ASTM D6639-01(2008)

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Designation:D6639–01 Designation: D 6639 – 01 (Reapproved 2008)
Standard Guide for
Using the Frequency Domain Electromagnetic Method for
Subsurface Investigations
This standard is issued under the fixed designation D 6639; 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 Purpose and Application:
1.1.1 This standard 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 investigations 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 ground water 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 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 when it is expected that the subsurface soil or rock, man-made
materials or geologic structure can be characterized by differences in electrical conductivity.
1.1.5 The FDEM method may be used instead of the Direct Current Resistivity method (Guide D 6431) when surface soils are
excessively insulating (for example, dry or frozen) or a layer of asphalt or plastic or other logistical constraints prevent electrode
to soil contact.
1.2 Limitations:
1.2.1 This standard guide provides an overview of the FDEM method using coplanar coils at or near ground level and has been
referred to by other names including Slingram, HLEM (horizontal loop electromagnetic) and Ground Conductivity methods. This
guide does not address the details of the electromagnetic theory, field procedures or interpretation of the data. References are
included that cover these aspects in greater detail and are considered an essential part of this guide (Grant and West, 1965; Wait,
1982; Kearey and Brook, 1991; Milsom, 1996; Ward, 1990). It is recommended that the user of the FDEM method review the
relevant material pertaining to their particular application. ASTM standards that should also be consulted include Guide D420,
Terminology D653, Guide D5730, Guide D5753, Practice D6235, Guide D6429, and Guide D6431D 420, Terminology D 653,
Guide D 5730, Guide D 5753, Practice D 6235, Guide D 6429, and Guide D 6431.
1.2.2 This guide is limited to frequency domain instruments using a coplanar orientation of the transmitting and receiving coils
in either the horizontal dipole (HD) mode with coils vertical, or the vertical dipole (VD) mode with coils horizontal (Fig. 2). It
does not include coaxial or asymmetrical coil orientations, which are sometimes used for special applications (Grant and West
1965).
1.2.3 This guide is limited to the use of frequency domain instruments in which the ratio of the induced secondary magnetic
field to the primary magnetic field is directly proportional to the ground’s bulk or apparent conductivity (see 5.1.4). Instruments
that give a direct measurement of the apparent ground conductivity are commonly referred to as Ground Conductivity Meters
(GCMs) that are designed to operate within the “low induction number approximation”.” Multi-frequency instruments operating
within and outside the low induction number approximation provide the ratio of the secondary to primary magnetic field, which
can be used to calculate the ground conductivity.
1.2.4 The FDEM (inductive) method has been adapted for a number of special uses within a borehole, on water, or airborne.
Discussions of these adaptations or methods are not included in this guide.
1.2.5 The approaches suggested in this guide for the frequency domain method are the most commonly used, widely accepted
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved Feb 10, 2001. Published May 2001.
Current edition approved Dec. 1, 2008. Published January 2009. Originally approved in 2001. Last previous edition approved in 2001 as D 6639 – 01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6639 – 01 (2008)
and proven; however other lesser-known or specialized techniques may be substituted if technically sound and documented.
1.2.6 Technical limitations and cultural interferences that restrict or limit the use of the frequency domain method are discussed
in section 5.4.
1.2.7 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, experience, and 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 without consideration of a project’s many unique aspects. The word
standard in the title of this document means that the document has been approved through the ASTM consensus process.
1.3 Precautions:
1.3.1 If the method is used at sites with hazardous materials, operations, or equipment, it is the responsibility of the user of this
guide to establish appropriate safety and health practices and to determine the applicability of regulations prior to use.
1.3.2 This standard guide does not purport to address all of the safety concerns that may be associated with its use. It is the
responsibility of the user of this standard guide to determine the applicability of regulations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D 420 Guide to Site Characterization for Engineering Design and Construction Purposes
D 653 Terminology Relating to Soil, Rock, and Contained Fluids
D 5730 Guide tofor Site Characterization for Environmental Purposes withWith Emphasis on Soil, Rock, the Vadose Zone and
Ground Water
D 5753 Guide for Planning and Conducting Borehole Geophysical Logging
D 6235 Practice for Expedited Site Characterization of Vadose Zone and Ground Water Contamination at Hazardous Waste
Contaminated Sites
D 6429 Guide for Selecting Surface Geophysical Methods
D 6431 Guide for Using the Direct Current Resistivity Method for Subsurface Investigation
3. Terminology
3.1 Definitions—Definitions shall be in accordance with the terms and symbols given in Terminology D 653.
3.2 The majority of the technical terms used in this document are defined in Sheriff (1991). An additional definition follows:
3.3 apparent conductivity, s — The conductivity that would be measured by a GCM when located over a homogeneous
a
isotropic half space that has the same ratio of secondary to primary magnetic fields (Hs/Hp) as measured by other frequency
domain instruments over an unknown subsurface. Apparent conductivity is measured in millisiemens per meter (mS/m).
4. Summary of Guide
4.1 Summary of the Method—An alternating current is generated in a transmitter coil producing an alternating primary
electromagnetic field, which induces an alternating current in any nearby conductive material. The alternating currents induced in
the earth material produce a secondary electromagnetic field, which is sensed by a nearby receiver coil (Fig. 1). The ratio of the
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FIG. 1 Principles of Electromagnetic Induction in Ground
Conductivity Measurements (Sheriff, 1989)
D 6639 – 01 (2008)
FIG. 2 Relative Response of Horizontal and Vertical Dipole Coil
Orientations (McNeill, 1980)
magnitude of this secondary magnetic field to the primary magnetic field is directly converted to a conductivity measurement of
the earth material in a GCM. The ratio of secondary to primary magnetic fields (Hs/Hp) in other frequency domain instruments
is interpreted in terms of the ground conductivity.
4.1.1 The depth of the investigation is related to the frequency of the alternating current, the distance between transmitter and
receiver coils (intercoil spacing) and coil orientation. For the GCM, the depth of investigation is related to the distance between
electrodes and the coil orientation.
4.1.2 The apparent conductivity measured by a GCM or calculated from the ratio of the secondary to primary magnetic fields
istheconductivityofahomogeneousisotropichalfspace,aslongasthelowinductionnumberconditionappliesandthesubsurface
is nonmagnetic. If the earth is horizontally layered, the apparent conductivity measured or calculated is the sum of the
conductivities of each layer, weighted by its thickness and depth, and is a function of the coil (dipole) orientation (Fig. 2). If the
earth is not layered, that is, a homogeneous isotropic half space, both the horizontal and vertical dipole measurements are equal.
In either case, if the true conductivities of the layered earth or the homogeneous half space are known, the apparent conductivity
that would be measured with a GCM can be calculated with a forward modeling program.
4.1.3 Any variation either in the electrical homogeneity of the half space, or the layers, or a physical deviation from a
horizontally layered earth, results in a change in the apparent conductivity measurement from the true conductivity. This
characteristic makes it possible to locate and identify many significant geological features, such as buried channels, some fractures
or faults (Fig. 3) or buried man-made objects. The signatures of FDEM measurements over troughs and dikes and similar features
are well covered in theory (Villegas-Garcia and West, 1983) and in practice.
4.1.4 While many ground conductivity surveys are carried out to determine simple lateral or areal changes in geologic
conditionssuchasthevariationinsoilsalinityorlocationofasubsurfaceconductivecontaminantplume,measurementsmadewith
a GCM with several intercoil spacings or different coil orientations can be used to identify up to two or three horizontal layers,
provided there is a sufficient conductivity contrast between the layers (Fig. 4), the layer thicknesses are appreciable, and the depth
of the layers falls within the depth range of the instrument used for the measurement.
4.1.5 Similarly, by taking both the horizontal and vertical dipole measurements at several heights above the surface resolved
with a rigid fixed transmitter-receiver configuration, two or three layers within the instrument depth of exploration can also
sometimes be resolved.
FIG. 3 Typical Vertical and Horizontal Dipole Profiles Over a
Fracture Zone (McNeill, 1990)
D 6639 – 01 (2008)
FIG. 4 Cross Section of Frequency Domain Soundings (Grady
and Haeni, 1984)
4.2 Complementary Data—Othercomplementarysurface(GuideD 6429)andborehole(GuideD 5753)geophysicaldata,along
with non-geophysical data related to the site, may be necessary, and are always useful, to properly interpret the subsurface
conditions from frequency domain data.
4.2.1 Frequency Domain as Complementary Method—In some cases, the frequency domain method is not able to provide
results in sufficient detail or resolution to meet the objectives of the investigation, although for a given depth of investigation, the
EM methods usually require less space than linear arrays of the DC method. It is, however, a fast, reliable method to locate the
objective of the investigation, which can then be followed up by a more detailed resistivity or time domain electromagnetic survey
(Hoekstra et al, 1992).
5. 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).
5.1.3 The transmitter coil, when placed on or near the earth’s surface and energized with an alternating current, induces small
currents in the near earth material proportional to the conductivity of the material. These induced alternating currents generate a
secondary magnetic field (H ), which is sensed with the primary field (H ) by the receiver coil.
s p
D 6639 – 01 (2008)
FIG. 5 Schematic of Frequency Domain Electromagnetic
Instrument
5.1.4 Under a constraint known as the “low induction number approximation” (McNeill, 1980) and when the subsurface is
nonmagnetic,thesecondarymagneticfieldisfullyout-of-phasewiththeprimaryfieldandisgivenbyafunctionofthesevariables.
s 5 4/vµ s H /H (1)
~ ! ~ !
a o s p
where:
s = apparent conductivity in siemens/meter, S/m,
a
v =2pf in radians/sec; f = frequency in Hz,
-7
µ = permeability of free space in henrys/meter 4p3 10 –7, /m,
o
s = intercoil spacing in meters, m, and
H /H = the ratio of the out-of-phase component of the secondary magnetic field to the primary magnetic field, both measured
s p
by the receiver coil.
Perhaps the most important constraint is that
...