Seeing into the Earth Noninvasive Characterization of the Shallow Subsurface for Environmental and Engineering Applications (2000) / Chapter Skim
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4 Methods of Characterization
4 Methods of Characterization
Pages 52-96
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From page 52... ...
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)
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From page 53... ...
Inadequate locational information has been rumored 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)
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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.
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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~.
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From page 56... ...
Magnetic remanence is the property that describes the ability of a material to retain magnetic field strength and direction in the absence of an external magnetic field. Magnetic fields are static vector fields with three-dimensional variation in direction over the surface of the earth with a small superimposed time-varying component.
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ELECTRICAL AND ELECTROMAGNETIC METHODS "Electrical methods" refer to measurements of natural or impressed electrical fields (potential fields) at low-frequency alternating current (ac)
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By contrast, electromagnetic (EM) methods measure magnetic fields associated with time-varying subsurface currents induced by a natural or artificial electromagnetic source (propagating fields)
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From page 59... ...
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, electrochemistry (e.g., the battery-like corrosion of naturally occurring sulfide minerals in water)
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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.
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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.
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Induced Polarization (IP) Using an electrode setup identical to that of the resistivity method, the response of the ground to the removal of an induced electrical signal can be investigated.
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From page 63... ...
Low-Frequency Electromagnetic Field Measurements Electromagnetic induction techniques operate at frequencies less than 1 MHz and are based on inducing eddy currents at the surface. Eddy currents diffuse into the earth at a rate that depends on the electrical conductivity and, to lesser extent, the magnetic susceptibility of the earth.
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From page 64... ...
These eddy currents and their associated secondary magnetic fields are directly proportional to the electrical properties of the shallow subsurface sediments and fluids beneath and between the two coils. The simplest frequency-domain EM instruments, known as terrain conductivity meters, yield depth-integrated measurements of soil conductivity from a depth of a meter to more than 30 m.
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From page 65... ...
Some VLF equipment also measures the electric field, allowing calculation of average ground conductivity. Applicability of Electrical and Electromagnetic Methods Electrical and electromagnetic methods have tremendous potential for significant advancements in the field of near-surface investigations.
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Electromagnetic methods can be particularly sensitive to the parameters of greatest interest in near-surface investigations. These methods include direct detection of contaminants in the subsurface, sensitivity to geological formation changes, and a correlation with parameters of interest in geotechnical studies.
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From page 67... ...
. GROUND PENETRATING RADAR GPR is similar to the seismic reflection method in the basic wave propagation physics, but uses high-frequency electromagnetic waves in the tens of megahertz to gigahertz range.
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From page 68... ...
Due to the smaller wavelengths used in the GPR method, resolution is commonly as much as one order of magnitude better than current seismic reflection techniques. The quality of GPR data and its usefulness in site characterization are determined by (1)
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From page 69... ...
METHODS OF CHARACTERIZATION 69 section reflectors with a distinct geometry or onentation, or packages of reflectors with a characteristic appearance. There are a number of examples in the literature in which GPR data have been used to reconstruct the geological setting by relating the GPR image to the subsurface stratigraphy and sedimentary facies.
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70 aft : .
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From page 71... ...
With current technologies, it is possible to collect and display three-dimensional data in a way that makes it relatively easy for the nonexpert to visualize useful information. This user-friendly aspect of GPR is likely to contribute significantly to the increased use of GPR in site characterization.
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From page 72... ...
Detection of Buried Objects Using GPR GPR has been found to be a useful technique for the detection of subsurface voids, buried drums, bodies, storage tanks, and utilities. In some cases, an object can be located using the changes in the dielectric properties in the surrounding zone disturbed during the digging and burial of the object.
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From page 73... ...
· Digital signal processing of GPR data using reflection seismology data processing software has been on the increase, but algorithm development is needed that accounts for the aspects of GPR data that are not common to seismic methods. For example, processing is required to account for dispersion due to frequency-dependent attenuation and scattering, both of which are much more dominant in GPR data than in seismic data.
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From page 74... ...
. An understanding of the link between the dielectric properties of the subsurface, as imaged in GPR data, and material properties (water content, porosity, permeability)
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From page 75... ...
ties related to the physical properties of the material in which they travel.1 The wave velocities are inversely proportional to the square root of density and directly proportional to the square root of the shear modulus for both types of elastic waves and, in addition, bulk modulus for compressional waves. Detectors (geophones)
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From page 76... ...
of geological layering and waste-zone geometry based on analysis of signal travel time and frequency content. The seismic reflection method is an image-based technique that produces a cross section of the volume of earth under investigation and shows acousticimpedance contrasts.
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The signals can be processed by a computer to produce an image a seismic reflection profile of the subsurface to depths of several kilometers. An uninterpreted profile is not a true geological cross section, although the gross geometry of the bedrock can be determined from it.
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From page 78... ...
Improvements in both acquisition and processing of data have allowed geophysicists to account for layer dip and spatial velocity variations of both the target refractor and the overburden soil velocities. Resolution of the geometry of the target refracting surface has been another source of improvement.
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. Locating channels in the bedrock surface and the fill material in these channels that differentially control the flow of fluids in the subsurface is another important use of the GRM refraction technique.
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Applications in Which Shallow Reflection Usually Works. Although one cannot tell in advance of field testing whether shallow seismic reflection will work at a particular site or for a particular objective, it often works in the applications discussed below.
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From page 81... ...
Determining gross geological structure is one of the classic uses of seismic reflection, and the technique works well in near-surface applications if the impedance contrast and frequency are sufficiently large. Examples of this application include determining depth to bedrock (see Figures 4.8 and 4.9)
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From page 82... ...
. Shallow seismic reflection profiles can provide a picture of geometric complexity and variability of contacts between different types of unconsolidated sediments and the sediment-bedrock interface.
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Shallow reflection techniques can sometimes detect and delineate facies changes in the shallow subsurface. The detection of facies changes requires a high signal-to-noise ratio and expert interpretation skills.
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From page 84... ...
Furthermore, frequencies at least an order of magnitude higher than those available in shallow seismic reflection are needed to detect such chemical saturation lenses at the thicknesses commonly encountered in real-world pollution situations. Direct detection of tunnels or other voids at depths of 100 m or more with surface-seismic reflection appears to be unlikely at this point.
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From page 85... ...
There is a need for collection and analysis of whole three-component seismograms that would also allow analysis of Swaves, mode converted waves, and Love waves. · In the petroleum industry, time-varying reflection surveys are now being used to monitor reservoir conditions during hydrocarbon production, including following velocity variations within the reservoir induced by enhanced production procedures such as steam injection.
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From page 86... ...
Technical instrument workshops on instruments are sponsored by the Jet Propulsion Laboratory, and substantial information and illustrative material are available on the Internet. Aerial Photography An ideal environmental remote-sensing system requires high spatial resolution, high sensitivity to changes in baseline characteristics, proven and accessible technologies, and low cost.
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From page 87... ...
Reflectance data have been successfully 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)
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From page 88... ...
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.
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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.
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From page 90... ...
Fresh groundwater has highly contrasting electrical properties to the intruding seawater, 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.
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From page 91... ...
Generic properties of the biota (identity, abundance, diversity, and ecology and their overall physiological status and activity) will be important in most site characterizations.
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From page 92... ...
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
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Zero phasing seismic data without wells offshore W Africa: Reducing uncertainty and variability of the wavelet, 62nd Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts 92, 241-243.
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Effects of short-term variations in near-surface moisture content on shallow seismic data, 65th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts 95, 419-421.
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From page 95... ...
Non-uniqueness in self-potential inversion, 66th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts 66, 950-953. Simms, J
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Similarities in modeling groundwater flow and do resistivity, 64th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts 94, 578-579. Wurmstich, B., D
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