1.1 目的和应用: 1.1.1 本指南是描述地球物理现场调查方法的一系列文件之一。 1.1.2 本指南总结了使用时域电磁法（TDEM）评估地下材料及其孔隙流体的设备、现场程序和解释方法。该方法也称为瞬态电磁（TEM）方法，在本指南中称为TDEM/TEM方法。时域和瞬态是指时间的测量- 变化的感应电磁场。 1.1.3 TDEM/TEM方法适用于各种条件下的地下现场调查。TDEM/TEM方法测量地下土壤或岩石的电阻率（或电导率的倒数）的变化，该变化由土壤或岩石的各种物理性质的横向和垂直变化引起。通过测量电阻率的横向和垂直变化，可以确定地下条件的变化。 1.1.4 本指南中所述的电阻率电磁测量应用于地质研究、岩土工程研究、水文现场调查，并用于绘制废物处置场的地下条件 ( 1. ) . 2. 电阻率测量可用于绘制地质变化图，如岩性、地质结构、裂缝、地层和基岩深度。此外，电阻率测量可用于水文现场调查，如地下水位深度、隔水层深度、沿海或内陆地下水盐度的存在，以及直接勘探地下水。 1.1.5 本标准不涉及TDEM/TEM方法用作金属探测器或其在未爆弹药（UXO）检测和表征中的使用。虽然应用了许多原则，但数据采集和解释与本标准指南中规定的不同。 1.1.6 使用该方法的一般参考文献为McNeill ( 2. ) ，Kearey和Brooks ( 3. ) ，和Telford等人 ( 4. ) . 1.2 限制: 1.2.1 本指南概述了TDEM/TEM方法。它没有提供或解决理论、现场程序或数据解释的细节。 为此目的，包含了大量参考文献，并被视为本指南的重要组成部分。建议TDEM/TEM方法的用户熟悉引用的参考文献和ASTM标准 D420 , D653 , D5088 , D5608 , D5730 , D5753 , D6235 , D6429 和 D6431 . 1.2.2 本指南仅限于在陆地上进行的TDEM/TEM测量。TDEM/TEM方法可适用于陆地、水、冰、钻孔内和空中的许多特殊用途。特殊TDEM/TEM配置用于金属和未爆弹药探测。 本指南中不讨论这些TDEM/TEM方法。 1.2.3 本指南中建议的TDEM/TEM方法是常用的、被广泛接受的和经过验证的方法。然而，可以替代TDEM/TEM方法的其他技术上合理的方法或修改。 1.2.4 本指南提供了有组织的信息收集或一系列选项，并不推荐具体的行动方案。本文件不能取代教育、经验，应与专业判断一起使用。 并非本指南的所有方面都适用于所有情况。本ASTM标准不代表或取代必须根据其判断给定专业服务的充分性的谨慎标准，也不应在不考虑项目的许多独特方面的情况下应用本文件。本文件标题中的“标准”一词仅表示该文件已通过ASTM共识程序获得批准。 1.3 注意事项: 1.3.1 本指南的用户有责任遵循设备制造商建议中的任何预防措施，并建立适当的健康和安全实践。 1.3.2 如果在有危险材料、操作或设备的现场使用该方法，则本指南的用户有责任在使用前制定适当的安全和健康做法，并确定任何法规的适用性。 1.3.3 本指南并非旨在解决与TDEM/TEM方法使用相关的所有安全问题。必须强调的是，许多TDEM/TEM变送器的输出端以及变送器回路（有时未绝缘）之间存在潜在致命电压。 本设备的用户有责任评估使用所选设备产生的潜在环境安全危害，制定适当的安全规程，并在使用前确定法规的适用性。 1.3.4 单位- 以国际单位制表示的数值视为标准值。括号中给出的值是英寸-磅单位的数学转换，仅供参考，不被视为标准值。以国际单位制以外的单位报告试验结果不应视为不符合本指南。 1.4 本国际标准是根据世界贸易组织技术性贸易壁垒（TBT）委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认标准化原则制定的。 ====意义和用途====== 5.1 概念: 5.1.1 所有TDEM/TEM仪器都基于这样一个概念，即地面大回路中流动的电流发生变化所产生的时变磁场将导致电流在其下方的地球中流动( 图3 ). 在典型的TDEM/TEM系统中，这些接地感应电流是通过突然终止变送器回路中的稳定电流而产生的 ( 2. ) . 土壤材料中感应的电流随时间向下和向外移动，在水平分层的土壤中，电流强度与该深度的地面电导率直接相关。这些电流呈指数衰减。衰变持续微秒，但在高导电矿体或导电层的情况下，衰变可以持续长达一秒。 因此，可以在短时间内进行许多测量，从而通过叠加提高数据质量。 5.1.2 大多数TDEM/TEM系统使用方波发射器电流，并在关闭时间进行测量( 图2 )总测量周期不到一分钟。由于信号强度取决于感应电流强度和二次磁场，因此现场调查的深度取决于发射器的磁矩。 5.1.3 均匀地下（一半）的典型瞬态响应或测得的接收器电压- 空格）如所示 图4 . 从后期响应中获得地下电阻率。如果有两个电阻不同的水平层，则响应或接收器输出电压类似于中所示的曲线 图5 . 5.2.8 由于孔隙流体电阻率的温度依赖性，冻结以上的温度变化将影响电阻率测量，其数量级为2 % 每摄氏度（1 % 每华氏度）。因此，冬季测量的数据可能与夏季测量的数据大不相同。 5.2.9 当地面温度降至冰点以下时，电阻率随着温度的降低而增加，对于细材料（其中大部分水未冻结，即使在相当低的温度下）缓慢增加，对于粗材料（其中水立即冻结）快速增加。 5.2.10 有关控制不同地质材料电阻率或电导率的因素的更多信息，请参阅Ward 1990 ( 7. ) . 5.2.11 由于TDEM/TEM技术测量地下电阻率，因此该技术仅检测引起电阻率空间变化的地质或水文结构。 如果不同地质材料或结构之间没有电阻率对比，如果电阻率对比太小，仪器无法检测，或者如果地下材料的电阻率非常高，则TDEM/TEM技术不会提供有用信息。 5.3 设备- 用于TDEM/TEM方法的地球物理设备包括发射器、导线发射器回路、发射器电源、接收器和一个或多个接收器线圈。 5.3.1 发射器的功率输出可能在几瓦到几十千瓦之间。 发射器的重要参数是发射干净的波形( 图2 )“关断”特性是众所周知的，并且非常稳定，因为它们影响瞬态响应的初始形状。 5.3.2 发射器电源的大小决定了探测的深度，范围从几个小电池到10千瓦的汽油驱动发电机。 5.3.3 为了安全起见，变送器回路导线通常是绝缘的。回路的大小和流过回路的电流量（以及导线的直径）决定了所需的探测深度。 线圈的重量通常存储在一个或多个卷筒上，可以在几公斤到100公斤（几磅到225磅）之间的任何地方。 5.3.4 接收器在衰减曲线上的多个点处测量接收器线圈输出电压的时变特性，并将该数据存储在内存中。由于电压小，且随时间快速变化，接收器必须具有良好的灵敏度、噪声抑制、线性、稳定性和带宽。发射器/接收器组合必须具有一些同步设施，以便接收器准确记录发射器电流终止或变化的时间。 该同步通过互连定时电缆或安装在每个单元中的高稳定性石英晶体振荡器来完成。TDEM/TEM接收器和发射器的特性足够专业化，因此不建议使用制造商未专门为TDEM/TEM设计的发射器和接收器。 5.3.5 接收器线圈必须与接收器本身的特性相匹配。它可能包含一个内置前置放大器，因此它可以位于距离接收器一定距离的位置。 线圈必须没有麦克风噪声，并且其构造必须使线圈金属和线圈屏蔽的瞬态响应可以忽略不计。 5.4 限制和干扰: 5.4.1 地球物理方法固有的一般局限性: 5.4.1.1 所有地球物理方法的一个基本限制是，一组给定的数据不能与一组独特的地下条件相关联。在大多数情况下，仅凭地表地球物理测量无法解决所有模糊问题，需要额外的信息，如钻孔数据。 由于地球物理方法的固有局限性，仅TDEM/TEM测量不被视为对地下条件的完整评估。TDEM/TEM测量与其他地质信息适当结合，是获取地下信息的高效方法。 5.4.1.2 此外，所有地表地球物理方法固有地受到分辨率随深度降低的限制。 5.4.2 TDEM/TEM方法的特定限制: 5.4.2.1 假设地下层在测量区域内水平。 5.4.2.2 背景条件和映射特征之间必须存在足够的电阻率对比度，才能检测特征。一些重要的地质或水文地质边界可能没有现场可测量的电阻率对比，因此无法使用该技术进行检测。 5.4.2.3 由于难以测量电导率的低值，TDEM/TEM方法在高电阻（极低电导率）材料中效果不佳。 5.4.2.4 仅对TDEM/TEM数据进行解释并不能在可能的地质模型和单个现场数据集之间产生唯一的相关性。 如中所述，通过进行等价性分析，可以显著减少这种歧义 6.12.3 可以通过使用足够的支持性地质数据和经验丰富的翻译来进一步解决。 5.4.3 自然和文化条件造成的干扰: 5.4.3.1 TDEM/TEM方法对各种自然环境和文化源的噪声非常敏感。地质因素引起的电阻率空间变化也可能产生噪声。文化噪声表现为非常明显的不稳定曲线行为，例如 图7 或者它可能是微妙的、可重复的，并且很难与有效的地下电阻率变化区分开来。 图7 电力线在接收器响应中引起的振荡 ( 5. ) 5.4.3.2 环境噪声源- 环境噪声源包括附近金属结构的辐射和感应响应，以及土壤和岩石电化学效应，包括激发极化。在TDEM/TEM测深中，在大多数测量时间范围内，信噪比通常良好。 然而，在后期，来自地面的瞬态响应衰减非常快，以至于在瞬态结束时，信号完全恶化，数据变得非常嘈杂。 5.4.3.3 辐射和感应噪声- 辐射噪声由无线电、雷达发射器和雷电产生的信号组成。前两个通常不是问题。然而，在夏季，当局部有广泛的雷暴活动时，雷击产生的电噪声可能会引起噪声问题。 可能需要增加积分（叠加）时间，或者在严重情况下，停止测量，直到风暴过去或减弱。 (1) 感应噪声的最重要来源是50/60 Hz电源线产生的强磁场。如果接收器增益设置过高，则接收器中从该源感应的大信号（其强度随与电源线的距离线性下降）可能会使接收器过载，从而导致严重错误。 补救措施是将接收器增益降低到不会发生过载的程度。在某些情况下，这可能会导致瞬态测量不太准确，因为接收器的可用动态范围没有得到充分利用。另一种选择是将测量阵列（尤其是接收器线圈）从电源线进一步移动。设备制造商的文件还可以提供有关哪些重复频率或基频（如有）可以最好地抑制电源线产生的噪声的信息。 (2) 如上所述，TDEM/TEM电阻率测深的优点之一是，由于测量是在发射器电流关闭后进行的，因此在没有主发射器场的情况下测量地面瞬态信号( 图2 ). 现代变送器使用极为有效的电子开关来终止较大的变送器电流。然而，非常敏感的接收器仍然可以检测到关闭后仍在回路中徘徊的小电流。这些电流的幅值及其时间行为可从设备制造商处获得，制造商可以向用户建议接收器线圈与实际发射器环路的距离。 (3) 铁氧体或铁心接收器线圈常见的另一个感应噪声源是接收器线圈在地球相对强的磁场中微小运动产生的麦克风噪声。这种运动通常是由风引起的，必须保护线圈免受风噪声的影响，或在夜间进行测量，此时噪声源最小。在极端情况下，可能需要埋设线圈。 5.4.3.4 附近存在金属结构- TDEM/TEM系统是优秀的金属探测器。 使用此类系统进行电阻率测深需要在不存在金属的情况下进行测量。这需要从测量仪器区域移除不属于测量设备的所有金属物体（金属椅子、工具箱等）。必须仔细遵循制造商关于接收器外壳本身相对于接收器线圈的位置的建议。 (1) 电力线通常可以被检测为金属目标以及感应噪声源。 在这种情况下，它们表现出振荡响应（来自包括地球在内的所有其他目标的响应单调衰减到零而不振荡）。由于振荡频率与接收器基频无关，因此电力线金属响应的影响会导致瞬态“噪声”( 图7 ). 由于这些振荡是由TDEM/TEM发射器对电力线中感应的涡流的响应引起的，因此重复测量会产生相同的响应，这是识别这些振荡器的一种方法。 另一种方法是在发射器关闭的情况下进行测量。如果噪声消失，则表明电源线响应是问题所在。唯一的补救措施是将变送器回路从电源线移开。 (2) 其他金属反应，例如来自埋置金属垃圾或管道的反应，可能会出现问题。如果响应较大，则必须选择另一个测深点。使用不同的地球物理仪器，如金属探测器或地面电导率仪，有助于快速测量埋置金属的探测现场。 5.4.3.5 地质噪声源- 地质噪声源于未预料到的地质结构或材料，导致地形电阻率变化。在粘性土壤中可能出现的一种罕见效应是激发极化。变送器电流和初级磁场的快速终止可以给土壤-颗粒界面的小电容器充电。这些电容器随后放电，产生与图1所示类似的电流 图3 ，但方向相反。 净效应是降低瞬态响应的幅度（从而增加视电阻率），或者在严重情况下，导致瞬态响应在测量时间范围的某些部分变为负值。因为这些反向电流源在变送器回路附近最为重要，使用偏移配置（如中所述 6.7.1.1 )通常会降低激发极化效应。 5.5 摘要- 在设计和执行TDEM/TEM调查的过程中，必须考虑环境、地质和文化噪声源，并注明发生时间和位置。 干扰的形式并不总是可预测的，因为它不仅取决于噪声的类型和噪声的大小，还取决于与噪声源的距离，可能还取决于一天中的时间。 5.6 替代方法- 在某些情况下，上述因素可能会妨碍TDEM/TEM方法和其他地表地球物理方法的有效使用，如常规直流电阻率测深（指南） D6431 )，频域电磁测量（指南 D6639 )或可能需要非地球物理方法来调查地下条件。

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. ====== 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 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.