5.1 Absolute and comparative methods provide a measure for sorting large quantities of nonferrous parts or stock with regard to composition or condition, or both.5.2 The comparative or two-coil method is used when high-sensitivity examination is required. The advantage of this method is that it almost completely suppresses interferences.5.3 The ability to accomplish these types of separations satisfactorily is dependent upon the relation of the electric characteristics of the nonferrous parts to their physical condition.5.4 These methods may be used for high-speed sorting in a fully automated setup where the speed of examination may approach many specimens per second depending on their size and shape.5.5 Successful sorting of nonferrous material depends mainly on the variables present in the sample and the proper selection of frequency and fill factor.5.6 The accuracy of a sort will be affected greatly by the coupling between the test coil field and the examined part during the measuring period.1.1 This practice describes a procedure for sorting nonferrous metals using the electromagnetic (eddy current) method. The procedure is intended for use with instruments using absolute or comparator-type coils for distinguishing variations in mass, shape, conductivity, and other variables such as alloy, heat treatment, or hardness that may be closely correlated with the electrical properties of the material. Selection of samples to evaluate sorting feasibility and to establish standards is also described.1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.3 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.

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5.1 The method described determines wet density and gravimetric water content by correlating complex impedance measurement data to an empirically developed model. The empirical model is generated by comparing the electrical properties of typical soils encountered in civil construction projects to their wet densities and gravimetric water contents determined by other accepted methods.5.2 The test method described is useful as a rapid, non-destructive technique for determining the in-place total density and gravimetric water content of soil and soil-aggregate mixtures and the determination of dry density.5.3 This method may be used for quality control and acceptance of compacted soil and soil-aggregate mixtures as used in construction and also for research and development. The non-destructive nature allows for repetitive measurements at a single test location and statistical analysis of the results.NOTE 2: The quality of the result produced by this standard test method is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the requirements of Practice D3740 are generally considered capable of competent and objective sampling/testing/inspection, and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluation some of those factors.1.1 This test method covers the procedures for determining in-place properties of non-frozen, unbound soil and soil aggregate mixtures such as total density, gravimetric water content and relative compaction by measuring the intrinsic impedance of the compacted soil.1.1.1 The method and device described in this test method are intended for in-process quality control of earthwork projects. Site or material characterization is not an intended result.1.2 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard.1.2.1 The gravitational system of inch-pound units is used when dealing with inch-pound units. In this system, the pound (lbf) represents a unit of force (weight) while the unit for mass is slugs. The rationalized slug unit is not given in this standard.1.2.2 In the engineering profession, it is customary practice to use, interchangeably, units representing both mass and force, unless dynamic calculations are involved. This implicitly combines two separate systems of units, that is, the absolute system and the gravimetric system. It is undesirable to combine the use of two separate systems within a single standard. The use of balances or scales recording pounds of mass (lbm), or the recording of density in lbm/ft3 should not be regarded as nonconformance with this standard.1.3 All observed and calculated values shall conform to the Guide for Significant Digits and Rounding established in Practice D6026.1.3.1 The procedures used to specify how data is collected, recorded, and calculated in this standard are regarded as industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or decrease the number of significant digits of reported data commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in the analysis methods for engineering design.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.NOTE 1: ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, are entirely their own responsibility.1.5 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.

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1. Scope This International Standard specifies test methods and acceptance criteria for evaluating the electromagnetic compa tibility of tractors and all kinds of mobile (including hand-held) agricultural machinery, forestry machinery, landscaping and

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4.1 General—Most thickness gauges are not applicable to all combinations of coating-substrate thicknesses and materials. The limitations of a particular instrument are generally delineated by its manufacturer. The substrate material and coating combination to be measured as well as the inherent variations in the substrate and coating shall be reviewed prior to selecting the instrument to be used and the measurement accuracy required.4.2 Magnetic—Magnetic-type gauges measure either magnetic attraction between a magnet and a coating or its substrate, or reluctance of a magnetic flux path passing through the coating and substrate. These gauges are designed to measure thickness of a nonmagnetic coating on a magnetic substrate. Some of them will also measure thickness of nickel coatings on a magnetic or nonmagnetic substrate.64.3 Eddy Current—Eddy current-type thickness gauges are electronic instruments that measure variations in impedance of an eddy current inducing coil caused by coating thickness variations. They can only be used if the electrical conductivity of the coating differs significantly from that of the substrate.4.4 Accuracy—The accuracy of a measurement depends on the instrument, the foils, its calibration and standardization, and its operating conditions. The accuracy is also affected by the interferences listed in Section 5, such as part geometry (curvature), magnetic permeability, electrical conductivity, and surface roughness.NOTE 2: This practice under ideal conditions may allow the coating thickness to be determined within ±10 % of its true thickness or to within ±2.5 μm (or ±0.0001 in.), whichever is the greater. (See exceptions in Appendix X2.)1.1 This practice covers the use of magnetic- and eddy current-type thickness instruments (gauges) for nondestructive thickness measurement of a coating on a metal (that is, electrically conducting) substrate. The substrate may be ferrous or nonferrous. The coating or plating being measured may be electrically conducting or insulating as well as ferrous or non-ferrous.1.2 More specific uses of these instruments are covered by Practice D7091 and the following test methods issued by ASTM: Test Methods B244, B499, and B530.1.3 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.1.4 Measurements made in accordance with this practice will be in compliance with the requirements of ISO 2178 as printed in 1982.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 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.

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4.1 This test method applies to the measurement of SE of planar materials under normal incidence, far-field, plane-wave conditions (E and H tangential to the surface of the material).4.2 The uncertainty of the measured SE values is a function of material, mismatches throughout the transmission line path, dynamic range of the measurement system, and the accuracy of the ancillary equipment. An uncertainty analysis is given in Appendix X1 to illustrate the probability of uncertainty achieved by an experienced operator using good equipment. Deviations from the procedure in this test method will increase this uncertainty.4.3 Approximate near-field values of SE can be calculated for both E or H sources by using measured values of far-field SE. A program can be generated from the source code in Appendix X2 that is suitable for use on a personal computer.4.4 This test method measures the net SE caused by reflection and absorption. The reflected and absorbed power measurement is accomplished by the addition of a calibrated bidirectional coupler to the input of the holder.1.1 This test method provides a procedure for measuring the electromagnetic (EM) shielding effectiveness (SE) of a planar material for a plane, far-field EM wave. From the measured data, near-field SE values can be calculated for magnetic (H) sources for electrically thin specimens.2,3 Electric (E) field SE values are also able to be calculated from this same far-field data, but their validity and applicability have not been established.1.2 The measurement method is valid over a frequency range of 30 MHz to 1.5 GHz. These limits are not exact, but are based on decreasing displacement current as a result of decreased capacitive coupling at lower frequencies and on overmoding (excitation of modes other than the transverse electromagnetic mode (TEM)) at higher frequencies for the size of specimen holder described in this test method. Select any number of discrete frequencies within this range. For electrically thin, isotropic materials with frequency independent electrical properties of conductivity, permittivity, and permeability, measurements will possibly be needed at only a few frequencies as the far-field SE values will be independent of frequency. If the material is not electrically thin or if any of the parameters vary with frequency, make measurements at several frequencies within the band of interest.1.3 This test method is not applicable to cables or connectors.1.4 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 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.

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5.1 Eddy current testing is a nondestructive method of locating discontinuities in a product. Changes in electromagnetic response caused by the presence of discontinuities are detected by the sensor, amplified and modified in order to actuate audio or visual indicating devices, or both, or a mechanical marker. Signals can be caused by outer surface, inner surface, or subsurface discontinuities. The eddy current examination is sensitive to many factors that occur as a result of processing (such as variations in conductivity, chemical composition, permeability, and geometry) as well as other factors not related to the tubing. Thus, all received indications are not necessarily indicative of defective tubing.1.1 This practice2 covers procedures for eddy current examination of seamless and welded tubular products made of relatively low conductivity materials such as titanium, stainless steel, and similar alloys, such as nickel alloys. Austenitic chromium-nickel stainless steels, which are generally considered to be nonmagnetic, are specifically covered as distinguished from the martensitic and ferritic straight chromium stainless steels which are magnetic.1.2 This practice is intended as a guide for eddy current examination of both seamless and welded tubular products using either an encircling coil or a probe-coil technique. Coils and probes are available that can be used inside the tubular product; however, their use is not specifically covered in this document. This type of examination is usually employed only to examine tubing which has been installed such as in a heat exchanger.1.3 This practice covers the examination of tubular products ranging in diameter from 0.125 to 5 in. (3.2 to 127.0 mm) and wall thicknesses from 0.005 to 0.250 in. (0.127 to 6.4 mm).1.4 For examination of aluminum alloy tubular products, see standard Practice E215.1.5 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.7 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.

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4.1 General—Ultrasonic testing is a widely used nondestructive method for the examination of a material. The majority of ultrasonic examinations are performed using transducers that directly convert electrical energy into acoustic energy through the use of piezoelectric crystals. This guide describes an alternate technique in which electromagnetic energy is used to produce acoustic energy inside an electrically conductive or ferromagnetic material. EMATs have unique characteristics when compared to conventional piezoelectric ultrasonic search units, making them a significant tool for some ultrasonic examination applications.4.2 Principle—An electromagnetic acoustic transducer (EMAT) generates and receives ultrasonic waves without the need to contact the material in which the acoustic waves are traveling. The use of an EMAT requires that the material to be examined be electrically conductive or ferromagnetic, or both. There are two basic components of an EMAT system, a magnet and a coil. The magnet may be an electromagnet or a permanent magnet, which is used to produce a magnetic field in the material under test. The coil is driven using alternating current at the desired ultrasonic frequency. The coil and AC current also induce a surface magnetic field in the material under test. In the presence of the static magnetic field, the surface current experiences Lorentz forces that produce the desired ultrasonic waves. Upon reception of an ultrasonic wave, the surface of the conductor oscillates in the presence of a magnetic field, thus inducing a voltage in the coil. The transduction process occurs within an electromagnetic skin depth. The EMAT forms the basis for a very reproducible noncontact system for generating and detecting ultrasonic waves.4.3 Specific Advantages—Since an EMAT technique does not have to be in contact with the material under examination, no fluid couplant is required. Important consequences of this include applications to moving objects, in remote or hazardous locations, to objects at elevated temperatures, or to objects with rough surfaces. The EMAT technique is environmentally safe since it does not use potentially polluting or hazardous chemicals. The technique facilitates the rapid scanning of components having complex geometries. EMAT signals are highly reproducible as a consequence of the manner in which the acoustic waves are generated. EMATs can also produce horizontally polarized shear (SH) waves without mode conversion and can accommodate scanning while using SH waves. (Note that in order to produce this wave mode by conventional ultrasonic techniques, either an epoxy or a highly viscous couplant is required. Thus, conventional ultrasonic techniques do not lend themselves easily to scanning when using SH wave modes.) Additionally, EMATs can allow the user to electronically steer shear waves.4.4 Specific Limitations—EMATs have very low efficiency as compared with conventional ultrasonic methods, with insertion losses of 40 dB or more. The EMAT technique can be used only on materials that are electrical conductors or are ferromagnetic. Highly corroded surfaces, especially inner surfaces, may render EMAT unsuitable for use if the surface disturbs the generation of the Lorentz forces. The design of EMAT probes is usually more complex than comparable piezoelectric search units, and are usually relatively large in size. Due to their low efficiency, EMATs usually require more specialized instrumentation for the generation and detection of ultrasonic signals. High transmitting currents, low-noise receivers, and careful electrical matching are imperative in system design. In general, EMAT probes are application-specific, in the same way as are piezoelectric transducers.1.1 This guide is intended primarily for tutorial purposes. It provides an overview of the general principles governing the operation and use of electromagnetic acoustic transducers (EMATs) for ultrasonic examination.1.2 This guide describes a non-contact technique for coupling ultrasonic energy into an electrically conductive or ferromagnetic material, or both, through the use of electromagnetic fields. This guide describes the theory of operation and basic design considerations as well as the advantages and limitations of the technique.1.3 This guide is intended to serve as a general reference to assist in determining the usefulness of EMATs for a given application as well as provide fundamental information regarding their design and operation. This guide provides guidance for the generation of longitudinal, shear, Rayleigh, and Lamb wave modes using EMATs.1.4 This guide does not contain detailed procedures for the use of EMATs in any specific applications; nor does it promote the use of EMATs without thorough testing prior to their use for examination purposes. Some applications in which EMATs have been applied successfully are outlined in Section 9.1.5 Units—The values stated in inch-pound units are to be regarded as the standard. The SI values given in parentheses are for information only.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.7 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.

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This specification covers the requirements for multilayer coatings of autocatalytic nickel-phosphorus over autocatalytic copper intended for electromagnetic interference or electrostatic discharge shielding to parts fabricated from either polymeric or metallic substrates or parts. Coatings are classified into four types based on thickness and testing requirements, and into two grades based on alloy composition, specifically phosphorus. Surfaces shall be prepared by cleaning it of surface contaminants, conditioning, and activation; identifying of base material suitability; and mechanical roughening. Coatings shouldl be inspected for process qualification, appearance, blisters, unplated areas, adhesion, electrical continuity, and DC resistance. Coatings should also be tested for thickness, which may be performed either by Coulometric, X-ray, or Beta backscatter methods. The requirements for process qualification shall include coating composition, electrical integrity, adhesion, thermal shock resistance, thermal cycling, and shielding effectiveness.1.1 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.

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5.1 Visual interpretation of gear teeth condition is different from examining for cracks or early signs of macro-pitting. Visual interpretation is referred to ASNI/AGMA 1010-F14.5.1.1 The purpose of using an eddy current array for mill girth gear tooth examination is it drastically reduces the examination time; covers a large area in one single pass; provides real-time cartography of the examined region, facilitating data interpretation; and improves reliability and probability of detection (POD). One tooth can be examined in less than 30 seconds.NOTE 3: In this practice, ECA is used as a discontinuity finding tool (see Fig. 4) and a presentation aid as support once problems are discovered and photographed. Colors and three-dimensional (3D) images (see Fig. 5) that help with visualization are invaluable in such circumstances.5.1.2 The purpose of using alternating current field measurement is to size surface-breaking cracks electronically.5.1.3 This practice is a useful tool for a condition-based monitoring program.5.2 The examination results may then be used by qualified personnel or organizations to assess the severity and potential consequences of the failure modes identified. This practice is not intended for the examination of non-surface-breaking discontinuities. Other methods should be considered to address examination for non-surface-breaking discontinuities.NOTE 1: Throughout the standard, “gear” means gear or pinion unless the gear is specifically identified.1.1 This practice describes a two-part procedure for electromagnetic evaluation on gear teeth on mill and kiln gear drives and pinions. The first part of this practice details the ability to detect 100 % of surface-breaking discontinuities in the flank and root area on both the drive side and non-drive (coast) side of the gear tooth using an eddy current array. The second part of the examination is to size or measure accurately the length and depth of any cracks found in these areas using electromagnetic methods. No other practice addresses the use of electromagnetic methods for the detection and sizing of surface-breaking discontinuities on mill and kiln ring gear teeth. For reference, Fig. 1 contains a schematic diagram labeling the areas of the gear teeth.FIG. 1 Schematic Image Labeling the Regions of the Gear Teeth and the Area (Shown in Green Shading) That is Scanned in One Pass With the Eddy Current Array Probe1.2 This practice is used only for the detection of surface breaking discontinuities including cracking, macropitting, and certain scuffing and wear patterns. It will not provide a full gear tooth analysis. Visual examination by an experienced gear specialist is the best way to characterize fully the failure modes present. It is imperative that the analysis of the gear teeth is completed at the time of examination. Sending data offsite for analysis later is not recommended, as potential failure modes could be missed from lack of in-situ visual examination.1.3 Two technicians, one lead technician, and a gear technician, are typically required for this practice. One technician guides the probe and the other technician operates the computer/software and analyzes the gear teeth condition.1.4 It is important that the appropriate method standards, such as Practice E3024 and Practice E2261, accompany the technician when performing the examination. If crack sizing is performed, then it shall be performed using an electromagnetic testing method such as the alternating current field measurement approach of Practice E2261.1.5 It is recommended that the technician review the appendixes in this practice in advance of starting the job.1.6 A clean gear is recommended for a complete gear analysis. Depending on the lubrication used, the technician, in discussion with the client, shall determine the appropriate cleaning procedure. If an oil bath lubrication system is used, ensure the gear teeth surfaces are clean. If an asphaltic-based or synthetic-based lubricant is used, refer to the annexes and appendices in this practice.1.7 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.9 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.

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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 investigation 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 (1 % per degree Fahrenheit). 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 resistivity increases with decreasing temperature, slowly for fine materials (in which a significant portion of the water remains unfrozen, even at quite low temperatures), and rapidly for coarse materials (in which the water freezes immediately).5.2.10 Further information about factors that control the electrical resistivity or conductivity of different geological materials can be found in Ward 1990 (7).5.2.11 Because the TDEM/TEM technique measures subsurface resistivity, only geological or hydrological structures that cause spatial variations in resistivity are detected by this technique. If there is no resistivity contrast between the different geological materials or structures, if the resistivity contrast is too small to be detected by the instrument, or if the resistivity of the subsurface material is very high, the TDEM/TEM technique gives no useful information.5.3 Equipment—Geophysical equipment used for the TDEM/TEM method includes a transmitter, a transmitter loop of wire, a transmitter power supply, a receiver and one or more receiver coils.5.3.1 The transmitter may have power output ranging from a few watts to tens of kilowatts. Important parameters of the transmitter are that it transmits a clean wave-form (Fig. 2), and that the “turn-off” characteristics are well known and extremely stable, because they influence the initial shape of the transient response.5.3.2 The size of the transmitter power supply determines the depth of exploration, and can range from a few small batteries to a 10-kW, gasoline-driven generator.5.3.3 The transmitter loop wire is usually insulated for safety. The size of the loop and the amount of current flowing through it (and thus the diameter of the wire) determines the desired depth of exploration. The weight of the loop, which is generally stored on one or more reels, can be anywhere from a few kilograms to over 100 kg (from a few pounds to over 225 lb).5.3.4 The receiver measures the time-varying characteristic of the receiver coil output voltage at a number of points along the decay curve and stores this data in memory. Because the voltage is small, and changes rapidly with time, the receiver must have excellent sensitivity, noise rejection, linearity, stability, and bandwidth. The transmitter/receiver combination must have some facility for synchronization so that the receiver accurately records the time of transmitter current termination or variation. This synchronization is done either with an interconnecting timing cable or with high-stability quartz crystal oscillators mounted in each unit. The characteristics of a TDEM/TEM receiver and transmitter are sufficiently specialized that use of transmitters and receivers not specifically designed for TDEM/TEM by their manufacturers is not recommended.5.3.5 The receiver coil must match the characteristics of the receiver itself. It may contain a built-in preamplifier so that it can be located some distance from the receiver. The coil must be free from microphone noise, and it must be constructed so that the transient response from the metal of the coil and the coil shielding is negligible.5.4 Limitations and Interferences: 5.4.1 General Limitations Inherent to Geophysical Methods: 5.4.1.1 A fundamental limitation of all geophysical methods is that a given set of data cannot be associated with a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all ambiguities, and additional information, such as borehole data, is required. Because of this inherent limitation in the geophysical methods, a TDEM/TEM survey alone is not considered a complete assessment of subsurface conditions. Properly integrated with other geologic information, TDEM/TEM surveying is a highly effective method of obtaining subsurface information.5.4.1.2 In addition, all surface geophysical methods are inherently limited by decreasing resolution with depth.5.4.2 Limitations Specific to the TDEM/TEM Method: 5.4.2.1 Subsurface layers are assumed horizontal within the area of measurement.5.4.2.2 A sufficient resistivity contrast between the background conditions and the feature being mapped must exist for the feature to be detected. Some significant geologic or hydrogeologic boundaries may have no field-measurable resistivity contrast across them and consequently cannot be detected with this technique.5.4.2.3 The TDEM/TEM method does not work well in highly resistive (very low conductivity) materials due to the difficulty in measuring low values of conductivity.5.4.2.4 An interpretation of TDEM/TEM data alone does not yield a unique correlation between possible geologic models and a single set of field data. This ambiguity can be significantly reduced by doing an equivalence analysis as discussed in 6.12.3 and can be further resolved through the use of sufficient supporting geologic data and by an experienced interpreter.5.4.3 Interferences Caused by Natural and Cultural Conditions: 5.4.3.1 The TDEM/TEM method is sensitive to noise from a variety of natural ambient and cultural sources. Spatial variations in resistivity caused by geologic factors may also produce noise. Cultural noise be manifested as very obviously erratic curve behavior such as in Fig. 7, or it may be subtle, repeatable, and difficult to distinguish from valid subsurface changes in resistivity.FIG. 7 Oscillations Induced in Receiver Response by Power Lines (5)5.4.3.2 Ambient Sources of Noise—Ambient sources of noise include radiated and induced responses from nearby metallic structures, and soil and rock electrochemical effects, including induced polarization. In TDEM/TEM soundings, the signal-to-noise ratio (SNR) is usually good over most of the measurement time range. However, at late times, the transient response from the ground decays extremely rapidly such that, towards the end of the transient, the signal deteriorates completely and the data become extremely noisy.5.4.3.3 Radiated and Induced Noise—Radiated noise consists of signals generated by radio, radar transmitters, and lightning. The first two are not generally a problem. However, on summer days when there is extensive local thunderstorm activity, the electrical noise from lightning strikes can cause noise problems. It may be necessary to increase the integration (stacking) time or, in severe cases, to discontinue the survey until the storms have passed by or abated.(1) The most important source of induced noise consists of intense magnetic fields arising from 50/60 Hz power lines. The large signals induced in the receiver from this source (the strength of which falls off more or less linearly with distance from the power line) can overload the receiver if the receiver gain is set too high, causing serious errors. The remedy is to reduce receiver gain to the point that overload does not occur. In some cases, this may result in less accurate measurement of the transient because the available dynamic range of the receiver is not fully utilized. Another alternative is to move the measurement array (particularly the receiver coil) further from the power line. The equipment manufacturer’s documentation may also provide information about which repetition rates or base frequencies (if any) provide the best rejection of the noise arising from power lines.(2) It was mentioned above that one of the advantages of TDEM/TEM resistivity sounding was that measurement of the transient signal from the ground was made in the absence of the primary transmitter field, since measurement is made after transmitter current turnoff (Fig. 2). Modern transmitters use extremely effective electronic switches to terminate the large transmitter current. Nevertheless very sensitive receivers can still detect small currents that linger in the loop after turn-off. The magnitude of these currents and their time behavior are available from the equipment manufacturer, who can advise the user as to how closely the receiver coil can be placed to the actual transmitter loop wire.(3) Another source of induced noise, common to ferrite or iron-cored receiver coils, is microphone noise arising from minute movements of the receiver coil in the earth's relatively strong magnetic field. Such movements are usually caused by the wind, and the coil must be shielded from the wind noise, or the measurements made at night when this source of noise is minimal. In extreme cases, it may be necessary to bury the coil.5.4.3.4 Presence of Nearby Metallic Structures—TDEM/TEM systems are excellent metal detectors. Use of such systems for resistivity sounding demands that measurements are not made in the presence of metal. This requires removal of all metallic objects not part of the survey equipment (metallic chairs, toolboxes, etc.) from the area of the survey instruments. The recommendations of the manufacturer with regard to the location of the receiver case itself with respect to the receiver coil must be followed carefully.(1) Power lines can often be detected as metallic targets as well as sources of induced noise. In this case, they exhibit an oscillatory response (the response from all other targets, including the earth, decays monotonically to zero without oscillation). Because the frequency of the oscillation is unrelated to the receiver base frequency, the effect of power line metallic response is to render the transient “noisy” (Fig. 7). Because these oscillations arise from response to eddy currents induced in the power line by the TDEM/TEM transmitter, repeating the measurement produces an identical response, which is one way that these oscillators are identified. Another way is to take a measurement with the transmitter turned off. If the noise disappears, it is a good indication that power line response is the problem. The only remedy is to move the transmitter loop further from the power line.(2) Other metallic responses, such as those from buried metallic trash or pipes can present a problem. If the response is large, another sounding site must be selected. Use of a different geophysical instrument such as a metal detector or ground conductivity meter is helpful to quickly survey the sounding site for buried metal.5.4.3.5 Geologic Sources of Noise—Geologic noise arises from the presence of unsuspected geological structures or materials, which cause variations in terrain resistivity. A rare effect that can occur in clayey soils, is induced polarization. Rapid termination of the transmitter current and thus primary magnetic field can charge up small electrical capacitors at soil particle interfaces. These capacitors subsequently discharge, producing current flow similar to that shown in Fig. 3, but reversed in direction. The net effect is to reduce the amplitude of the transient response (thus increasing the apparent resistivity) or, in severe situations, to cause the transient response to become negative over some portion of the measurement time range. Because these sources of reverse current are most significant in the vicinity of the transmitter loop, using the offset configuration (described in 6.7.1.1) usually reduces the induced polarization effect.5.5 Summary—During the course of designing and carrying out a TDEM/TEM survey, the sources of ambient, geologic and cultural noise must be considered and the time of occurrence and location noted. The form of the interference is not always predictable, as it not only depends upon the type of noise and the magnitude of the noise but upon the distance from the source of noise and possibly the time of day.5.6 Alternate Methods—In some cases, the factors discussed above may prevent the effective use of the TDEM/TEM method, and other surface geophysical methods such as conventional direct current (DC) resistivity sounding (Guide D6431), frequency domain electromagnetic surveying (Guide D6639) or non-geophysical methods may be required to investigate subsurface conditions.1.1 Purpose and Application: 1.1.1 This guide is one in a series of documents that describe geophysical site investigation methods.1.1.2 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.1.1.3 The TDEM/TEM method is applicable to the subsurface site investigation 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.1.1.4 Electromagnetic measurements of resistivity as described in this guide are applied in geologic studies, geotechnical studies, hydrologic site investigations, 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 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.1.1.5 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.1.1.6 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 recommended that the user of the TDEM/TEM method be familiar with the references cited and with the ASTM standards D420, D653, D5088, D5608, D5730, D5753, D6235, D6429 and D6431.1.2.2 This guide is limited to TDEM/TEM measurements made on land. The TDEM/TEM method can be adapted for a number of special uses on land, water, ice, within a borehole, and airborne. Special TDEM/TEM configurations are used for metal and unexploded ordnance detection. These TDEM/TEM methods are not discussed in this guide.1.2.3 The approaches suggested in this guide for the TDEM/TEM method are commonly used, widely accepted, and proven. However, other approaches or modifications to the TDEM/TEM method that are technically sound may be substituted.1.2.4 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 should be used in conjunction with 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, nor should this document be applied without consideration of a project's many unique aspects. The word standard in the title of this document means only that the document has been approved through the ASTM consensus process.1.3 Precautions: 1.3.1 It is the responsibility of the user of this guide to follow any precautions in the equipment manufacturer's recommendations and to establish appropriate health and safety practices.1.3.2 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 any regulations prior to use.1.3.3 This guide does not purport to address all of the safety concerns that may be associated with the use of the TDEM/TEM method. It must be emphasized that potentially lethal voltages exist at the output terminals of many TDEM/TEM transmitters, and also across the transmitter loop, which is sometimes uninsulated. It is the responsibility of the user of this equipment to assess potential environmental safety hazards resulting from the use of the selected equipment, establish appropriate safety practices and to determine the applicability of regulations prior to use.1.3.4 Units—The values stated in SI units are regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units, which are provided for information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this guide.1.4 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.

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