Triad Issue Paper:
Using Geophysical Tools to Develop the
Conceptual Site Model
December 2008
73
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PURPOSE AND AUDIENCE
This technology bulletin explains how hazardous-
waste site professionals can use geophysical tools to
provide information about subsurface conditions to
create a more representative conceptual site model
(GSM). The GSM is a tool for gaining a synergistic
understanding of the site, improve cost effectiveness,
and improve decision-making within the Triad
approach. Geophysical tools can be applied to create
more robust CSMs with more complete data sets that
result in a more representative and accurate depiction
of the site characteristics at Brownfields and other
hazardous waste sites.
1. GEOPHYSICS AND THE TRIAD APPROACH
The Triad approach (www.triadcentral.org) blends
traditional analytical laboratory data (using strict data
quality assurances) with rapid turnaround field
methods to streamline the site assessment and
cleanup process without sacrificing overall data
quality. Geophysical tools may fit into the Triad
approach as a "rapid" turnaround field method
that has value throughout the site
characterization process—from initial scoping
though monitoring of remedial processes, and
post-remediation monitoring. Relatively low-cost
geophysical surveys can streamline the site
assessment process and lead to higher
"information value" by providing a more complete
data-set. Other available field methods include,
but are not limited to, direct push technologies,
X-ray fluorescence, field-portable gas
chromatography, and immunoassay kits.
professionals are realizing the full benefits of
characterizing sites using geophysical tools during all
phases of site investigation.
Geophysical tools use non-invasive or invasive
investigative techniques for measuring and
interpreting material physical properties to determine
subsurface conditions. The investigative tools can be
as simple as a metal detection sweep; or as
comprehensive as full site digital data collection of
thousands of georeferenced data points collected with
global positioning system (GPS) survey equipment,
and integrated into what geologists may call a (site-
specific, near-surface) Common Earth Model (GEM)
or Shared Earth Model. The GEM can be a key piece
of the GSM. The GEM is a quantitative subsurface
model depicting all known data, which is continuously
The use of applied geophysics for subsurface
investigation dates back to the mid-19th century.
By the early- to mid-20th century, geophysics
became the primary method for mining and
petroleum exploration. Over the past several
decades, geophysics has increasingly been
adopted for environmental site-management and
decision-making. Today, environmental site
Prevailing wind direction
Transport Medum(air]
• J
Retease
mechanism
(volatilization)
Figure 1: A Simplified Conceptual Site Model. Site investigation and
CSM design assess the viability of exposure pathways from
contaminant release points to human and environmental receptors.
Shown here are potential exposure concerns from transport
mechanisms including air, soil, and groundwater, which may lead to
contaminant concerns with indoor air and drinking water. The
usefulness of a CSM is in generating consensus and assisting with
site-closure decisions by depicting a model representative of all data
known about a site.
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edited and refined as the collection of new field data
proceeds.
The CSM is a central element of the Triad approach
and any successful site cleanup. The CSM presents
all of the relevant data collected at the site, and is not
limited to the product the geologist may deliver as a
"GEM." There is no prescribed format for the CSM.
Information may be presented in various graphical or
textual formats with more than one possible
representation for a given site, where each
component represents a different facet of the site
characteristics. For example, Figure 1 depicts a
simplified graphical CSM that shows contamination
sources, pathways, and receptors. A more complex
CSM representing the "final" synthesis of data
collection at a site is shown in Figure 2. Each of
these CSMs represents different site characteristics
and levels of complexity; however, each one is
valuable in communicating information about the site.
The CSM provides an interpretation tool, acts as a
communication device, identifies gaps where more
information is needed, and can be a tool to direct
future work (Crumbling 2001). All stakeholders
Using Geophysical Tools to Develop the Conceptual Site Model
involved in the site, including technical staff and
private citizens, reference the CSM for a clear
understanding of site conditions and dynamics. The
CSM is updated as the project team processes site
data and the understanding of site conditions evolve.
Updating the CSM continuously is a means of
maximizing value obtained through data collection
efforts. Site professionals use geophysical survey
data to direct invasive sampling efforts and contribute
to the CSM by providing information about subsurface
conditions and through the sampling strategy.
Often the geophysicist is not the person responsible
for overall CSM development and maintenance and,
therefore, communication between the geophysicist
conducting surveys and the CSM developer is crucial.
Without clear communication, the CSM, and all of its
dependent stakeholders, will not benefit from the
added value of geophysical tools.
Exposure points include air. soil, aroundwater
LEGEND
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LAMINATED, FRACTURED SILTY SANDSTONE (PIERRE SHALE)
LANDFILL MATERIAL
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HIGHLY WEMHESED FRACTURED SANDSTONE {PEFWE SHALE)
DENSE KQN-AOUEOUS PHASE LOUD (DUA.PL)
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Figure 2: Poudre River Case Study Conceptual Site Model Cross-Section. Electromagnetic and resistivity geophysical surveys were used
in conjunction with site characterization tools to map subsurface conditions across the site; including definition of the bedrock surface and
identification of the presence or absence of preferential pathways such as bedrock fractures, subsurface channels in alluvium, and
underground pipelines. Geophysical data aids the preparation of a more robust CSM. The updated CSM is then used to make decisions
about characterization strategy and sampling design, and to assess site concerns related to human health and environmental cleanup.
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Using Geophysical Tools to Develop the Conceptual Site Model
2. GEOPHYSICS WITHIN A COLLABORATIVE
DATA MANAGEMENT PROGRAM
The Triad approach minimizes sampling and
analytical uncertainty by leveraging the strengths of
each type of analysis in the GSM toolbox.
Collaborative data management is a central part of
this goal. In order to maximize project cost-
effectiveness and efficiency, less expensive field
methods are coupled with high-precision analytical
methods. Multiple lines of evidence may be
developed, with the field methods efficiently covering
large spatial areas, and the highly precise analytical
methods confirming the results. To understand and
manage the relationship between these two data
sources, data are classified into operational groups:
(1) data for rapid initial GSM development, and (2)
data to manage analytical uncertainty. Geophysical
data can be used to guide sampling (drilling, etc.),
and as a layer of subsurface information in the GSM
that can help classify information into the proper risk
context for decision-making. Other sampling methods
are used to ground-truth the GSM geophysical survey
data. Furthermore, the use of geophysics in GSM
development may be broken down into two distinct
types of uses: (1) as an initial characterization tool for
getting a basic understanding of site's physical
characteristics, or as a continuation of this use, and
(2) as a tool to gain a synergistic understanding of the
site. Each of these uses is discussed in detail in the
following sections. No matter the application, the
fundamental goal of using geophysical tools in GSM
development is to obtain the most information from
available site resources and direct sampling to
maximize coverage and cost-effectiveness. A
synergistic understanding may lead to using
collaborative strategies throughout the cleanup,
monitoring, and closure process; so it is important to
think of using geophysics beyond the "assessment"
phase of site cleanup.
2.1 Initial Characterizations Using Geophysics
Using geophysical tools during an initial
characterization is an intuitive process offering rapid
insight to subsurface physical properties. When used
in this capacity, often only limited amounts of data
have already been collected onsite. Data are usually
collected in one or two field mobilizations, and
geophysical data add detail to the overall GSM. The
goal of the initial characterization may be defining site
geology (highly porous and permeable channels,
fractures, clay layers, etc.), locating buried objects
(drums, underground storage tanks, etc), or mapping
utilities. The results present a framework around
which the GSM is constructed, or identify gaps or
uncertainties in the initial GSM that must then be
addressed.
2.2 A Synergistic Approach Using Geophysics
In many initial characterizations, the goal of the
geophysical survey is achieved when a target is
identified and site investigation can proceed to
invasive sampling methods. This scope of work is
appropriate at some sites, and no further geophysical
investigations may be warranted. For example, a site
assessment using geophysics may uncover locations
where drums are buried and excavation can proceed
accordingly. However, in other cases, an initial
geophysical survey may only be the first step in a
broader geophysical investigation for continual GSM
refinement.
This CSM-building approach requires a broader
scope of work. Rather than using geophysical
methods in the one-time contribution described
above, geophysical tools should be used in
conjunction with sampling to strategically collect
samples, compare analytical data with geophysical
results, and repeat the process as necessary. At
sites where monitoring wells are used for sampling,
an example of this iterative approach may be to: (1)
carry out an initial characterization to identify well
placement locations, (2) compare sample analytical
data, borehole geophysical data, and well log
information to initial geophysical data, and (3) place
additional monitoring wells based on the collaborative
data sets and any additional characterization surveys
necessary.
This approach produces a more complete picture of
the site by leveraging the data from each step of the
process. Because data are ground-truthed through
collaborative analysis, the features observed in the
geophysical data set may be attributed to subsurface
properties with a higher level of confidence. A result
can be increased confidence in the characterization of
a contaminant plume or preferential flow pathway.
The collaborative analysis may identify additional
areas of concern and the iterative process takes over
each time resulting in a better understanding of the
geophysical data and how it relates to characteristics
at that particular site.
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Using Geophysical Tools to Develop the Conceptual Site Model
2.3 Real-time Analysis and a Dynamic Work
Strategy
In accordance with the Triad's incorporation of
dynamic work strategies, survey design must be
flexible to accommodate data results as they become
available. Data are often available from geophysical
surveys before the processing and modeling that
follows a field mobilization effort. Efforts should be
made to use these data products to direct and adapt
the ongoing geophysical survey. However, generally
speaking, geophysical methods are not truly "real-
time" field screening techniques. Most methods
require complex processing routines and strict
scrutiny for a reliable, final interpretation. Data
reviewed in the field are useful for providing a rough-
cut idea of the final results and generally should not
be taken as a final product. Whenever possible, data
should be ground-truthed to verify any modeling or
interpretation before preparing the final product. For
example, data from resistivity profiling that require
inversion can be considered in the field by processing
data immediately after collection on a field computer.
Although the models generated during field data
processing may not be of high enough confidence to
include in a final report, these data often provide
sufficient information to immediately direct additional
field surveys without the expense of an additional field
mobilization.
2.4 Cost
Projects that follow the Triad approach have
demonstrated an overall project cost savings of up to
fifty percent over traditional management approaches
(Crumbling et al 2001). The dynamic and continually
evolving GSM created through the use of geophysical
tools contributes significantly to this savings through
improved site characterization that results in a more
effective remediation and/or management plan.
Although overall project costs are significantly
decreased with the creation of an accurate GSM, the
initial costs of site investigation may be higher than
traditional approaches, potentially creating an
administrative hurdle when initiating a geophysical
investigation for GSM development. It is important to
remember that investing in GSM development using
the most appropriate geophysical tools will pay off by
providing a clearer understanding of the site
dynamics and result in an improved, cost-effective,
site-specific assessment or remedy designed.
The actual cost of a geophysical investigation varies
with the type and scale of the survey being
conducted. Instrument specifics, terrain, labor
requirements, size of survey, grid spacing, and other
elements will all contribute to survey costs. Standard
equipment rental prices can vary from less than $100
per day to over $250 per day, but these estimates do
not include other mobilization and data interpretation
costs and generally do not include the necessary
processing software, which may be costly for some
instruments. In most instances, a contractor is used
to provide geophysical services and expertise. One
day of work from a contractor using standard
geophysical equipment may start at around $1,500
and may rise significantly depending upon survey
specifics. These costs may be comparable to one
day of drilling; however, the benefits of a drilling
program supported by geophysical services may far
outweigh the value of data from drilling alone by both
guiding the drilling operation and potentially
minimizing the number of required borings.
3. CHOOSING THE RIGHT TOOLS
The sections below provide environmental site
professionals with a general understanding of
different geophysical methods and the basics for
carrying out a survey. A knowledgeable and
experienced geophysicist should always conduct
each step of a geophysical survey, from planning to
interpretation, and should contribute to the Systematic
Project Planning (SPP) concept using a dynamic work
strategy approach.
While geophysics offers great promise in overall
project cost savings and improved data coverage,
poor planning and communication will result in wasted
money or data that contribute little value to achieving
overall objectives. Planning for a successful survey
requires a working knowledge of various methods and
a clear understanding of project objectives by all
stakeholders.
Even though geophysics may be used in the earliest
stages of the GSM development, choosing the
appropriate tools to use at a site requires some
understanding of the physical properties of the site
that are expected to affect geophysical
measurements. For example, ground penetrating
radar (GPR) is often employed in an attempt to locate
buried drums; however, this method is not suitable for
use in many clayey soils that exhibit a high cation
exchange capacity (CEC), because the CEC of the
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Using Geophysical Tools to Develop the Conceptual Site Model
clay quickly attenuates the radar signal. Another
common problem is buried electrical utilities. An
electrical geophysical method may be the perfect tool
for a specific problem, but electrical utilities may
preclude its use at a site depending on the survey
objectives and survey geometry. There is no "one-
size-fits-all" approach for geophysical investigations,
and each site will inevitably have a scientifically-
based, site-specific work plan. The level of
knowledge of site characteristics needed before
conducting a geophysical survey will vary with the
investigation's goals and the scale of the project.
No matter what tools are chosen, the most
comprehensive data sets are born through the
integration of several methods (Haeni et al 2001,
Shapiro et al 1999). This geophysical "toolbox"
approach reflects an understanding that a number of
physical properties affect the data collected and the
inherent heterogeneity of the subsurface can be
better managed by a multi-method approach. Each
data set, affected by different physical properties, can
be compared and interpreted collaboratively. In
addition, if one method fails to detect the target of
interest, another method, already planned for use at
the site, may provide detection.
4. COMMON GEOPHYSICAL METHODS
Geophysical methods involve a response to some
physical property of the subsurface. In most
environmental investigations, the main purpose of
applying geophysical methods is to delineate some
subsurface structure or other discreet target. To be
able to resolve a target, there must be a sufficient
physical contrast between the target and the
surrounding matrix to cause a response in the
measured signal. Table 1 (page 10) provides an
overview of common surface geophysical methods
applied to environmental problems.
Different methods rely on different physical
properties. For the purposes of this document, the
geophysical methods have been broken down into
broad categories to provide an overview of the types
of methods in use at environmental sites:
magnetometry and gravity methods, electrical
methods, electromagnetic methods, seismic methods,
and borehole methods; all of which are discussed in
the following subsections. These broad categories
each encompass several specific methods. Methods
such as magnetic resonance sounding and induced
polarization, which are not yet in common practice in
the environmental field, are omitted from the
discussion. References containing detailed
information on any method including theory, field
operation, and data interpretation are provided below.
Regardless of the method, geophysical data are
being integrated more often with digital positioning
methods, such as differential GPS or robotic total
station (RTS). This approach increases the overall
positioning accuracy of the data and provides
georeferenced data (that is, field data for which
spatial coordinates are known) that can be overlaid
with other site data, enhancing the quality of the
overall GSM.
4.1 Magnetometry and Gravity Methods
Magnetometry and gravity methods measure small-
scale variations in a larger field that is constantly
changing with position and time. Both methods can
take measurements using handheld instruments or,
for large area surveys, an airborne 1-dimensional
profile line. The two methods may be used to
complement each other and facilitate interpretation.
In gravity surveying, a gravimeter is used to measure
the strength of a gravitational field. After applying a
series of corrections to account for latitude, elevation,
terrain, etc., any anomalies (that is, a geophysical
reading considered a deviation from normal data)
present that are the result of subsurface density
Additional Resources
Books/Publications:
ASTM D6429-99, Milsom 1996, Reynolds 1997, Sharma 1997,
Telford1990, U.S. EPA 1991
Internet sites:
U.S. Geological Survey, Office of Ground Water, Branch of
Geophysics
(http://water.usqs.qov/oqw/bqas/)
U.S. Geological Survey, Toxic Substances Hydrology Program,
Geophysical Characterization
(http://toxics.usqs.qov/topics/qeophvsics.htmn
U.S. Environmental Protection Agency, Field-Based
Geophysical Technologies Online Seminar
(http://www.clu-in.orq/conf/tio/qeophvsical 121201/)
U.S. Environmental Protection Agency, Region 5, Field
Services Section
(http://www.epa.gov/region5superfund/sfdfss/htm/geophy.html)
U.S. Army Corps of Engineers, EM1110-1-1802, Geophysical
Exploration for Engineering and Environmental Investigation
(htlp://140.194.76.129/publications/enq-manuals/em1110-1-
1802/)
Natural Resources Canada, Borehole Geophysics and
Petrophysics (http://qsc.nrcan.qc.ca/borehole/index e.php]
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Using Geophysical Tools to Develop the Conceptual Site Model
variations are recorded. Any subsurface target that
creates a sufficient density contrast with the matrix
(for example, unconsolidated sediments, voids, or
rock) can be detected. Gravity surveying requires
precise field instrumentation, a precise geographic
location system (with elevation), and a detailed
processing routine.
Magnetic surveys are more common in the
environmental field than gravity surveys. Iron objects
and magnetic minerals cause local variations in the
earth's magnetic field. Magnetic surveys are often
used to locate buried ferrous metal drums and
underground storage tanks. The two common
magnetic surveying measurement techniques are
total field magnetometry (TFM) and gradient
magnetometry. TFM uses one sensor and helps
locate near surface objects and detect local trends
due to larger scale geologic features and changes in
soil magnetic susceptibility that may also be of
interest to the site investigation. Gradient
magnetometry measures the difference in the total
magnetic field between two sensors separated by a
small vertical or horizontal distance. Gradient
magnetometry emphases near surface anomalies and
does not require corrections for diurnal variations
(variations due to solar activity) since both sensors
are equally affected. TFM does require a correction
for diurnal variation, but this correction is simplified
with the use of modern digital data collection and
software, and it does not add significantly to the field
time. Magnetic surveys will not directly respond to
non-ferrous metal or non-metallic objects, such as
non-metal drums and composite tanks, or to container
contents and spillage. Magnetometer and gravity
data are most often presented in the form of plan view
contour and/or color filled maps showing lateral
changes in response.
4.2 Electrical Methods
Electrical geophysical methods measure the
distribution of electrical resistance in the subsurface
by applying direct current (DC) to the ground using,
for example, two electrodes (current sources) and
measuring the potential difference (voltage) between
a second pair of electrodes (current receptors). The
resistance data may be inverted to produce
conductivity estimates. Multiple voltages can be
measured at once using several pairs of source and
receptor electrodes. Electrical methods such as
direct current (DC) resistivity have become popular in
environmental surveys as field resistivity systems
have become more user-friendly and computing
advances have made processing steps easier.
Investigations to detect contaminant plumes, fluid-
filled bedrock fractures, or landfill boundaries and
voids, all lend themselves to the resistivity method if a
sufficient resistivity contrast exists. Furthermore,
resistivity surveys can be used periodically during a
site remediation process to monitor changes in the
subsurface electrical properties because of plume or
contaminant attenuation. Field data must be
processed to create a model of resistivity distribution
in order to approximate the true resistivity of the
measured subsurface.
The earliest resistivity surveys were conducted as
soundings or profiles, and inverted into 1-dimension.
Improvements in computing capabilities and
advances in field equipment have increased the ease
of performing more complex, 2- and even 3-
dimensional imaging surveys. For these surveys, a
multi-electrode system is typically used to collect a
series of sounding data over a profile line or lines.
Field acquired (apparent resistivity) data are inverted
in 2- (or 3-) dimensions to produce a subsurface
model. The final product may be as simple as a 2-
dimensional profile, called an "earth section" or a
"pseudo-section," to a full 3-dimensional image
contoured to illustrate vertical and horizontal changes
in subsurface resistivity. A multi-channel system can
be used to collect near-continuous data and the
system can be deployed as a towed survey on either
land or water.
Electrical resistivity tomography (ERT) is a cross-
borehole or surface to borehole application that
produces a tomogram (an image constructed from ER
data at the site) of subsurface resistivity. ERT
surveys have been used for monitoring subsurface
conditions that are expected to change over time.
Electrodes are temporarily or permanently placed in
boreholes and data are automatically collected using
a surface multi-electrode system. ERT surveys
produce a 2- or 3-dimensional model, often performed
repeatedly over time as part of a monitoring
investigation.
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Using Geophysical Tools to Develop the Conceptual Site Model
4.3 Electromagnetic Methods
The methods in the electromagnetic (EM) category
are perhaps the most varied and diverse. EM
induction (EMI) and ground penetrating radar (GPR)
both fall within this category, although their methods
of operation are quite different. EMI methods operate
by inducing an EM pulse (referred to as an 'EM field')
into the subsurface and measuring the secondary
field generated by conductive and/or magnetic media
or objects in order to map the distribution of these
magnetic materials in the subsurface. Conversely,
GPR relies on the propagation of EM waves in the
subsurface and records reflections off subsurface
structures and/or objects due to the contrast in
electrical properties of these structures and objects.
EMI data are depicted commonly as factors of
frequency (in millivolts), depth (in feet or meters) or
time between current induction and reception (in
milliseconds). Methods may use a local, active
source of induced EM energy (for example, terrain
conductivity, time domain EM) or a remote source of
induced EM energy (for example, very low frequency
[VLF]). If an active source is used, an EMI instrument
consists of a transmitter and receiver coil. A VLF unit
only requires a receiver. With an active source, the
transmitter coil produces a magnetic field that induces
an EM pulse into the ground. If there are subsurface
conductive materials, this primary EM wave
generates a secondary wave that the instrument
measures at the receiver coil. The fundamental
principle of electromagnetic induction is the
measurement of the change in impedance (the ratio
of the electrical field to the magnetic-field strength)
between the transmitting and receiving of coils above
the earth after the EM energy passes through
subsurface conditions with various electromagnetic
properties. The strength and direction of the fields is
used to determine the conductive nature of
subsurface materials. Examples of these conductive
media include subsurface soils and ferrous and non-
ferrous metals.
EMI surveys have been used to map plumes and
locate metal (ferrous and non-ferrous) objects such
as drums, tanks, and utilities (Figure 3). EMI surveys
are often the chosen method when the goal of a
survey is to locate a buried anthropogenic object.
Surveys by hand-held instruments (Figure 4),
borehole surveys, and airborne surveys are all
common applications. Some instruments have a
fixed-coil spacing (giving it a fixed maximum
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Radar Profile -500 mHz antenna
Figure 3: GPR survey of underground storage tanks. GPR produces
an underground cross-sectional, two-dimensional image of the soils
and subsurface features. GPR provides a high resolution, cross-
sectional image of the shallow subsurface. A good example of
geophysical tools, GPR works by sending a short pulse of
electromagnetic energy into the subsurface. When this pulse strikes
an interface between layers of material with different electrical
properties, part of the wave reflects back, and the remaining energy
continues to the next interface. Depth measurements to interfaces
are determined from travel time of the reflected pulse and the velocity
of the radar signal. GPR works well to detect fiberglass tanks or tanks
beneath highly cluttered sites or building floors.
investigation depth, unless variable frequencies are
used), while others have adjustable coil spacing so
the operator may change the depth of exploration.
Instruments have trade-offs. A fixed-coil instrument,
for example, may require fewer operators in the field
and may be more rapid in data collection.
One advantage of the EMI system over resistivity
systems is that direct contact with the ground is not
required. EMI data is most often presented as a plan
view contour and/or color filled maps showing lateral
changes in response.
GPR transmits EM waves, or pulses, into the
subsurface and records the reflections of these
pulses off subsurface structures that have contrasting
electrical properties. The amount of EM energy
reflected back to the surface at an interface is
determined by the material's ability to transmit an
electric field (relative electric permittivity) between the
two layers and the electrical conductivity of a layer.
Therefore, only targets that have dielectric properties
that contrast with surrounding media will be
detectable using GPR. Dielectric materials are non-
conductive, but can sustain an electric field; so
"dielectric" relates to a layer's ability to transmit
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Using Geophysical Tools to Develop the Conceptual Site Model
electric energy by induction and not conduction.
Thus, EM waves are quickly attenuated by conductive
media such as fine silts and clays with high cation
exchange capacity, thereby greatly reducing
exploration depth under these conditions. However,
GPR is often used to detect areas of high conductivity
in this way, such as a biodegrading petroleum plume.
As seen in Figure 5, the final product is similar to that
produced by seismic reflection. The amount of data
processing necessary before final interpretation
depends upon the complexity of data and subsurface
conditions.
Best penetration using GPR is achieved in dry sandy
soils or massive dry materials such as granite,
limestone, and concrete. GPR provides the greatest
resolution of currently available surface geophysical
methods and is a method for detecting buried plastic
containers. GPR has been used in environmental
field work to locate buried objects such as drums,
tanks, and utilities; although for these purposes EMI
is often better suited, faster, and less expensive for
initial or reconnaissance surveys. GPR has also
been used to map or identify subsurface stratigraphy,
disturbed zones, or conductive or resistive
groundwater plumes.
Specialized GPR systems can also be used to collect
borehole radar data either between two boreholes
(radar tomography) or in a single borehole
(reflection). Borehole radar can be used in
polyvinylchloride (PVC) cased or open holes and can
locate fractures or other targets that do not physically
intersect the borehole. GPR data is most often
FIGURE 10
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ELECTROMAGNETIC
SURVEY RESULTS M*P
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I
.
Figure 4: EMI survey results at the Poudre River site Case Study.
Higher conductivity readings (shown in red) indicate denser
subsurface lithology, influenced by bedrock topography and locations
of subsurface metal objects. In general, the lower apparent
conductivity materials are located at the northwest (lower ten) half of
the site. The range of conductivities here would suggest loose fill,
sand, and other porous materials (such as landfill debris). The higher
conductivity areas of the east and northeast (upper left) portion of the
site nearer the buildings suggest engineering fill and fine-grained soil
such as clay or clay-like silts. Data and interpretations from this
survey were added to the CSM to produce the updated cross-section
(Figure 2). The black circle indicates the boundary of a playground.
Buried Waste Metal Former LIST Urcaions
nit lint red Concrete
LJ. s. EnvirannunlA) pnrieOKn Agency
Figure 5: Electromagnetic (EM) Induction Time Domain Metal Detection Survey Showing Multiple Targets. EM systems can be used for
near-surface environmental investigations, including detection of buried structures such as building foundations, as well as for the detection
of highly conductive metallic objects like steel drums, tanks, large metallic utilities and other nondescript buried iron metallic objects.
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Using Geophysical Tools to Develop the Conceptual Site Model
presented in the form of profiles or cross sections
similar to seismic reflection surveys (see Section 4.4
below) but can also be presented as 3-dimensional
images and plan view depth or time slices showing
lateral trends in instrument response.
4.4
Seismic Methods
Seismic refraction and seismic reflection are used in
GSM development primarily to determine depth to
bedrock, to determine depth to water table, and to
map subsurface stratigraphy. Advances in equipment
and automated processing have led to the
increasingly common use of seismic reflection
methods in both terrestrial and marine settings. Both
methods transmit a seismic wave into the subsurface
and measure the refraction of the wave off subsurface
layers (seismic refraction method) or the reflection of
the wave off these layers (seismic reflection method).
These refractions and reflections are caused by either
the seismic velocity contrasts in the subsurface
(Figure 6) or the acoustic impedance of the
subsurface stratigraphy. The contrasts can be
caused by material contrasts as well as by the
presence or lack of subsurface fluids, such as
groundwater or contaminants. Refraction surveys
can provide information about depth to bedrock and
other subsurface layers such as the water saturated
zone. Minimal data processing is needed for seismic
refraction surveys. However, more detailed
information about subsurface structure and
stratigraphy can be obtained from seismic reflection
surveys, which require significant data processing
before final interpretation. The equipment used for
each type of survey may be similar, but setup and
acquisition time may be quite different. Final results
may be presented as a profile of the subsurface or a
contour map indicating depth to some layer, such as
the bedrock surface.
4.5 Borehole Methods
Borehole geophysical methods are used to obtain
subsurface information such as lithology, well
construction, fracture orientation, or vertical
groundwater flow. The combined use of borehole and
surface methods described earlier at a site often
yields the most complete geophysical data set
possible through all phases of an investigation or
remediation. Some borehole methods are discussed
above in their respective method categories. Other
standard borehole methods are presented in Table 2
(Page 12). Table 2 is not a comprehensive list of all
methods; rather, it represents some of the more
common methods used in the development of a GSM.
USGS field-scale examples of using borehole radar to monitor
remediation:
• Vegetable Oil Biostimulation
htto://pubs.usqs.qov/sir/2006/5199/pdf/SIR2006-5199.pdf
• Steam-enhanced Remediation
http://pubs.usgs.gov/sir/2006/5191/SIR2006-5191.pdf
Figure 6: Seismic Velocity Subsurface Model. The top of each of the six cross-sections indicates ground surface with increasing
depth on the y-axis. Each cross-section represents a parallel seismic 'slice' through the subsurface investigation area. The varied
topography and presence or absence of interpreted geologic horizons indicate possible pathways for contaminants. For example,
identification of subsurface channel morphology is clearly delineated in this vertical cross-section seismic velocity model. Spacing
between sections is 8.75m. Violet represents seismic wave travel times at -200 meters per second (m/s) and red at-1000 m/s. In
addition, shallow subsurface seismic models can be used to isolate lithology surfaces, the thickness oflithologic units, and even the
competency of a unit at limiting vertical groundwater and contaminant migration.
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Using Geophysical Tools to Develop the Conceptual Site Model
Table 1: Common surface geophysical methods applied to environmental problems
Category
Operation
Common Methods
Typical Application
Typical Final Product
Magnetometry
(See Section 4.1)
Measures the total magnetic field intensity
that changes or is disturbed by subsurface
features of contrasting magnetic properties.
Typical units of measure: nanolesla (nT), or
nanoTesla/meter (nT/m) for gradient. Some
environmental geophysics users prefer
gammas and gammas/meter. Sensing
technologies vary and will determine speed of
operation. Range of detection increases with
size of buried anomalies.
Total Field
Magnetometry
(uses one sensor - and
base station
recommended)
Locating buried ferrous metal objects such as MEC, drums, tanks,
utilities landfills, waste pits, and foundations. Requires corrections
to diurnal changes.
Gradient Magnetometry
(uses two sensors)
Locating buried ferrous metal objects such as tanks, drums, utilities,
MEC*, landfills, waste pits, and foundations. When used in
combination with EM methods, can help delineate metal by ferrous
and non-ferrous.
Gravity
(See Section 4.1)
Measures total attraction of the earth's gravity
field that changes over subsurface media of
contrasting density. Units of measure:
Milligals (mgals) or Microgals (ugals).
Gravimetry
Mapping subsurface structural features such as voids and
sinkholes.
Color contoured or color filled plan view maps
showing characteristic magnetic intensity
responses from targets of interest (anomalies)
in contrasting colors to background (ambient)
responses. Data profiles along survey lines
may also be produced, showing response
curves that can be compared to standard
models. Product may also indicate the
amount of mass present (i.e., how much
contamination, mappable by magnetic
methods, is below ground). Other methods
cannot provide this information.
Microgravimetry
Mapping subsurface structural features such as voids and
sinkholes.
Electrical
Resistivity
(See Section 4.2)
Electrical current is applied to the ground by a
series of surface electrodes and the potential
field (voltage) is measured at the surface
between another set of electrodes. Electrode
position, applied current, and the measured
electric field are used to calculate resistivity.
Unit of measure: Ohm-meter
Direct Current (DC)
Resistivity
Mapping subsurface structural features and stratigraphy; identifying
disturbed zones, significantly conductive or resistive groundwater
plumes, and depth to groundwater and bedrock.
2D cross sections showing lateral and vertical
changes in resistivity of subsurface features
along a single survey line. The cross sections
are mathematically derived from raw data
pseudo sections and must be interpreted in
light of available geologic information. 3D
models can be derived from several cross
sections.
Electromagnetics
(See Section 4.3)
Measures the ratio of the transmitted electric
and magnetic fields compared to received
(induced) electric and magnetic fields from
subsurface media. This ratio is converted into
a relative response and conductivity, or
resistivity. Units: millivolts, milliSiemens per
meter (mS/m)
Range of detection (frequency domain)
dependent on coil spacing. Range of
detection limited to about 10-15 feet max.
Best in sands; poorest in clays.
Not recommended to operate two EM
instruments at same time because they will
interfere.
Frequency Domain
Terrain Conductivity
Mapping lateral changes in soil, ground conductivity, contaminant
plumes (only if significant thickness and difference exists between
background conditions), and both geologic and anthropogenic
features. Also useful in locating buried metal objects, such as
drums, tanks, landfills, waste pits, foundations, and utilities.
Averages large bulk area within range of transmitter and receiver.
Contour maps similar to magnetic data.
Time Domain Metal
Detection
Locating ferrous and nonferrous metal objects such as tanks,
drums, utilities, MEC, landfills, waste pits, and foundations.
Measures area directly under coils—which allows operator to detect
shape of anomaly (i.e., for a tank, operator can detect lateral
extents of tank).
Measures radar (electromagnetic) travel time,
which is converted into velocity contrasts in
subsurface media.
Units of measure: Travel time/wave velocity
in nanoseconds and nanoseconds per meter
(ns/m) Often must test-run area to determine
depth of penetration. Signals may not
penetrate past first metallic objects.
Ground Penetrating
Radar
Mapping subsurface structural features and stratigraphy; identifying
disturbed zones, conductive or resistive groundwater plumes, and
depth to groundwater and bedrock. Secondary application in
locating buried objects such as MEC, drums, tanks, landfills, waste
pits, foundations, and utilities. May be good at determining if buried
objects have rounded or flat surface.
Profiles or cross sections similar to seismic
records. Several GPR lines can be used to
create 2D plan view and full 3D displays.
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Category
Seismic
(See Section 4.4)
Operation
Measures seismic energy travel time that is
converted into velocity contrasts in subsurface
medium.
Units of measure: Travel time/wave velocity in
milliseconds and milliseconds per meter
(ms/m). Range of detection determined by
geology and type of sound source to generate
energy.
Common Methods
Seismic Refraction
Seismic Reflection
Typical Application
Mapping subsurface stratigraphy in bedrock, low velocity
unconsolidated materials and structural features such as voids and
sinkholes. Particularly useful for finding depth to bedrock and
groundwater.
Mapping subsurface bedrock stratigraphy and fine geologic
structural features such as voids and sinkholes.
Typical Final Product
Travel time curves in which 2D and 3D
models are created.
Seismic cross sections showing reflectors
from rock interfaces in alternating black and
white lines or shades of color. Several cross
sections can be used to create a 3D model.
*MEC = Munitions and explosives of concern Some information for this table derived from Hoover et al, 1996.
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Using Geophysical Tools to Develop the Conceptual Site Model
Table 2: Common borehole geophysical methods
Method
Optical Televiewer (OTV)
Acoustic Televiewer (ATV)
EM Induction Logging
Gamma Logging
Normal/Lateral Resistivity
(electric logs)
Caliper / Acoustic Caliper
Heat Pulse Flow Meter
(HPFM)
Colloidal Borescope (lateral
flow meter)
Cross-hole/Tomography
Spontaneous Potential
Casing Status/Type Required for Operation
open borehole, PVC or metal casing,
open borehole
open borehole or PVC casing
open borehole, PVC or metal casing,
open borehole
open borehole
open borehole or screened
open borehole or screened
various
open borehole
Operation
Oriented 360° digital photo of borehole wall.
Some optical units only show video view of hole
(not orientated).
Oriented 360° acoustic image of borehole wall
Records the electrical conductivity or resistivity
of the rocks and water surrounding the borehole.
Records natural gamma radiation emission from
formation.
Uses variably spaced electrodes to measure
resistivity of borehole and materials surrounding
a borehole. Logs are affected by bed thickness,
borehole diameter, and borehole fluid.
Mechanical arms / acoustic waves measure
variation in borehole diameter.
Measures vertical flow of water by tracking the
movement of a pulse of heated water.
Measures naturally occurring particles in
groundwater moving through a well's screened
interval. Observes flow at the pore scale,
measure velocities ranging
from 0 to 25 mm/sec.
Measures physical properties of subsurface
media between two or more boreholes.
Commonly EMI, resistivity, and seismic methods
are used.
Records potentials or voltages developed
between the borehole fluid and the surrounding
rock and fluids.
Typical application
Fracture/void zones, orientation of fractures, orientation of
strata, lithology, well construction, casing condition, screen
condition or elevation location. Requires clear fluids for
camera to view through.
Open fracture zones, orientation of open fractures,
orientation of strata, well construction. Does not require
clear fluids, can work in holes filled with mud.
Significantly conductive or non-conductive contaminants
that contrast substantially from background, fracture
zones, lithology (clay layers). Locate steel central izers
outside PVC casing (Caution: centralizers could be
interpreted as clay or conductive interval).
Lithology (clay layers)
Resistivity of borehole conditions, surrounding rock, and
surrounding water
Fracture zones, lithology changes, well construction casing
joints, voids, changes in casing diameter
Transmissive zones, vertical groundwater flow
Groundwater velocity, direction, capture zones, particle
size, tidal influences
Lithology, fracture zones, and conductive contamination
Lithology, water quality
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Using Geophysical Tools to Develop the Conceptual Site Model
5. CASE STUDIES
Increasingly, traditional geophysical techniques have
found new and innovative uses at hazardous waste
sites. Geophysical techniques have been used for
decades in other industries, principally the petroleum
and mining industries, for their ability to describe
geological structures deep within the earth's crust.
This proven track record has been applied to
subsurface characterization at hazardous waste sites.
Examples of how geophysical surveys have been
used to improve CSMs are described in the following
case studies.
Poudre River: EPA issued a Targeted Brownfields
Assessment (TBA) grant in 2003 to evaluate the
potential for official landfill closure to support the
planned expansion of a recreation center built on the
former landfill. Geophysical techniques were used at
the Poudre River site to support the GSM and to
establish a connection between potential landfill
source areas and coal tar contamination found in a
nearby river. A geophysical subcontractor conducted
high-resolution resistivity (HRR) and GPR
geophysical surveys to better define the bedrock
surface and to identify the presence or absence of
preferential pathways such as bedrock fractures,
subsurface channels in alluvium, or underground
pipelines. The survey was conducted prior to drilling
and sampling mobilization to allow time for
geophysical data evaluation, GSM refinement, and
subsequent refining of the sampling strategy.
The use of geophysical techniques at the Poudre
River Site provided a relatively inexpensive approach
for addressing the project team's presumptions about
the distribution of geologic features, such as
preferential pathways at the site, and allowed the
development of a more accurate GSM. Collaborative
data analysis and a synergistic site approach were
used to create a detailed soil and groundwater GSM
based on geophysical and site sampling data.
Collaborative data analysis and a synergistic site
approach were used to create a detailed soil and
groundwater GSM based on geophysical and site
sampling data.
UConn Landfill: The U.S. Geological Survey, Office
of Ground Water, Branch of Geophysics, conducted
an intensive investigation to characterize groundwater
contamination in fractured bedrock at the University of
Connecticut Landfill. The investigation was
undertaken as part of a collaborative effort with state
and federal regulators, private consulting firms, and
the public. A suite of surface and borehole
geophysical techniques were used in conjunction with
hydraulic and sampling methods to interpret data in
an integrated and iterative process. The development
of a groundwater GSM was a main goal of the
investigation.
Surface geophysical methods (including several
resistivity methods, EM induction, and seismic
methods) were successfully used to identify fractures,
to define the dominant fracture orientation, and to
locate potential leachate plumes. Based on results
from surface geophysical analysis, several boreholes
were drilled and logged with multiple borehole
geophysical methods including conventional methods.
Borehole geophysical logging successfully
differentiated high and low conductivity areas, which
were attributed to lithologic changes and those
caused by fluid-filled fractures. Further surface
geophysical surveys coupled with hydraulic and
chemical data interpretation helped to define the
extent of the conductive contaminant plume and to
produce an accurate GSM. The investigation
occurred in stages, with the first comprehensive
round of geophysical analysis, including surface
methods, drilling, and borehole methods, completed
in six months. The full scale, integrated geophysical
investigation occurred over several years, continually
refining the groundwater GSM.
Collaborative data analysis and a synergistic site
approach were used to create a detailed groundwater
GSM based on geophysical and hydraulic data.
Johnson, C.D., Dawson, C.B., Bel aval, M., and Lane,
J.W., Jr., 2002, An integrated surface-geophysical
investigation of the University of Connecticut landfill,
Storrs, Connecticut—2000: U.S. Geological Survey,
Water Resources Investigations Report 02-4008, 39
P.
Johnson, C.D., Joesten, P.K., and Mondazzi, R.A.,
2005, Integrated borehole-geophysical and hydraulic
investigation of the fractured-rock aquifer near the
University of Connecticut landfill, Storrs,
Connecticut—2000 to 2001: U.S. Geological Survey
Water-Resources Investigations Report 03-4125,133
P.
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Brunswick Naval Air Station: In order to refine a GSM
for the Brunswick Naval Air Station in Brunswick, ME,
the site contractor employed a suite of surface
geophysical tools to map the bedrock surface, identify
fracture zones, and to map the continuity and extent
of key stratigraphic horizons. The contamination is
attributed to past solvent disposal and is known to
extend deep into an aquifer. Existing data from
boreholes and cone penetrometer studies were
integrated with geophysical data from seismic
refraction and reflection, GPR, and resistivity to
produce an integrated database and construct a 2-
dimensional grid model.
A total of 28 geophysical survey lines using the above
methods were carried out during the study. Data sets
were processed individually and merged for
conceptual modeling purposes. Each geophysical
technique contributed to achieving the goals of the
survey. GPR data were used primarily for modeling
stratigraphy and depth to bedrock. Resistivity profiles
aided in locating fractures and, to a lesser degree,
identified depth to bedrock and stratigraphy. Seismic
methods provided information on depth to bedrock
and possible fracture locations. Final results were
presented in integrated models showing depth to
bedrock, fractures, and major stratigraphic units.
The entire geophysical investigation took
approximately 31/2 months to complete, including
geophysical data collection and compiling and
integrating almost ten years of previous site data from
various consulting firms. A new extraction well was
sited based on the improved GSM.
Creosote Investigation Using EM Methods: The U.S.
EPA Region 5 and the Department of Geological
Sciences at Ohio State University used EM induction
and GPR to investigate creosote contamination at the
former Baker Wood Creosoting Company industrial
site in Ohio. Data compiled from the geophysical
investigation were used to design an efficient,
comprehensive, and cost-effective remediation plan.
The study went beyond standard application of these
methods by successfully detecting high levels of
organic contamination that produced a resistive
response, rather than the typical inorganic, electrically
conductive plumes usually detected by EM methods.
Although clay-rich deposits are typical of the site
geology, GPR still provided valuable sub-surface
information. GPR data were collected in a cross-pole
configuration (transmitter and receiver arranged
orthogonal to each other) to improve the signal to
noise ratio. GPR penetration was sufficient to detect
back-filled trenches and a creosote-filled vault
beneath a foundation. EM data were used to map the
extent and depth of creosote compounds.
Contamination predictions based on the geophysical
data were validated through sampling analytical
methods.
Through integration of geophysics, an accurate
delineation of site contamination was completed with
as little invasive sampling as possible. In addition,
locating previously unknown buried waste pits
prevented possible future contamination.
Guy, E.D., Daniels, J.J., Holt, J., Radzevicius, S.J.,
Vendl, M.A., 2000, Electromagnetic induction and
GPR measurements for creosote contaminant
investigation: Journal of Environmental and
Engineering Geophysics, v 5, n 2, June 2000: 11-19.
(http://www.geology.ohio-
state.edu/~jeff/Library/emjeegOO.pdf)
6. REFERENCES
ASTM D6429-99, Standard Guide for Selecting
Surface Geophysical Methods, ASTM International.
Crumbling, D.M., Groenjes, C., Lesnik, B., Lynch, K.,
Shockley, J., van Ee, J., Howe, R.A., Keith, L.H.,
McKenna, J., 2001, Managing uncertainty in
environmental decisions: applying the concept of
effective data at contaminated sites could reduce
costs and improve cleanups: Environmental Science
and Technology, 35: 404A-409A.
Haeni, P.P., Lane, J.W. Jr., Williams, J.W., and
Johnson, C.D., 2001, Use of a geophysical tool box to
characterize ground-water flow in fractured rock: in
Proceedings, Fractured Rock 2001 Conference,
Toronto, Ontario, March 26-28, 2001, CD-ROM.
Milsom J., 1996, Field Geophysics, 2nded., The
Geological Field Guide Series: New York, John Wiley
and Sons. 187 p.
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Reynolds, J.M., 1997, An Introduction to Applied and
Environmental Geophysics, John Wiley and Sons,
Ltd: New York, 796 p.
Shapiro, A.M., Hsieh, P.A., and Haeni, F. P., 1999,
Integrating multidisciplinary investigations in the
characterization of fractured rock, in Morganwalp,
D.W., and Buxton, H.T., (Eds.), U.S. Geological
Survey Toxic Substances Hydrology Program-
Proceedings of the Technical Meeting, Charleston,
South Carolina, March 8-12,1999: U.S. Geological
Survey Water-Resources Investigation Report 99-
4018C.V. 3, 669-680.
Sharma, P.V., 1997, Environmental and Engineering
Geophysics, New York: Cambridge University Press,
475 p.
Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990,
Applied Geophysics, 2nded., Cambridge University
Press, New York, 770 p.
U.S. Environmental Protection Agency (U.S. EPA),
1991, Use of airborne, surface, and borehole
geophysical techniques at contaminated sites,
EPA/625/R-92/007, Washington, DC: Office of
Research and Development, U.S. EPA.
(http://www.hanford.gov/dqo/project/level5/
borehole.pdf)
U.S. Environmental Protection Agency (U.S. EPA),
2004, Participant Manual, Streamlined investigations
and cleanups using the Triad Approach, Summer/Fall
2004, CERCLA Education Center, unpublished class
slides.
U.S. Environmental Protection Agency (U.S. EPA),
2005, Case Study: The role of a conceptual site
model for expedited site characterization using the
triad approach at the Poudre River Site, Fort Collins,
Colorado, case study in press. Prepared in
cooperation with Tetra Tech EM Inc.
Using Geophysical Tools to Develop the Conceptual Site Model
NOTICE AND DISCLAIMER
This publication was prepared by EPA's Office of
Solid Waste and Emergency Response under EPA
Contract No. EP-W-07-078. The information in this
bulletin is not intended to revise or update EPA policy
or guidance on how to investigate or cleanup
Brownfields or other revitalization sites. Mention of
trade names or commercial products does not
constitute endorsement or recommendation for use.
This publication can be downloaded from EPA's
Brownfields and Land Revitalization Technology
Support Center at www.brownfieldstsc.org. For
further information about this publication or about the
Triad approach, please contact Michael Adam of EPA
at 703-603-9915, or adam.michael@epa.gov.
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