1、Designation: D6820 18Standard Guide forUse of the Time Domain Electromagnetic Method forSubsurface Site Characterization1This standard is issued under the fixed designation D6820; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 Purpose and Application:1.1.1 This guide summarizes the equipment, fieldprocedures, and interpretation metho
3、ds for the assessment ofsubsurface materials and their pore fluids using the TimeDomain Electromagnetic (TDEM) method. This method is alsoknown as theTransient Electromagnetic (TEM) Method, and inthis guide is referred to as the TDEM/TEM method. TimeDomain and Transient refer to the measurement of a
4、 time-varying induced electromagnetic field.1.1.2 The TDEM/TEM method is applicable to the subsur-face site characterization for a wide range of conditions.TDEM/TEM methods measure variations in the electricalresistivity (or the reciprocal, the electrical conductivity) of thesubsurface soil or rock
5、caused by both lateral and verticalvariations in various physical properties of the soil or rock. Bymeasuring both lateral and vertical changes in resistivity,variations in subsurface conditions can be determined.1.1.3 Electromagnetic measurements of resistivity as de-scribed in this guide are appli
6、ed in geologic studies, geotech-nical studies, hydrologic site characterizations, and for map-ping subsurface conditions at waste disposal sites (1).2Resistivity measurements can be used to map geologic changessuch as lithology, geological structure, fractures, stratigraphy,and depth to bedrock. In
7、addition, measurement of resistivitycan be applied to hydrologic site characterizations such as thedepth to water table, depth to aquitard, presence of coastal orinland groundwater salinity, and for the direct exploration forgroundwater.1.1.4 This standard does not address the use of TDEM/TEMmethod
8、for use as metal detectors or their use in unexplodedordnance (UXO) detection and characterization. While manyof the principles apply the data acquisition and interpretationdiffer from those set forth in this standard guide.1.1.5 General references for the use of the method areMcNeill (2), Kearey an
9、d Brooks (3), and Telford et al (4).1.2 Limitations:1.2.1 This guide provides an overview of the TDEM/TEMmethod. It does not provide or address the details of the theory,field procedures, or interpretation of the data. Numerousreferences are included for that purpose and are considered anessential p
10、art of this guide. It is recommended that the user ofthe TDEM/TEM method be familiar with the references citedand with the ASTM standards D420, D653, D5088, D5608,D5730, D5753, D6235, D6429 and D6431.1.2.2 This guide is limited to TDEM/TEM measurementsmade on land. The TDEM/TEM method can be adapted
11、 for anumber of special uses on land, water, ice, within a borehole,and airborne. Special TDEM/TEM configurations are used formetal and unexploded ordnance detection. These TDEM/TEMmethods are not discussed in this guide.1.2.3 The approaches suggested in this guide for the TDEM/TEM method are common
12、ly used, widely accepted, andproven. However, other approaches or modifications to theTDEM/TEM method that are technically sound may be sub-stituted.1.2.4 This guide offers an organized collection of informa-tion or a series of options and does not recommend a specificcourse of action. This document
13、 cannot replace education,experience, and should be used in conjunction with profes-sional judgment. Not all aspects of this guide may be appli-cable in all circumstances. This ASTM standard is not intendedto represent or replace the standard of care by which theadequacy of a given professional serv
14、ice must be judged, norshould this document be applied without consideration of aprojects many unique aspects. The word standard in the title ofthis document means only that the document has been ap-proved through the ASTM consensus process.1.3 Precautions:1.3.1 It is the responsibility of the user
15、of this guide tofollow any precautions in the equipment manufacturers rec-ommendations and to establish appropriate health and safetypractices.1.3.2 If the method is used at sites with hazardous materials,operations, or equipment, it is the responsibility of the user of1This guide is under the juris
16、diction ofASTM Committee D18 on Soil and Rockand is the direct responsibility of Subcommittee D18.01 on Surface and SubsurfaceCharacterization.Current edition approved Feb. 1, 2018. Published March 2018. Originallyapproved in 2002. Last previous edition approved in 2007 as D6820 02(2007),which was w
17、ithdrawn January 2016 and reinstated February 2018. DOI: 10.1520/D6820-18.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
18、 West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the Worl
19、d Trade Organization Technical Barriers to Trade (TBT) Committee.1this guide to establish appropriate safety and health practicesand to determine the applicability of any regulations prior touse.1.3.3 This guide does not purport to address all of the safetyconcerns that may be associated with the us
20、e of the TDEM/TEM method. It must be emphasized that potentially lethalvoltages exist at the output terminals of many TDEM/TEMtransmitters, and also across the transmitter loop, which issometimes uninsulated. It is the responsibility of the user ofthis equipment to assess potential environmental saf
21、ety hazardsresulting from the use of the selected equipment, establishappropriate safety practices and to determine the applicabilityof regulations prior to use.1.3.4 UnitsThe values stated in SI units are regarded asstandard. No other units of measurement are included in thisstandard. Reporting of
22、test results in units other than SI shallnot be regarded as nonconformance with this guide.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standar
23、ds, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3D420 Guide to Site Characterization for Engineering Designand Construction PurposesD653 Terminology Relating to Soil, Rock, and ContainedFluids
24、D5088 Practice for Decontamination of Field EquipmentUsed at Waste SitesD5608 Practices for Decontamination of Sampling and NonSample Contacting Equipment Used at Low Level Radio-active Waste SitesD5730 Guide for Site Characterization for EnvironmentalPurposes With Emphasis on Soil, Rock, the Vadose
25、 Zoneand Groundwater (Withdrawn 2013)4D5753 Guide for Planning and Conducting GeotechnicalBorehole Geophysical LoggingD6235 Practice for Expedited Site Characterization of Va-dose Zone and Groundwater Contamination at HazardousWaste Contaminated SitesD6429 Guide for Selecting Surface Geophysical Met
26、hodsD6431 Guide for Using the Direct Current ResistivityMethod for Subsurface InvestigationD6639 Guide for Using the Frequency Domain Electromag-netic Method for Subsurface Site Characterizations3. Terminology3.1 Definitions:3.1.1 For definitions of common technical terms used in thisstandard, refer
27、 to Terminology D653.3.1.2 The majority of the technical terms used in thisdocument are defined in Sheriff (5) and Bates and Jackson (6).4. Summary of Guide4.1 SummaryA typical TDEM/TEM survey configurationfor resistivity sounding (Fig. 1) consists of a transmitterconnected to a (usually single-turn
28、) square loop of wire(generally but not necessarily insulated), laid on the ground. Amulti-turn receiver coil, usually located at the center of thetransmitter loop, is connected to a receiver through a shortlength of cable. In some scenarios, it is advantageous to alsomeasure the horizontal componen
29、t(s) (called Hx and Hy) of thereceived signal. In addition, depending upon the project goals,measurements may be made both inside and outside of thetransmitter loop, sometimes called a fixed-loop array.4.1.1 The transmitter current waveform is usually aperiodic, symmetrical square wave (Fig. 2).Afte
30、r every secondquarter-period the transmitter current (typically between 1 and40 amps) is abruptly reduced to zero for one quarter period,after which it flows in the opposite direction to the previousflow.4.1.2 Other TDEM/TEM configurations use triangular wavecurrent waveforms and measure the time-va
31、rying magneticfield while the current is on.4.1.3 The process of abruptly reducing the transmittercurrent to zero induces, in accord with Faradays Law, ashort-duration voltage pulse in the ground that causes a currentto flow in the vicinity of the transmitter wire (Fig. 3). After thetransmitter curr
32、ent is abruptly turned off, the current loop canbe thought of as an image, just below the surface of the ground,of the transmitter loop. However, because of the resistivity ofthe ground, the magnitude of the current flow immediatelydecays. This decaying current induces a voltage pulse in the3For ref
33、erenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4The last approved version of this historical standard is referenced
34、onwww.astm.org.FIG. 1 Typical TDEM/TEM Survey Configuration (7)D6820 182ground, which causes more current to flow at larger distancesfrom the transmitter loop and at greater depths (Fig. 3). Thedeeper current flow also decays, due to the resistivity of theground, inducing even deeper current flow. T
35、o determine theresistivity as a function of depth, the magnitude of the currentflow in the ground as a function of time is determined bymeasuring the voltage induced in the receiver coil. The voltageis proportional to the time rate of change of the magnetic fieldarising from the subsurface current f
36、low. The magnetic field isdirectly proportional to the magnitude of the subsurfacecurrent. By measuring the receiver coil voltage at successivelylater times, measurement is effectively made of the currentflow, and thus the electrical resistivity of the earth, at succes-sively greater depths.4.1.4 Da
37、ta resulting from a TDEM/TEM sounding consistof a curve of receiver coil output voltage as a function of time.Analysis of this curve produces a layered earth model of thevariation of earth resistivity as a function of depth.The analysiscan be done graphically or with commercially availableTDEM/TEM d
38、ata inversion programs.4.1.5 To determine lateral variations of resistivity in thesubsurface, both transmitter and receiver are moved alongprofile lines on a survey grid. In this way, a three-dimensionalpicture of the terrain resistivity is developed.4.1.6 TDEM/TEM surveys for geologic, engineering,
39、 hy-drologic and environmental applications are carried out todetermine depths of layers or lateral changes in geologicalconditions to a depth of tens of meters. Using larger transmit-ters and more sensitive receivers, it is possible to achievedepths up to 1000 m.4.2 Complementary DataGeologic and w
40、ater table dataobtained from borehole logs, geologic maps, data from out-crops or other geological or surface geophysical methods(Guide D6429) and borehole geophysical methods (GuideD5753) are always helpful in interpreting subsurface condi-tions from TDEM/TEM survey data.5. Significance and Use5.1
41、Concepts:5.1.1 All TDEM/TEM instruments are based on the conceptthat a time-varying magnetic field generated by a change in thecurrent flowing in a large loop on the ground will cause currentto flow in the earth below it (Fig. 3). In the typicalTDEM/TEMsystem, these earth-induced currents are genera
42、ted by abruptlyterminating a steady current flowing in the transmitter loop (2).The currents induced in the earth material move downward andoutward with time and, in a horizontally layered earth, theFIG. 2 Typical Time Domain Electromagnetic Waveforms (2)FIG. 3 Time Domain Electromagnetic Eddy Curre
43、nt Flow at (a) Early Time and (b) Late Time (2)D6820 183strength of the currents is directly related to the groundconductivity at that depth. These currents decay exponentially.The decay lasts microseconds, except in the cases of a highlyconductive ore body or conductive layer when the decay canlast
44、 up to a second. Hence, many measurements can be madein a short time period allowing the data quality to be improvedby stacking.5.1.2 Most TDEM/TEM systems use a square wave trans-mitter current with the measurements taken during the off-time(Fig. 2) with the total measurement period of less than am
45、inute. Because the strength of the signal depends on theinduced current strength and secondary magnetic field, thedepth of site characterization depends on the magnetic momentof the transmitter.5.1.3 A typical transient response, or receiver voltagemeasured, for a homogeneous subsurface (half-space)
46、 is shownin Fig. 4. The resistivity of the subsurface is obtained from thelate stage response. If there are two horizontal layers withdifferent resistivities, the response or receiver output voltage issimilar to the curves shown in Fig. 5.5.2 Parameter Measured and Representative Values:5.2.1 The TD
47、EM/TEM technique is used to measure theresistivity of subsurface materials. Although the resistivity ofmaterials can be a good indicator of the type of material, it isnever a unique indicator. Fig. 6 shows resistivity values forvarious earth materials. Each soil or rock type has a wide rangeof resis
48、tivity values and many ranges overlap. It is theinterpreter who, based on knowledge of the local geology andother conditions, must interpret the resistivity data and arrive ata reasonable interpretation. Very often, it is the shape of aresistivity anomaly that is diagnostic, rather than the actualva
49、lues of interpreted resistivity.5.2.2 In the TDEM/TEM technique, the measured quantityis the time-varying voltage induced in the receiver coil andgenerated by the time-varying magnetic flux (field) of thedecaying currents as they move to successively greater depthsin the earth. This time rate of change of magnetic flux, and thusthe receiver output voltage, has units of volts per square meterof receiver coil area (which area is supplied by the equipmentmanufacturer). Since the voltage is usually extremely small itis measured in nanovolts (nV) per square meter of receivercoi
