ASTM D6429-23

Standard Guide for Selecting Surface Geophysical Methods

SIGNIFICANCE AND USE
5.1 This guide applies to commonly used surface geophysical methods for those applications listed in Table 1. The rating system used in Table 1 is based upon the ability of each method to produce results under average field conditions when compared to other methods applied to the same application. An “A” rating implies a preferred method and a “B” rating implies an alternate method. There may be a single method or multiple methods that can be successfully applied. There may also be a method or methods that will be successful technically at a lower cost. Selection of the most appropriate method(s) must be made based on the scale and setting of the target. The final selection must be made considering site specific conditions and project objectives; therefore, it is critical to have a qualified professional make the final decision as to the method(s) selected.  
5.1.1 Benson et al (1)  provides one of the earlier guides to the application of geophysics to environmental problems.  
5.1.2 Ward (2) is a three-volume compendium that deals with geophysical methods applied to geotechnical and environmental problems.  
5.1.3 Butler (3) provides detailed technical explanations of near-surface geophysical methods and includes several detailed case histories.  
5.1.4 The U.S. Army Corps of Engineers manual (4) provides introductory chapters for the methods of Geophysical Exploration for Engineering and Environmental Investigations. This manual can be downloaded for no charge from the Corps of Engineers website.  
5.1.5 Olhoeft (5) provides an expert system for helping select geophysical methods to be used at hazardous waste sites.  
5.1.6 The U.S. EPA (6) provides an excellent literature review of the theory and use of geophysical methods for use at contaminated sites.  
5.2 An Introduction to Geophysical Measurements:  
5.2.1 Geophysical measurements provide a means of mapping lateral and vertical variations of one or more physical properties or monitoring temporal cha...
SCOPE
1.1 This guide covers the selection of surface geophysical methods, as commonly applied to geologic, geotechnical, hydrologic, and environmental site investigations and subsequent site characterization, as well as forensic and archaeological applications. These geophysical methods are rarely the sole method used in the site investigation and are often used for pre-screening to guide how and where drilling, sampling or other targeted in situ testing are conducted. This guide does not describe the specific procedures for conducting geophysical surveys. Individual guides have been developed for many surface geophysical methods.  
1.2 Surface geophysical methods yield direct and indirect measurements of the physical properties of soil and rock and pore fluids, as well as buried objects.  
1.3 This guide provides an overview of applications for which surface geophysical methods are appropriate. It does not address the details of the theory underlying specific methods, 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 this guide be familiar with the references cited (1-27)2 and with Guides D420, D5730, D5753, D5777, D6285, D6430, D6431, D6432, D6820, D7046, and D7128, as well as Practices D5088, D5608, D6235, and Test Methods D4428/D4428M, D7400/D7400M, and G57.  
1.4 To obtain detailed information on specific geophysical methods, ASTM standards, other publications, and references cited in this guide, should be consulted.  
1.5 The success of a geophysical survey is dependent upon many factors. One of the most important factors is the competence of the person(s) responsible for planning, carrying out the survey, and interpreting the data. An understanding of the method's theory, field procedures, and interpretation along with an understanding of the site geology, is necessary to successful...

General Information

• Status: Published
• Publication Date: 14-Feb-2023
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.01 - Surface and Subsurface Investigation

Relations

• Refers: ASTM D3740-23 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Nov-2023
• Refers: ASTM D5088-20 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-May-2020
• Refers: ASTM D6820-20 - Standard Guide for Use of the Time Domain Electromagnetic Method for Geophysical Subsurface Site Investigation
• Effective Date: 01-Jan-2020
• Refers: ASTM D6432-19 - Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation
• Effective Date: 15-Nov-2019
• Refers: ASTM D3740-19 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Oct-2019
• Refers: ASTM D7400/D7400M-19 - Standard Test Methods for Downhole Seismic Testing
• Effective Date: 01-Feb-2019
• 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 D7128-18 - Standard Guide for Using the Seismic-Reflection Method for Shallow Subsurface Investigation
• Effective Date: 15-Jul-2018
• Refers: ASTM D6639-18 - Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Site Characterizations
• Effective Date: 01-Feb-2018
• Refers: ASTM D420-18 - Standard Guide for Site Characterization for Engineering Design and Construction Purposes
• Effective Date: 01-Feb-2018
• Refers: ASTM D5753-18 - Standard Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
• Effective Date: 01-Feb-2018
• Refers: ASTM D6431-18 - Standard Guide for Using the Direct Current Resistivity Method for Subsurface Site Characterization
• Effective Date: 01-Feb-2018
• Refers: ASTM D6430-18 - Standard Guide for Using the Gravity Method for Subsurface Site Characterization
• Effective Date: 01-Feb-2018
• Refers: ASTM D6820-18 - Standard Guide for Use of the Time Domain Electromagnetic Method for Subsurface Site Characterization
• Effective Date: 01-Feb-2018
• Refers: ASTM D6285-99(2016) - Standard Guide for Locating Abandoned Wells
• Effective Date: 01-Dec-2016

Overview

ASTM D6429-23 is the Standard Guide for Selecting Surface Geophysical Methods, developed by ASTM International. This guide aids professionals in the selection of appropriate surface geophysical techniques for geologic, geotechnical, hydrologic, environmental site investigations, as well as forensic and archaeological applications. The standard emphasizes the importance of qualified professionals in method selection and stresses that surface geophysical methods often complement other investigative techniques such as drilling or sampling.

Surface geophysical methods provide both direct and indirect measurements of physical properties in soils, rocks, pore fluids, and buried objects. They are typically used during the initial phase of site investigations to guide the efficient allocation of more invasive or targeted analyses.

Key Topics

• Method Selection Criteria: ASTM D6429-23 provides a framework for choosing among various surface geophysical methods using a rating system based on suitability for specific site conditions and project goals. Factors such as the scale, target setting, and unique site conditions are critical in determining the best approach.
• Professional Judgment: The guide stresses the necessity for a qualified professional-such as engineers, geophysicists, or geologists with appropriate credentials-to lead the selection, execution, and interpretation of geophysical surveys.
• Applications Covered: The standard covers a broad scope of applications, including but not limited to:
  • Mapping soil layers, rock strata, and groundwater table
  • Identifying fractures, faults, voids, and sinkholes
  • Detecting buried utilities, drums, tanks, and unexploded ordnance
  • Locating archaeological features and forensic evidence
  • Assessing contamination (inorganic/organic) and soil salinity
• Data Quality and Interpretation: Successful projects depend on data quality, which is influenced by the competence of personnel, equipment selection, and survey design. Interpretive results improve when geophysical data is correlated with other site information.
• Practical Considerations: Surface geophysical surveys are minimally invasive, facilitate high sample density, and support continuous monitoring or temporal change detection, providing significant value for large or complex sites.

Applications

Surface geophysical methods addressed by ASTM D6429-23 include:

• Environmental and Hydrologic Investigations: Identifying contamination plumes, mapping leachate migration, or monitoring saltwater intrusion in aquifers.
• Geologic and Geotechnical Investigations: Determining the depth to bedrock, delineating rock layers and faults, or evaluating dam and lagoon leakage.
• Infrastructure and Utility Location: Detecting buried utilities (pipes, cables), tanks, and drums-essential for construction, remediation, and safety planning.
• Archaeology and Forensics: Locating graves, foundations, or forensic evidence in legal or historic research.
• Pre-Construction Assessment: Rapidly screening sites to optimize the location of boreholes, samples, or further in situ tests, thus reducing costs and improving the effectiveness of subsequent investigations.

Related Standards

The selection and application of surface geophysical methods are often supported by additional ASTM standards and references, which offer detailed procedures, terminology, and best practices. Key referenced standards include:

• ASTM D420 - Guide for Site Characterization for Engineering Design and Construction Purposes
• ASTM D5777 - Guide for Using the Seismic Refraction Method for Subsurface Investigation
• ASTM D6432 - Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation
• ASTM D6235 - Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Sites
• ASTM G57 - Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method

For thorough site assessments and high-confidence results, ASTM D6429-23 should be implemented in conjunction with these related standards and authoritative references.

By assisting in the systematic selection of geophysical methods, ASTM D6429-23 streamlines the site investigation process, enhances cost-effectiveness, and improves data reliability for geoscience and environmental professionals.

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: D6429 − 23
Standard Guide for
Selecting Surface Geophysical Methods
This standard is issued under the fixed designation D6429; 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* the method’s theory, field procedures, and interpretation along
with an understanding of the site geology, is necessary to
1.1 This guide covers the selection of surface geophysical
successfully complete a survey. Personnel not having special-
methods, as commonly applied to geologic, geotechnical,
ized training or experience should be cautious about using
hydrologic, and environmental site investigations and subse-
geophysical methods and should solicit assistance from quali-
quent site characterization, as well as forensic and archaeologi-
fied professionals. All references in this standard to the
cal applications. These geophysical methods are rarely the sole
“qualified professional” refers to individuals (such as
method used in the site investigation and are often used for
engineers, soil scientists, geophysicists, engineering geologists
pre-screening to guide how and where drilling, sampling or
or geologists), who have the appropriate experience and, if
other targeted in situ testing are conducted. This guide does not
required by local regulations, applicable certification, licensure
describe the specific procedures for conducting geophysical
or registration. The term “engineering” must be understood to
surveys. Individual guides have been developed for many
be associated with the practices or activities of that qualified
surface geophysical methods.
professional.
1.2 Surface geophysical methods yield direct and indirect
1.6 Units—The values stated in SI units are to be regarded
measurements of the physical properties of soil and rock and
as standard. The values given in parentheses are for informa-
pore fluids, as well as buried objects.
tion only and are not considered standard. Reporting of test
1.3 This guide provides an overview of applications for
results in units other than SI shall not be regarded as noncon-
which surface geophysical methods are appropriate. It does not
formance with this standard.
address the details of the theory underlying specific methods,
1.7 This guide offers an organized collection of information
field procedures, or interpretation of the data. Numerous
or a series of options and does not recommend a specific
references are included for that purpose and are considered an
essential part of this guide. It is recommended that the user of course of action. This document cannot replace education or
experience and should be used in conjunction with professional
this guide be familiar with the references cited (1-27) and with
Guides D420, D5730, D5753, D5777, D6285, D6430, D6431, judgment. Not all aspects of this guide may be applicable in all
circumstances. This ASTM standard is not intended to repre-
D6432, D6820, D7046, and D7128, as well as Practices
D5088, D5608, D6235, and Test Methods D4428/D4428M, sent or replace the standard of care by which the adequacy of
a given professional service must be judged, nor should this
D7400/D7400M, and G57.
document be applied without consideration of a project’s many
1.4 To obtain detailed information on specific geophysical
unique aspects. The word “Standard” in the title of this
methods, ASTM standards, other publications, and references
document means only that the document has been approved
cited in this guide, should be consulted.
through the ASTM consensus process.
1.5 The success of a geophysical survey is dependent upon
1.8 This standard does not purport to address all of the
many factors. One of the most important factors is the
safety concerns, if any, associated with its use. It is the
competence of the person(s) responsible for planning, carrying
responsibility of the user of this standard to establish appro-
out the survey, and interpreting the data. An understanding of
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock
1.9 This international standard was developed in accor-
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
dance with internationally recognized principles on standard-
Investigation.
Current edition approved Feb. 15, 2023. Published March 2023. Originally
ization established in the Decision on Principles for the
approved in 1999. Last previous edition approved in 2020 as D6429 – 20. DOI:
Development of International Standards, Guides and Recom-
10.1520/D6429-23.
2 mendations issued by the World Trade Organization Technical
The boldface numbers given in parentheses refer to a list of references at the
end of this standard. Barriers to Trade (TBT) Committee.
*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
D6429 − 23
2. Referenced Documents 4. Summary of Guide
2.1 ASTM Standards: 4.1 This guide applies to surface geophysical techniques
D420 Guide for Site Characterization for Engineering De- that are commonly used in site investigations, as well as
sign and Construction Purposes forensic and archaeological applications.
D653 Terminology Relating to Soil, Rock, and Contained
4.2 The selection of preferred geophysical methods for a
Fluids
number of common applications is summarized in Table 1. The
D3740 Practice for Minimum Requirements for Agencies
table is followed by brief descriptions of each application.
Engaged in Testing and/or Inspection of Soil and Rock as
4.3 A brief description of each geophysical method along
Used in Engineering Design and Construction
with some of the field considerations and limitations also are
D4428/D4428M Test Methods for Crosshole Seismic Test-
provided.
ing (Withdrawn 2023)
D5088 Practice for Decontamination of Field Equipment
4.4 It is recommended that personnel consult appropriate
Used at Waste Sites
references on each of the methods, applications, and their
D5608 Practices for Decontamination of Sampling and Non
interpretations. All geophysical measurements should be car-
Sample Contacting Equipment Used at Low Level Radio-
ried out by knowledgeable professionals who have experience
active Waste Sites
and training in theory and application of the method, and the
D5730 Guide for Site Characterization for Environmental
interpretation of the data resulting from the use of the specific
Purposes With Emphasis on Soil, Rock, the Vadose Zone
method.
and Groundwater (Withdrawn 2013)
NOTE 1—The quality of the result produced by this standard is
D5753 Guide for Planning and Conducting Geotechnical
dependent on the competence of the personnel performing it, and the
Borehole Geophysical Logging
suitability of the equipment and facilities used. Agencies that meet the
D5777 Guide for Using the Seismic Refraction Method for
criteria of Practice D3740 are generally considered capable of competent
Subsurface Investigation
and objective testing/sampling/inspection, and the like. Users of this
D6235 Practice for Expedited Site Characterization of Va- standard are cautioned that compliance with Practice D3740 does not in
itself assure reliable results. Reliable results depend on many factors;
dose Zone and Groundwater Contamination at Hazardous
Practice D3740 provides a means of evaluating some of those factors.
Waste Contaminated Sites
D6285 Guide for Locating Abandoned Wells
5. Significance and Use
D6430 Guide for Using the Gravity Method for Subsurface
Site Characterization
5.1 This guide applies to commonly used surface geophysi-
D6431 Guide for Using the Direct Current Resistivity
cal methods for those applications listed in Table 1. The rating
Method for Subsurface Site Characterization system used in Table 1 is based upon the ability of each method
D6432 Guide for Using the Surface Ground Penetrating
to produce results under average field conditions when com-
Radar Method for Subsurface Investigation pared to other methods applied to the same application. An “A”
D6639 Guide for Using the Frequency Domain Electromag-
rating implies a preferred method and a “B” rating implies an
netic Method for Subsurface Site Characterizations alternate method. There may be a single method or multiple
D6820 Guide for Use of the Time Domain Electromagnetic
methods that can be successfully applied. There may also be a
Method for Geophysical Subsurface Site Investigation method or methods that will be successful technically at a
D7046 Guide for Use of the Metal Detection Method for
lower cost. Selection of the most appropriate method(s) must
Subsurface Exploration (Withdrawn 2020) be made based on the scale and setting of the target. The final
D7128 Guide for Using the Seismic-Reflection Method for
selection must be made considering site specific conditions and
Shallow Subsurface Investigation project objectives; therefore, it is critical to have a qualified
D7400/D7400M Test Methods for Downhole Seismic Test- professional make the final decision as to the method(s)
ing selected.
G57 Test Method for Measurement of Soil Resistivity Using 5.1.1 Benson et al (1) provides one of the earlier guides to
the Wenner Four-Electrode Method
the application of geophysics to environmental problems.
5.1.2 Ward (2) is a three-volume compendium that deals
3. Terminology with geophysical methods applied to geotechnical and envi-
ronmental problems.
3.1 Definitions:
5.1.3 Butler (3) provides detailed technical explanations of
3.1.1 For definitions of common technical terms used in this
near-surface geophysical methods and includes several detailed
standard, refer to Terminology D653.
case histories.
5.1.4 The U.S. Army Corps of Engineers manual (4) pro-
vides introductory chapters for the methods of Geophysical
Exploration for Engineering and Environmental Investigations.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
This manual can be downloaded for no charge from the Corps
Standards volume information, refer to the standard’s Document Summary page on
of Engineers website.
the ASTM website.
5.1.5 Olhoeft (5) provides an expert system for helping
The last approved version of this historical standard is referenced on www.ast-
m.org. select geophysical methods to be used at hazardous waste sites.
D6429 − 23
A,B
TABLE 1 Selection of Geophysical Methods for Common Applications
Geophysical Methods
(Section 6)
Seismic Electrical Electromagnetic
Ground
DC Frequency Time Pipe/Cable Metal
Applications
Refraction Reflection MASW SP Penetrating Magnetics Gravity
Resistivity Domain Domain VLF (6.8) Locator Detectors
(6.1) (6.2) (6.3) (6.5) Radar (6.12) (6.13)
(6.4) (6.6) (6.7) (6.9) (6.10)
C
(6.11)
Natural Geologic and Hydrologic
Conditions
Soil/unconsolidated layers A B B A B A B A
Rock layers B A B B B B
Depth to bedrock A A B B B B B A B
Depth to water table A B B B B A
Fractures and fault zones B A B B A B A B B B
Voids and sinkholes B B A A B B A A
Soil and rock properties A A A B
Dam and lagoon leakage B B A B B
Inorganic Contaminants
Landfill leachate A A A B B
Saltwater intrusion A A A B B
Soil salinity A A
Organic Contaminants
Light, nonaqueous phase liquids B B B B
D
Dissolved phase
Dense, nonaqueous phase
D
liquids
Manmade Buried Objects
Utilities B A B A
Drums and USTs A A A A A
UXO B B B A B A
Abandoned wells B B B A
Landfill and trench boundaries B B B A B A
Forensics B A B B A B
Archaeological features B B B B A A A B
A
“A” implies primary choice of method for a given set of site-specific conditions.
B
“B” implies secondary choice or alternate method.
C
Selection implication is applicable where mineralogic clays or conductive pore fluids are not predominant.
D
Also see natural geologic and hydrologic conditions to characterize contaminant pathways.

D6429 − 23
5.1.6 The U.S. EPA (6) provides an excellent literature a cross-section produced from the geophysical data may
review of the theory and use of geophysical methods for use at resemble a geological cross-section, although the two are not
necessarily identical.
contaminated sites.
5.4 Geophysical methods should be used in the following
5.2 An Introduction to Geophysical Measurements:
instances:
5.2.1 Geophysical measurements provide a means of map-
5.4.1 Surface geophysical methods can and should be used
ping lateral and vertical variations of one or more physical
early in a site investigation program to aid in identifying
properties or monitoring temporal changes in conditions, or
background conditions, as well as anomalous conditions so that
both. In the absence of prior information about the site,
borings, soundings, and sampling points can be located to be
reconnaissance-level geophysical investigations may be appro-
representative of site conditions and to investigate anomalies.
priate as a precursor to refined surveys. A primary factor
Geophysical methods also can be used later in the site
affecting the accuracy of site investigation results is the number
investigation program after an initial study is completed to
of test locations. Insufficient spatial sampling to adequately
confirm and improve the site investigation findings and provide
characterize the conditions at a site can result if the number of
fill-in data between other measurements. General site knowl-
samples is too small. Interpolation between these sample points
edge (for example, depth to bedrock, site use history) is a
may be difficult and may lead to an inaccurate site character-
useful precursor for designing a geophysical survey.
ization.
5.4.2 The level of success of a geophysical survey is
5.2.2 Geophysical measurements generally can be made
improved if the survey objectives are well defined. In some
relatively quickly, are minimally intrusive, and enable interpo-
cases, the objective may be refined as the survey uncovers new
lation between known points of control. Continuous data
or unknown data about the site conditions. The flexibility to
acquisition can be obtained with certain geophysical methods
change or add to the technical approach should be built into the
at speeds up to several km/h (mph). In some cases, total site
program to account for changes in interpretation of site
coverage is economically possible. Because of the greater
conditions as a site investigation progresses.
sample density, geophysical methods can be used to define
5.5 Profiling and Sounding Measurements:
background (ambient) conditions and detect anomalous condi-
5.5.1 Profiling by stations or by continuous measurements
tions resulting in a more accurate site characterization than
provides a means of assessing lateral changes in subsurface
using borings or soundings alone. Geophysical survey design
conditions.
considerations vary according to the intended distribution of
5.5.2 Soundings provide a means of assessing depth and
measurements. Data may be collected along individual tran-
thickness of geologic layers or other targets. Most surface
sects to investigate linear features (dams, levees, roadways),
geophysical sounding measurements can resolve three and
while multiple transects or 3D survey geometries are required
possibly four layers.
to identify areal trends over larger sites and non-linear targets.
These geophysical methods are especially important to pre-
5.6 Ease of Use and Interpretation of Data:
screen large sites prior to detailed planning of further site
5.6.1 The theory of applied geophysics is quantitative,
investigation such as other drilling, sampling and testing
however, in application, geophysical methods often yield
methods.
interpretations that are qualitative.
5.6.2 Some geophysical methods provide data from which a
5.3 A contrast in material properties must be present for
preliminary interpretation can be made in the field, for
geophysical measurements to be successful.
example, ground penetrating radar (GPR), frequency domain
5.3.1 Geophysical methods measure the physical, electrical,
electromagnetic profiling, direct current (DC) resistivity
or chemical properties of soil, rock, and pore fluids. To detect
profiling, magnetic profiling, and metal detector profiling. A
an anomaly, a soil to rock contact, the presence of inorganic
map of GPR anomalies or a contour map of the EM
contaminants, or a buried drum, there must be a contrast in the
(electromagnetic), resistivity, magnetic or metal detector data
property being measured. For example, the target to be
often can be created in the field.
detected or geologic feature to be defined must have properties
5.6.3 Some methods, (for example, time domain electro-
significantly different from “background” conditions.
magnetics and DC resistivity soundings, seismic refraction,
5.3.2 For example, the interface between fresh water and
seismic reflection, and gravity), require that the data be
saltwater in an aquifer can be detected by the differences in
processed before any quantitative interpretation can be done.
electrical properties of the pore fluids. The contact between soil
5.6.4 Any preliminary interpretation of field data should be
and unweathered bedrock can be detected by the differences in
treated with caution. Such preliminary analysis should be
acoustic velocity of the materials. In some cases, the differ-
confirmed by correlation with other information from known
ences in measured physical properties may be too small for
points of control, such as borings or outcrops. Such preliminary
anomaly detection by geophysical methods.
analysis is subject to change after data processing and is
performed mostly as a means of quality control (QC).
5.3.3 Because physical properties of soil and rock vary
widely, some by many orders of magnitude, one or more of
5.6.5 It is the interpretation and integration of all site data
these properties usually will correspond to a geologic discon- that results in useful information for site characterization. The
tinuity; therefore, boundaries determined by the geophysical
conversion of raw data to useful information is a value-added
methods will usually coincide with geological boundaries, and process that experienced professionals achieve by careful
D6429 − 23
analysis. Such analysis must be conducted by a competent method selected will be determined by the specific property to
professional to ensure that the interpretation is consistent with be measured. ASTM standards pertinent to those properties
geologic and hydrologic conditions. should be consulted. For example, rippability and acoustic
velocities of rock are discussed in Guide D5777, the wave
5.7 Discussion of Applications—Applications listed in Table
velocities measured down a single borehole in Test Method
1 are discussed below.
D7400/D7400M and between boreholes in Test Methods
5.7.1 Natural Geologic and Hydrologic Conditions:
D4428/D4428M. Soil resistivity measurements are discussed
5.7.1.1 Soil/Unconsolidated Layers—This application in-
in Test Method G57. Density, porosity measurements and
cludes determining the depth to, thickness of, and areal extent
seismic velocity measurements in boreholes are discussed in
of unconsolidated layers. These layers may be discontinuous or
Guide D5753.
include lenses of various materials. These layers can be
5.7.1.8 Dam and Lagoon Leakage—This application refers
detected because of differences in their physical properties as
to the detection and mapping of fluids leaking along preferen-
compared to adjacent materials.
tial flow pathways from a dam or lagoon. The application of
5.7.1.2 Rock Layers—This application includes determining
surface geophysical methods to detect leakage is contingent
the contact between different rock layers, for example, lime-
upon the presence of localized flow or difference in conduc-
stone over granite or sandstone over shale, discontinuous
tivity.
bedding planes, and unconformities and the thicknesses of
5.7.2 Inorganic Contaminants:
these layers. Several geophysical methods can be used to
5.7.2.1 Landfill Leachate—This application includes all
delineate rock layers depending on the physical properties and
types of waste disposal sites in which the primary leachate is
the depths and thicknesses of the layers.
likely to be inorganic and electrically conductive. This includes
5.7.1.3 Depth to Bedrock—This application includes deter-
municipal landfill sites, hazardous waste sites, and mine
mining depth to the top of competent rock covered by
tailings. Inorganic contaminants can be detected using electri-
unconsolidated overburden. The choice of geophysical method
cal or electromagnetic geophysical methods.
depends on whether there is a physical property contrast
5.7.2.2 Saltwater Intrusion—Saltwater intrusion refers to
between the rock and the overlying material. In areas where the
movement of saline water into fresh water aquifers, and
top of rock is weathered or highly fractured, top of rock may be
although this is primarily a coastal problem, it can occur
difficult to determine. Highly irregular rock surfaces may
naturally in inland aquifers or by man-made contamination, for
present additional problems.
example, brine ponds. Saline water is highly conductive and
5.7.1.4 Depth to Water Table—This application includes
can be detected by DC resistivity and electromagnetic meth-
determining the depth at which a subsurface unit is fully
ods. The lateral boundary of the saltwater/fresh water interface
saturated. The water table (top of the saturated zone) can be
can be mapped and the depth of the saline water estimated.
detected because of the changes in physical properties that are
5.7.2.3 Soil Salinity—Soil salinity is a condition in which
caused by saturated conditions. The ability to detect the water
salt concentrations within soils have reached levels affecting
table may depend on the geologic unit in which it occurs.
the growth and yields of crops. DC resistivity and electromag-
Seismic methods can be used to detect the water table in most
netic conductivity measurements provide means for measuring
unconsolidated materials; electrical, electromagnetic, or GPR
the soil salinity over a large area and at various depths.
methods may be used to detect the water table in either
consolidated or unconsolidated materials. 5.7.3 Organic Contaminants:
5.7.1.5 Fractures and Fault Zones—This application in- 5.7.3.1 Light, Nonaqueous Phase Liquids (LNAPL)—This
cludes the location and characterization of joints, fractures, and application includes petroleum products present as discrete,
faults. These features range from individual joints and fracture measurable contaminants with concentrations greater than their
zones to larger regional structural features. Joints, fractures and solubility in water. The contaminants are lighter than water and
fault zones may be dry, fluid-filled or filled with clays or “float” on the surface of an unconfined aquifer in porous
weathered rock. The detectability of these features increases media. The geometry of their occurrence in fractured soil or
with the size of the feature and with the presence of distinctive rock is more complex and less well defined. LNAPL dissolves
pore fluids or conductive fill material.
into water and acts as a source of dissolved contaminant
plumes (see dissolved organic contaminants). LNAPL can be
5.7.1.6 Voids and Sinkholes—This application includes karst
features, such as weathered depressions in rock, open, water- detected in some cases because its electrical properties are
different from those of ground water; it depresses the ground
filled, or sediment-filled sinkholes, and cavities or larger cave
systems. In many cases, the target of concern may be beyond water surface if present in sufficient quantities; and, it can alter
the capillary properties of soil.
the effective resolution or depth range of some or all of the
surface geophysical methods; however, deep cavities often
5.7.3.2 Dense, Nonaqueous Phase Liquids (DNAPL):
show signs of their presence in the near surface and may be
(1) This application includes chlorinated organic solvents
interpreted using shallow geophysical data. The ability to
and other contaminants that are present as a discrete, measur-
detect a given size cavity decreases with increasing depth for
able contaminant phase with concentrations greater than their
all surface geophysical methods.
solubility in water. The contaminants are denser than water and
5.7.1.7 Soil and Rock Properties—This application refers to “sink” below the water table. The distribution of DNAPL in the
the measurement of the physical properties of soil and rock, for subsurface is complex and is controlled by gravity and the
example, elastic, plastic, and electrical. The geophysical capillary properties of subsurface materials, rather than by
D6429 − 23
ground water flow direction. DNAPL dissolves into water and 5.7.4.3 Unexploded Ordnance (UXO)—This application in-
acts as a source of dissolved contaminant plumes (see dis- cludes a wide range of materials that were designed to explode,
such as bombs, mines, and antipersonnel weapons. UXO occur
solved organic contaminants). Moreover,“ residual” DNAPL
in a variety of sizes from a few centimeters to meters and are
(immobile contaminant left behind during migration) also can
made of a wide variety of metals and other materials. Shape,
act as a source of dissolved organic contamination. Residual
size, depth, composition and orientation of the UXO can limit
concentrations of DNAPL do not significantly alter the prop-
detectability.
erties measured by most geophysical methods.
5.7.4.4 Abandoned Wells—This application includes aban-
(2) Some DNAPLs have dielectric properties that may
doned wells that may be uncased or cased with steel, PVC, or
allow their detection using GPR if temporal measurements are
concrete. Abandoned wells can be detected by various methods
made before the DNAPL is introduced to compare with
depending upon construction, associated surface pits and other
properties that exist after the DNAPL is present; thus, GPR
facilities, leaking fluids, and the method of abandonment.
may be useful to monitor the movement of DNAPL during
Guide D6285 provides a discussion of geophysical and other
remediation.
methods to locate abandoned wells.
(3) The geophysical methods listed in Table 1 under natural
5.7.4.5 Landfill and Trench Boundaries—This application
geologic and hydrologic conditions are appropriate to charac-
includes landfills, pits, and trenches. Those that contain buried
terize the hydrogeology of a site; therefore, an attempt can be
metallic materials can be detected because of the presence of
made to predict DNAPL occurrence and distribution based
the metal. Boundaries of trenches and pits can sometimes be
upon an understanding of site geology.
detected by changes in electrical conductivity, disturbance of
5.7.3.3 Dissolved Phase:
subsurface layers, or the presence of fill material. Determining
(1) This application includes fuels, solvents, and other
the depth to the bottom of a landfill or trench is much more
organic contaminants dissolved in ground water. Sources can
difficult than defining the lateral boundaries.
be leaks and spills of LNAPL or DNAPL or can be leaks and
5.7.4.6 Forensics—This application includes buried bodies
spills of such small volume that the contaminant is dissolved as
and a variety of metallic and nonmetallic objects. These objects
it reaches ground water.
can sometimes be detected directly or may be detected indi-
(2) Dissolved organic contaminants are of regulatory con-
rectly by disturbed soil conditions.
cern at very low concentrations (parts per billion) in ground
5.7.4.7 Archaeological Features—This application includes
water. The properties of the dissolved organic plumes that can
a wide range of targets, including stone foundations, walls,
be measured by most geophysical methods are not sufficiently
roads, fire pits, caves, and graves, as well as metallic and
different from those of ambient ground water to be detectable.
nonmetallic objects. These targets and objects can sometimes
Some organic contaminants, such as alcohol, are highly
be detected directly or may be detected indirectly by changes
soluble, and are not detectable even at high concentrations.
in soil conditions.
(3) When sources of dissolved organic contaminants have
6. Discussion of the Geophysical Methods
been identified, geophysical methods can be used to character-
6.1 Seismic Refraction:
ize the hydrogeology of a site so that pathways for migration of
6.1.1 Introduction—Seismic refraction measurements are
dissolved plumes can be identified. The appropriate methods
made by measuring the travel time of direct and refracted
are discussed in the sections of this guide that pertain to
acoustic waves as they travel from the surface through one
geologic and hydrologic conditions.
layer to another and back to the surface where their arrival
5.7.4 Man-Made Buried Objects:
times are recorded. The travel time is a function of the seismic
5.7.4.1 Utilities—This application includes a very wide
or acoustic velocity and the geometry of subsurface layers of
range of targets including pipes, cables, and utilities.
soil and rock.
Fortunately, most utilities are buried near the ground surface,
6.1.2 Applications—The primary application for seismic
making them relatively easy targets to detect. The geophysical
refraction is for determination of depth and thickness of
method selected will depend on the material of which the pipes
geologic layers, for example, depth to bedrock and water table,
or utilities are made (ferrous or nonferrous metals or nonme-
and to delineate geologic structure. Velocity measurements are
tallic materials). Nonmetallic utilities, that is, concrete or
a measure of the material properties and can be used as an aid
plastic, can sometimes be detected with GPR.
in assessing rock quality and rippability of rock. If compres-
5.7.4.2 Underground Storage Tanks and Drums—This ap-
sional P-wave and shear S-wave velocities are measured, in
plication includes underground storage tanks (UST) and
situ elastic moduli of soil and rock can be determined.
drums. Since most underground storage tanks are large (more
6.1.3 Depth—Typical depths of measurements are less than
than 2000 L (500 gal)), buried shallow, and often made of steel,
30 m (100 ft), but measurements can be made to much greater
they are relatively easy to detect. If the tank is made of
depths, if necessary. Shallow measurements may be made
non-metallic material (for example, concrete or fiberglass), it is
using the energy of a sledgehammer, a weight drop hammer,
more difficult to detect. Drums of various sizes (typically 4 to
accelerated drop weights, vibratory systems, or a shotgun
200 L (1 to 55 gal)) are manufactured from either non-metallic
source while deeper measurements will require larger mechani-
or metallic materials. While groups of drums may be detected, cal energy sources and possibly explosives.
a single 200-L (55-gal) drum and smaller drums are more
6.1.4 Ease of Use—Seismic refraction measurements are
difficult to locate. labor intensive. Refraction measurements require that the
D6429 − 23
geophones and the energy source be in contact with the ground. propagated. Resolution may be as good as 1 m with frequen-
Extensive cable handling and moving of the source is required. cies of 500 Hz. The optimum conditions for shallow reflection
The resulting data must be analyzed before a quantitative surveys are saturated fine-grained soils that enable higher
interpretation can be made. The travel time of the P-wave frequency energy to be coupled with the ground. Lateral
arrivals are picked and then a time distance plot is drawn from
resolution is a function of geophone spacing, which is com-
which depths and velocities are determined. A variety of monly 0.3 to 3 m (1 to 10 ft). The reflection method provides
interpretive methods can be used ranging from the simple time
a high resolution cross section of soil or rock layers along a
intercept method to delay time, ray tracing, and the generalized profile line. Although two-dimensional reflection surveys are
reciprocal method. Each interpretive method requires specific
common, three-dimensional reflection surveys also can be
data acquisition in the field. The results of seismic refraction
conducted.
data commonly are displayed as interpreted depth cross-
6.2.6 Limitations—Measurements are sensitive to acoustic
sections or as contour maps of stratigraphic layers.
noise and vibrations. The distance between the source and the
6.1.5 Resolution—Vertical resolution requires that a layer
farthest geophone usually is 1 to 2 times the desired depth of
have a thickness that is a substantial fraction of the depth to its
investigation, much less than that required for refraction
upper surface. Seismic refraction measurements can typically
measurements.
resolve three to four layers. Lateral resolution is a function of
6.2.7 References—Steeples and Miller (8) provide an intro-
geophone spacing, typically 2 to 6 m (5 to 20 ft) or more. Large
duction to the reflection method with emphasis on the common
spacings between source and geophones are used for deeper
depth point method. Pullan and Hunter (9) provide a case
measurements.
history using the common offset method. Guide D7128 is the
6.1.6 Limitations—Measurements are sensitive to acoustic
standard guide for use of this method.
noise and vibrations. Seismic velocity of layers must increase
6.3 Multichannel Analysis of Surface Waves (MASW):
with depth. The method will not detect thin layers. A source to
6.3.1 Introduction—MASW technique estimates a shear
geophone distance of up to three to five times the desired depth
wave velocity profile within the ground through measurement
of investigation is needed.
of surface wave propagation through a surface array of
6.1.7 References—Haeni (7) provides an excellent introduc-
geophones and then inversion using computer algorithms.
tion to the method with case histories. Guide D5777 is the
standard guide for the use of this method. 6.3.2 Applications—Shear wave velocity profiles can be
used directly in analyses requiring soil stiffness and related
6.2 Seismic Reflection:
properties, such as settlement, liquefaction, and seismic site
6.2.1 Introduction—The seismic reflection technique mea-
response analyses. These profiles are also used to locate
sures the two way travel time of seismic waves from the
discontinuities in the ground profile such as bedrock, changes
ground surface downward to a geologic contact at which part
in soil consistency, voids, and buried anomalies. Shear wave
of the seismic energy is reflected back to geophones at the
velocity profiles have been used in construction quality control
surface. Reflections occur when there is a contrast in material
of soil compaction, grouting, and in-situ densification.
density or velocity, or both, between two layers.
6.3.3 Depth—Depending on the length of the geophone
6.2.2 Applications—The primary application for the seismic
array and the seismic wave source, MASW generates profiles
reflection method is to identify and determine the depth and
from 10 m (30 ft) to more than 100 m (300 ft) depth below
thickness of geologic layers. The top of bedrock may be
ground surface. Shallow measurements often can be made
mapped along with overlying layers. The method also can be
using a sledge hammer, a weight drop hammer, accelerated
used to locate and characterize geologic structure.
drop weights, vibratory systems, shotgun, or rifle as seismic
6.2.3 Depth—Reflection measurements detect layers from
sources. Larger mechanical sources or explosives may be
about 15 to 300 m (50 to 1000 ft) deep. Shallow measurements
required for deeper investigations or in highly attenuative
often can be made using a sledge hammer, a weight drop
material.
hammer, accelerated drop weights, vibratory systems, shotgun,
6.3.4 Ease of Use—MASW surveys require multiple
or rifle as seismic sources. Larger mechanical sources or
geophones, cables, and specialized data collection equipment.
explosives may be required for deeper investigations or in
MASW surveys are typically conducted by specialized person-
highly attenuative material.
nel.
6.2.4 Ease of Use—Seismic reflection measurements are
6.3.5 Resolution—Horizontal resolution depends on the ap-
relatively difficult to make and are labor intensive. Reflection
erture of the receiver array used for analysis. For standard
measurements require that the geophones and the energy
geologic applications, the limit on horizontal resolution has
source be in contact with the ground. Extensive cable handling
been estimated to be about 10 m (30 ft). Vertical resolution
and moving of the source is required. Two different approaches
decreases with depth.
to data acquisition are used, the common offset method and the
common depth point (CDP) method. The CDP method has
6.3.6 Limitations—MASW geophone arrays require access
become more common for use with modern seismographs. The
to the ground surface along a straight line to place the
resulting field data must be processed prior to quantitative
geophones in contact with the ground. MASW surveys can be
interpretation.
conducted on hard surfaces including pavement and concrete.
6.2.5 Resolution—Vertical resolution is proportional to the Accuracy of processed data is difficult without evaluation of
frequency of the seismic energy that can be generated and dispersion curve picks.
D6429 − 23
6.3.7 References—Park et al (10) provides an introduction 6.4.5 Resolution—Lateral resolution is a function of elec-
to the method. trode spacing, as well as, the spacing between station measure-
ments. Resistivity soundings typically can resolve three to four
6.4 DC Resistivity:
layers.
6.4.1 Introduction—DC resistivity measurements are made
6.4.6 Limitations—Measurements are susceptible to inter-
by injecting a DC current into the ground through two current
ference from nearby metal pipes, cables, or fences. The spacing
electrodes and measuring the resulting voltage at the surface
between electrodes must extend three to five times the depth of
between two potential electrodes. This method measures bulk
interest, which results in long electrode spreads and cables.
electrical resistivity that is a function of the soil and rock
Finding sufficient accessible space can sometimes be a prob-
matrix, percentage of fluid saturation, and the conductivity of
lem. Obtaining a good connection with the ground can some-
pore fluids.
times be a problem in areas with high resistivity soils.
6.4.2 Applications—Resistivity measurements can be made
6.4.7 References—Ward (12) and Zonge (13) provide an
as soundings to determine depth and thickness of geologic
introduction to the method and Zohdy et al (14) provides an
layers, or as profiles to locate lateral changes in geologic
introduction and applications of the method. Guide D6431 is
conditions, detecting and mapping inorganic contaminant the standard guide for use of this method.
plumes, and locating buried wastes. Sounding measurements
6.5 Spontaneous Potential (SP):
are made by incrementally increasing the spacing between
6.5.1 Introduction—The spontaneous, or self, potential (SP)
electrodes to make a sequence of measurements at increasing
method measures the natural voltage that exists at the ground
depths. Soundings generally are applicable to defining geologic
surface. Measurements are made between two nonpolarizing
layers where the geology is laterally homogeneous and layers
electrodes, usually copper-copper sulfate cells, in contact with
are flat or gently dipping. Profile measurements are made with
the ground. Usually one electrode is fixed as a reference
a fixed electrode spacing. Profiling is used to locate and map
electrode and measurements are made with the second roving
areas of significant lateral variations in resistivity at a given
electrode. SP voltages are produced by two different sources, as
depth, for example, a conductive inorganic contaminant plume.
a result of the electrochemical differences between soils, rock,
6.4.2.1 Earth Resistivity Tomography—Earth resistivity to-
pore fluids, or minerals and their oxidation or reduction state,
mography (ERT) is a related method of obtaining
as well as by the electrokinetic effect of the presence of flowing
2-dimensional resistivity cross sections from a series of sound-
water, sometimes called streaming potential. Voltages pro-
ings. A series of electrodes, generally 48 to 256, are placed in
duced can be as great as a few 100 millivolts, but are more
a linear array and then select electrodes are automatically
commonly a few tens of millivolts.
activated as current or potential electrodes to produce a series
6.5.2 Applications—The primary application for SP mea-
of soundings across the length of the array. Data are then
surements is assessing seepage from dams and embankments.
processed (inverted) to obtain a best fitting finite element grid
Environmental and engineering applications of the self-
of earth resistivities. This can also be extended to 3-D arrays.
potential method have been investigations of subsurface water
This process is not within the scope of this Standard Guide.
movement. These include landslide investigations, location of
References include Daily (11).
faults, location and study of drainage structures, location of
6.4.3 Depth—The depth of measurements is related primar-
shafts, tunnels and sinkholes, and the mapping of coal mine
ily to electrode spacing and the electrical properties of the
fires. Time series SP measurements also can be made to
subsurface. Measurements can be made to depths of a few
monitor changes in seepage. It is possible that SP can be used
hundred meters or more. There is no theoretical limit to the
to map geochemical variations associated with contaminant
depth of investigation if sufficient space is available to lay out
plumes.
the electrode array and sufficient energy is injected into the
6.5.3 Depth—SP is a potential field technique, so source
ground.
parameters cannot be changed to vary the depth of investiga-
6.4.4 Ease of Use—Resistivity measurements are relatively tion. The size, depth, orientation, and magnitude of subsurface
slow and labor intensive since the method requires ground targets all affect the magnitude of the detected SP anomaly.
Depth of investigation is usually less than 30 m (100 ft).
contact. This is achieved by driving metal electrodes into the
ground and deploying connecting cables. Measurements are
6.5.4 Ease of Use—SP measurements are made on a station
made on a station by station basis. Measurements also can be
to station basis and are relatively easy to make; however, the
made by placing a grid of electrodes in the ground and making
electrodes must be in good electrical contact with the ground.
measurements between various electrodes to achieve different Results can be plotted as profiles or contoured and often can be
electrode spacings and geometries (as in azimuthal surveys). used with little processing. Corrections are applied to improve
Profile data can be plotted as apparent resistivity versus the signal to noise ratio and geometric curve matching and
distance along a profile line with little if any processing. analytical models may be used for analysis; however, interpre-
tation of self-potential data is often qualitative, using the
Sounding data must be processed to obtain depth, thickness,
anomalies observed in profile data or contour patterns to
and resistivity of layers. Processing can be done by curve
identify seepage flow paths or other sources.
matching or by using forward and inverse modeling. Results of
grid surveys are modeled to provide an image of the subsur-
6.5.5 Resolution—Lateral resolution is a function of station
face. spacing.
D6429 − 23
6.5.6 Limitations—Measurements are susceptible to inter- 6.7.1 Introduc
...

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: D6429 − 20 D6429 − 23
Standard Guide for
Selecting Surface Geophysical Methods
This standard is issued under the fixed designation D6429; 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*
1.1 This guide covers the selection of surface geophysical methods, as commonly applied to geologic, geotechnical, hydrologic,
and environmental site investigations and subsequent site characterization, as well as forensic and archaeological applications.
These geophysical methods are rarely the sole method used in the site investigation and are often used for pre-screening to guide
how and where drilling, sampling or other targeted in-situ in situ testing are conducted. This guide does not describe the specific
procedures for conducting geophysical surveys. Individual guides have been developed for many surface geophysical methods.
1.2 Surface geophysical methods yield direct and indirect measurements of the physical properties of soil and rock and pore fluids,
as well as buried objects.
1.3 This guide provides an overview of applications for which surface geophysical methods are appropriate. It does not address
the details of the theory underlying specific methods, 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 this guide be familiar
with the references cited (1-27) and with Guides D420, D5730, D5753, D5777, D6285, D6430, D6431, D6432, D6820, D7046,
and D7128, as well as Practices D5088, D5608, D6235, and Test Methods D4428/D4428M, D7400/D7400M, and G57.
1.4 To obtain detailed information on specific geophysical methods, ASTM standards, other publications, and references cited in
this guide, should be consulted.
1.5 The success of a geophysical survey is dependent upon many factors. One of the most important factors is the competence
of the person(s) responsible for planning, carrying out the survey, and interpreting the data. An understanding of the method’s
theory, field procedures, and interpretation along with an understanding of the site geology, is necessary to successfully complete
a survey. Personnel not having specialized training or experience should be cautious about using geophysical methods and should
solicit assistance from qualified practitioners.professionals. All references in this standard to the “qualified professional” refers to
individuals (such as engineers, soil scientists, geophysicists, engineering geologists or geologists), who have the appropriate
experience and, if required by local regulations, applicable certification, licensure or registration. The term “engineering” must be
understood to be associated with the practices or activities of that qualified professional.
1.6 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only
and are not considered standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this
standard.
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
CharacterizationInvestigation.
Current edition approved Aug. 1, 2020Feb. 15, 2023. Published September 2020March 2023. Originally approved in 1999. Last previous edition approved in 20112020
ɛ1
as D6429 – 99 (2011)20. , which was withdrawn in March 2020 and reinstated in August 2020. DOI: 10.1520/D6429-20.DOI: 10.1520/D6429-23.
The boldface numbers given in parentheses refer to a 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
D6429 − 23
1.7 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 judgment. 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.
1.8 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.9 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 for Site Characterization for Engineering Design and Construction Purposes
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D4428/D4428M Test Methods for Crosshole Seismic Testing (Withdrawn 2023)
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
D5777 Guide for Using the Seismic Refraction Method for Subsurface Investigation
D6235 Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste
Contaminated Sites
D6285 Guide for Locating Abandoned Wells
D6430 Guide for Using the Gravity Method for Subsurface Site Characterization
D6431 Guide for Using the Direct Current Resistivity Method for Subsurface Site Characterization
D6432 Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation
D6639 Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Site Characterizations
D6820 Guide for Use of the Time Domain Electromagnetic Method for Geophysical Subsurface Site Investigation
D7046 Guide for Use of the Metal Detection Method for Subsurface Exploration (Withdrawn 2020)
D7128 Guide for Using the Seismic-Reflection Method for Shallow Subsurface Investigation
D7400/D7400M Test Methods for Downhole Seismic Testing
G57 Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
3. Terminology
3.1 Definitions:
3.1.1 For definitions of common technical terms used in this standard, refer to Terminology D653.
3.1.2 site investigation, n—in geologic, geotechnical, hydrologic, and environmental evaluations, the effort(s) to plan a scope of
work and collect data to support the assessment of the acquired data, such as engineering, chemical and index properties, of rock,
soil and/or groundwater, and possibly their spatial variability, at the area of interest.
3.1.2.1 Discussion—
One or more parties may have responsibility for various elements of the site investigation.
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.
D6429 − 23
3.1.3 site characterization, n—in geologic, geotechnical, hydrologic, and environmental evaluations, to analyze the data collected
during the site investigation; then prepare various presentations/reports covering the purpose/intent of the site investigation(s).
3.1.3.1 Discussion—
The site characterization findings and conclusions may be based on a combination of engineering judgment, risk management
considerations and standard of care appropriate at the time of the investigation(s).
3.1 Definitions:
3.1.1 For definitions of common technical terms used in this standard, refer to Terminology D653.
4. Summary of Guide
4.1 This guide applies to surface geophysical techniques that are commonly used in site investigations, as well as forensic and
archaeological applications.
4.2 The selection of preferred geophysical methods for a number of common applications is summarized in Table 1. The table is
followed by brief descriptions of each application.
4.3 A brief description of each geophysical method along with some of the field considerations and limitations also are provided.
4.4 It is recommended that personnel consult appropriate references on each of the methods, applications, and their interpretations.
All geophysical measurements should be carried out by knowledgeable professionals who have experience and training in theory
and application of the method, and the interpretation of the data resulting from the use of the specific method.
NOTE 1—The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the
equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective
testing/sampling/inspection, and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results.
Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
5. Significance and Use
5.1 This guide applies to commonly used surface geophysical methods for those applications listed in Table 1. The rating system
used in Table 1 is based upon the ability of each method to produce results under average field conditions when compared to other
methods applied to the same application. An “A” rating implies a preferred method and a “B” rating implies an alternate method.
There may be a single method or multiple methods that can be successfully applied. There may also be a method or methods that
will be successful technically at a lower cost. Selection of the most appropriate method(s) must be made based on the scale and
setting of the target. The final selection must be made considering site specific conditions and project objectives; therefore, it is
critical to have an experienceda qualified professional make the final decision as to the method(s) selected.
5.1.1 Benson et al (1) provides one of the earlier guides to the application of geophysics to environmental problems.
5.1.2 Ward (2) is a three-volume compendium that deals with geophysical methods applied to geotechnical and environmental
problems.
5.1.3 Butler (3) provides detailed technical explanations of near-surface geophysical methods and includes several detailed case
histories histories.
5.1.4 The U.S. Army Corps of Engineers manual (4) provides introductory chapters for the methods of Geophysical Exploration
for Engineering and Environmental Investigations. This manual can be downloaded for no charge from the Corps of Engineers
website.
5.1.5 Olhoeft (5) provides an expert system for helping select geophysical methods to be used at hazardous waste sites.
5.1.6 The U.S. EPA (6) provides an excellent literature review of the theory and use of geophysical methods for use at
contaminated sites.
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A,B
TABLE 1 Selection of Geophysical Methods for Common Applications
Geophysical Methods
(Section 6)
Seismic Electrical Electromagnetic
Ground
DC Frequency Time Pipe/Cable Metal
Applications
Refraction Reflection MASW SP Penetrating Magnetics Gravity
Resistivity Domain Domain VLF (6.8) Locator Detectors
(6.1) (6.2) (6.3) (6.5) Radar (6.12) (6.13)
(6.4) (6.6) (6.7) (6.9) (6.10)
C
(6.11)
Natural Geologic and Hydrologic
Conditions
Soil/unconsolidated layers A B A A B A B A
Soil/unconsolidated layers A B B A B A B A
Rock layers B A A B B B
Rock layers B A B B B B
Depth to bedrock A A A B B B B A B
Depth to bedrock A A B B B B B A B
Depth to water table A A B B B B A
Depth to water table A B B B B A
Fractures and fault zones B A B B A B A B B B
Voids and sinkholes B B A B B B A A
Voids and sinkholes B B A A B B A A
Soil and rock properties A B A B
Soil and rock properties A A A B
Dam and lagoon leakage A B A B B
Dam and lagoon leakage B B A B B
Inorganic Contaminants
Landfill leachate A A A B B
Saltwater intrusion A A A B B
Soil salinity A A
Organic Contaminants
Light, nonaqueous phase liquids B B B B
D
Dissolved phase
Dense, nonaqueous phase
D
liquids
Manmade Buried Objects
Utilities B A B A
Drums and USTs A A A A A
UXO B B B A B A
Abandoned wells B B B A
Landfill and trench boundaries B B B A B A
Forensics B A B B A B
Archaeological features B B B B A A A B
A
“A” implies primary choice of method.method for a given set of site-specific conditions.
B
“B” implies secondary choice or alternate method.
C
Selection implication is applicable where mineralogic clays or conductive pore fluids are not present.predominant.
D
Also see natural geologic and hydrologic conditions to characterize contaminant pathways.

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5.2 An Introduction to Geophysical Measurements:
5.2.1 Geophysical measurements provide a means of mapping lateral and vertical variations of one or more physical properties
or monitoring temporal changes in conditions, or both. In the absence of prior information about the site, reconnaissance-level
geophysical investigations may be appropriate as a precursor to refined surveys. A primary factor affecting the accuracy of site
investigation results is the number of test locations. Insufficient spatial sampling to adequately characterize the conditions at a site
can result if the number of samples is too small. Interpolation between these sample points may be difficult and may lead to an
inaccurate site characterization.
5.2.2 Geophysical measurements generally can be made relatively quickly, are minimally intrusive, and enable interpolation
between known points of control. Continuous data acquisition can be obtained with certain geophysical methods at speeds up to
several km/h (mph). In some cases, total site coverage is economically possible. Because of the greater sample density, geophysical
methods can be used to define background (ambient) conditions and detect anomalous conditions resulting in a more accurate site
characterization than using borings or soundings alone. Geophysical survey design considerations vary according to the intended
distribution of measurements. Data may be collected along individual transects to investigate linear features (dams, levees,
roadways), while multiple transects or 3D survey geometries are required to identify areal trends over larger sites and non-linear
targets. These geophysical methods are especially important to pre-screen large sites prior other drilling, sampling and testing
methods, and additionally are relevant and important for use on levees and other linear structures.to detailed planning of further
site investigation such as other drilling, sampling and testing methods.
5.2.3 Geophysical measurements provide a means of mapping lateral and vertical variations of one or more physical properties
or monitoring temporal changes in conditions, or both.
5.3 A contrast in material properties must be present for geophysical measurements to be successful.
5.3.1 Geophysical methods measure the physical, electrical, or chemical properties of soil, rock, and pore fluids. To detect an
anomaly, a soil to rock contact, the presence of inorganic contaminants, or a buried drum, there must be a contrast in the property
being measured, formeasured. For example, the target to be detected or geologic feature to be defined must have properties
significantly different from “background” conditions.
5.3.2 For example, the interface between fresh water and saltwater in an aquifer can be detected by the differences in electrical
properties of the pore fluids. The contact between soil and unweathered bedrock can be detected by the differences in acoustic
velocity of the materials. In some cases, the differences in measured physical properties may be too small for anomaly detection
by geophysical methods.
5.3.3 Because physical properties of soil and rock vary widely, some by many orders of magnitude, one or more of these properties
usually will correspond to a geologic discontinuity; therefore, boundaries determined by the geophysical methods will usually
coincide with geological boundaries, and a cross-section produced from the geophysical data may resemble a geological
cross-section, although the two are not necessarily identical.
5.4 Geophysical methods should be used in the following instances:
5.4.1 Surface geophysical methods can and should be used early in a site investigation program to aid in identifying background
conditions, as well as anomalous conditions so that borings, soundings, and sampling points can be located to be representative
of site conditions and to investigate anomalies. Geophysical methods also can be used later in the site investigation program after
an initial study is completed to confirm and improve the site investigation findings and provide fill-in data between other
measurements. General site knowledge (for example, depth to bedrock, site use history) is a useful precursor for designing a
geophysical survey.
5.4.2 The level of success of a geophysical survey is improved if the survey objectives are well defined. In some cases, the
objective may be refined as the survey uncovers new or unknown data about the site conditions. The flexibility to change or add
to the technical approach should be built into the program to account for changes in interpretation of site conditions as a site
investigation progresses.
5.5 Profiling and Sounding Measurements:
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5.5.1 Profiling by stations or by continuous measurements provides a means of assessing lateral changes in subsurface conditions.
5.5.2 Soundings provide a means of assessing depth and thickness of geologic layers or other targets. Most surface geophysical
sounding measurements can resolve three and possibly four layers.
5.6 Ease of Use and Interpretation of Data:
5.6.1 The theory of applied geophysics is quantitative, however, in application, geophysical methods often yield interpretations
that are qualitative.
5.6.2 Some geophysical methods provide data from which a preliminary interpretation can be made in the field, for example,
ground penetrating radar (GPR), frequency domain electromagnetic profiling, direct current (DC) resistivity profiling, magnetic
profiling, and metal detector profiling. A map of GPR anomalies or a contour map of the EM (electromagnetic), resistivity,
magnetic or metal detector data often can be created in the field.
5.6.3 Some methods, (for example, time domain electromagnetics and DC resistivity soundings, seismic refraction, seismic
reflection, and gravity), require that the data be processed before any quantitative interpretation can be done.
5.6.4 Any preliminary interpretation of field data should be treated with caution. Such preliminary analysis should be confirmed
by correlation with other information from known points of control, such as borings or outcrops. Such preliminary analysis is
subject to change after data processing and is performed mostly as a means of quality control (QC).
5.6.5 It is the interpretation and integration of all site data that results in useful information for site characterization. The
conversion of raw data to useful information is a value-added process that experienced professionals achieve by careful analysis.
Such analysis must be conducted by a competent professional to ensure that the interpretation is consistent with geologic and
hydrologic conditions.
5.7 Discussion of Applications—Applications listed in Table 1 are discussed below.
5.7.1 Natural Geologic and Hydrologic Conditions:
5.7.1.1 Soil/Unconsolidated Layers—This application includes determining the depth to, thickness of, and areal extent of
unconsolidated layers. These layers may be discontinuous or include lenses of various materials. These layers can be detected
because of differences in their physical properties as compared to adjacent materials.
5.7.1.2 Rock Layers—This application includes determining the contact between different rock layers, for example, limestone over
granite or sandstone over shale, discontinuous bedding planes, and unconformities and the thicknesses of these layers. Several
geophysical methods can be used to delineate rock layers depending on the physical properties and the depths and thicknesses of
the layers.
5.7.1.3 Depth to Bedrock—This application includes determining depth to the top of competent rock covered by unconsolidated
overburden. The choice of geophysical method depends on whether there is a physical property contrast between the rock and the
overlying material. In areas where the top of rock is weathered or highly fractured, top of rock may be difficult to determine. Highly
irregular rock surfaces may present additional problems.
5.7.1.4 Depth to Water Table—This application includes determining the depth at which a subsurface unit is fully saturated. The
water table (top of the saturated zone) can be detected because of the changes in physical properties that are caused by saturated
conditions. The ability to detect the water table may depend on the geologic unit in which it occurs. Seismic methods can be used
to detect the water table in most unconsolidated materials; electrical, electromagnetic, or GPR methods may be used to detect the
water table in either consolidated or unconsolidated materials.
5.7.1.5 Fractures and Fault Zones—This application includes the location and characterization of joints, fractures, and faults.
These features range from individual joints and fracture zones to larger regional structural features. Joints, fractures and fault zones
may be dry, fluid-filled or filled with clays or weathered rock. The detectability of these features increases with the size of the
feature and with the presence of distinctive pore fluids or conductive fill material.
5.7.1.6 Voids and Sinkholes—This application includes karst features, such as weathered depressions in rock, open, water-filled,
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or sediment-filled sinkholes, and cavities or larger cave systems. In many cases, the target of concern may be beyond the effective
resolution or depth range of some or all of the surface geophysical methods; however, deep cavities often show signs of their
presence in the near surface and may be interpreted using shallow geophysical data. The ability to detect a given size cavity
decreases with increasing depth for all surface geophysical methods.
5.7.1.7 Soil and Rock Properties—This application refers to the measurement of the physical properties of soil and rock, for
example, elastic, plastic, and electrical. The geophysical method selected will be determined by the specific property to be
measured. ASTM standards pertinent to those properties should be consulted. For example, rippability and acoustic velocities of
rock are discussed in Guide D5777, the wave velocities measured down a single borehole in Test Method D7400/D7400M and
between boreholes in Test Methods D4428/D4428M. Soil resistivity measurements are discussed in Test Method G57. Density,
porosity measurements and seismic velocity measurements in boreholes are discussed in Guide D5753.
5.7.1.8 Dam and Lagoon Leakage—This application refers to the detection and mapping of fluids leaking along preferential flow
pathways from a dam or lagoon. The application of surface geophysical methods to detect leakage is contingent upon the presence
of localized flow or difference in conductivity.
5.7.2 Inorganic Contaminants:
5.7.2.1 Landfill Leachate—This application includes all types of waste disposal sites in which the primary leachate is likely to be
inorganic and electrically conductive. This includes municipal landfill sites, hazardous waste sites, and mine tailings. Inorganic
contaminants can be detected using electrical or electromagnetic geophysical methods.
5.7.2.2 Saltwater Intrusion—Saltwater intrusion refers to movement of saline water into fresh water aquifers, and although this
is primarily a coastal problem, it can occur naturally in inland aquifers or by man-made contamination, for example, brine ponds.
Saline water is highly conductive and can be detected by DC resistivity and electromagnetic methods. The lateral boundary of the
saltwater/fresh water interface can be mapped and the depth of the saline water estimated.
5.7.2.3 Soil Salinity—Soil salinity is a condition in which salt concentrations within soils have reached levels affecting the growth
and yields of crops. DC resistivity and electromagnetic conductivity measurements provide means for measuring the soil salinity
over a large area and at various depths.
5.7.3 Organic Contaminants:
5.7.3.1 Light, Nonaqueous Phase Liquids (LNAPL)—This application includes petroleum products present as discrete, measurable
contaminants with concentrations greater than their solubility in water. The contaminants are lighter than water and “float” on the
surface of an unconfined aquifer in porous media. The geometry of their occurrence in fractured soil or rock is more complex and
less well defined. LNAPL dissolves into water and acts as a source of dissolved contaminant plumes (see dissolved organic
contaminants). LNAPL can be detected in some cases because its electrical properties are different from those of ground water;
it depresses the ground water surface if present in sufficient quantities; and, it can alter the capillary properties of soil.
5.7.3.2 Dense, Nonaqueous Phase Liquids (DNAPL):
(1) This application includes chlorinated organic solvents and other contaminants that are present as a discrete, measurable
contaminant phase with concentrations greater than their solubility in water. The contaminants are denser than water and “sink”
below the water table. The distribution of DNAPL in the subsurface is complex and is controlled by gravity and the capillary
properties of subsurface materials, rather than by ground water flow direction. DNAPL dissolves into water and acts as a source
of dissolved contaminant plumes (see dissolved organic contaminants). Moreover,“ residual” DNAPL (immobile contaminant left
behind during migration) also can act as a source of dissolved organic contamination. Residual concentrations of DNAPL do not
significantly alter the properties measured by most geophysical methods.
(2) Some DNAPLs have dielectric properties that may allow their detection using GPR if temporal measurements are made
before the DNAPL is introduced to compare with properties that exist after the DNAPL is present; thus, GPR may be useful to
monitor the movement of DNAPL during remediation.
(3) The geophysical methods listed in Table 1 under natural geologic and hydrologic conditions are appropriate to characterize
the hydrogeology of a site; therefore, an attempt can be made to predict DNAPL occurrence and distribution based upon an
understanding of site geology.
5.7.3.3 Dissolved Phase:
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(1) This application includes fuels, solvents, and other organic contaminants dissolved in ground water. Sources can be leaks
and spills of LNAPL or DNAPL or can be leaks and spills of such small volume that the contaminant is dissolved as it reaches
ground water.
(2) Dissolved organic contaminants are of regulatory concern at very low concentrations (parts per billion) in ground water.
The properties of the dissolved organic plumes that can be measured by most geophysical methods are not sufficiently different
from those of ambient ground water to be detectable. Some organic contaminants, such as alcohol, are highly soluble, and are not
detectable even at high concentrations.
(3) When sources of dissolved organic contaminants have been identified, geophysical methods can be used to characterize the
hydrogeology of a site so that pathways for migration of dissolved plumes can be identified. The appropriate methods are discussed
in the sections of this guide that pertain to geologic and hydrologic conditions.
5.7.4 Man-Made Buried Objects:
5.7.4.1 Utilities—This application includes a very wide range of targets including pipes, cables, and utilities. Fortunately, most
utilities are buried near the ground surface, making them relatively easy targets to detect. The geophysical method selected will
depend on the material of which the pipes or utilities are made (ferrous or nonferrous metals or nonmetallic materials). Nonmetallic
utilities, that is, concrete or plastic, can sometimes be detected with GPR.
5.7.4.2 Underground Storage Tanks and Drums—This application includes underground storage tanks (UST) and drums. Since
most underground storage tanks are large (more than 2000 L (500 gal)), buried shallow, and often made of steel, they are relatively
easy to detect. If the tank is made of non-metallic material (for example, concrete or fiberglass), it is more difficult to detect. Drums
of various sizes (typically 4 to 200 L (1 to 55 gal)) are manufactured from either non-metallic or metallic materials. While groups
of drums may be detected, a single 200-L (55-gal) drum and smaller drums are more difficult to locate.
5.7.4.3 Unexploded Ordnance (UXO)—This application includes a wide range of materials that were designed to explode, such
as bombs, mines, and antipersonnel weapons. UXO occur in a variety of sizes from a few centimeters to meters and are made of
a wide variety of metals and other materials. Shape, size, depth, composition and orientation of the UXO can limit detectability.
5.7.4.4 Abandoned Wells—This application includes abandoned wells that may be uncased or cased with steel, PVC, or concrete.
Abandoned wells can be detected by various methods depending upon construction, associated surface pits and other facilities,
leaking fluids, and the method of abandonment. Guide D6285 provides a discussion of geophysical and other methods to locate
abandoned wells.
5.7.4.5 Landfill and Trench Boundaries—This application includes landfills, pits, and trenches. Those that contain buried metallic
materials can be detected because of the presence of the metal. Boundaries of trenches and pits can sometimes be detected by
changes in electrical conductivity, disturbance of subsurface layers, or the presence of fill material. Determining the depth to the
bottom of a landfill or trench is much more difficult than defining the lateral boundaries.
5.7.4.6 Forensics—This application includes buried bodies and a variety of metallic and nonmetallic objects. These objects can
sometimes be detected directly or may be detected indirectly by disturbed soil conditions.
5.7.4.7 Archaeological Features—This application includes a wide range of targets, including stone foundations, walls, roads, fire
pits, caves, and graves, as well as metallic and nonmetallic objects. These targets and objects can sometimes be detected directly
or may be detected indirectly by changes in soil conditions.
6. Discussion of the Geophysical Methods
6.1 Seismic Refraction:
6.1.1 Introduction—Seismic refraction measurements are made by measuring the travel time of direct and refracted acoustic waves
as they travel from the surface through one layer to another and back to the surface where their arrival times are recorded. The
travel time is a function of the seismic or acoustic velocity and the geometry of subsurface layers of soil and rock.
6.1.2 Applications—The primary application for seismic refraction is for determination of depth and thickness of geologic layers,
for example, depth to bedrock and water table, and to delineate geologic structure. Velocity measurements are a measure of the
material properties and can be used as an aid in assessing rock quality and rippability of rock. If compressional P-wave and shear
S-wave velocities are measured, in situ elastic moduli of soil and rock can be determined.
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6.1.3 Depth—Typical depths of measurements are less than 30 m (100 ft), but measurements can be made to much greater depths,
if necessary. Shallow measurements may be made using the energy of a sledgehammer sledgehammer, a weight drop hammer,
accelerated drop weights, vibratory systems, or a shotgun source while deeper measurements will require larger mechanical energy
sources and possibly explosives.
6.1.4 Ease of Use—Seismic refraction measurements are labor intensive. Refraction measurements require that the geophones and
the energy source be in contact with the ground. Extensive cable handling and moving of the source is required. The resulting data
must be analyzed before a quantitative interpretation can be made. The travel time of the P-wave arrivals are picked and then a
time distance plot is drawn from which depths and velocities are determined. A variety of interpretive methods can be used ranging
from the simple time intercept method to delay time, ray tracing, and the generalized reciprocal method. Each interpretive method
requires specific data acquisition in the field. The results of seismic refraction data commonly are displayed as interpreted depth
cross-sections or as contour maps of stratigraphic layers.
6.1.5 Resolution—Vertical resolution requires that a layer have a thickness that is a substantial fraction of the depth to its upper
surface. Seismic refraction measurements can typically resolve three to four layers. Lateral resolution is a function of geophone
spacing, typically 2 to 6 m (5 to 20 ft) or more. Large spacings between source and geophones are used for deeper measurements.
6.1.6 Limitations—Measurements are sensitive to acoustic noise and vibrations. Seismic velocity of layers must increase with
depth. The method will not detect thin layers. A source to geophone distance of up to three to five times the desired depth of
investigation is needed.
6.1.7 References—Haeni (7) provides an excellent introduction to the method with case histories. Guide D5777 is the standard
guide for the use of this method.
6.2 Seismic Reflection:
6.2.1 Introduction—The seismic reflection technique measures the two way travel time of seismic waves from the ground surface
downward to a geologic contact at which part of the seismic energy is reflected back to geophones at the surface. Reflections occur
when there is a contrast in material density or velocity, or both, between two layers.
6.2.2 Applications—The primary application for the seismic reflection method is to identify and determine the depth and thickness
of geologic layers. The top of bedrock may be mapped along with overlying layers. The method also can be used to locate and
characterize geologic structure.
6.2.3 Depth—Reflection measurements detect layers from about 15 to 300 m (50 to 1000 ft) deep. Shallow measurements often
can be made using a sledge hammer, a weight drop hammer, accelerated drop weights, vibratory systems, shotgun, or rifle as
seismic sources. Larger mechanical sources or explosives may be required for deeper investigations or in highly attenuative
material.
6.2.4 Ease of Use—Seismic reflection measurements are relatively difficult to make and are labor intensive. Reflection
measurements require that the geophones and the energy source be in contact with the ground. Extensive cable handling and
moving of the source is required. Two different approaches to data acquisition are used, the common offset method and the
common depth point (CDP) method. The CDP method has become more common for use with modern seismographs. The resulting
field data must be processed prior to quantitative interpretation.
6.2.5 Resolution—Vertical resolution is proportional to the frequency of the seismic energy that can be generated and propagated.
Resolution may be as good as 1 m with frequencies of 500 Hz. The optimum conditions for shallow reflection surveys are saturated
fine-grained soils that enable higher frequency energy to be coupled with the ground. Lateral resolution is a function of geophone
spacing, which is commonly 0.3 to 3 m (1 to 10 ft). The reflection method provides a high resolution cross section of soil or rock
layers along a profile line. Although two-dimensional reflection surveys are common, three-dimensional reflection surveys also can
be conducted.
6.2.6 Limitations—Measurements are sensitive to acoustic noise and vibrations. The distance between the source and the farthest
geophone usually is 1 to 2 times the desired depth of investigation, much less than that required for refraction measurements.
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6.2.7 References—Steeples and Miller (8) provide an introduction to the reflection method with emphasis on the common depth
point method. Pullan and Hunter (9) provide a case history using the common offset method. Guide D7128 is the standard guide
for use of this method.
6.3 Multichannel Analysis of Surface Waves (MASW):
6.3.1 Introduction—MASW technique estimates a shear wave velocity profile within the ground through measurement of surface
wave propagation through a surface array of geophones and then inversion using computer algorithms.
6.3.2 Applications—Shear wave velocity profiles can be used directly in analyses requiring soil stiffness and related properties,
such as settlement, liquefaction, and seismic site response analyses. These profiles are also used to locate discontinuities in the
ground profile such as bedrock, changes in soil consistency, voids, and buried anomalies. Shear wave velocity profiles have been
used in construction quality control of soil compaction, grouting, and in-situ densification.
6.3.3 Depth—Depending on the sizelength of the geophone array, array and the seismic wave source, MASW generates profiles
from 10 m (30 ft) to more than 100 m (300 ft) depth below ground surface. Shallow measurements often can be made using a
sledge hammer, a weight drop hammer, accelerated drop weights, vibratory systems, shotgun, or rifle as seismic sources. Larger
mechanical sources or explosives may be required for deeper investigations or in highly attenuative material.
6.3.4 Ease of Use—MASW surveys require multiple geophones, cables, and specialized data collection equipment. MASW
surveys are typically conducted by specialized personnel.
6.3.5 Resolution—Horizontal resolution depends on the frequencies analyzed using the MASW technique. For practical purposes,
aperture of the receiver array used for analysis. For standard geologic applications, the limit on horizontal resolution has been
estimated to be about 10 m. m (30 ft). Vertical resolution decreases with depth due to increasing surface wavelength with depth.
6.3.6 Limitations—MASW geophone arrays require access to the ground surface along a straight line to place the geophones in
contact with the ground. MASW surveys can be conducted on hard surfaces including pavement and concrete. Accuracy of
processed data is difficult without evaluation of dispersion curve picks.
6.3.7 References—Park et al (10) provides an introduction to the method.
6.4 DC Resistivity:
6.4.1 Introduction—DC resistivity measurements are made by injecting a DC current into the ground through two current
electrodes and measuring the resulting voltage at the surface between two potential electrodes. This method measures bulk
electrical resistivity that is a function of the soil and rock matrix, percentage of fluid saturation, and the conductivity of pore fluids.
6.4.2 Applications—Resistivity measurements can be made as soundings to determine depth and thickness of geologic layers, or
as profiles to locate lateral changes in geologic conditions, detecting and mapping inorganic contaminant plumes, and locating
buried wastes. Sounding measurements are made by incrementally increasing the spacing between electrodes to make a sequence
of measurements at increasing depths. Soundings generally are applicable to defining geologic layers where the geology is laterally
homogeneous and layers are flat or gently dipping. Profile measurements are made with a fixed electrode spacing. Profiling is used
to locate and map areas of significant lateral variations in resistivity at a given depth, for example, a conductive inorganic
contaminant plume.
6.4.2.1 Earth Resistivity Tomography—Earth resistivity tomography (ERT) is a related method of obtaining 2-dimensional
resistivity cross sections from a series of soundings. A series of electrodes, generally 48 to 256, are placed in a linear array and
then select electrodes are automatically activated as current or potential electrodes to produce a series of soundings across the
length of the array. Data are then processed (inverted) to obtain a best fitting finite element grid of earth resistivities. This can also
be extended to 3-D arrays. This process is not within the scope of this Standard Guide. References include Daily (11).
6.4.3 Depth—The depth of measurements is related primarily to electrode spacing and the electrical properties of the subsurface.
Measurements can be made to depths of a few hundred meters or more. There is no theoretical limit to the depth of investigation
if sufficient space is available to lay out the electrode array and sufficient energy is injected into the ground.
6.4.4 Ease of Use—Resistivity measurements are relatively slow and labor intensive since the method requires ground contact.
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This is achieved by driving metal electrodes into the ground and deploying connecting cables. Measurements are made on a station
by station basis. Measurements also can be made by placing a grid of electrodes in the ground and making measurements between
various electrodes to achieve different electrode spacings and geometries (as in azimuthal surveys). Profile data can be plotted as
apparent resistivity versus distance along a profile line with little if any processing. Sounding data must be processed to obtain
depth, thickness, and resistivity of layers. Processing can be done by curve matching or by using forward and inverse modeling.
Results of grid surveys are modeled to provide an image of the subsurface.
6.4.5 Resolution—Lateral resolution is a function of electrode spacing, as well as, the spacing between station measurements.
Resistivity soundings typically can resolve three to four layers.
6.4.6 Limitations—Measurements are susceptible to interference from nearby metal pipes, cables, or fences. The spacing between
electrodes must extend three to five times the depth of interest, which results in long electrode spreads and cables. Finding sufficient
accessible space can sometimes be a problem. Obtaining a good connection with the ground can sometimes be a problem in areas
with high resistivity soils.
6.4.7 References—Ward (12) and Zonge (13) provide an introduction to the method and Zohdy et al (14) provides an introduction
and applications of the method. Guide D6431 is the standard guide for use of this method.
6.5 Spontaneous Potential (SP):
6.5.1 Introduction—The spontaneous, or self, potential (SP) method measures the natural voltage that exists at the ground surface.
Measurements are made between two nonpolarizing electrodes, usually copper-copper sulfate cells, in contact with the ground.
Usually one electrode is fixed as a re
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