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OCR for page 31
~2
What Is Characterized?
As discussed in Chapter 2, characterization of the subsurface provides the
information required for numerous applications from resource exploration to
basic science. Although the applications and the motivations vary, in the broadest
sense the specific characterization objectives often are similar. In most cases,
information is required about the materials, their boundaries, and their properties
(see Table 3.1~; in many cases, knowledge is also needed about the physical,
chemical, and biological processes in the subsurface and their variation in space
and time.
PROPERTIES AND PROCESSES
Noninvasive determinations of subsurface properties and processes are indi-
rect. Many properties are interpreted from measured perturbations in fields that
are generated artificially or naturally. Passive investigations measure variations
in naturally occurring fields (the earth's gravity, magnetic, electric, thermal,
radiometric, stress, solar irradiation, and hydraulic fields). For example, pertur-
bations in the earth's gravity field can be used to infer subsurface changes in the
material density or the presence of voids. Active investigations use a source of
energy that creates a known field, and measurements are made of the perturba-
tions in this field or in the response of the earth to it. For example, seismic
investigations use vibratory or explosive sources to propagate elastic waves and
observe their travel times, wavelet changes, and scattering to describe the hetero-
geneity of the interior of the earth.
Many of the properties and most of the processes within the earth occur not
in isolation but in relation to one another. Fluid flow through a porous material
31
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SEEING INTO THE EARTH
TABLE 3.1 Example of Properties Often Needed for Characterization
Physical Properties Transport electrical, thermal or hydraulic conductivity,
permeability, elastic attenuation
Storage dielectric permittivity, magnetic permeability, hydraulic
storativity, elastic moduli
Strength mechanical, dielectric breakdown
Textural density, porosity, pore or grain size and shape
distribution, water content
Morphological pore lining/bridging/blocking clays
Chemical Properties Concentration, diffusion coefficient, reactivity, kinetics, solubility,
mineralogy, phase
Biological Properties Identity, abundance, diversity, ecology and overall physiological
status and activity potential
Geological Properties Stratigraphy, depth/thickness, dip/strike/azimuth, fracture presence/
concentration/orientation, state of stress, migration pathways, water
table depth
often will create an electrical current flow that generates a voltage called a stream-
ing potential. A measurement of streaming potential sometimes can be used to
locate flowing water (e.g., dam leaks). Many transport properties are dominated
by the presence of water-filling pore spaces, which causes positive correlative
behavior (or response) between conduction properties and environmental factors
such as rainfall or freeze-thaw. Not all desired physical, chemical, and biological
properties and processes can be determined noninvasively. Some have been mea-
sured for centuries (e.g., the earth's magnetic and gravity fields), whereas others
are still on the horizon (e.g., biological activity).
Most subsurface physical processes involve either movement or storage of
energy or mass; they can be described by either the diffusion or wave propagation
equations. 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. The attenuation of the propagating wave is related to energy loss
(and energy transport), whereas the velocity of propagation is related to the
ability of the material to store energy.
A good deal of characterization has simply been anomaly detection (e.g.,
detecting where things differ from normal background or from the surface mate-
rials). From such measurements, the location and size of an anomaly can be
determined. Detection of anisotropy (measurements in different directions giving
different values for the same property) is especially important in systems dealing
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WHATIS CHARACTERIZED?
33
with fracture-dominated fluid flow. Determination of connectivity is important in
mining clay and coal seams and environmental cleanup. Inadequate ability to
describe and understand heterogeneity is probably the single largest reason for
the failure of groundwater cleanup methods at hazardous waste sites. Measure-
ments made at different scales are known to produce different responses. For
example, the mechanical strength of a rock is inversely related to the size of the
sample measured, and the hydraulic conductivity of fractured rock usually in-
creases with the size of the sample measured. Such behavior often is not properly
taken into account when such measurements are transferred from field surveys to
site characterization models.
Many properties and processes are known to change with time, and knowing
when a measurement was performed can be vital to its interpretation. This tempo-
ral perspective is especially important with regard to seasonal, freeze-thaw, and
wet-dry variations that can affect not only properties but processes (e.g., erosion,
stream flow, landslides, sinkholes, frost heaving, swelling clay). Contaminant
plumes can move through the subsurface for long periods and can be disturbed or
remobilized by site remediation activities. Stresses can build up over long periods
and be released over shorter periods, as in earthquakes. Water tables rise and fall
with tidal events, water well pumping, and climate changes.
EXAMPLES OF CHARACTERIZATION
Some selected examples of characterization follow. The discussion of each
example focuses on a specific characterization objective (which might be common
to many applications) and reviews the noninvasive techniques that can be used.
Geological Characterization
A site's geology defines the overall framework within which study of the
subsurface environment is carried out. Questions relating to, for example, the
occurrence and movement of groundwater, geotechnical investigations, resource
exploration, the migration of chemical contaminants, and the subsurface
environment's microbiology, must all be posed in the context of the site's geol-
ogy. The nature and extent of the related physical, chemical, and biological
processes are constrained by the structure and Ethology of the bedrock and over-
lying surficial materials. All aspects of site characterization and remedial investi-
gations 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. Commonly used
measurements include differences in seismic velocity, electrical resistivity, and
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SEEING INTO THE EARTH
dielectric permittivity. In most cases there is not a unique relationship between a
measured physical property and Ethology; however, the combined use of differ-
ent noninvasive techniques to measure complementary properties can help deter-
mine and analyze a site's Ethology.
Different rock types, each with a characteristic mineralogy and geochemis-
try, react differently with water, solutes, suspended solids, and microorganisms.
Often these reactions are poorly understood. Rock types also have typical physi-
cal characteristics or engineering properties and, thus, compact and deform in
particular ways. In addition, there is a strong correlation between Ethology and
the occurrence of certain types of resources.
Detailed lithological maps can help evaluate the impact of aquifer contami-
nation and various remediation schemes. The likelihood of migration of a dis-
solved contaminant in groundwater, for example, is influenced by adsorption to
mineral surfaces, the dissolution or precipitation reactions of minerals, and oxi-
dation-reduction reactions, which are often mediated by microorganisms. The
reactions possible in a given aquifer are defined largely by the aquifer's lithol-
ogy. Further, certain hydraulic properties may be characteristic of certain rock
types.
Knowledge of Ethology is also essential for engineering and construction in
the subsurface. Lithological characteristics (e.g., hard rock, soft rock, intact rock,
jointed rock) greatly affect such things as a site's suitability for foundation sup-
port, applicable methods for excavation, and groundwater flow conditions.
Some lithologies are especially important to identify. Limestone and other
soluble rock types may be extensively dissolved at depth, creating secondary
porosity and permeability. Being heterogeneous in their distribution, such sub-
surface 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).
Shale and clay are important to site characterization for engineering and
environmental applications. Low-permeability shale layers that are not fractured
can confine transmissive sandstone aquifers, trap migrating hydrocarbons, or
prevent migration of landfill leachate. Exact knowledge of their location and
continuity in the subsurface is critical to properly assessing their role in site or
regional hydrogeology. Clay can occur dispersed as lenses within another lithol-
ogy. For instance, clay lenses in a sandy aquifer can create perched water table
conditions that could confound our understanding of flow conditions. Clay lenses
also can act as reservoirs of immobile groundwater into which contaminants can
diffuse and be retained in an aquifer undergoing traditional pump-and-treat
remediation. The ability to recognize relatively small clay layers or lenses within
an aquifer system would improve our ability to develop and protect groundwater
resources. The presence of certain clays is also of concern in foundation design
because it can lead to extensive settlement or heave (e.g., Chleborad et al., 1996~.
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WHATIS CHARACTERIZED?
Structure and Stratigraphy
35
Porosity and hydraulic conductivity set the broad constraints on fluid migra-
tion in the subsurface, an important issue in environmental and engineering stud
ies. These properties depend on structural features such as faults, fractures, folds,
and lithological contacts (see Figure 3.1~. Further, the actual location of ground-
water, contaminants, ore deposits, and planes of weakness for engineering pur-
poses may be constrained by subsurface structural features.
Fractures as well as contacts between different lithologies are often path-
ways for groundwater flow. Some rock types (primarily poorly cemented sand-
stone) 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. Bed-
ding planes in sedimentary sequences and fractures in sedimentary, igneous, and
metamorphic rocks may offer significant conduits for fluid migration.
Structural features largely control communication among various water-bear-
ing units as well. Even in a simple layer-cake sedimentary sequence (e.g., a
water-saturated sandstone confined by low-transmissivity, clay-rich shales), as-
sessing the fate of contaminants is difficult, if not impossible, without under-
standing cross-strata transport pathways. A confining layer can be breached by
flow along faults and fractures, which can dramatically influence predictions
about contaminant containment (see Figure 3.5~. Many of the groundwater con-
tamination sites that require restoration today resulted from mistaken pre-
sumptions about the integrity of engineered or geological barriers to fluid flow
(National Research Council, 1984~.
Noninvasive detection of these structural features (faults, fractures, folds,
lithologic contacts) relies on the existence of a contrast in properties across these
features or a unique response associated with them. Lithologies on either side of
a feature, can have significantly different physical properties such as seismic
velocity, dielectric constant, and electrical resistivity. Given that these features
are often continuous over meters to tens of meters or kilometers, it is generally
possible to locate such features with existing technology if the conditions at the
site are appropriate. One example of mapping a lithological contact the top of
the bedrock is given in Davis and Annan (1989), where ground penetrating
radar (GPR) was used to image the interface between the granodioritic bedrock
and overlying fine sands. The contrast in dielectric constant coupled with the
continuity of the contrast made this an ideal target for GPR.
The noninvasive technologies can produce high-quality images of the near-
surface structure and stratigraphy; however, their success can be highly variable.
There can be a large influence of the very near surface on most noninvasive
methods. For example, weathering within the vadose (or water-unsaturated) zone
can produce submeter-scale heterogeneities in physical properties that cause sig-
nificant problems with seismic reflection data; overcoming these "statics" re
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36
SEEING INTO THE EARTH
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FIGURE 3.1 Examples of stratigraphic interpretations using subsurface geophysical
surveys: (a) ground penetrating radar (from Benson et al., 1982~; (b) delineating a
bedrock channel by seismic reflection (from Benson, 1991~; (c) relationship of EM
quires mixing of data that can lose resolution. In the use of GPR the most com-
mon limiting problem is the occurrence of clays, with a high electrical conductiv-
ity (>30 mS/m) that prevents the penetration of radar signals. The groundwater
table can have a similar limiting effect if the conductivity of the water is high.
Fractures
Understanding the presence, distribution, and connectiveness of fractures is
critical to site characterization. Fractures play a fundamental role in where and
OCR for page 37
WHATIS CHARACTERIZED?
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1991~; and (d) electrical resistivity profile of karst terrain (from Hoekstra and Hoekstra,
1991~. (Figure adapted from Cohen and Mercer, 1993~.
how rapidly fluids can move through the subsurface and to the surface. A recent
National Research Council report (NRC, 1996) 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~. In addition to observing
topographic expression using images and photographs, various remote sensing
methods (including multispectral reflectance, imaging spectroscopy, thermal in
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38
SEEING INTO THE EARTH
FIGURE 3.2 Semihorizontal fracture zones observed by ground penetrating radar
along a profile measured on granitic outcrops at the Underground Research Labora-
tory, Manitoba, Canada, showing distribution of fractures along borehole WB2.
Reflectors S-4 and S-5, seen at depths of 40 to 50 m and 65 m, respectively, are
verified by increased fracture frequency observed in the slanted borehole. (From
Holloway et al., 1992~.
frared, and radar) have been used to detect juxtaposed lithologic contrasts at the
surface. Thermal infrared images also have been used to infer fractures where
moisture content differences in soil cause associated surface temperature changes.
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. Frequently used methods for detailed work have been GPR (see
Figure 3.2) and seismology. Resistivity surveys and detailed magnetic surveys
also have had limited success. Resistivity soundings repeated over a range of
azimuths at one location often can indicate the gross vertical fracture directions
(see Figure 3.3~. Generally, remote sensing methods lack the detailed follow-up
work to verify results.
The connectedness of fractures is important to characterization because these
connections affect whether and where fluids can flow as well as the flow rates.
Hydraulically significant fractures may comprise only a small fraction of the total
fractures present. Detailed three-dimensional mapping of properties (at scales
OCR for page 39
WHATIS CHARACTERIZED?
39
that depend on the problem) is required to evaluate connectedness between frac-
tures. Fractures may vary in length from hand specimen size to kilometers, with
widths generally a couple of orders of magnitude less. Their presence at a site
may be the source of anisotropy in an otherwise isotropic background. Because
fractures are conduits for fluids, anomalous mineralization may occur with them.
Where these minerals outcrop, they may be detected by imaging spectroscopy. If
the minerals produce an electrical conductivity contrast, this can provide three-
dimensional information about the fracture. In almost all cases, surface geophysi-
cal 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 usu-
ally are required.
Heterogeneity
Spatial heterogeneities in the physical properties of rock units prevent com-
plete characterization of subsurface rock formations from observations made in
outcrops or in cores. In environmental and engineering studies, properties of
:_,
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FIGURE 3.3 Resistivity measurements made in 16 different directions define a
resistivity ellipse whose major axis is aligned with the fracture orientation. This
example, from an open-pit quarry in southern Indiana, demonstrates that the domi-
nant fracture direction is east-west. (From Cohn and Rudman, 1995.)
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40
SEEING INTO THE EARTH
interest, such as porosity, hydraulic conductivity, and chemical or mineralogical
composition might vary over short distances within a single geological unit. No
reliable mathematical model for interpolating between observations exists. Most
mapping of Ethology is done by assuming continuity between observation points.
Yet a single important discontinuity in material properties may dictate the fate
and transport of contaminants or the stability of a rock slope. Many site
remediation failures result from inadequate characterization of site heterogeneity
(EPA, 1992~. An ability to more fully describe the location and character of
heterogeneities throughout an aquifer would yield a better description of hydrau-
lic, geochemical, and biological responses to contamination or remediation.
The importance of minor geological details in geotechnical engineering is
well known (e.g., Terzaghi, 1929~. Thin clay layers may serve as slip surfaces,
impairing the stability of both natural slopes and excavations. Sand lenses may
act both as drains or sources of artesian pressure and water flow into an area,
depending on the regional hydrogeology. The natural heterogeneity of sand and
gravel deposits is the source of nonuniform settlements and uncertainties about
resistance to liquefaction during earthquakes.
The key to effectively describing the subsurface's heterogeneous nature is
most likely the integration of different types of information at different scales.
Although noninvasive techniques can determine large-scale lithologic units, high-
resolution, often invasive, measurements are required to detect meter- or
submeter-scale changes in rock and/or fluid properties. There is considerable
interest in the use of GPR to noninvasively image this small-scale spatial vari-
ability. Very closely spaced electrical and electromagnetic sounding techniques
also have the potential to provide increased lateral resolution (see Figure 3.4~. In
addition, arrays of sensors and multicomponent measurements may provide more
detail on the spatial variations in resistivity and electrical polarization.
Fluids
Subsurface fluids play a large role in resource recovery and storage, environ-
mental protection and remediation, and civil engineering projects. In the unsatur-
ated (or vadose) zone above the water table, there is generally a two-phase fluid
system consisting of an aqueous phase and a gaseous phase. In areas contami-
nated with organic chemicals, a third nonaqueous-phase liquid (NAPL) may also
be present. (The most frequently encountered NAPL contaminants are organic
solvents and hydrocarbon fuels.) The aqueous phase may contain various dis-
solved natural and human-made constituents, such as salts, pesticides, and or-
ganic chemicals. 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
OCR for page 41
WHATIS CHARACTERIZED?
/
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DATA OBTAINED FROM
STATION
MEASUREMENTS
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DATA OBTAINED FROM
CONTINUOUS
MEASUREMENTS
41
FIGURE 3.4 Comparison of station and continuous surface EM conductivity mea-
surements made along the same transect. The electrical conductivity peaks are due
to fractures in gypsum bedrock. (From Benson et al., 1982.)
ous phase or a two-phase (aqueous and NAPL) system. The interfaces between
these fluid phases are often biologically active.
Generally speaking, all aspects of the presence and behavior of fluids in the
subsurface are of interest their distribution (e.g., Figure 3.5) and composition,
their rates of migration, and the hydraulic properties of the subsurface media.
Hydraulic properties include permeability, porosity, and when multiple fluid
phases are present, relative permeability and capillary pressure characteristics.
Hydraulic parameters tend to vary spatially; they can also depend on the scale of
investigation. The desired level of detail for characterizing these parameters de-
pends strongly on the engineering or remediation applications. Demands on spa-
tial 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. In addition, common charac
OCR for page 42
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OCR for page 43
WHATIS CHARACTERIZED?
43
terization tasks specific to addressing the distribution and migration of fluids
include the depth to the water table and the chemical composition of the fluids.
For some applications it is sufficient to know changes in fluid distribution over
time rather than the current distribution of fluids.
Depth to the Water Table
Knowing the location of the water table is essential for almost every environ-
mental, resource recovery, and engineering application. There are significant
contrasts in transport properties, chemical and microbiological reactions, and
strength and deformation properties between the unsaturated vadose zone and the
water-saturated zone.
The water table is an interface across which there may be a change in several
physical properties (electrical conductivity, seismic wave velocity, dielectric con-
stant), making it a viable target for detection with geophysical techniques. How-
ever, in some situations (e.g., coarse-grained sands and gravels), the contrast in
physical properties is between the saturated and the unsaturated zone, so geo-
physical techniques may locate the top of the saturated zone, which might be
different from the true water table. Complications in detecting this interface arise
when air is trapped below the "water table" due to annual fluctuations in the
level.
Geophysical methods commonly used include direct current (dc) resistivity,
time- and frequency-domain electromagnetic soundings, seismic refraction, and
GPR. Each of these methods, whether model based (e.g., do resistivity) or image
based (e.g., GPR), requires ancillary data often a by-product of data processing
(e.g., radar wave propagation velocity) or from drill holes for complete inter-
pretation. 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).
Fluid Composition
Knowledge of the chemical composition of fluids in the subsurface often is
required to assess groundwater quality, to track the movement of contaminants,
and to monitor containment remediation. Fluid composition can affect the physi-
cal, chemical, and biological properties of these confining geological or soil units
in ways that allow remote detection of the fluid' s composition. Characterization
involves assessing the nature and amount of dissolved and suspended inorganic
and organic constituents.
In some cases, it is possible to directly detect the contaminant using electro-
magnetic methods. For example, recognizing electrically conductive water in the
near surface (see Plate 5) caused by chloride ions from salt water is a relatively
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SEEING INTO THE EARTH
easy procedure using commercially available equipment and routine geophysical
interpretation procedures. Similarly, chloride ions in soils from improper dis-
posal of water co-produced from petroleum production can be detected easily.
The signal levels associated with electrically conductive contaminants are often
one to two orders of magnitude higher than background levels, which leads to a
high degree of confidence (see Plate 6~. The distribution of such near-surface
contaminants often can be modeled in three dimensions (Danbom, 1995~. With
such a three-dimensional model, a limited direct sampling program could con-
firm and calibrate the electrical geophysical anomalies.
Many contaminants, such as petroleum hydrocarbons, are electrically
insulative and, therefore, much more difficult to detect. However, Benson et al.
(1997) provide an example of successfully detecting petroleum hydrocarbons
using the offset sounding procedure variant in do resistivity.
Immiscible fluids such as gasoline and chlorinated solvents can sometimes
be found using complex resistivity (induced polarization) measurements to detect
electrochemical reactions exhibited by these solvents in the presence of clay
minerals.
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 con-
taminants can be detected using GPR, which detects contrasts in the dielectric
constants between materials such as pore water and organic compounds. An
experimentally controlled spill of perchloroethylene (PCE), a dense nonaqueous-
phase liquid (DNAPL), was successfully monitored using GPR and other tech-
niques (Sander et al., 1992; Greenhouse et al., 1993~. This study points out the
need for time-differential measurements to remove background effects and allow
the detection of small dielectric changes. The technique may be most useful for
monitoring contaminant movement during remediation efforts.
Noninvasive geophysical techniques determine subsurface fluid distributions
by finding a contrast in physical properties. A fundamental difficulty arises from
the fact that geological media are often heterogeneous for a range of scales;
single-method geophysical measurements cannot establish whether observed
property variations are due to nonuniform fluid distributions or to formation
heterogeneities. This nonuniqueness of the interpretation is reduced or eliminated
if diverse data sets are available and if data can be collected over time to monitor
changes associated with the movement of the fluid.
Increased biological activity often is found at the boundary of contaminant
plumes. Evidence for this activity can be found in decreased concentrations of
electron acceptors such as oxygen, nitrate, and sulfate, and in increased produc-
tion of ferrous iron minerals and methane. Noninvasive detection of such micro-
bial activity is not possible now, but is very desirable hence, another research
need. Minimally invasive sensing of microbial activity is possible through soil-
gas surveys.
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WHATIS CHARACTERIZED?
45
Biology
A wide range of organisms inhabit the soil and subsurface. More complex
eukaryotic biota such as plant roots, earthworms, nematodes, insect larvae, and
soil algae are limited to the upper regions of biologically active soil (Killham,
1994~; simpler life forms such as bacteria, fungi, protozoa (and probably viruses)
extend into deeper regions of the subsurface (Ghiorse and Wilson, 1988; Madsen
and Ghiorse, 1993; Frederickson and Onstott, 1996; Amy and Haldeman, 1997;
Ghiorse, 1997~.
The properties of the biota of most interest to site characterization biologists
may be the most difficult to determine noninvasively. The identity, abundance,
diversity, and ecology of the resident organisms, as well as their overall physi-
ological status, are the most important general properties to assess. Some of these
properties can be assessed by minimally invasive methods such as soil-gas analy-
sis and selective culturing techniques. Noninvasive remote sensing technology
shows some promise in such assessments, but until more research is done to
develop other methods, characterization of site biology will still depend to a large
degree on analysis of samples obtained by invasive methods.
There is a possibility that some biologically mediated environmental proper-
ties might be detected by noninvasive or minimally invasive geophysical tech-
niques. These properties could be targeted to indicate near-surface biological
activity.
Buried Objects
The location of buried objects is a relatively common objective in subsurface
and site characterization. The information required about the object usually in-
cludes the following:
Where is it (lateral position)?
How deep is it (vertical position)?
How large is it?
What is around it (context)?
What shape is it?
What is its composition (metal, plastic, void)?
What is it (pipe, bomb, drum, etch?
The specific set of parameters (physical, chemical, and biological) that need
to be measured at a site to characterize buried objects depends on the defined
target of interest and the host medium in which it is buried. The measurements must
also take into account any sources of noise or interference. If the goal is to detect the
presence of the object itself, the set of crucial parameters is determined by the
contrast between the properties of the object and the medium in which it is buried.
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46
SEEING INTO THE EARTH
(In addition to considering the initial state of the object at the time of burial,
investigators must consider the possibility that there can be time-dependent changes
in the object and the geological background due to processes such as weathering or
corrosion.) These can produce distinct physical, chemical, and biological changes
that can be monitored and used in locating and identifying the object. The detection
of an object may also rely on more indirect measurements. One common example
is the detection of the disturbed ground surrounding a buried object.
To review the parameters used in the location of buried objects, it is conve-
nient to divide the topic into metallic and nonmetallic materials. Location of
underground cavities and voids is also treated in this section, because many of the
. · . · ·.
principles are slmllar.
Metallic Objects
Metallic objects, which include buried drums, underground storage tanks,
well casing, metal pipes, and UXO (see Figure 2.2), can range in size from
millimeters to meters and can be buried at depths up to 10 m.
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 electro-
magnetic methods. Adaptations of these methods are hand-held terrain conduc-
tivity meters, trolley-mounted transient electromagnetic "radiometers, and metal
detectors used by the utility industry in locating underground cables and also
used by "treasure hunters" (see Box 3.11. There are limitations in the use of any
of these methods with respect to the accuracy with which the size and location of
the object can be determined. An additional limitation is that near-surface anoma-
lies can mask the presence of a deeper target.
A buried ferromagnetic object will also exhibit a magnetic anomaly that can
be modeled to locate the object. The magnetic anomaly will have an induced
component (proportional to the earth's magnetic field) as well as a permanent
"remanent" component. However, magnetic properties (as well as electrical con-
ductivity) can change with time if oxidation to nonmagnetic oxides occurs, re-
sulting in noticeable difference between "new" objects and rusted objects. Also,
as with other potential-field methods, other material distributions sometimes
found in the subsurface can produce similar anomalies.
Metallic (and nonmetallic) objects in the subsurface will interact with high-
frequency electromagnetic waves in such a way as to cause diffraction hyperbo-
las in unprocessed GPR data. These distinct patterns in GPR data often are used
to locate buried objects.
It is often more feasible to detect the disturbed zone around the buried object
than the object itself. The disturbed zone may differ from the surrounding region
in its density, dielectric constant, and electrical conductivity. The small-scale
OCR for page 47
WHATIS CHARACTERIZED?
47
structure will also be disrupted in the zone, which can produce a "jumbled"
appearance in the GPR or high-resolution magnetic response from the area (see
Figure 3.6~.
Nonmetallic Objects
Nonmetallic objects of interest in site characterization include containers,
pipes, UXO, and other waste. These objects commonly range in size from milli-
meters to meters and are buried at depths up to 10 m. Noninvasively detectable
physical properties of nonmetallic objects include electrical conductivity, den-
sity, dielectric permittivity, and seismic velocity.
The detection of nonmetallic objects using electrical conductivity is possible
only when there is a contrast with the background material. However, it is diffi-
cult 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) to detect the contrast in electrical
and dielectric properties between the objects and the background. High-frequency
antennae, with theoretical resolution on the order of centimeters, could poten-
tially be very useful for detecting small objects such as pipes.
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48
SEEING INTO THE EARTH
_ EN coNDucT'v~r
f :'
Hordpan
Cherry
Cay
~He rER
Jo_ PETAL OETECTOIl
~ Approx. 5 meters - ~
:!.
-~1 c' l~R
. .
c
-
FIGURE 3.6 Buried metallic drums with representative magnetic and EM signa-
tures; GPR signals show disturbed ground and surrounding stratigraphy. (After Benson
and Glaccum, 1980.)
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WHATIS CHARACTERIZED?
Cavities
49
It is important to locate subsurface cavities in karst areas prior to building or
road construction in order to avoid potential future collapse (Franklin et al.,
1981~. Cavern systems can also provide preferential Towpaths for water and
contaminants, knowledge of which may be important in water resource investiga-
tions or hazardous waste charactenzation. Noninvasive techniques helpful in
locating cavities include certain geophysical techniques (e.g., microgravity) and
fracture-trace analysis using aerial photographs.
Cavities in the subsurface can be either natural, such as caves in karst areas,
or human-made, such as tunnels or shafts in mines. Detecting cavities in the
subsurface involves locating a region with properties close to those of air or water
surrounded by a region with properties of the background geology. Cavities can
produce contrasts in a number of physical properties including gravity (see Fig-
ure 2.3), dielectric constant, seismic velocity, and electrical conductivity. In addi-
tion, cavities may contain increased biological activity due to steep geochem~cal
gradients and interfaces within the cavity. As a result, there may be biogeochemi-
cal indicators of the presence of the cavity, such as gases, microbial mats, or
bionic mineral accumulations, that can be detected remotely. As in the detection
of other objects, the problem of resolution must be considered for any method
that is used. The cavities of interest usually range in size from centimeters to tens
of meters.
Microgravity surveys are also useful in detecting cavities (Butler, 1984; Hinze,
1994~. The contrast in P-wave velocity between the water or air in the cavity and
the background geology makes cavities targets for seismic reflection and diffrac-
tion methods as well (Steeples and Miller, 1987; Branham and Steeples, 1988~.
There can be a distinct GPR response associated with the presence of cavities (e.g.,
Gourry et al., 1995~. The dielectric constant of a void filled with air or water will be
significantly different from that of the surrounding matenal.
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Representative terms from entire chapter:
water table
