ASTM D6639-18

Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Site Characterizations

SIGNIFICANCE AND USE
5.1 Concepts:  
5.1.1 This guide summarizes the equipment, field procedures and interpretation methods used for the characterization of subsurface materials and geological structure as based on their properties to conduct, enhance or obstruct the flow of electrical currents as induced in the ground by an alternating electromagnetic field.  
5.1.2 The frequency domain method requires a transmitter or energy source, a transmitter coil, receiver electronics, a receiver coil, and interconnect cables (Fig. 5).  
Perhaps the most important constraint is that the depth of penetration (skin depth, see section 6.5.3.1) of the electromagnetic wave generated by the transmitter be much greater than the intercoil spacing of the instrument. The depth of penetration is inversely proportional to the ground conductivity and instrument frequency. For example, an instrument with an intercoil spacing of 10 m and a frequency of 6400 Hz, using the vertical dipole, meets the low induction number assumption for earth conductivities less than 200 mS/m.  
5.1.5 Multi-frequency domain instruments usually measure the two components of the secondary magnetic field: a component in-phase with the primary field and a component 90° out-of-phase (quadrature component) with the primary field (Kearey and Brook 1991). Generally, instruments do not display either the in-phase or out-of-phase (quadrature) components but do show either the apparent conductivity or the ratio of the secondary to primary magnetic fields.  
5.1.6 When ground conditions are such that the low induction number approximation is valid, the in-phase component is much less than the quadrature phase component. If there is a relatively large in-phase component, the low induction number approximation is not valid and there is likely a very conductive buried body or layer, that is, ore body or man-made metal object.  
5.1.7 The transmitter and receiver coils are almost always aligned in a plane either parallel to the earth's sur...
SCOPE
1.1 Purpose and Application:  
1.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface conditions using the frequency domain electromagnetic (FDEM) method.  
1.1.2 FDEM measurements as described in this standard guide are applicable to mapping subsurface conditions for geologic, geotechnical, hydrologic, environmental, agricultural, archaeological and forensic site characterizations as well as mineral exploration.  
1.1.3 The FDEM method is sometimes used to map such diverse geologic conditions as depth to bedrock, fractures and fault zones, voids and sinkholes, soil and rock properties, and saline intrusion as well as man-induced environmental conditions including buried drums, underground storage tanks (USTs), landfill boundaries and conductive groundwater contamination.  
1.1.4 The FDEM method utilizes the secondary magnetic field induced in the earth by a time-varying primary magnetic field to explore the subsurface. It measures the amplitude and phase of the induced field at various frequencies. FDEM instruments typically measure two components of the secondary magnetic field: a component in-phase with the primary field and a component 90° out-of-phase (quadrature component) with the primary field (Kearey and Brook 1991). Generally, the in-phase response is more sensitive to metallic items (either above or below the ground surface) while the quadrature response is more sensitive to geologic variations in the subsurface. However, both components are, to some degree, affected by both metallic and geologic features. FDEM measurements therefore are dependent on the electrical properties of the subsurface soil and rock or buried man-made objects as well as the orientation of any subsurface geological features or man-made objects. In many cases, the FDEM measurements can be used to identify the subsurface structure or object. This method is used only wh...

General Information

• Status: Published
• Publication Date: 31-Jan-2018
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.01 - Surface and Subsurface Investigation

Relations

• Replaces: ASTM D6639-01(2008) - Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Investigations (Withdrawn 2017)
• Effective Date: 01-Feb-2018
• 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 D6431-18 - Standard Guide for Using the Direct Current Resistivity Method for Subsurface Site Characterization
• 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 D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM D6429-99(2011)e1 - Standard Guide for Selecting Surface Geophysical Methods (Withdrawn 2020)
• Effective Date: 01-Jul-2011
• Refers: ASTM D6235-04(2010) - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 01-May-2010
• Refers: ASTM D6431-99(2010) - Standard Guide for Using the Direct Current Resistivity Method for Subsurface Investigation
• Effective Date: 01-May-2010
• Refers: ASTM D5753-05(2010) - Standard Guide for Planning and Conducting Borehole Geophysical Logging
• Effective Date: 01-May-2010
• Refers: ASTM D653-09 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Jan-2009
• Refers: ASTM D653-08a - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Dec-2008
• Refers: ASTM D653-08 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Nov-2008
• Refers: ASTM D653-07f - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 15-Dec-2007
• Refers: ASTM D653-07e - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Nov-2007

Overview

ASTM D6639-18: Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Site Characterizations provides comprehensive guidance for evaluating subsurface materials and geological structures using the Frequency Domain Electromagnetic (FDEM) method. Published and maintained by ASTM International, this standard outlines recommended practices for equipment setup, field procedures, and interpretation techniques when using EM induction tools to characterize subsurface conditions.

This guide is designed to ensure reliable and consistent subsurface site characterizations in a wide range of applications, including geotechnical, environmental, hydrologic, archaeological, forensic, and mineral exploration projects. It establishes general principles and practical considerations for successfully using FDEM instruments, commonly known as ground conductivity meters (GCMs), at near-surface sites.

Key Topics

• Equipment and Setup
  The FDEM method requires a transmitter and receiver, each with coils and electronic components, to induce and measure electromagnetic fields in subsurface materials. Instrument configuration (coil orientation, intercoil spacing, and frequency) is a key consideration affecting the depth and resolution of investigations.

• Measurement Principles
  FDEM instruments induce a primary electromagnetic field, which causes secondary currents in the ground. The response-measured by the receiver-is interpreted in terms of apparent electrical conductivity. Instruments typically record two components: in-phase (more sensitive to metallic objects) and quadrature (more sensitive to geologic variations).

• Interpretation and Limitations
  Accurate characterization depends on soil and rock conductivity contrasts, coil orientation, and survey design. The method is most effective in environments that fit the “low induction number approximation.” Limitations include decreasing resolution with depth, sensitivity to surface and buried metal, and potential interference from cultural (e.g., power lines, buried utilities) and natural (e.g., atmospheric) sources.

• Data Analysis
  Collected data supports mapping of lateral and vertical conductivity variations-indicative of geological layers, groundwater conditions, or anthropogenic features-using profiles, mapping grids, or soundings. Integration with other geophysical or borehole data is often recommended for robust site interpretation.

Applications

The FDEM method, as formalized in ASTM D6639-18, is highly versatile and widely used for:

• Geological Site Mapping: Identifying depth to bedrock, fractures, faults, voids, and sinkholes.
• Hydrologic Studies: Delineating saline intrusion, groundwater contamination plumes, and mapping water table variations.
• Environmental Investigations: Detecting and mapping buried drums, underground storage tanks (USTs), landfill boundaries, and conductive leachates.
• Agricultural and Archaeological Surveys: Assessing soil salinity, stratigraphy, and uncovering subsurface relics or artifacts.
• Forensic and Engineering Uses: Locating buried utilities, metallic infrastructure, or other manmade objects; assessing suitability for construction or remediation.

The FDEM approach is especially valuable when direct electrode contact is impractical, such as across dry, frozen, asphalted, or otherwise non-conductive surfaces, offering a non-invasive and rapid means to gather subsurface data.

Related Standards

ASTM D6639-18 references and works in concert with several other ASTM guides and standards:

• ASTM D420: Guide to Site Characterization for Engineering Design and Construction Purposes
• ASTM D653: Terminology Relating to Soil, Rock, and Contained Fluids
• ASTM D5730: Guide for Site Characterization for Environmental Purposes (withdrawn)
• ASTM D5753: Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
• ASTM D6235: Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination
• ASTM D6429: Guide for Selecting Surface Geophysical Methods
• ASTM D6431: Guide for Using the Direct Current Resistivity Method for Subsurface Characterization

Professionals are encouraged to consult these related documents for a broader understanding of subsurface site investigation techniques and terminology, ensuring comprehensive and authoritative site assessments.

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Keywords: ASTM D6639-18, frequency domain electromagnetic, FDEM, site characterization, subsurface investigation, ground conductivity meter, environmental assessment, geotechnical survey, hydrologic mapping, ASTM standards, electromagnetic induction, geophysical methods

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: D6639 − 18
Standard Guide for
Using the Frequency Domain Electromagnetic Method for
Subsurface Site Characterizations
This standard is issued under the fixed designation D6639; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* can be used to identify the subsurface structure or object. This
methodisusedonlywhenitisexpectedthatthesubsurfacesoil
1.1 Purpose and Application:
or rock, man-made materials or geologic structure can be
1.1.1 This guide summarizes the equipment, field
characterized by differences in electrical conductivity.
procedures, and interpretation methods for the assessment of
1.1.5 The FDEM method may be used instead of the Direct
subsurface conditions using the frequency domain electromag-
Current Resistivity method (Guide D6431) when surface soils
netic (FDEM) method.
are excessively insulating (for example, dry or frozen) or a
1.1.2 FDEM measurements as described in this standard
layerofasphaltorplasticorotherlogisticalconstraintsprevent
guide are applicable to mapping subsurface conditions for
electrode to soil contact.
geologic, geotechnical, hydrologic, environmental,
agricultural, archaeological and forensic site characterizations
1.2 Limitations:
as well as mineral exploration.
1.2.1 This standard guide provides an overview of the
1.1.3 The FDEM method is sometimes used to map such
FDEMmethodusingcoplanarcoilsatorneargroundleveland
diverse geologic conditions as depth to bedrock, fractures and
has been referred to by other names including Slingram,
fault zones, voids and sinkholes, soil and rock properties, and
HLEM(horizontalloopelectromagnetic)andGroundConduc-
saline intrusion as well as man-induced environmental condi-
tivity methods. This guide does not address the details of the
tions including buried drums, underground storage tanks
electromagnetictheory,fieldproceduresorinterpretationofthe
(USTs), landfill boundaries and conductive groundwater con-
data. References are included that cover these aspects in
tamination.
greater detail and are considered an essential part of this guide
1.1.4 The FDEM method utilizes the secondary magnetic
(Grant and West, 1965; Wait, 1982; Kearey and Brook, 1991;
field induced in the earth by a time-varying primary magnetic
Milsom,1996;Ward,1990).Itisrecommendedthattheuserof
field to explore the subsurface. It measures the amplitude and
the FDEM method review the relevant material pertaining to
phase of the induced field at various frequencies. FDEM
their particular application. ASTM standards that should also
instruments typically measure two components of the second-
be consulted include Guide D420, Terminology D653, Guide
arymagneticfield:acomponentin-phasewiththeprimaryfield
D5730, Guide D5753, Practice D6235, Guide D6429, and
and a component 90° out-of-phase (quadrature component)
Guide D6431.
withtheprimaryfield(KeareyandBrook1991).Generally,the
1.2.2 Thisguideislimitedtofrequencydomaininstruments
in-phase response is more sensitive to metallic items (either
using a coplanar orientation of the transmitting and receiving
above or below the ground surface) while the quadrature
coils in either the horizontal dipole (HD) mode with coils
response is more sensitive to geologic variations in the
vertical, or the vertical dipole (VD) mode with coils horizontal
subsurface. However, both components are, to some degree,
(Fig. 2). It does not include coaxial or asymmetrical coil
affected by both metallic and geologic features. FDEM mea-
orientations,whicharesometimesusedforspecialapplications
surements therefore are dependent on the electrical properties
(Grant and West 1965).
of the subsurface soil and rock or buried man-made objects as
1.2.3 This guide is limited to the use of frequency domain
well as the orientation of any subsurface geological features or
instruments in which the ratio of the induced secondary
man-made objects. In many cases, the FDEM measurements
magnetic field to the primary magnetic field is directly propor-
tionaltotheground’sbulkorapparentconductivity(see5.1.4).
Instruments that give a direct measurement of the apparent
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
ground conductivity are commonly referred to as Ground
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Conductivity Meters (GCMs) that are designed to operate
Current edition approved Feb. 1, 2018. Published March 2018. Originally
within the “low induction number approximation.” Multi-
approved in 2001. Last previous edition approved in 2008 as D6639–01(2008),
frequency instruments operating within and outside the low
which was withdrawn January 2017 and reinstated February 2018. DOI: 10.1520/
D6639-18. induction number approximation provide the ratio of the
*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
D6639 − 18
FIG. 1 Principles of Electromagnetic Induction in Ground Con-
ductivity Measurements (Sheriff, 1989)
FIG. 2 Relative Response of Horizontal and Vertical Dipole Coil
Orientations (McNeill, 1980)
secondary to primary magnetic field, which can be used to unique aspects. The word standard in the title of this document
calculate the ground conductivity.
means that the document has been approved through theASTM
1.2.4 The FDEM (inductive) method has been adapted for a consensus process.
numberofspecialuseswithinaborehole,onwater,orairborne.
1.3 Units—The values stated in SI units are to be regarded
Discussionsoftheseadaptationsormethodsarenotincludedin
asstandard.Nootherunitsofmeasurementareincludedinthis
this guide.
standard. Reporting of test results in units other than SI shall
1.2.5 The approaches suggested in this guide for the fre-
not be regarded as nonconformance with this test method.
quency domain method are the most commonly used, widely
accepted and proven; however other lesser-known or special-
1.4 Precautions:
ized techniques may be substituted if technically sound and
1.4.1 Ifthemethodisusedatsiteswithhazardousmaterials,
documented.
operations, or equipment, it is the responsibility of the user of
1.2.6 Technical limitations and cultural interferences that
this guide to establish appropriate safety and health practices
restrict or limit the use of the frequency domain method are
and to determine the applicability of regulations prior to use.
discussed in section 5.4.
1.5 This standard does not purport to address all of the
1.2.7 This guide offers an organized collection of informa-
safety concerns, if any, associated with its use. It is the
tion or a series of options and does not recommend a specific
responsibility of the user of this standard to establish appro-
course of action. This document cannot replace education,
priate safety, health, and environmental practices and deter-
experience, and professional judgment. Not all aspects of this
mine the applicability of regulatory limitations prior to use.
guide may be applicable in all circumstances. This ASTM
standard is not intended to represent or replace the standard of 1.6 This international standard was developed in accor-
care by which the adequacy of a given professional service dance with internationally recognized principles on standard-
must be judged without consideration of a project’s many ization established in the Decision on Principles for the
D6639 − 18
Development of International Standards, Guides and Recom- measurement of the earth material in a GCM. The ratio of
mendations issued by the World Trade Organization Technical secondary to primary magnetic fields (Hs/Hp) in other fre-
Barriers to Trade (TBT) Committee. quency domain instruments can be interpreted in terms of the
ground conductivity. When operating under the low induction
2. Referenced Documents
number approximation, most of the response will be in the
quadrature component. When this assumption does not hold,
2.1 ASTM Standards:
such as in the presence of metal, there will be a significant
D420GuidetoSiteCharacterizationforEngineeringDesign
and Construction Purposes in-phase component to the response, and the direct correlation
of the signal response to apparent conductivity breaks down.
D653Terminology Relating to Soil, Rock, and Contained
Fluids
4.1.1 The depth of the site characterization is related to the
D5730Guide for Site Characterization for Environmental frequency of the alternating current, the distance between
Purposes With Emphasis on Soil, Rock, the Vadose Zone transmitter and receiver coils (intercoil spacing) and coil
and Groundwater (Withdrawn 2013) orientation.FortheGCM,thedepthofthesitecharacterization
D5753Guide for Planning and Conducting Geotechnical is related to the distance between electrodes and the coil
Borehole Geophysical Logging orientation.
D6235Practice for Expedited Site Characterization of Va-
4.1.2 The apparent conductivity measured by a GCM or
dose Zone and Groundwater Contamination at Hazardous
calculated from the ratio of the secondary to primary magnetic
Waste Contaminated Sites
fields is the conductivity of a homogeneous isotropic half
D6429Guide for Selecting Surface Geophysical Methods
space, as long as the low induction number condition applies
D6431Guide for Using the Direct Current Resistivity
and the subsurface is nonmagnetic. If the earth is horizontally
Method for Subsurface Characterization
layered, the apparent conductivity measured or calculated is
the sum of the conductivities of each layer, weighted by its
3. Terminology
thickness and depth, and is a function of the coil (dipole)
3.1 Definitions: orientation (Fig. 2). If the earth is not layered, that is, a
3.1.1 Fordefinitionsofcommontechnicaltermsusedinthis
homogeneous isotropic half space, both the horizontal and
standard, refer to Terminology D653. vertical dipole measurements are equal. In either case, if the
3.1.2 The majority of the technical terms used in this true conductivities of the layered earth or the homogeneous
document are defined in Sheriff (1991). An additional defini- half space are known, the apparent conductivity that would be
tion follows: measured with a GCM can be calculated with a forward
modeling program.
3.2 apparent conductivity, σ —The conductivity that would
a
4.1.3 Any variation either in the electrical homogeneity of
be measured by a GCM when located over a homogeneous
the half space, or the layers, or a physical deviation from a
isotropic half space that has the same ratio of secondary to
horizontally layered earth, results in a change in the apparent
primary magnetic fields (Hs/Hp) as measured by other fre-
conductivity measurement from the true conductivity. This
quency domain instruments over an unknown subsurface.
characteristic makes it possible to locate and identify many
Apparent conductivity is measured in millisiemens per meter
significant geological features, such as buried channels, some
(mS/m).
fractures or faults (Fig. 3) or buried man-made objects. The
4. Summary of Guide
signaturesofFDEMmeasurementsovertroughsanddikesand
similarfeaturesarewellcoveredintheory(Villegas-Garciaand
4.1 Summary of the Guide—An alternating current is gen-
West, 1983) and in practice.
erated in a transmitter coil producing an alternating primary
4.1.4 While many ground conductivity surveys are carried
electromagnetic field, which induces an alternating current in
out to determine simple lateral or areal changes in geologic
any nearby conductive material. The alternating currents in-
conditions such as the variation in soil salinity or location of a
duced in the earth material produce a secondary electromag-
subsurface conductive contaminant plume, measurements
netic field, which is sensed by a nearby receiver coil (Fig. 1).
made with a GCM with several intercoil spacings or different
Common FDEM instruments operate under the “low induction
coil orientations can be used to identify up to two or three
number approximation”, which is a function of the separation
horizontal layers, provided there is a sufficient conductivity
between the transmitter and receiver, the electrical permeabil-
contrast between the layers (Fig. 4), the layer thicknesses are
ity and conductivity of the ground, and the frequency of the
appreciable, and the depth of the layers falls within the depth
transmitter signal. Essentially, this means that, in the absence
range of the instrument used for the measurement.
of any metallic objects in the subsurface, the ratio of the
4.1.5 Similarly, by taking both the horizontal and vertical
magnitude of this secondary magnetic field to the primary
dipole measurements at several heights above the surface
magneticfieldisdirectlyconvertedtoanapparentconductivity
resolved with a rigid fixed transmitter-receiver configuration,
two or three layers within the instrument depth of exploration
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
can also sometimes be resolved.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
4.2 Complementary Data—Other complementary surface
the ASTM website.
(Guide D6429) and borehole (Guide D5753) geophysical data,
The last approved version of this historical standard is referenced on
www.astm.org. along with non-geophysical data related to the site, may be
D6639 − 18
FIG. 3 Typical Vertical and Horizontal Dipole Profiles Over a Frac-
ture Zone (McNeill, 1990)
FIG. 4 Cross Section of Frequency Domain Soundings (Grady
and Haeni, 1984)
necessary, and are always useful, to properly interpret the 4.2.1 Frequency Domain as Complementary Method—In
subsurface conditions from frequency domain data. some cases, the frequency domain method is not able to
D6639 − 18
provide results in sufficient detail or resolution to meet the Perhaps the most important constraint is that the depth of
objectives of the site characterization, although for a given penetration (skin depth, see section 6.5.3.1) of the electromag-
depth of investigation, the EM methods usually require less netic wave generated by the transmitter be much greater than
space than linear arrays of the DC method. It is, however, a the intercoil spacing of the instrument. The depth of penetra-
fast, reliable method to locate the objective of the site tion is inversely proportional to the ground conductivity and
characterization, which can then be followed up by a more instrument frequency. For example, an instrument with an
detailed resistivity or time domain electromagnetic survey intercoilspacingof10mandafrequencyof6400Hz,usingthe
(Hoekstra et al, 1992). verticaldipole,meetsthelowinductionnumberassumptionfor
earth conductivities less than 200 mS/m.
5. Significance and Use
5.1.5 Multi-frequency domain instruments usually measure
5.1 Concepts:
the two components of the secondary magnetic field: a com-
5.1.1 This guide summarizes the equipment, field proce-
ponent in-phase with the primary field and a component 90°
dures and interpretation methods used for the characterization
out-of-phase (quadrature component) with the primary field
of subsurface materials and geological structure as based on
(Kearey and Brook 1991). Generally, instruments do not
their properties to conduct, enhance or obstruct the flow of
display either the in-phase or out-of-phase (quadrature) com-
electrical currents as induced in the ground by an alternating ponents but do show either the apparent conductivity or the
electromagnetic field.
ratio of the secondary to primary magnetic fields.
5.1.2 The frequency domain method requires a transmitter 5.1.6 When ground conditions are such that the low induc-
or energy source, a transmitter coil, receiver electronics, a
tion number approximation is valid, the in-phase component is
receiver coil, and interconnect cables (Fig. 5). much less than the quadrature phase component. If there is a
5.1.3 The transmitter coil, when placed on or near the
relativelylargein-phasecomponent,thelowinductionnumber
earth’s surface and energized with an alternating current, approximationisnotvalidandthereislikelyaveryconductive
inducessmallcurrentsinthenearearthmaterialproportionalto
buried body or layer, that is, ore body or man-made metal
the conductivity of the material. These induced alternating object.
currents generate a secondary magnetic field (H ), which is
5.1.7 The transmitter and receiver coils are almost always
s
sensed with the primary field (H ) by the receiver coil. aligned in a plane either parallel to the earth’s surface (axis of
p
5.1.4 Under a constraint known as the “low induction
the coils vertical) and generally called the vertical dipole (VD)
number approximation” (McNeill, 1980) and when the subsur- mode or aligned in a plane perpendicular to the earth surface
face is nonmagnetic, the secondary magnetic field is fully
(axis of the coils horizontal) generally called the horizontal
out-of-phase with the primary field and is given by a function dipole (HD) mode (Fig. 3).
of these variables.
5.1.8 The vertical and horizontal dipole orientations mea-
2 sure distinctly different responses to the subsurface material
σ 5 4/ωµ s H /H (1)
~ !~ !
a o s p
(Fig. 2). When these vertical and horizontal dipole mode
where:
measurements are made with several intercoil spacings or
σ = apparent conductivity in siemens/meter, S/m,
appropriate frequencies, they can be combined to resolve
a
ω =2πf in radians/sec; f = frequency in Hz,
multipleearthlayersofvaryingconductivitiesandthicknesses.
–7
µ = permeability of free space in henrys/meter 4π×10 ,
o
This FDEM method is generally limited to only 2 or 3 layers
/m,
with good resolution of depth and conductivity and only if
s = intercoil spacing in meters, m, and
there is a strong conductivity contrast between layers that are
H = the out-of-phase component of the secondary magnetic
s
relatively thick and relatively shallow (in terms of the intercoil
field, both measured by the receiver coil.
spacing).
H = the out-of-phase component of the primary magnetic
p
5.1.9 The conductivity value obtained in 5.1.4 is referred to
field measured by the receiver coil.
as the apparent conductivity σ . For a homogeneous and
a
FIG. 5 Schematic of Frequency Domain Electromagnetic Instru-
ment
D6639 − 18
isotropic earth or half space (in which no layering is present), “low induction number approximation” the measurement is
the apparent conductivity will be the same for both the given by the ratio of the secondary magnetic field to the
measurements. Since the horizontal dipole (HD) is more primary magnetic field (H /H ).
s p
sensitive to the near surface material than the vertical dipole 5.2.2 Some GCMs also give an in-phase measurement
(VD), these two measurements can be used together to tell corresponding to the in-phase component of the secondary
whether the conductivity is increasing or decreasing with magnetic field in parts per thousand (ppt) of the primary field.
depth. The in-phase component is especially useful for mineral
5.1.10 For instruments referred to as Ground Conductivity exploration,detectingburiedman-mademetallicobjects,orfor
Meters (GCMs), the system parameters and constants in 5.1.4 measuring the soil or rock magnetic susceptibility and verify-
are included in the measurement process, giving a calculated ing the assumption that the subsurface is nonmagnetic
reading of σ , usually in mS/m. In some instruments, the ratio (McNeill, 1983).
a
of the in-phase components of the secondary to primary 5.2.3 Fig. 6 shows the electrical conductivities for typical
magnetic fields (H /H ) is displayed in ppt (parts per thou- earth materials varying over five decades from 0.01 mS/m to a
s pp
sand). fewthousandmS/m.Evenaspecificearthmaterial(Fig.6)can
5.1.11 For other frequency domain instruments, the mea- have a large variation in conductivity, which is related to its
surements for both the in-phase and quadrature phase of the temperature, particle size, porosity, pore fluid saturation, and
secondary magnetic field are given as ratios. pore fluid conductivity. Some of these variations, such as a
5.1.12 For a homogeneous horizontally layered earth, the conductive contaminant pore fluid, may be detected by the
measuredapparentconductivitycalculatedbytheinstrumentis FDEM method.
the sum of each layer’s conductivity weighted by the appro-
5.3 Equipment:
priate HD or VD response function (Fig. 2).
5.3.1 The FDEM equipment consists of a transmitter elec-
5.1.13 Whenthesubsurfaceisnothomogeneousorhorizon-
tronics and transmitter coil, a receiver electronics and receiver
tally layered (such as when there is a geologic anomaly or
coil, and interconnect cables. Generally these vary only from
man-made object present), the apparent conductivity may not
one instrument to another in transmitter power, coil size,
be representative of the bulk conductivity of the subsurface
intercoil separation and transmitter frequency.
material. Some anomalous features can, because of their
5.3.2 Some instruments are designed with a rigid, fixed
orientation relative to the instrument coils, produce a negative
intercoil separation usually less than about 4 meters and are
apparent conductivity.While this negative value is not valid as
usedforrelativelyshallowmeasurementsoflessthan6meters.
a conductivity measurement, it is an indication of the presence
5.3.3 For deeper measurements of up to 100 meters, de-
of a geologic anomaly or buried object.
pending on the instrument, the instrument consists of separate
5.1.14 Many common geologic features such as fracture
coilsinterconnectedbycable,(Fig.5)andgenerallyoperatesat
zones, buried channels, dikes and faults, and man-made buried
several intercoil spacings. Instruments using the “low induc-
objects, can be detected and identified by relatively well-
tion number approximation” usually have a single frequency
known anomalous survey signatures (Fig. 3).
for each intercoil spacing and are generally referred to as
5.2 Parameters Measured and Representative Values: Ground Conductivity Meters (GCMs). Measurements of ap-
5.2.1 The FDEM method provides a measure of the appar- parent conductivity, σ , are calculated and displayed in mil-
a
ent electrical conductivity of the subsurface materials. For lisiemens per meter (mS/m).
ground conductivity meters (GCMs), this apparent conductiv- 5.3.4 FDEM instruments taking multiple frequency mea-
ity is read or recorded directly. For instruments not using the surements at a fixed intercoil separation usually give their
FIG. 6 Earth Material Conductivity Ranges (Sheriff, 1991)
D6639 − 18
results as a ratio of the secondary to primary magnetic fields 5.4.2.4 Equivalence problems occur when more than one
(H /H ).These instruments usually have some frequencies that layered model fits the data because combinations of layer
s p
satisfy the low induction number approximation from which conductivities and thicknesses produce the same sounding
theapparentconductivityiscalculated.Thelargermultiplecoil responses. For example, a thin highly conductive layer will
look much like a thicker, less conductive layer of approxi-
separation, multiple frequency instruments are mainly used for
mineral exploration, whereas the smaller multiple frequency mately the same conductivity thickness product. These prob-
lemsaresometimesresolvedbyusingboreholeconductivityor
instruments are used for much the same applications as the
GCMs. resistivity data, knowing the general geology of the area, or by
knowing what is being looked for and what response is
5.4 Limitations and Interferences:
expected. FDEM systems give the best results when searching
5.4.1 General Limitations Inherent to Geophysical Meth-
for a conductive layer in a resistive medium. It is difficult to
ods:
resolveresistivethinlayersinaconductivemediumevenifthe
5.4.1.1 Afundamental limitation inherent to all geophysical layers have a significant electrical contrast.
methods lies in the fact that a given set of data cannot be
5.4.2.5 Frequency domain instruments are best used under
associated with a unique set of subsurface conditions. In most
relatively high electrical conductivity conditions (greater than
situations, surface geophysical measurements alone cannot
1 mS/m). For low conductivity materials (less than 1 mS/m),
resolve all ambiguities, and some additional information, such
useful measurements are better obtained with resistivity meth-
as borehole data, is required. Because of this inherent limita-
ods (Guide D6431).
tion in geophysical methods, a frequency domain or ground
5.4.2.6 Ground conductivity meters (GCMs) have a
conductivity survey alone can never be considered a complete
straight-line (linear) relationship between the true bulk con-
assessment of subsurface conditions. It should be noted that
ductivity of a homogeneous half space and the apparent
multiple methods of measuring electrical conductivity in the
conductivity read by the instrument, provided that the true
earth (that is, FDEM, TDEM, DC Resistivity) will only
conductivity is within the region controlled by the low induc-
produce the same answers for the ideal conditions of a
tion number approximation for the physical parameters of the
nonmagnetic, frequency-independent, isotropic homogeneous
particularinstrument-intercoilseparationandfrequency.Asthe
half-space. The presence of heterogeneities (for example,
conductivity of the half space increases, making the approxi-
layering, objects), anisotropy, magnetic materials, and fre-
mation less and less valid, the apparent conductivity measured
quencydependentmechanismswillresultinvaryinggeometric
by the GCM or calculated using the low induction number
patternsofelectricalcurrentflowinthegroundandconsequent
approximation (5.1.4) deviates more and more from the true
differentvaluesofmeasuredapparentconductivitybetweenthe
ground conductivity. Fig. 7 shows this nonlinearity for a short
methods. Properly integrated with other information, conduc-
one-meter (3.3 ft) intercoil spaced instrument operating at 13
tivity surveying can be an effective method of obtaining
kHz, and shows that, for this spacing, nonlinearity of response
subsurface information.
is not a problem for most earth materials.
5.4.1.2 In addition, all surface geophysical methods are
5.4.2.7 The deviation from linearity, however, can be quite
inherently limited by decreasing resolution with depth.
significant for instruments with large intercoil spacings (up-
5.4.2 Limitations Specific to the FDEM Method: wards of 20 m) and relatively high frequency of operation.
Here the nonlinearity can start at relatively low values of
5.4.2.1 The interpretation of subsurface conditions from
conductivityandcanresultinnegativevaluesathighvaluesof
frequency domain measurements assumes a nonmagnetic ho-
the true conductivity (Fig. 8).
mogeneous horizontally layered earth.Any variation from this
5.4.3 NaturalandCulturalSourcesofNoise(Interferences):
ideal results in variations in the interpretation from the actual
subsurface. There are areas with soils that contain significant
5.4.3.1 Sources of noise referred to here do not include
quantities of ferromagnetic or superparamagnetic minerals or
thoseofaphysicalnaturesuchasdifficultterrainorman-made
metal fragments in which this assumption is no longer valid.
obstructionsbutratherthoseofageologic,ambient,orcultural
This can be tested with electromagnetic instruments (see
nature that adversely affect the measurements and hence the
5.2.2). If the assumption is incorrect, then the apparent
interpretation.
conductivity will be higher than it should be.
5.4.3.2 The project’s objectives in many cases determine
5.4.2.2 Ground conductivity meters (GCMs) using a single
what is characterized as noise. If the survey is attempting to
frequency and one intercoil spacing are limited to detecting
characterize geologic conditions, responses due to buried
lateral variations. With two coil orientations, (horizontal and pipelines and man-made objects are considered noise.
vertical dipole modes), a qualitative interpretation of whether
However, if the survey were attempting to locate such objects,
the conductivity is increasing or decreasing with depth is variations in the measurements due to varying geologic con-
available.Informationastothelayeringorverticaldistribution
ditions would be considered noise. In general, noise is any
of the subsurface conductivity can be derived from measure- variation in the measured values not attributable to the object
ments at different heights above the surface.
of the survey.
5.4.2.3 For soundings, using both coil orientations and 5.4.3.3 Natural Sources of Noise—Themajornaturalsource
multiple intercoil separations, only two or three layers can be of noise in FDEM measurements is naturally occurring atmo-
reasonably interpreted. There must still be a significant con- spheric electricity (spherics). This interference is caused by
ductivity contrast between layers and layer thicknesses. solar activity or electrical storms. Information about solar
D6639 − 18
FIG. 7 Non-linearity for a Short-spaced Instrument
5.4.3.5 Surveys should not be made in close proximity to
buildings, metal fences or buried metal pipelines that can be
detected by frequency domain, unless detection of the buried
pipeline, for example, is the object of the survey. It is
sometimes difficult to predict the appropriate distance from
potential noise sources. Measurements made on-site can
quickly identify the magnitude of the problem and the survey
design should incorporate this information (see 6.3.2.2).
5.4.4 Alternate Methods—In some instances, the preceding
factors may prevent the effective use of the FDEM method.
Other surface geophysical (see Guide D6429)ornon-
geophysical methods may be required to investigate the sub-
surface conditions. Alternate methods, such as DC Resistivity
(Guide D6431) or TDEM, which may not be affected by the
specific source of interference affecting the frequency domain
method may be used to show an electrical contrast.
6. Procedure
FIG. 8 Non-linearity for a Long-spaced Instrument
6.1 Qualification of Personnel—The success of a FDEM
survey, as with most geophysical techniques, is dependent
activitycanbeobtainedontheInternetattheNationalOceanic
upon many factors. Among the most important is the compe-
and Atmospheric Administration web site (http://
tency of the persons responsible for planning, carrying out the
www.noaa.gov). Electr
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