ASTM D6431-18

Standard Guide for Using the Direct Current Resistivity Method for Subsurface Site Characterization

SIGNIFICANCE AND USE
5.1 Concepts—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.  
5.2 Parameter Being Measured and Representative Values:  
5.2.1 Table 1 shows some general trends for resistivity values. Fig. 2 shows ranges in resistivity values for subsurface materials.    
5.6.2 Schlumberger 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:
    where:
  AB  =  distance between current electrodes, and    MN  =  distance between potential electrodes.    
5.6.3 Dipole-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:
    where:
  na  =  distance between innermost electrodes measured as a number (n) of a-spacings, and   a  =  distance between the current electrodes and also the potential electrodes.    
5.6.4 Comparison of the Arrays:  
5.6.4.1 Schlumberger 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.
5.6.4.2 Wenner Arrays:  
(1) The Wenner array provid...
SCOPE
1.1 Purpose 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),2 Zohdy et al  (2), Koefoed (3), EPA(4), Ward (5), Griffiths and King (6), and Telford et al (7).  
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.  
1.2 Limitations:  
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 fo...

General Information

• Status: Published
• Publication Date: 31-Jan-2018
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.01 - Surface and Subsurface Investigation

Relations

• Replaces: ASTM D6431-99(2010) - Standard Guide for Using the Direct Current Resistivity Method for Subsurface Investigation
• Effective Date: 01-Feb-2018
• Refers: ASTM D5088-20 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-May-2020
• Refers: ASTM D6235-18 - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 15-Dec-2018
• Refers: ASTM D5608-16 - Standard Practices for Decontamination of Sampling and Non Sample Contacting Equipment Used at Low Level Radioactive Waste Sites
• Effective Date: 01-Feb-2016
• Refers: ASTM D5088-15a - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-Aug-2015
• Refers: ASTM D5088-15 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 15-Jan-2015
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM D6429-99(2011)e1 - Standard Guide for Selecting Surface Geophysical Methods (Withdrawn 2020)
• Effective Date: 01-Jul-2011
• Refers: ASTM D6235-04(2010) - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 01-May-2010
• Refers: ASTM D5608-10 - Standard Practices for Decontamination of Field Equipment Used at Low Level Radioactive Waste Sites
• Effective Date: 01-May-2010
• Refers: ASTM D653-09 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Jan-2009
• Refers: ASTM D653-08a - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Dec-2008
• Refers: ASTM D653-08 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Nov-2008
• Refers: ASTM D5088-02(2008) - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 15-Sep-2008

Overview

ASTM D6431-18: Standard Guide for Using the Direct Current Resistivity Method for Subsurface Site Characterization provides comprehensive guidelines for assessing the electrical properties of subsurface materials using the direct current (DC) resistivity method. Developed by ASTM International, this standard outlines procedures for measuring apparent resistivity from the surface and offers recommendations on equipment, field procedures, data interpretation, and safety. The guide is widely used in geological, geotechnical, environmental, and hydrologic investigations to evaluate subsurface conditions such as lithology, structure, fractures, groundwater features, and potential contaminants.

Key Topics

• Principles of DC Resistivity: The standard introduces the concept of measuring the resistivity of earth materials using electrode arrays. While resistivity can suggest the type of subsurface material, it is not a unique indicator and must be interpreted with supplementary geologic and hydrologic data.
• Apparent Resistivity: Measurements yield an apparent resistivity, representing a bulk average of earth material. True resistivity values require interpretation, often aided by additional data.
• Electrode Array Methods: The guide covers standard electrode configurations:
  • Wenner: Equally spaced in-line electrodes, suitable for high-noise environments.
  • Schlumberger: Unequally spaced electrodes, efficient in the field and less susceptible to contact issues.
  • Dipole-Dipole: Closely spaced current and potential electrode pairs, informative for profiling and imaging.
• Survey Methods:
  • Sounding (Depth) Surveys: Determining changes in resistivity with depth at a single location.
  • Profiling (Lateral Variations): Mapping lateral changes at a constant depth, useful for defining the extent of geological features.
• Limitations and Interferences: The method is sensitive to electrical interference from natural, geological, and cultural sources. Non-unique solutions necessitate integration with complementary methods and borehole data for accurate interpretation.

Applications

The DC resistivity method described in ASTM D6431-18 is applicable across a range of subsurface investigations, including:

• Geological Mapping: Identifying subsurface stratigraphy, lithology, and structural features such as faults and fractures.
• Geotechnical Site Characterization: Evaluating soil and rock properties to inform construction and foundation design.
• Hydrogeologic Assessment: Locating the water table, identifying aquitards, determining groundwater quality or salinity, and delineating contaminant plumes.
• Environmental Investigations: Detecting and mapping subsurface contaminant migration, especially relevant in hazardous waste site assessments.
• Infrastructure Planning: Supporting the design and placement of utilities and monitoring systems by revealing hidden subsurface features.

By following standardized procedures in ASTM D6431-18, practitioners can obtain reliable resistivity data, minimize errors due to equipment, site conditions, or field procedures, and ensure safer operations.

Related Standards

Integrating ASTM D6431-18 with other ASTM standards enhances the reliability and context of subsurface investigations. Key related documents include:

• ASTM D420: Guide to Site Characterization for Engineering Design and Construction Purposes
• ASTM D653: Terminology Relating to Soil, Rock, and Contained Fluids
• ASTM D5088 / D5608: Practices for Decontamination of Field Equipment at Waste Sites
• ASTM D5730: Guide for Site Characterization for Environmental Purposes
• ASTM D5753: Geotechnical Borehole Geophysical Logging
• ASTM D6235: Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination
• ASTM D6429: Guide for Selecting Surface Geophysical Methods
• ASTM G57: Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method

By employing ASTM D6431-18 as part of a suite of geophysical standards, professionals in geotechnical, environmental, and hydrological fields can enhance the accuracy and value of subsurface site characterization projects, guiding effective decision-making and risk management.

Keywords: ASTM D6431-18, DC resistivity method, subsurface site characterization, geophysical survey, Wenner array, Schlumberger array, dipole-dipole array, groundwater assessment, environmental site investigation, ASTM standards.

Standards Content (Sample)

This 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.
Designation: D6431 − 18
Standard Guide for
Using the Direct Current Resistivity Method for Subsurface
Site Characterization
This standard is issued under the fixed designation D6431; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 1.2.2 This guide is limited to the commonly used approach
for resistivity measurements using sounding and profiling
1.1 Purpose and Application:
techniques with the Schlumberger, Wenner, or dipole-dipole
1.1.1 This guide summarizes the equipment, field
arrays and modifications to those arrays. It does not cover the
procedures, and interpretation methods for the assessment of
use of a wide range of specialized arrays. It also does not
the electrical properties of subsurface materials and their pore
include the use of spontaneous potential (SP) measurements,
fluids, using the direct current (DC) resistivity method. Mea-
induced polarization (IP) measurements, or complex resistivity
surements of the electrical properties of subsurface materials
methods.
are made from the land surface and yield an apparent resistiv-
1.2.3 The resistivity method has been adapted for a number
ity. These data can then be interpreted to yield an estimate of
of special uses, on land, within a borehole, or on water.
the depth, thickness, voids, and resistivity of subsurface
Discussions of these adaptations of resistivity measurements
layer(s).
are not included in this guide.
1.1.2 Resistivity measurements as described in this guide
1.2.4 The approaches suggested in this guide for the resis-
are applied in geological, geotechnical, environmental, and
tivity method are the most commonly used, widely accepted
hydrologic investigations. The resistivity method is used to
and proven; however, other approaches or modifications to the
map geologic features such as lithology, structure, fractures,
resistivitymethodthataretechnicallysoundmaybesubstituted
and stratigraphy; hydrologic features such as depth to water
if technically justified and documented.
table, depth to aquitard, and groundwater salinity; and to
1.2.5 This guide offers an organized collection of informa-
delineate groundwater contaminants. General references are,
tion or a series of options and does not recommend a specific
Keller and Frischknecht (1), Zohdy et al (2), Koefoed (3),
course of action. This document cannot replace education or
EPA(4), Ward (5), Griffiths and King (6), and Telford et al (7).
experienceandshouldbeusedinconjunctionwithprofessional
1.1.3 This guide does not address the use tomographic
judgements. Not all aspects of this guide may be applicable in
interpretation methods, commonly referred to as electrical
all circumstances. This ASTM standard is not intended to
resistivity tomography (ERT) or electrical resistivity imaging
represent or replace the standard of care by which the
(ERI). While many of the principles apply the data acquisition
adequacy of a given professional service must be judged, nor
and interpretation differ from those set forth in this guide.
should this document be applied without consideration of a
1.2 Limitations:
project’s many unique aspects. The word “Standard” in the
1.2.1 This guide provides an overview of the Direct Current
title of this document means only that the document has been
Resistivity Method. It does not address in detail the theory,
approved through the ASTM consensus process.
field procedures, or interpretation of the data. Numerous
1.3 Units—The values stated in SI units are to be regarded
references are included for that purpose and are considered an
as standard. No other units of measurement are included in this
essential part of this guide. It is recommended that the user of
standard. Reporting of test results in units other than SI shall
the resistivity method be familiar with the references cited in
not be regarded as nonconformance with this test method.
the text and with the Guide D420, Practice D5088, Practice
D5608, Guide D5730, Test Method G57, D6429, and D6235.
1.4 Precautions:
1.4.1 It is the responsibility of the user of this guide to
follow any precautions in the equipment manufacturer’s rec-
This guide is under the jurisdiction ofASTM CommitteeD18 on Soil and Rock
ommendations and to consider the safety implications when
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
high voltages and currents are used.
Characterization.
Current edition approved Feb. 1, 2018. Published March 2018. Originally
1.4.2 If this guide is used at sites with hazardous materials,
approved in 1999. Last previous edition approved in 2010 as D6431–99(2010).
operations, or equipment, it is the responsibility of the user of
DOI: 10.1520/D6431-18.
this guide to establish appropriate safety and health practices
The boldface numbers in parentheses refer to the list of references at the end of
this standard. and to determine the applicability of regulations prior to use.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6431 − 18
1.5 This standard does not purport to address all of the 3.2.3 resistance, n—opposition to the flow of direct current.
safety concerns, if any, associated with its use. It is the 3.2.3.1 Discussion—Resistance is measured in ohm (Ω).
responsibility of the user of this standard to establish appro-
3.2.4 resistivity, n—the property of a material that resists the
priate safety, health, and environmental practices and deter-
flow of electrical current.
mine the applicability of regulatory limitations prior to use.
3.2.4.1 Discussion—Resistivity is measured in ohm-meter
1.6 This international standard was developed in accor-
(Ωm).
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
4. Summary of Guide
Development of International Standards, Guides and Recom-
4.1 Summary—The measurement of electrical resistivity
mendations issued by the World Trade Organization Technical
requires that four electrodes be placed in contact with the
Barriers to Trade (TBT) Committee.
surface materials (Fig. 1). The geometry and separation of the
electrode array are selected on the basis of the application and
2. Referenced Documents
3 required depth of the site characterization.
2.1 ASTM Standards:
4.1.1 In an electrical resistivity survey, a direct current or a
D420 Guide to Site Characterization for Engineering Design
very low frequency alternating current is passed into the
and Construction Purposes
ground through a pair of current electrodes (C1 and C2), and
D653 Terminology Relating to Soil, Rock, and Contained
the resulting potential drop is measured across a pair of
Fluids
potential electrodes (P1 and P2) as shown in Fig. 1. The
D5088 Practice for Decontamination of Field Equipment
resistance is then derived as the ratio of the voltage measured
Used at Waste Sites
across the potential electrodes and the measured applied
D5608 Practices for Decontamination of Sampling and Non
current. The apparent resistivity of subsurface materials is
Sample Contacting Equipment Used at Low Level Radio-
derived as the resistance multiplied by a geometric factor that
active Waste Sites
is determined by the geometry and spacing of the electrode
D5730 Guide for Site Characterization for Environmental
array.
Purposes With Emphasis on Soil, Rock, the Vadose Zone
4 4.1.2 The calculated apparent resistivity measurement rep-
and Groundwater (Withdrawn 2013)
resents a bulk average resistivity of the volume of earth
D5753 Guide for Planning and Conducting Geotechnical
determined by the geometry of the array and the resistivity of
Borehole Geophysical Logging
the subsurface material. This apparent resistivity is different
D6235 Practice for Expedited Site Characterization of Va-
from true resistivity unless the subsurface materials are elec-
dose Zone and Groundwater Contamination at Hazardous
trically uniform. Representative resistivity values of layers are
Waste Contaminated Sites
interpreted from apparent resistivity values obtained from a
D6429 Guide for Selecting Surface Geophysical Methods
series of measurements made with variable electrode spacing.
G57 Test Method for Field Measurement of Soil Resistivity
Increasing electrode spacing may permit distinction among
Using the Wenner Four-Electrode Method
layers that vary in electrical properties with depth.
4.1.3 Most resistivity surveys for geologic, engineering,
3. Terminology
hydrologic, and environmental applications are carried out to
3.1 Definitions:
determine depths of specific layers or lateral changes in
3.1.1 Forcommondefinitionsoftermsusedinthisstandard,
geologic conditions at depths of less than a hundred metres.
see Terminology D653.
However, with sufficient power and instrument sensitivity,
3.1.2 The majority of the technical terms used in this
resistivitymeasurementsaremadetodepthsofseveralhundred
document are defined in Sheriff (1991).
metres.
3.2 Additional Definitions:
4.2 Complementary Data—Other complementary surface
3.2.1 apparent resistivity, n—the resistivity of
geophysical methods (D6429) or borehole geophysical meth-
homogeneous, isotropic ground that would give the same
ods (Guide D5753) and non-geophysical methods may be
voltage-current relationship as measured.
necessary to properly interpret subsurface conditions.
3.2.1.1 Discussion—Apparent resistivity is expressed in
ohm-meter (Ωm).
5. Significance and Use
3.2.2 conductivity, n—the ability of a material to conduct an
electrical current. In isotropic material, it is the reciprocal of
5.1 Concepts—The resistivity technique is used to measure
resistivity. the resistivity of subsurface materials. Although the resistivity
3.2.2.1 Discussion—Conductivity is measured in Siemens
of materials can be a good indicator of the type of subsurface
per meter (S/m). material present, it is not a unique indicator. While the
resistivity method is used to measure the resistivity of earth
materials,itistheinterpreterwho,basedonknowledgeoflocal
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
geologic conditions and other data, must interpret resistivity
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on data and arrive at a reasonable geologic and hydrologic
the ASTM website.
interpretation.
The last approved version of this historical standard is referenced on
www.astm.org. 5.2 Parameter Being Measured and Representative Values:
D6431 − 18
FIG. 1 Diagram Showing Basic Concept of Resistivity Measurement (from Benson et al, (8))
5.2.1 Table 1 shows some general trends for resistivity 5.2.5 The dependence of resistivity on water saturation is
values. Fig. 2 shows ranges in resistivity values for subsurface not linear. Resistivity increases relatively little as saturation
materials. decreases from 100 % to somewhere between 40 and 60 % and
5.2.2 Materials with either a low effective porosity or that then increases much more as saturation continues to decrease.
lack conductive pore fluids have a relatively high resistivity AnempiricalrelationshipknownasArchie’sLawdescribesthe
(>1000 Ωm). These materials include massive limestones, relationship between pore fluid resistivity, porosity, and bulk
most unfractured igneous rocks, unsaturated unconsolidated resistivity (McNeill (10)).
materials, and ice.
5.3 Equipment—Geophysical apparatus used for surface
5.2.3 Materialsthathavehighporositywithconductivepore
resistivity measurement includes a source of power, a means to
fluids or that consist of or contain clays usually have low
measure the current, a high impedance voltmeter, electrodes to
resistivity. These include clay soil and weathered rock.
make contact with the ground, and the necessary cables to
5.2.4 Materials whose pore water has low salinity have
connect the electrodes to the power sources and the volt meter
moderately high resistivity.
(Fig. 1).
5.3.1 While resistivity measurements can be made using
common electronic instruments, it is recommended that com-
TABLE 1 Representative Resistivity Values for Soil, Water, and
mercial resistivity instruments specifically designed for the
Rock (Mooney (4))
purpose be used for resistivity measurements in the field.
Regional Soil Resistivity Ωm
5.3.2 Caremustbetakentoensuregoodelectricalcontactof
- wet regions 50 to 200
the electrodes with the ground. Electrodes should be driven
- dry regions 100 to 500
- arid regions 200 to 1000 (sometimes as low as 50 if the
into the ground until they are in firm contact. If connections
soil is saline)
between electrodes and the insulated wire are not waterproof,
Water Type Ωm
care must be taken to ensure that they will not be shorted out
- soil water 1 to 100
- rain water 30 to 1000
by moisture. Special waterproof cables and connectors are
- sea water order of 0.2
required for wet areas.
- ice 105 to 108
Earth Material Types Ωm 5.3.3 A large variety of resistivity systems are available
- igneous and metamorphic 100 to 10,000
from different manufacturers. Relatively inexpensive battery-
- consolidated sediments 10 to 100
powered units are available for shallow surveys. The current
- unconsolidated sediments 1 to 100
source (transmitter) and the potential measurement instrument
D6431 − 18
FIG. 2 Typical Ranges of Resistivities of Earth Materials (from Sheriff, (9))
(receiver) are often assembled into a single, portable unit. In 5.4.2.4 Ambiguities in interpretation arising from suppres-
some cases, the transmitter and receiver units are separate. sion (where resistant layers are sandwiched between more
High power units capable of deep survey work are powered by conductive layers).
generators. The current used in dc resistivity surveys varies 5.4.2.5 Extremely resistive materials will prevent current
from a few milliamps to several amps, depending on the depth injection into the ground.
of the site characterization. 5.4.3 Interferences Caused by Ambient and Geologic Con-
5.3.4 Signal Enhancement—Signal enhancement capability ditions:
is available in many resistivity systems. It is a significant aid 5.4.3.1 The resistivity method is sensitive to electrical
when working in noisy areas or with low power sources. interference from a variety of sources. Spatial variables caused
Enhancement is accomplished by adding the results from a by geologic factors and cultural factors may also produce
number of measurements at the same station. This process noise.
increases the signal-to-noise ratio. 5.4.3.2 Ambient Sources of Noise—Natural (ambient)
sources of noise include lightning or natural earth currents,
5.4 Limitations and Interferences:
which may induce a voltage in resistivity cables.
5.4.1 Limitations Inherent to Geophysical Methods:
5.4.3.3 Geologic Sources of Noise—Geologic sources of
5.4.1.1 Afundamental limitation of all geophysical methods
noise include local inhomogeneities near electrodes that may
liesinthefactthatagivensetofdatacannotbeassociatedwith
result in measurement error and variations in the subsurface
a unique set of subsurface conditions. In most situations,
that are not the object of the survey.
surface geophysical measurements alone cannot resolve all
5.4.3.4 Sources of Noise—Resistivity measurements may be
ambiguities,andsomeadditionalinformation,suchasborehole
influenced by nearby features (such as power lines, radio
data, is required. Because of this inherent limitation in geo-
stations, cathodic pipeline protection, and other geophysical
physicalmethods,aresistivitysurveyaloneisneverconsidered
equipment) that generate electrical or electromagnetic fields.
a complete assessment of subsurface conditions. Properly
Pipelines, fences, and metal buildings may also affect them.
integrated with other information, resistivity surveying is an
5.4.3.5 Leakage—A resistivity measurement may also be
effective method of obtaining subsurface information.
affected by leakage from the insulated wire used to connect the
5.4.1.2 All surface geophysical methods are inherently lim-
instrument to the electrodes. Tests for leakage can be made at
ited by decreasing resolution with depth.
the time of the measurement.
5.4.2 Limitations Specific to the Resistivity Method:
5.4.4 Cultural Interference—During the course of designing
5.4.2.1 Interpretation methods assume horizontal (or paral-
survey locations, potential cultural interferences should be
lel) layered conditions where each layer has a uniform electri-
considered. Cultural interference may emanate from pipes,
cal resistivity. If subsurface conditions cannot be reasonably
fences, buried utilities and other grounded linear electric
approximated by this assumption, then results will be in error.
conductors in the vicinity of the survey. These objects may
5.4.2.2 Thin layers or multiple layers with similar resistivi-
“short circuit” the measured currents assumed to be passing
ties may not be detected.
through the ground.
5.4.2.3 Ambiguities in interpretation arising from equiva-
lence (where two resistive layers carry nearly the same electric 5.5 Alternate Methods—The limitations previously dis-
current if the products of their resistivity and thickness equal). cussed may prohibit the effective use of the resistivity method,
D6431 − 18
and other methods may be required to resolve the problem.An where:
alternative to the resistivity method is the EM method, which
A = electrode spacing,
is preferred in high-resistivity (low-conductivity) materials,
V = measured voltage, and
I = current.
and may require less space to conduct the survey.
5.6.2 Schlumberger Array—The Schlumberger array con-
5.6 Electrode Array Geometry—Usually the electrodes are
sists of unequally spaced in-line electrodes (Fig. 3), where AB
arranged in a collinear array in one of several fixed geometries.
>5 MN. The formula for calculating apparent resistivity from
Several standard electrode geometries have been developed for
a Schlumberger measurement is:
various applications (Fig. 3). For engineering, environmental,
and groundwater studies, the Wenner, Schlumberger, and 2 2
AB/2 2 MN/2 V
~ ! ~ !
R 5π 3 (2)
dipole-dipole arrays are the most commonly used.
MN I
5.6.1 Wenner Array—The Wenner array consists of equally
where:
spaced, in-line electrodes (Fig. 3). The formula for calculating
AB = distance between current electrodes, and
apparent resistivity from a Wenner measurement is:
MN = distance between potential electrodes.
R 5 2πA~V/I! (1)
FIG. 3 Standard Electrode Geometries
D6431 − 18
5.6.3 Dipole-Dipole Array—The dipole-dipole array con- technique. Soundings provide a means of measuring changes
sists of a pair of closely spaced current electrodes and a pair of of electrical resistivity with depth at a single location. Several
closely spaced potential electrodes (Fig. 3). The formula for
measurements are made with increasing electrode spacings.As
calculating apparent resistivity from a dipole-dipole measure- the spacing of the electrodes is increased, there is an increase
ment is:
in the depth and volume of material measured (Fig. 4). The
center point of the array remains fixed as the electrical spacing
R 5πna n11 n12 V/I (3)
~ !~ !~ !
is increased.
where:
5.7.1 Sounding measurements result in a series of apparent
na = distance between innermost electrodes measured as a
electricalresistivityvaluesatvariouselectrodespacings.These
number (n) of a-spacings, and
apparent resistivity values are plotted against electrode spacing
a = distance between the current electrodes and also the
using a log-log scale (Fig. 5) and are interpreted using
potential electrodes.
inversion techniques to derive true resistivity and thickness of
5.6.4 Comparison of the Arrays:
subsurface layers.
5.6.4.1 Schlumberger Arrays:
5.7.2 Successive electrode spacings should be (approxi-
(1) Schlumberger arrays are less susceptible to contact
mately) equally spaced on a logarithmic scale. Normally, 3 to
problems and the influence of nearby geologic conditions that
6 data points per decade should be measured. A resistivity
mayaffectreadings.Themethodprovidesameanstorecognize
sounding curve obtained from measurements of a uniform
theeffectsoflateralvariationsandtopartiallycorrectforthem.
layered medium should follow a smooth curve, (Fig. 5). By
(2) Schlumberger arrays are slightly faster in field opera-
using six points per decade, noise is generally less significant
tions since only the current electrodes must be moved between
and a smooth sounding curve may be obtained. Data should be
readings.
plotted in the field to ensure that an adequate number of
5.6.4.2 Wenner Arrays:
noise-free measurements are made.
(1) TheWennerarrayprovidesahighersignaltonoiseratio
5.7.3 The depth of penetration for an inhomogeneous strati-
than other arrays because its potential electrodes are always
fied earth depends upon the electrode separation and the
farther apart and located between the current electrodes. As a
resistivities of the earth’s layers. In general, the overall array
result, the Wenner array measures a larger voltage for a given
length could be many times the exploration depth.
current than is measured with other arrays.
(2) This array is good in high-noise environments such as
5.8 Profiling Measurements—A series of profile measure-
urban areas. ments along a line is used to assess lateral changes in
(3) This array requires less current for a given depth
subsurface conditions at a given depth (Fig. 6). Electrical
capability. This translates into less severe instrumentation
resistivity profiling is accomplished by making measurements
requirements for a given depth capability.
with fixed electrode spacing and array geometry at several
5.6.4.3 Dipole-Dipole Arrays:
stations along a profile line (Fig. 7). A single profile measure-
(1) Relatively short cable lengths are required to explore
ment results in an apparent electrical resistivity value at a
large depths.
station. Several profiles over an area can be used to produce a
(2) Short cable lengths reduce current leakage.
contour map of changes in subsurface conditions (Fig. 8).
(3) More detailed information on the direction of dip of
These apparent resistivity profiles or maps cannot be inter-
electrical horizons is obtainable.
pretedintermsoflayerresistivityvalueswithoutsoundingdata
5.6.5 Other Arrays—There are several other arrays: Lee-
or other additional information.
partitioning array (Zohdy et al (2)), square array (Lane et al
5.8.1 Vertical soundings are used to determine the appropri-
(11)), gradient array (Ward (2)) and pole-dipole (Ward (5)) and
ate electrode spacing for profiling. Small electrode spacings
automated data acquisition and imaging systems that are not
can be used to emphasize shallow variations in resistivity that
discussed in this guideline.
may affect the interpretation of deeper data. Spacing between
5.7 Sounding (Depth) Measurements—Sounding measure- measurements controls the lateral resolution that can be ob-
ments are one of the most widespread uses for the resistivity
tained from a series of profile measurements.
FIG. 4 Increased Electrode Spacing Samples Greater Depth and Volume of Earth (from Benson et al, (8))
D6431 − 18
FIG. 5 Resistivity Sounding Curve (from Benson et al, (8))
FIG. 6 Profiling to Assess Lateral Changes (from Zohdy et al, (12))
6. Procedure resistivity data along with an understanding of the site geology
is necessary. Personnel not having specialized training or
6.1 Qualification of Personnel—The success of a resistivity
experience should be cautious about using this technique and
survey, as with most geophysical techniques, is dependent
solicit assistance from qualified practitioners.
upon many factors. One of the most important is the compe-
tence of the persons responsible for planning, carrying out the 6.2 Planning the Survey—Successful use of the resistivity
survey, and interpreting the data. An understanding of the method depends to a great extent on careful and detailed
theory, field procedures, and methods for interpretation of the planning.
D6431 − 18
FIG. 7 Stations Along a Profile (from Benson et al, (8))
6.2.1 Objectives of the Resistivity Survey: resistivity method assessed.Aresistivity or induction (conduc-
6.2.1.1 Planninganddesignofaresistivitysurveyshouldbe tivity) log may be run in boreholes on or near the site to
done with due consideration to the objectives of the survey and provide resistivity (conductivity) values with depth and aid in
the characteristics of the site. These factors will determine the assessing the potential success of the surface resistivity
survey design, the equipment used, the level of effort, the method.
interpretation method selected, and the budget necessary to 6.2.2.3 A forward model can be used to plot the apparent
achieve the desired results. Important considerations include resistivity versus electrode spacing for an assumed thickness
sitegeology,depthofthesitecharacterization,topography,and and resistivity of subsurface layers. These results are used to
access. The presence of noise-generating activities (on-site predict the success of the resistivity method in resolving the
utilities, man-made structures) and operational constraints desired layer or layers and to help determ
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6431 − 99 (Reapproved 2010) D6431 − 18
Standard Guide for
Using the Direct Current Resistivity Method for Subsurface
InvestigationSite Characterization
This standard is issued under the fixed designation D6431; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 Purpose 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), Zohdy et al (2), Koefoed (3), EPA(4), Ward (5), Griffiths and
King (6), and Telford et al (7).
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.
1.2 Limitations:
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.
1.2.5 This 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.
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved May 1, 2010Feb. 1, 2018. Published September 2010March 2018. Originally approved in 1999. Last previous edition approved in 20052010 as
D6431–99(2005).D6431–99(2010). DOI: 10.1520/D6431-99R10.10.1520/D6431-18.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6431 − 18
1.3 Units—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.
1.4 Precautions:
1.4.1 It 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.
1.4.2 If 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.
1.5 This 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This 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.
2. Referenced Documents
2.1 ASTM Standards:
D420 Guide to Site Characterization for Engineering Design and Construction Purposes (Withdrawn 2011)
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D5088 Practice for Decontamination of Field Equipment Used at Waste Sites
D5608 Practices for Decontamination of Sampling and Non Sample Contacting Equipment Used at Low Level Radioactive
Waste Sites
D5730 Guide for Site Characterization for Environmental Purposes With Emphasis on Soil, Rock, the Vadose Zone and
Groundwater (Withdrawn 2013)
D5753 Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
D6235 Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste
Contaminated Sites
D6429 Guide for Selecting Surface Geophysical Methods
G57 Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
3. Terminology
3.1 Definitions:
3.1.1 For common definitions of terms used in this standard, see Terminology D653.
3.1.2 The majority of the technical terms used in this document are defined in Sheriff (1991).
3.2 Additional Definitions:
3.1.1 Definitions shall be in accordance with the terms and symbols given in Terminology D653.
3.2.1 apparent resistivity, n—the resistivity of homogeneous, isotropic ground that would give the same voltage-current
relationship as measured.
3.2.1.1 Discussion—
Apparent resistivity is expressed in ohm-meter (Ωm).
3.1.2 The majority of the technical terms used in this document are defined in Sheriff (1991).
3.2.2 conductivity, n—the ability of a material to conduct an electrical current. In isotropic material, it is the reciprocal of
resistivity.
3.2.2.1 Discussion—
Conductivity is measured in Siemens per meter (S/m).
3.1.3 Additional Definitions:
3.1.3.1 apparent resistivity—the resistivity of homogeneous, isotropic ground that would give the same voltage-current
relationship as measured.
3.1.3.2 conductivity—The ability of a material to conduct an electrical current. In isotropic material, it is the reciprocal of
resistivity. The units of conductivity are siemens per metre.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
D6431 − 18
3.1.3.3 resistance—opposition to the flow of direct current. The unit of resistance is ohms.
3.1.3.4 resistivity—the property of a material that resists the flow of electrical current. The units of resistivity are ohmmetres
or ohm-feet (1 Ωm = 3.28 Ω-ft).
3.2.3 resistance, n—opposition to the flow of direct current.
3.2.3.1 Discussion—
Resistance is measured in ohm (Ω).
3.2.4 resistivity, n—the property of a material that resists the flow of electrical current.
3.2.4.1 Discussion—
Resistivity is measured in ohm-meter (Ωm).
4. Summary of Guide
4.1 Summary—The measurement of electrical resistivity requires that four electrodes be placed in contact with the surface
materials (Fig. 1). The geometry and separation of the electrode array are selected on the basis of the application and required depth
of investigation.the site characterization.
4.1.1 In an electrical resistivity survey, a direct current or a very low frequency alternating current is passed into the ground
through a pair of current electrodes, and electrodes (C1 and C2), and the resulting potential drop is measured across a pair of
potential electrodes ((P1 and P2) as shown in Fig. 1). The resistance is then derived as the ratio of the voltage measured across
the potential electrodes and the current electrodes. measured applied current. The apparent resistivity of subsurface materials is
derived as the resistance multiplied by a geometric factor that is determined by the geometry and spacing of the electrode array.
4.1.2 The calculated apparent resistivity measurement represents a bulk average resistivity of the volume of earth determined
by the geometry of the array and the resistivity of the subsurface material. This apparent resistivity is different from true resistivity
unless the subsurface materials are electrically uniform. Representative resistivity values of layers are interpreted from apparent
resistivity values obtained from a series of measurements made with variable electrode spacing. Increasing electrode spacing may
permit distinction among layers that vary in electrical properties with depth.
FIG. 1 Diagram Showing Basic Concept of Resistivity Measurement (from Benson et al, (8))
D6431 − 18
4.1.3 Most resistivity surveys for geologic, engineering, hydrologic, and environmental applications are carried out to determine
depths of specific layers or lateral changes in geologic conditions at depths of less than a hundred metres. However, with sufficient
power and instrument sensitivity, resistivity measurements are made to depths of several hundred metres.
4.2 Complementary Data—Other complementary surface geophysical methods (D6429) or borehole geophysical methods
(Guide D5753) and non-geophysical methods may be necessary to properly interpret subsurface conditions.
5. Significance and Use
5.1 Concepts—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.
5.2 Parameter Being Measured and Representative Values:
5.2.1 Table 1 shows some general trends for resistivity values. Fig. 2 shows ranges in resistivity values for subsurface materials.
5.2.2 Materials with either a low effective porosity or that lack conductive pore fluids have a relatively high resistivity (>1000
Ωm).Ωm). These materials include massive limestones, most unfractured igneous rocks, unsaturated unconsolidated materials, and
ice.
5.2.3 Materials that have high porosity with conductive pore fluids or that consist of or contain clays usually have low resistivity.
These include clay soil and weathered rock.
5.2.4 Materials whose pore water has low salinity have moderately high resistivity.
5.2.5 The dependence of resistivity on water saturation is not linear. Resistivity increases relatively little as saturation decreases
from 100 % to 40-60 % and somewhere between 40 and 60 % and then increases much more as saturation continues to decrease.
An empirical relationship known as Archie’s Law describes the relationship between pore fluid resistivity, porosity, and bulk
resistivity (McNeill (10)).
5.3 Equipment—Geophysical apparatus used for surface resistivity measurement includes a source of power, a means to
measure the current, a high impedance voltmeter, electrodes to make contact with the ground, and the necessary cables to connect
the electrodes to the power sources and the volt meter (Fig. 1).
5.3.1 While resistivity measurements can be made using common electronic instruments, it is recommended that commercial
resistivity instruments specifically designed for the purpose be used for resistivity measurements in the field.
5.3.2 Commonly used equipment includes the following elements:
5.3.2.1 A source of current consisting of batteries or a generator,
5.3.2.2 A high-impedance voltmeter or resistivity unit,
5.3.2.3 Metal stakes for the current and potential electrodes, and
5.3.2.4 Insulated wire to connect together all of the preceding components.
5.3.2 Care must be taken to ensure good electrical contact of the electrodes with the ground. Electrodes should be driven into
the ground until they are in firm contact. If connections between electrodes and the insulated wire are not waterproof, care must
be taken to ensure that they will not be shorted out by moisture. Special waterproof cables and connectors are required for wet
areas.
TABLE 1 Representative Resistivity Values for Soil, Water, and
Rock (Mooney (4))
Regional Soil Resistivity Ωm
Regional Soil Resistivity m
- wet regions 50–200
- wet regions 50 to 200
- dry regions 100–500
- dry regions 100 to 500
- arid regions 200–1000 (sometimes as low as 50 if the soil
is saline)
- arid regions 200 to 1000 (sometimes as low as 50 if the
soil is saline)
Waters Ωm
Water Type m
- soil water 1 to 100
- rain water 30 to 1000
- sea water order of 0.2
- ice 105 to 108
Rock Types Ωm
Earth Material Types m
- igneous and metamorphic 100 to 10,000
- consolidated sediments 10 to 100
- unconsolidated sediments 1 to 100
D6431 − 18
FIG. 2 Typical Ranges of Resistivities of Earth Materials (from Sheriff, (9))
5.3.3 A large variety of resistivity systems are available from different manufacturers. Relatively inexpensive battery-powered
units are available for shallow surveys. The current source (transmitter) and the potential measurement instrument (receiver) are
often assembled into a single, portable unit. In some cases, the transmitter and receiver units are separate. High power units capable
of deep survey work are powered by generators. The current used in dc resistivity surveys varies from a few milliamps to several
amps, depending on the depth of the investigation.site characterization.
5.3.4 Signal Enhancement—Signal enhancement capability is available in many resistivity systems. It is a significant aid when
working in noisy areas or with low power sources. Enhancement is accomplished by adding the results from a number of
measurements at the same station. This process increases the signal-to-noise ratio.
5.4 Limitations and Interferences : Interferences:
5.4.1 Limitations Inherent to Geophysical Methods:
5.4.1.1 A fundamental limitation of all geophysical methods lies in the fact that a given set of data cannot be associated with
a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all ambiguities,
and some additional information, such as borehole data, is required. Because of this inherent limitation in geophysical methods,
a resistivity survey alone is never considered a complete assessment of subsurface conditions. Properly integrated with other
information, resistivity surveying is an effective method of obtaining subsurface information.
5.4.1.2 All surface geophysical methods are inherently limited by decreasing resolution with depth.
5.4.2 Limitations Specific to the Resistivity Method:
5.4.2.1 Interpretation methods assume horizontal (or parallel) layered conditions where each layer has a uniform electrical
resistivity. If subsurface conditions cannot be reasonably approximated by this assumption, then results will be in error.
5.4.2.2 Thin layers or multiple layers with similar resistivities may not be detected.
5.4.2.3 Ambiguities in interpretation arising from equivalence (where two resistive layers carry nearly the same electric current
if the products of their resistivity and thickness equal).
5.4.2.4 Ambiguities in interpretation arising from suppression (where resistant layers are sandwiched between more conductive
layers).
5.4.2.5 Extremely resistive materials will prevent current injection into the ground.
5.4.3 Interferences Caused by Ambient and Geologic Conditions:
5.4.3.1 The resistivity method is sensitive to electrical interference from a variety of sources. It is inherently sensitive to
electrical interference. Spatial variables caused by geologic factors and cultural factors may also produce noise.
5.4.3.2 Ambient Sources of Noise—Natural (ambient) sources of noise include lightning or natural earth currents, which may
induce a voltage in resistivity cables.
5.4.3.3 Geologic Sources of Noise—Geologic sources of noise include local inhomogeneities near electrodes that may result in
measurement error and variations in the subsurface that are not the object of the survey.
5.4.3.4 Cultural Sources of Noise—Resistivity measurements may be influenced by nearby cultural features (such as power
lines, radio stations, cathodic pipeline protection, and other geophysical equipment) that generate electrical or electromagnetic
fields. Pipelines, fences, and metal buildings may also affect them.
5.4.3.5 Leakage—A resistivity measurement may also be affected by leakage from the insulated wire used to connect the
instrument to the electrodes. Tests for leakage can be made at the time of the measurement.
D6431 − 18
5.4.4 Summary—Cultural Interference—During the course of designing survey locations, potential cultural interferences should
be considered. During the survey, the occurrence of electrical interferences should be noted.Cultural interference may emanate
from pipes, fences, buried utilities and other grounded linear electric conductors in the vicinity of the survey. These objects may
“short circuit” the measured currents assumed to be passing through the ground.
5.5 Alternate Methods—The limitations previously discussed may prohibit the effective use of the resistivity method, and other
methods may be required to resolve the problem. An alternative to the resistivity method is the EM method, which is preferred
in high-resistivity (low-conductivity) materials, and may require less space to conduct the survey.
5.6 Electrode Array Geometry—Usually the electrodes are arranged in a collinear array in one of several fixed geometries.
Several standard electrode geometries have been developed for various applications (Fig. 3). For engineering, environmental, and
groundwater studies, the Wenner, Schlumberger, and dipole-dipole arrays are the most commonly used.
5.6.1 Wenner Array—The Wenner array consists of equally spaced, in-line electrodes (Fig. 3). The formula for calculating
apparent resistivity from a Wenner measurement is:
R 5 2πa~V/I! (1)
R 5 2πA~V/I! (1)
where:
a = electrode spacing,
A = electrode spacing,
V = measured voltage, and
FIG. 3 Standard Electrode Geometries
D6431 − 18
I = current.
5.6.2 Schlumberger 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:
2 2
~AB/2! 2 ~MN/2! V
R 5 π 3 (2)
MN I
where:
AB = distance between current electrodes, and
MN = distance between potential electrodes.
5.6.3 Dipole-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:
R 5 πna n11 n12 V/I (3)
~ ! ~ ! ~ !
where:
n = distance between innermost electrodes, and
na = distance between innermost electrodes measured as a number (n) of a-spacings, and
a = distance between the current electrodes and also the potential electrodes.
5.6.4 Comparison of the Arrays:
5.6.4.1 The advantages of Schlumberger arrays:Schlumberger 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.
5.6.4.2 Advantages of Wenner 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.
5.6.4.3 Advantages of Dipole-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.
5.6.5 Other Arrays—There are several other arrays: Lee-partitioning array (Zohdy et al (2)), square array (Lane et al (11)),
gradient array (Ward (2)) and pole-dipole (Ward (5)) and automated data acquisition and imaging systems that are not discussed
in this guideline.
5.7 Sounding (Depth) Measurements —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.
5.7.1 This method can be used to determine changes in lithology, stratigraphy, and depth to water table. These depths are
interpreted from measurements of apparent resistivity.
FIG. 4 Increased Electrode Spacing Samples Greater Depth and Volume of Earth (from Benson et al, (8))
D6431 − 18
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.
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.
5.8 Profiling 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 can not cannot be interpreted in terms of layer resistivity values
without sounding data or other additional information.
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.
6. Procedure
6.1 Qualification of Personnel —Personnel—The success of a resistivity survey, as with most geophysical techniques, is
dependent upon many factors. One of the most important is the competence of the persons responsible for planning, carrying out
the survey, and interpreting the data. An understanding of the theory, field procedures, and methods for interpretation of the
resistivity data along with an understanding of the site geology is necessary. Personnel not having specialized training or
experience should be cautious about using this technique and solicit assistance from qualified practitioners.
6.2 Planning the Survey—Successful use of the resistivity method depends to a great extent on careful and detailed planning.
6.2.1 Objectives of the Resistivity Survey:
6.2.1.1 Planning and design of a resistivity survey should be done with due consideration to the objectives of the survey and
the characteristics of the site. These factors will determine the survey design, the equipment used, the level of effort, the
interpretation method selected, and the budget necessary to achieve the desired results. Important considerations include site
geology, depth of investigation, the site characterization, topography, and access. The presence of noise-generating activities
FIG. 5 Resistivity Sounding Curve (from Benson et al, (8))
D6431 − 18
FIG. 6 Profiling to Assess Lateral Changes (from Zohdy et al, (12))
FIG. 7 Stations Along a Profile (from Benson et al, (8))
(on-site utilities, man-made structures) and operational constraints (impervious surfaces) must also be considered. It is good
practice to obtain as much relevant information as possible about the site prior to designing a survey and mobilizing to the field.
6.2.1.2 A simple conceptual model of hydrogeologic conditions at the site should be developed early in the design phase and
should include thickness and type of soil cover, depth and type of rock, depth to water table, and stratigraphy.
6.2.1.3 The intent of the survey may be for reconnaissance of subsurface conditions or to provide detailed subsurface
information. In reconnaissance surveys, station spacing is large and topographic maps are usually sufficient for location. Under
these conditions, the effort to obtain resistivity data is relatively low, but the resulting subsurface data are not detailed. In a detailed
survey, station spacing is small and locations of measurements are more accurately determined using surveying methods or GPS
techniques. Under these conditions, the effort to obtain resisti
...