninvasively; this vexing environmental problem is an opportunity for continued research. Under certain circumstances, nonconductive organic contaminants can be detected using GPR, which detects contrasts in the dielectric constants between materials such as pore water and organic compounds.
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There is a possibility that some biologically mediated environmental properties might be detected by noninvasive or minimally invasive geophysical techniques. These properties could be targeted to indicate near-surface biological activity.
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Given present geophysical techniques, the most useful physical property in terms of detection is the high electrical conductivity and magnetic permeability of these objects. Electrical conductivity can be measured remotely using electromagnetic methods.
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However, it is difficult to detect a resistive object within a conductive medium using electromagnetics. There is currently much interest in the location of nonmetallic objects using GPR (e.g., Bradford et al., 1996)
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. c FIGURE 3.6 Buried metallic drums with representative magnetic and EM signatures; GPR signals show disturbed ground and surrounding stratigraphy.
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, dielectric constant, seismic velocity, and electrical conductivity. In addition, cavities may contain increased biological activity due to steep geochem~cal gradients and interfaces within the cavity.
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3D diagnosis of a tunnel through infrared thermography combined with ground penetrating radar, in Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society, 139-148. Greenhouse, J., M
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NRC, 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications, National Academy Press, Washington, D.C.
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