ASTM D6820-18
(Guide)Standard Guide for Use of the Time Domain Electromagnetic Method for Subsurface Site Characterization
Standard Guide for Use of the Time Domain Electromagnetic Method for Subsurface Site Characterization
SIGNIFICANCE AND USE
5.1 Concepts:
5.1.1 All TDEM/TEM instruments are based on the concept that a time-varying magnetic field generated by a change in the current flowing in a large loop on the ground will cause current to flow in the earth below it (Fig. 3). In the typical TDEM/TEM system, these earth-induced currents are generated by abruptly terminating a steady current flowing in the transmitter loop (2). The currents induced in the earth material move downward and outward with time and, in a horizontally layered earth, the strength of the currents is directly related to the ground conductivity at that depth. These currents decay exponentially. The decay lasts microseconds, except in the cases of a highly conductive ore body or conductive layer when the decay can last up to a second. Hence, many measurements can be made in a short time period allowing the data quality to be improved by stacking.
5.1.2 Most TDEM/TEM systems use a square wave transmitter current with the measurements taken during the off-time (Fig. 2) with the total measurement period of less than a minute. Because the strength of the signal depends on the induced current strength and secondary magnetic field, the depth of site characterization depends on the magnetic moment of the transmitter.
5.1.3 A typical transient response, or receiver voltage measured, for a homogeneous subsurface (half-space) is shown in Fig. 4. The resistivity of the subsurface is obtained from the late stage response. If there are two horizontal layers with different resistivities, the response or receiver output voltage is similar to the curves shown in Fig. 5.
5.2.8 Variations in temperature above freezing will affect resistivity measurements as a result of the temperature dependence of the resistivity of the pore fluid, which is of the order of 2 % per degree Celsius. Thus, data from measurements made in winter can be quite different from those made in summer.
5.2.9 As the ground temperature decreases below freezing, the ...
SCOPE
1.1 Purpose and Application:
1.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface materials and their pore fluids using the Time Domain Electromagnetic (TDEM) method. This method is also known as the Transient Electromagnetic (TEM) Method, and in this guide is referred to as the TDEM/TEM method. Time Domain and Transient refer to the measurement of a time-varying induced electromagnetic field.
1.1.2 The TDEM/TEM method is applicable to the subsurface site characterization for a wide range of conditions. TDEM/TEM methods measure variations in the electrical resistivity (or the reciprocal, the electrical conductivity) of the subsurface soil or rock caused by both lateral and vertical variations in various physical properties of the soil or rock. By measuring both lateral and vertical changes in resistivity, variations in subsurface conditions can be determined.
1.1.3 Electromagnetic measurements of resistivity as described in this guide are applied in geologic studies, geotechnical studies, hydrologic site characterizations, and for mapping subsurface conditions at waste disposal sites (1).2 Resistivity measurements can be used to map geologic changes such as lithology, geological structure, fractures, stratigraphy, and depth to bedrock. In addition, measurement of resistivity can be applied to hydrologic site characterizations such as the depth to water table, depth to aquitard, presence of coastal or inland groundwater salinity, and for the direct exploration for groundwater.
1.1.4 This standard does not address the use of TDEM/TEM method for use as metal detectors or their use in unexploded ordnance (UXO) detection and characterization. While many of the principles apply the data acquisition and interpretation differ from those set forth in this standard guide.
1.1.5 General references for the use of the method are McNeill (2), Kearey an...
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Designation:D6820 −18
Standard Guide for
Use of the Time Domain Electromagnetic Method for
1
Subsurface Site Characterization
This standard is issued under the fixed designation D6820; 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* 1.1.5 General references for the use of the method are
McNeill (2), Kearey and Brooks (3), and Telford et al (4).
1.1 Purpose and Application:
1.1.1 This guide summarizes the equipment, field
1.2 Limitations:
procedures, and interpretation methods for the assessment of
1.2.1 This guide provides an overview of the TDEM/TEM
subsurface materials and their pore fluids using the Time
method.Itdoesnotprovideoraddressthedetailsofthetheory,
DomainElectromagnetic(TDEM)method.Thismethodisalso
field procedures, or interpretation of the data. Numerous
knownastheTransientElectromagnetic(TEM)Method,andin
references are included for that purpose and are considered an
this guide is referred to as the TDEM/TEM method. Time
essential part of this guide. It is recommended that the user of
Domain and Transient refer to the measurement of a time-
the TDEM/TEM method be familiar with the references cited
varying induced electromagnetic field.
and with the ASTM standards D420, D653, D5088, D5608,
1.1.2 The TDEM/TEM method is applicable to the subsur-
D5730, D5753, D6235, D6429 and D6431.
face site characterization for a wide range of conditions.
1.2.2 This guide is limited to TDEM/TEM measurements
TDEM/TEM methods measure variations in the electrical
made on land. The TDEM/TEM method can be adapted for a
resistivity (or the reciprocal, the electrical conductivity) of the
number of special uses on land, water, ice, within a borehole,
subsurface soil or rock caused by both lateral and vertical
and airborne. Special TDEM/TEM configurations are used for
variations in various physical properties of the soil or rock. By
metal and unexploded ordnance detection. These TDEM/TEM
measuring both lateral and vertical changes in resistivity,
methods are not discussed in this guide.
variations in subsurface conditions can be determined.
1.2.3 TheapproachessuggestedinthisguidefortheTDEM/
1.1.3 Electromagnetic measurements of resistivity as de-
TEM method are commonly used, widely accepted, and
scribed in this guide are applied in geologic studies, geotech-
proven. However, other approaches or modifications to the
nical studies, hydrologic site characterizations, and for map-
2 TDEM/TEM method that are technically sound may be sub-
ping subsurface conditions at waste disposal sites (1).
stituted.
Resistivitymeasurementscanbeusedtomapgeologicchanges
1.2.4 This guide offers an organized collection of informa-
such as lithology, geological structure, fractures, stratigraphy,
tion or a series of options and does not recommend a specific
and depth to bedrock. In addition, measurement of resistivity
course of action. This document cannot replace education,
can be applied to hydrologic site characterizations such as the
experience, and should be used in conjunction with profes-
depth to water table, depth to aquitard, presence of coastal or
sional judgment. Not all aspects of this guide may be appli-
inland groundwater salinity, and for the direct exploration for
cable in all circumstances. ThisASTM standard is not intended
groundwater.
to represent or replace the standard of care by which the
1.1.4 ThisstandarddoesnotaddresstheuseofTDEM/TEM
adequacy of a given professional service must be judged, nor
method for use as metal detectors or their use in unexploded
should this document be applied without consideration of a
ordnance (UXO) detection and characterization. While many
project’smanyuniqueaspects.Thewordstandardinthetitleof
of the principles apply the data acquisition and interpretation
this document means only that the document has been ap-
differ from those set forth in this standard guide.
proved through the ASTM consensus process.
1
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock 1.3 Precautions:
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
1.3.1 It is the responsibility of the user of this guide to
Characterization.
follow any precautions in the equipment manufacturer’s rec-
Current edition approved Feb. 1, 2018. Published March 2018. Originally
approved in 2002. Last previous edition approved in 2007 as D6820–02(2007), ommendations and to establish appropriate health and safety
which was withdrawn January 2016 and reinstated February 2018. DOI: 10.1520/
practices.
D6820-18.
2 1.3.2 Ifthemethodisusedatsiteswithhazardousmaterials,
Theboldfacenumbersinpare
...
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