ASTM D6820-02(2007)
(Guide)Standard Guide for Use of the Time Domain Electromagnetic Method for Subsurface Investigation (Withdrawn 2016)
Standard Guide for Use of the Time Domain Electromagnetic Method for Subsurface Investigation (Withdrawn 2016)
SIGNIFICANCE AND USE
Concepts:
This guide summarizes the equipment, field procedures, and interpretation methods for using the TDEM/TEM method for determination of those subsurface conditions that cause variations in subsurface resistivity. Personnel requirements are as discussed in Practice D3740.
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.
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 investigation depends on the magnetic moment of the transmitter.
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.
Parameter Measured and Representative Values:
The TDEM/TEM technique is used to measure the resistivity of subsurface materials. ...
SCOPE
1.1 Purpose and Application—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 Method (TEM), 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.
1.1.1 The TDEM/TEM method is applicable to investigation of a wide range of subsurface 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.
1.1.2 Electromagnetic measurements of resistivity as described in this guide are applied in geologic studies, geotechnical studies, hydrologic investigations, and for mapping subsurface conditions at waste disposal sites (1). 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 investigations such as the depth to water table, depth to aquitard, presence of coastal or inland groundwater salinity, and for the direct exploration for groundwater.
1.1.3 General references for the use of the method are McNeill (2), Kearey and Brooks (3), and Telford et al (4).
1.2 Limitations:
1.2.1 This guide provides an overview of the TDEM/TEM method. It does not provide or address the details of the theory, field procedures, or interpretation of the data. Numerous references are included for that purpose and are considered an essential part of this guide. It is r...
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D6820 −02(Reapproved 2007)
Standard Guide for
Use of the Time Domain Electromagnetic Method for
Subsurface Investigation
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 references are included for that purpose and are considered an
essential part of this guide. It is recommended that the user of
1.1 Purpose and Application—This guide summarizes the
the TDEM/TEM method be familiar with the references cited
equipment,fieldprocedures,andinterpretationmethodsforthe
and with the ASTM standards D420, D653, D5088, D5608,
assessment of subsurface materials and their pore fluids using
D5730, D5753, D6235, D6429 and D6431.
the Time Domain Electromagnetic (TDEM) method. This
1.2.2 This guide is limited to TDEM/TEM measurements
methodisalsoknownastheTransientElectromagneticMethod
made on land. The TDEM/TEM method can be adapted for a
(TEM), and in this guide is referred to as the TDEM/TEM
number of special uses on land, water, ice, within a borehole,
method. Time Domain and Transient refer to the measurement
and airborne. Special TDEM/TEM configurations are used for
of a time-varying induced electromagnetic field.
metal and unexploded ordnance detection. These TDEM/TEM
1.1.1 The TDEM/TEM method is applicable to investiga-
methods are not discussed in this guide.
tion of a wide range of subsurface conditions. TDEM/TEM
1.2.3 TheapproachessuggestedinthisguidefortheTDEM/
methods measure variations in the electrical resistivity (or the
TEM method are commonly used, widely accepted, and
reciprocal, the electrical conductivity) of the subsurface soil or
proven. However, other approaches or modifications to the
rock caused by both lateral and vertical variations in various
TDEM/TEM method that are technically sound may be sub-
physical properties of the soil or rock. By measuring both
stituted.
lateral and vertical changes in resistivity, variations in subsur-
1.2.4 This guide offers an organized collection of informa-
face conditions can be determined.
tion or a series of options and does not recommend a specific
1.1.2 Electromagnetic measurements of resistivity as de-
course of action. This document cannot replace education,
scribed in this guide are applied in geologic studies, geotech-
experience, and should be used in conjunction with profes-
nical studies, hydrologic investigations, and for mapping
sional judgment. Not all aspects of this guide may be appli-
1). Resistivity
subsurface conditions at waste disposal sites (
cable in all circumstances. This ASTM standard is not intended
measurements can be used to map geologic changes such as
to represent or replace the standard of care by which the
lithology, geological structure, fractures, stratigraphy, and
adequacy of a given professional service must be judged, nor
depth to bedrock. In addition, measurement of resistivity can
should this document be applied without consideration of a
be applied to hydrologic investigations such as the depth to
project’s many unique aspects. The word standard in the title of
water table, depth to aquitard, presence of coastal or inland
this document means only that the document has been ap-
groundwatersalinity,andforthedirectexplorationforground-
proved through the ASTM consensus process.
water.
1.1.3 General references for the use of the method are
1.3 Precautions:
McNeill (2), Kearey and Brooks (3), and Telford et al (4).
1.3.1 It is the responsibility of the user of this guide to
1.2 Limitations:
follow any precautions in the equipment manufacturer’s rec-
1.2.1 This guide provides an overview of the TDEM/TEM ommendations and to establish appropriate health and safety
method.Itdoesnotprovideoraddressthedetailsofthetheory,
practices.
field procedures, or interpretation of the data. Numerous
1.3.2 Ifthemethodisusedatsiteswithhazardousmaterials,
operations, or equipment, it is the responsibility of the user of
this guide to establish appropriate safety and health practices
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
and to determine the applicability of any regulations prior to
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
use.
Characterization.
Current edition approved Sept. 1, 2007. Published October 2007. Originally
1.3.3 Thisguidedoesnotpurporttoaddressallofthesafety
approved in 2002. Last previous edition approved in 2002 as D6820–02. DOI:
concerns that may be associated with the use of the TDEM/
10.1520/D6820-02R07.
TEM method. It must be emphasized that potentially lethal
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. voltages exist at the output terminals of many TDEM/TEM
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6820−02 (2007)
transmitters, and also across the transmitter loop, which is 3.1.1 See Terminology D653.The majority of the technical
sometimes uninsulated. It is the responsibility of the user of termsusedinthisdocumentaredefinedinSheriff (5)andBates
this equipment to establish appropriate safety practices and to and Jackson (6).
determine the applicability of regulations prior to use.
1.3.4 The values stated in SI units are regarded as standard.
4. Summary of Guide
The values given in parentheses are inch-pound units, which
4.1 Summary of the Method—AtypicalTDEM/TEMsurvey
are provided for information only and are not considered
configuration for resistivity sounding (Fig. 1) consists of a
standard.
transmitter connected to a (usually single-turn) square loop of
wire (generally but not necessarily insulated), laid on the
2. Referenced Documents
ground.Amulti-turnreceivercoil,usuallylocatedatthecenter
2.1 ASTM Standards:
of the transmitter loop, is connected to a receiver through a
D420GuidetoSiteCharacterizationforEngineeringDesign
short length of cable.
and Construction Purposes (Withdrawn 2011)
4.1.1 The transmitter current waveform is usually a
D653Terminology Relating to Soil, Rock, and Contained
periodic,symmetricalsquarewave(Fig.2).Aftereverysecond
Fluids
quarter-period the transmitter current (typically between 1 and
D3740Practice for Minimum Requirements for Agencies
40 amps) is abruptly reduced to zero for one quarter period,
Engaged in Testing and/or Inspection of Soil and Rock as
after which it flows in the opposite direction to the previous
Used in Engineering Design and Construction
flow.
D5088Practice for Decontamination of Field Equipment
4.1.2 OtherTDEM/TEMconfigurationsusetriangularwave
Used at Waste Sites
current waveforms and measure the time-varying magnetic
D5608Practices for Decontamination of Field Equipment
field while the current is on.
Used at Low Level Radioactive Waste Sites
D5730Guide for Site Characterization for Environmental 4.1.3 The process of abruptly reducing the transmitter
Purposes With Emphasis on Soil, Rock, the Vadose Zone current to zero induces, in accord with Faraday’s Law, a
and Groundwater (Withdrawn 2013) short-durationvoltagepulseinthegroundthatcausesacurrent
D5753Guide for Planning and Conducting Borehole Geo- to flow in the vicinity of the transmitter wire (Fig. 3).After the
physical Logging transmitter current is abruptly turned off, the current loop can
D6235Practice for Expedited Site Characterization of Va-
bethoughtofasanimage,justbelowthesurfaceoftheground,
dose Zone and Groundwater Contamination at Hazardous of the transmitter loop. However, because of the resistivity of
Waste Contaminated Sites
the ground, the magnitude of the current flow immediately
D6429Guide for Selecting Surface Geophysical Methods
decays. This decaying current induces a voltage pulse in the
D6431Guide for Using the Direct Current Resistivity
ground, which causes more current to flow at larger distances
Method for Subsurface Investigation
from the transmitter loop and at greater depths (Fig. 3). The
D6639Guide for Using the Frequency Domain Electromag-
deeper current flow also decays, due to the resistivity of the
netic Method for Subsurface Investigations
ground, inducing even deeper current flow. To determine the
resistivity as a function of depth, the magnitude of the current
3. Terminology
flow in the ground as a function of time is determined by
measuringthevoltageinducedinthereceivercoil.Thevoltage
3.1 Definitions:
is proportional to the time rate of change of the magnetic field
arising from the subsurface current flow. The magnetic field is
directly proportional to the magnitude of the subsurface
current. By measuring the receiver coil voltage at successively
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
later times, measurement is effectively made of the current
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
flow, and thus the electrical resistivity of the earth, at succes-
the ASTM website.
sively greater depths.
The last approved version of this historical standard is referenced on
www.astm.org.
FIG. 1Typical TDEM/TEM Survey Configuration (7)
D6820−02 (2007)
FIG. 2Typical Time Domain Electromagnetic Waveforms (2)
FIG. 3Time Domain Electromagnetic Eddy Current Flow at (a) Early Time and (b) Late Time (2)
4.1.4 Data resulting from a TDEM/TEM sounding consist 5. Significance and Use
of a curve of receiver coil output voltage as a function of time.
5.1 Concepts:
Analysis of this curve produces a layered earth model of the
5.1.1 This guide summarizes the equipment, field
variationofearthresistivityasafunctionofdepth.Theanalysis
procedures, and interpretation methods for using the TDEM/
can be done graphically or with commercially available
TEM method for determination of those subsurface conditions
TDEM/TEM data inversion programs.
that cause variations in subsurface resistivity. Personnel re-
4.1.5 To determine lateral variations of resistivity in the
quirements are as discussed in Practice D3740.
subsurface, both transmitter and receiver are moved along
5.1.2 AllTDEM/TEMinstrumentsarebasedontheconcept
profile lines on a survey grid. In this way, a three-dimensional
thatatime-varyingmagneticfieldgeneratedbyachangeinthe
picture of the terrain resistivity is developed.
currentflowinginalargelooponthegroundwillcausecurrent
4.1.6 TDEM/TEM surveys for geologic, engineering, hy-
toflowintheearthbelowit(Fig.3).InthetypicalTDEM/TEM
drologic and environmental applications are carried out to
system, these earth-induced currents are generated by abruptly
determine depths of layers or lateral changes in geological
terminatingasteadycurrentflowinginthetransmitterloop (2).
conditions to a depth of tens of meters. Using larger transmit-
Thecurrentsinducedintheearthmaterialmovedownwardand
ters and more sensitive receivers, it is possible to achieve
outward with time and, in a horizontally layered earth, the
depths up to 1000 m.
strength of the currents is directly related to the ground
4.2 Complementary Data—Geologic and water table data conductivity at that depth. These currents decay exponentially.
obtained from borehole logs, geologic maps, data from out- The decay lasts microseconds, except in the cases of a highly
crops or other geological or surface geophysical methods conductive ore body or conductive layer when the decay can
(Guide D6429) and borehole geophysical methods (Guide last up to a second. Hence, many measurements can be made
D5753) are always helpful in interpreting subsurface condi- in a short time period allowing the data quality to be improved
tions from TDEM/TEM survey data. by stacking.
D6820−02 (2007)
5.1.3 Most TDEM/TEM systems use a square wave trans- reciprocal of resistivity is conductivity (usually designated by
mittercurrentwiththemeasurementstakenduringtheoff-time the symbol σ, where σ=1/ρ), which represents the absolute
(Fig. 2) with the total measurement period of less than a ability of the same substance to allow the flow of electrical
minute. Because the strength of the signal depends on the current. Resistive terrain has a low value of conductivity and
induced current strength and secondary magnetic field, the vice versa. Throughout this guide, the term resistivity is used .
depth of investigation depends on the magnetic moment of the Theresistivityofamaterialdependsonthephysicalproperties
transmitter. of the material and is independent of the geometry. Units of
5.1.4 A typical transient response, or receiver voltage resistivity are ohmmeters or ohm-ft (1 ohmmeter = 3.28 ohm
measured,forahomogeneoussubsurface(half-space)isshown ft). Units of conductivity are siemens/meter (S/m) or more
in Fig. 4.The resistivity of the subsurface is obtained from the commonly millisiemens/meter (mS/m), where 1 S/m = 1000
late stage response. If there are two horizontal layers with mS/m. Thus ρ (ohmmeters) = 1/ σ (siemens/meter) = 1000/σ
different resistivities, the response or receiver output voltage is (mS/m).
similar to the curves shown in Fig. 5.
5.2.4 For most applications in engineering and
hydrogeology, the pore fluid dominates the flow of electrical
5.2 Parameter Measured and Representative Values:
currentandthus,theresistivity.Asageneralrule,materialsthat
5.2.1 The TDEM/TEM technique is used to measure the
lack porosity show high resistivity (examples are massive
resistivity of subsurface materials. Although the resistivity of
limestone, most igneous and metamorphic rocks); materials
materials can be a good indicator of the type of material, it is
whose pore space lacks water show high resistivity (examples
never a unique indicator. Fig. 6 shows resistivity values for
aredrysandorgravel,ice);materialswhoseporewaterisfresh
variousearthmaterials.Eachsoilorrocktypehasawiderange
show high resistivity (examples are clean gravel or sand, even
of resistivity values and many ranges overlap. It is the
whensaturated);andmaterialswhoseporewaterissalineshow
interpreter who, based on knowledge of the local geology and
very low resistivity.
otherconditions,mustinterprettheresistivitydataandarriveat
5.2.5 The relationship between resistivity and water satura-
a reasonable geologic and hydrologic interpretation. Very
tion is not linear. The resistivity increases relatively slowly as
often,itisthelshapeofaresistivityanomalythatisdiagnostic,
saturation decreases from 100% to 40 to 60%, and then
rather than the actual values of interpreted resistivity.
increases m
...
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