Keywords: electrical method, geophysics, resistivity, subsurface site characterization, surface geophysics,, ICS Number Code 07.060 (Geology. Meteorology. Hydrology)
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Significance and Use
5.1Concepts—The resistivity technique is
used to measure the resistivity of subsurface materials. Although
the resistivity of materials can be a good indicator of the type of
subsurface material present, it is not a unique indicator. While
the resistivity method is used to measure the resistivity of earth
materials, it is the interpreter who, based on knowledge of local
geologic conditions and other data, must interpret resistivity data
and arrive at a reasonable geologic and hydrologic
interpretation.
5.2Parameter Being Measured and Representative
Values:
5.2.1Table 1 shows some general trends for
resistivity values. Fig. 2
shows ranges in resistivity values for subsurface materials.
5.6.2Schlumberger Array—The Schlumberger array
consists of unequally spaced in-line electrodes (Fig. 3), where AB > 5 MN. The formula for calculating apparent
resistivity from a Schlumberger measurement is:
5.6.3Dipole-Dipole Array—The dipole-dipole
array consists of a pair of closely spaced current electrodes and a
pair of closely spaced potential electrodes (Fig. 3). The formula for calculating
apparent resistivity from a dipole-dipole measurement is:
5.6.4Comparison of the Arrays:
5.6.4.1Schlumberger Arrays:
(1) Schlumberger arrays are
less susceptible to contact problems and the influence of nearby
geologic conditions that may affect readings. The method provides a
means to recognize the effects of lateral variations and to
partially correct for them.
(2) Schlumberger arrays are
slightly faster in field operations since only the current
electrodes must be moved between readings.
5.6.4.2Wenner Arrays:
(1) The Wenner array provides
a higher signal to noise ratio than other arrays because its
potential electrodes are always farther apart and located between
the current electrodes. As a result, the Wenner array measures a
larger voltage for a given current than is measured with other
arrays.
(2) This array is good in
high-noise environments such as urban areas.
(3) This array requires less
current for a given depth capability. This translates into less
severe instrumentation requirements for a given depth
capability.
5.6.4.3Dipole-Dipole Arrays:
(1) Relatively short cable
lengths are required to explore large depths.
(2) Short cable lengths
reduce current leakage.
(3) More detailed information
on the direction of dip of electrical horizons is obtainable.
5.6.5Other Arrays—There are several other
arrays: Lee-partitioning array (Zohdy et al (2)), square array (Lane et al
5.7Sounding (Depth) Measurements—Sounding
measurements are one of the most widespread uses for the
resistivity technique. Soundings provide a means of measuring
changes of electrical resistivity with depth at a single location.
Several measurements are made with increasing electrode spacings.
As the spacing of the electrodes is increased, there is an increase
in the depth and volume of material measured (Fig. 4). The center point of the array
remains fixed as the electrical spacing is increased.
FIG.
4 Increased Electrode Spacing Samples Greater Depth and
Volume of Earth (from Benson et al, 5.7.1 Sounding measurements result in a
series of apparent electrical resistivity values at various
electrode spacings. These apparent resistivity values are plotted
against electrode spacing using a log-log scale (Fig. 5) and are interpreted using
inversion techniques to derive true resistivity and thickness of
subsurface layers.
FIG.
5 Resistivity Sounding Curve (from Benson et al,
5.7.2 Successive electrode spacings
should be (approximately) equally spaced on a logarithmic scale.
Normally, 3 to 6 data points per decade should be measured. A
resistivity sounding curve obtained from measurements of a uniform
layered medium should follow a smooth curve, (Fig. 5). By using six points per decade,
noise is generally less significant and a smooth sounding curve may
be obtained. Data should be plotted in the field to ensure that an
adequate number of noise-free measurements are made.
5.7.3 The depth of penetration for an
inhomogeneous stratified earth depends upon the electrode
separation and the resistivities of the earth's layers. In general,
the overall array length could be many times the exploration
depth.
5.8Profiling Measurements—A series of
profile measurements along a line is used to assess lateral changes
in subsurface conditions at a given depth (Fig. 6). Electrical resistivity profiling
is accomplished by making measurements with fixed electrode spacing
and array geometry at several stations along a profile line
(Fig. 7). A single profile
measurement results in an apparent electrical resistivity value at
a station. Several profiles over an area can be used to produce a
contour map of changes in subsurface conditions (Fig. 8). These apparent resistivity
profiles or maps cannot be interpreted in terms of layer
resistivity values without sounding data or other additional
information.
FIG.
6 Profiling to Assess Lateral Changes (from Zohdy et
al, FIG.
7 Stations Along a Profile (from Benson et al,
FIG.
8 Apparent Resistivity Contour Map (from Zohdy et al,
5.8.1 Vertical soundings are used to
determine the appropriate electrode spacing for profiling. Small
electrode spacings can be used to emphasize shallow variations in
resistivity that may affect the interpretation of deeper data.
Spacing between measurements controls the lateral resolution that
can be obtained from a series of profile measurements.
1. Scope
1.1Purpose and Application:
1.1.1 This guide summarizes the
equipment, field procedures, and interpretation methods for the
assessment of the electrical properties of subsurface materials and
their pore fluids, using the direct current (DC) resistivity
method. Measurements of the electrical properties of subsurface
materials are made from the land surface and yield an apparent
resistivity. These data can then be interpreted to yield an
estimate of the depth, thickness, voids, and resistivity of
subsurface layer(s).
1.1.2 Resistivity measurements as
described in this guide are applied in geological, geotechnical,
environmental, and hydrologic investigations. The resistivity
method is used to map geologic features such as lithology,
structure, fractures, and stratigraphy; hydrologic features such as
depth to water table, depth to aquitard, and groundwater salinity;
and to delineate groundwater contaminants. General references are,
Keller and Frischknecht 1.1.3 This guide does not address the use
tomographic interpretation methods, commonly referred to as
electrical resistivity tomography (ERT) or electrical resistivity
imaging (ERI). While many of the principles apply the data
acquisition and interpretation differ from those set forth in this
guide.
1.2Limitations:
1.2.1 This guide provides an overview of
the Direct Current Resistivity Method. It does not address in
detail the theory, field procedures, or interpretation of the data.
Numerous references are included for that purpose and are
considered an essential part of this guide. It is recommended that
the user of the resistivity method be familiar with the references
cited in the text and with the Guide D420, Practice D5088, Practice D5608, Guide D5730, Test Method G57, D6429, and D6235.
1.2.2 This guide is limited to the
commonly used approach for resistivity measurements using sounding
and profiling techniques with the Schlumberger, Wenner, or
dipole-dipole arrays and modifications to those arrays. It does not
cover the use of a wide range of specialized arrays. It also does
not include the use of spontaneous potential (SP) measurements,
induced polarization (IP) measurements, or complex resistivity
methods.
1.2.3 The resistivity method has been
adapted for a number of special uses, on land, within a borehole,
or on water. Discussions of these adaptations of resistivity
measurements are not included in this guide.
1.2.4 The approaches suggested in this
guide for the resistivity method are the most commonly used, widely
accepted and proven; however, other approaches or modifications to
the resistivity method that are technically sound may be
substituted if technically justified and documented.
1.2.5This guide offers an organized collection of
information or a series of options and does not recommend a
specific course of action. This document cannot replace education
or experience and should be used in conjunction with professional
judgements. Not all aspects of this guide may be applicable in all
circumstances. This ASTM standard is not intended to represent or
replace the standard of care by which the adequacy of a given
professional service must be judged, nor should this document be
applied without consideration of a project's many unique aspects.
The word “Standard” in the title of this document means only that
the document has been approved through the ASTM consensus
process.
1.3Units—The values stated in SI units are
to be regarded as standard. No other units of measurement are
included in this standard. Reporting of test results in units other
than SI shall not be regarded as nonconformance with this test
method.
1.4Precautions:
1.4.1It
is the responsibility of the user of this guide to follow any
precautions in the equipment manufacturer's recommendations and to
consider the safety implications when high voltages and currents
are used.
1.4.2If
this guide is used at sites with hazardous materials, operations,
or equipment, it is the responsibility of the user of this guide to
establish appropriate safety and health practices and to determine
the applicability of regulations prior to use.
1.5This standard does not purport to
address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to
establish appropriate safety, health, and environmental practices
and determine the applicability of regulatory limitations prior to
use.
1.6This international standard was
developed in accordance with internationally recognized principles
on standardization established in the Decision on Principles for
the Development of International Standards, Guides and
Recommendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
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