1.1 目的和应用 : 1.1.1本指南总结了使用频域电磁法（FDEM）评估地下条件的设备、现场程序和解释方法。 1.1.2本标准指南中所述的FDEM测量适用于地质、岩土工程、水文、环境、农业、考古和法医调查以及矿产勘探的地下条件测绘。 1.1.3 FDEM方法有时用于绘制各种地质条件的地图，如基岩深度、裂缝和断层带、孔隙和天坑、土壤和岩石性质、盐分侵入以及人类活动- 诱发的环境条件包括埋桶、地下储罐（UST）、垃圾填埋场边界和传导地下水污染。 1.1.4 FDEM方法利用随时间变化的一次磁场在地球中感应的二次磁场来探测地下。它测量不同频率下感应场的振幅和相位。因此，FDEM测量取决于地下土壤和岩石或埋藏人造物体的电特性，以及任何地下地质特征或人造物体的方向。在许多情况下，FDEM测量可用于识别地下结构或物体。 只有当地下土壤或岩石、人造材料或地质结构的导电性存在差异时，才使用该方法。 1.1.5可以使用FDEM方法代替直流电阻率法（指南 D6431 )当表土过度绝缘（例如，干燥或冻结）或一层沥青或塑料或其他物流约束阻止电极与土壤接触时。 1.2 局限性 : 1.2.1本标准指南概述了在地面或近地面使用共面线圈的FDEM方法，并用其他名称提及，包括Slingram、HLEM（水平回路电磁）和地面电导率方法。 本指南不涉及电磁理论、现场程序或数据解释的细节。包括更详细地涵盖这些方面的参考文献，并被视为本指南的重要组成部分（Grant和West，1965；Wait，1982；Kearey和Brook，1991；Milsom，1996；Ward，1990）。建议FDEM方法的用户审查与其特定应用相关的材料。还应参考ASTM标准，包括指南 D420 术语 D653 指导 D5730 指导 D5753 实践 D6235 指导 D6429 ，和指南 D6431 . 1.2.2本指南仅限于使用发射和接收线圈共面定向的频域仪器，在水平偶极子（HD）模式下线圈垂直，或垂直偶极子（VD）模式下线圈水平（图。 2). 它不包括有时用于特殊应用的同轴或不对称线圈方向（Grant和West 1965）。 1.2.3本指南仅限于使用频域仪器，其中感应二次磁场与一次磁场的比率与地面的体积电导率或视电导率成正比（见5.1.4）。直接测量视在地面电导率的仪器通常被称为地面电导率仪（GCM），其设计用于在 “ 低感应数近似。 ” 在低感应数近似范围内外运行的多频仪器提供了二次磁场与一次磁场的比值，可用于计算地面电导率。 1.2.4 FDEM（感应）方法已适用于钻孔、水上或空中的许多特殊用途。本指南不包括对这些调整或方法的讨论。 1.2.5本指南中建议的频域方法是最常用、最广泛接受和证明的方法；然而，如果技术上合理且有文件记录，则可以替代其他鲜为人知或专业化的技术。 1.2.6第5.4节讨论了限制或限制频域方法使用的技术限制和文化干扰。 1.2.7 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、经验和专业判断。并非本指南的所有方面都适用于所有情况。本ASTM标准不代表或取代在不考虑项目的许多独特方面的情况下判断给定专业服务是否充分的谨慎标准。本文件标题中的“标准”一词表示该文件已通过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制定适当的安全和健康实践，并在使用前确定法规的适用性。 1.3.2 本标准指南并非旨在解决与其使用相关的所有安全问题。本标准指南的用户有责任在使用前确定法规的适用性。 图1地面电导率测量中的电磁感应原理（Sheriff，1989） 图2水平和垂直偶极线圈方向的相对响应（McNeill，1980） ====意义和用途====== 概念 : 本指南总结了用于表征地下材料和地质结构的设备、现场程序和解释方法，这些材料和地质结构基于其传导、增强或阻碍由交变电磁场在地面感应的电流的特性。 频域方法需要发射器或能源、发射器线圈、接收器电子设备、接收器线圈和互连电缆（图5）。 当发射器线圈放置在地球表面或其附近并用交流电通电时，会在近地材料中感应与材料电导率成比例的小电流。这些感应交流电产生二次磁场（H s )，由主场（H）感应 p )通过接收器线圈。 在称为 “ 低感应数近似 ” （McNeill，1980）当次表面无磁性时，二次磁场完全消失- 由这些变量的函数给出。 哪里: σ 一 = 视电导率，单位:西门子/米，秒/米， ω = 2. π f（弧度/秒）；f=频率（Hz）， µ o = 亨利/米4的自由空间渗透率 π × 10 – 7. ，/m， s = 线圈间距，单位为米、米和 H s /H p = 由接收器线圈测量的二次磁场与一次磁场的异相分量之比。 也许最重要的约束是发射器产生的电磁波的穿透深度（趋肤深度，见第6.5.3.1节）远大于仪器的线圈间距。 穿透深度与地面电导率和仪器频率成反比。例如，使用垂直偶极子、线圈间距为10 m（33 ft）、频率为6400 Hz的仪器符合地球电导率小于200 mS/m的低感应数假设。 多频域仪器通常测量二次磁场的两个分量:与一次磁场同相的分量和90分量 ° 与主场异相（正交分量）（Kearey和Brook 1991）。通常，仪器既不显示同相分量，也不显示异相分量（正交分量），但显示表观电导率或次级磁场与初级磁场的比值。 当接地条件使得低感应数近似有效时，同相分量远小于正交相位分量。如果存在相对较大的同相分量，则低感应数近似无效，并且可能存在非常导电的埋体或埋层，即矿体或人造金属物体。 发射器和接收器线圈几乎总是在平行于地球表面的平面上对齐（线圈轴垂直），通常称为垂直偶极子（VD）模式，或在垂直于地球表面的平面上对齐（线圈轴水平），通常称为水平偶极子（HD）模式（图。 3). 垂直和水平偶极子方向测量了对地下材料明显不同的响应（图2）。当使用多个线圈间距或适当的频率进行这些垂直和水平偶极子模式测量时，可以将它们结合起来，以解析具有不同电导率和厚度的多个土层。这种FDEM方法通常仅限于2或3层，具有良好的深度和电导率分辨率，并且仅当相对较厚和相对较浅的层之间存在强烈的电导率对比时（就线圈间距而言）。 5.1.4中获得的电导率值称为表观电导率 σ 一 . 对于均匀各向同性地球或半空间（其中不存在分层），两次测量的视电导率相同。由于水平偶极子（HD）比垂直偶极子（VD）对近表面材料更敏感，因此这两个测量可以一起用于判断电导率是否随深度增加或减少。 对于被称为地面电导率仪（GCM）的仪器，5.1.4中的系统参数和常数包括在测量过程中，给出了计算读数 σ 一 ，通常以mS/m为单位。在某些仪器中，二次磁场与一次磁场（H）的同相分量之比 s /H p p )以ppt显示（单位:千分之一）。 对于其他频域仪器，二次磁场同相和正交相位的测量值均以比率形式给出。 对于均匀的水平分层土壤，仪器计算的实测视电导率是通过适当的HD或VD响应函数加权的每层电导率之和（图2）。 当地下不均匀或水平分层时（例如存在地质异常或人造物体），视电导率可能无法代表地下材料的整体电导率。 由于其相对于仪器线圈的方向，一些异常特征可能会产生负的视电导率。虽然该负值作为电导率测量无效，但它表明存在地质异常或埋藏物。 许多常见的地质特征，如断裂带、埋藏通道、岩墙和断层以及人造埋藏物，可以通过相对知名的异常测量特征来检测和识别（图3）。 测量参数和代表值 : FDEM方法提供了地下材料表观电导率的测量。对于地面电导率仪（GCM），直接读取或记录该表观电导率。 对于不使用 “ 低感应数近似 ” 通过二次磁场与一次磁场（H）的比值进行测量 s /H p ). 一些GCM还提供了对应于二次磁场同相分量的同相测量，单位为一次磁场的千分之一（ppt）。同相分量对于矿产勘探、探测埋藏的人造金属物体、测量土壤或岩石磁化率以及验证地下非磁性假设特别有用（McNeill，1983）。 图6显示了典型接地材料的电导率在50年内从0变化。 01 mS/m到几千mS/m。即使是特定的地球材料（图6）的电导率也可能有很大的变化，这与其温度、粒度、孔隙度、孔隙流体饱和度和孔隙流体电导率有关。其中一些变化，例如导电污染物孔隙流体，可以通过FDEM方法检测。 设备 : FDEM设备由发射器电子设备和发射器线圈、接收器电子设备和接收器线圈以及互连电缆组成。通常，这些参数仅在发射器功率、线圈尺寸、线圈间距和发射器频率方面因仪器而异。 一些仪器设计为刚性、固定的线圈间距，通常小于4米（13英尺），用于小于6米（20英尺）的相对较浅测量。 对于高达100米（330英尺）的深度测量，根据仪器的不同，仪器由通过电缆互连的独立线圈组成（图5），通常以几个线圈间距运行。使用 “ 低感应数近似 ” 每个线圈间距通常有一个单一频率，通常称为地面电导率仪（GCM）。表观电导率测量， σ 一 ，以毫西门子/米（mS/m）计算和显示。 在固定的线圈间隔处进行多次频率测量的FDEM仪器通常将其结果表示为次级磁场和初级磁场（H）的比值 s /H p ). 这些仪器通常具有一些满足低感应数近似的频率，从中计算表观电导率。较大的多线圈分离、多频仪器主要用于矿产勘探，而较小的多频仪器用于与GCMs几乎相同的应用。 限制和干扰 : 地球物理方法固有的一般局限性 : 所有地球物理方法固有的一个基本局限性在于，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，需要一些额外的信息，如钻孔数据。 由于地球物理方法的固有局限性，单独的频域或地面电导率测量永远不能被视为对地下条件的完整评估。应注意的是，多种测量地球电导率的方法（即FDEM、TDEM、直流电阻率）只能在非磁性、频率独立、各向同性均匀半空间的理想条件下得出相同的答案。异质性（例如，分层、物体）、各向异性、磁性材料和频率相关机制的存在将导致地面电流的几何模式不同，从而导致不同方法之间测量的视电导率值不同。 与其他信息适当结合，电导率测量可以成为获取地下信息的有效方法。 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。 FDEM方法的特定限制 : 从频域测量中解释地下条件假设为非磁性均匀水平层状地球。这种理想的任何变化都会导致实际地下解释的变化。有些地区的土壤中含有大量铁磁性或超顺磁性矿物或金属碎片，这一假设不再有效。 这可以用电磁仪器进行测试（见5.2.2）。如果假设不正确，则表观电导率将高于其应有值。 使用单一频率和一个线圈间距的地面电导率仪仅限于检测横向变化。通过两个线圈方向（水平和垂直偶极子模式），可以定性解释电导率是否随深度增加或减少。有关地下电导率分层或垂直分布的信息可以从地表以上不同高度的测量中得出。 对于测深，使用两个线圈方向和多个线圈间分离，只能合理解释两层或三层。 各层和各层厚度之间仍必须存在显著的导电性差异。 当多个分层模型拟合数据时，会出现等效问题，因为层电导率和厚度的组合会产生相同的测深响应。例如，一个薄的高导电层看起来很像一个较厚、导电性较差的层，其导电性厚度乘积大致相同。这些问题有时可以通过使用钻孔电导率或电阻率数据、了解该地区的一般地质情况或了解正在寻找的内容和预期的响应来解决。在电阻介质中搜索导电层时，FDEM系统提供最佳结果。 即使导电介质中的电阻薄层具有显著的电反差，也很难解析。 频域仪器最好在相对较高的电导率条件下（大于1ms/m）使用。对于低电导率材料（小于1 mS/m），使用电阻率法可以更好地获得有用的测量结果（指南 D6431 ). 地面电导率仪（GCM）在均匀半空间的真实体积电导率和仪器读取的表观电导率之间具有直线（线性）关系，前提是真实电导率在特定仪器物理参数的低感应数近似值控制的区域内- 线圈间距和频率。随着半空间电导率的增加，使得近似值越来越不有效，由GCM测量或使用低感应数近似值（5.1.4）计算的表观电导率越来越偏离真实地面电导率。图7显示了在13 kHz下工作的一米（3.3英尺）短线圈间隔仪器的非线性，并表明，对于这种间隔，大多数接地材料的响应非线性不是问题。 然而，对于具有较大的线圈间距（大于20 m[66 ft]）和相对较高的操作频率的仪器，线性偏差可能非常显著。 在这里，非线性可以从相对较低的电导率值开始，并且可以在较高的真实电导率值下产生负值（图8）。 自然和文化噪声源（干扰） : 此处提及的噪声源不包括物理性质的噪声源，如困难地形或人为障碍物，而是对测量和解释产生不利影响的地质、环境或文化性质的噪声源。 在许多情况下，项目的目标决定了什么是噪音。如果调查试图描述地质条件，则由于埋地管道和人为因素引起的响应- 人造物体被视为噪声。然而，如果调查试图定位此类物体，则由于地质条件不同而导致的测量变化将被视为噪声。一般来说，噪声是指测量值中的任何变化，不归因于调查对象。 自然噪声源 — FDEM测量中的主要自然噪声源是自然发生的大气电（spherics）。这种干扰是由太阳活动或电风暴引起的。有关太阳活动的信息可以在国家海洋和大气管理局网站的互联网上获得(http://www.noaa.gov). 许多英里外的电风暴仍然会导致测量结果发生很大变化。 当这些条件存在时，最好放弃调查，直到更好的时候。增加发射器功率可以显著降低球形效应。这会增加二次场强度，从而提高信噪比。不幸的是，这样的过程是以更大更重的发射器线圈为代价的。 文化噪声源 — 文化噪声源包括来自电力线路、通信设备、附近建筑物、金属围栏、地表或近地表金属、管道、地下储罐、垃圾填埋场和导电渗滤液的干扰。来自电力线的干扰与线圈间距成正比，主要只影响较大的线圈间距（大于15或20 m[50或66 ft]）。 具有较小线圈间距的频域仪器通常不受影响。 不应在可通过频域检测的建筑物、金属围栏或埋地金属管道附近进行测量，除非测量的对象是埋地管道。有时很难预测与潜在噪声源的适当距离。现场测量可以快速确定问题的严重程度，调查设计应包含这些信息（见6.3.2.2）。 替代方法 — 在某些情况下，上述因素可能会妨碍FDEM方法的有效使用。 其他地表地球物理（见指南 D6429 )或可能需要非地球物理方法来调查地下条件。替代方法，如直流电阻率（指南 D6431 )或TDEM，其可能不受影响频域方法的特定干扰源的影响，可用于显示电对比度。 图5频域电磁仪器示意图 图6土壤材料电导率范围（Sheriff，1991） 图7短间距仪器的非线性 图8长间距仪器的非线性

1.1 Purpose and Application : 1.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface conditions using the frequency domain electromagnetic (FDEM) method. 1.1.2 FDEM measurements as described in this standard guide are applicable to mapping subsurface conditions for geologic, geotechnical, hydrologic, environmental, agricultural, archaeological and forensic investigations as well as mineral exploration. 1.1.3 The FDEM method is sometimes used to map such diverse geologic conditions as depth to bedrock, fractures and fault zones, voids and sinkholes, soil and rock properties, and saline intrusion as well as man-induced environmental conditions including buried drums, underground storage tanks (USTs), landfill boundaries and conductive groundwater contamination. 1.1.4 The FDEM method utilizes the secondary magnetic field induced in the earth by a time-varying primary magnetic field to explore the subsurface. It measures the amplitude and phase of the induced field at various frequencies. FDEM measurements therefore are dependent on the electrical properties of the subsurface soil and rock or buried man-made objects as well as the orientation of any subsurface geological features or man-made objects. In many cases, the FDEM measurements can be used to identify the subsurface structure or object. This method is used only when it is expected that the subsurface soil or rock, man-made materials or geologic structure can be characterized by differences in electrical conductivity. 1.1.5 The FDEM method may be used instead of the Direct Current Resistivity method (Guide D6431 ) when surface soils are excessively insulating (for example, dry or frozen) or a layer of asphalt or plastic or other logistical constraints prevent electrode to soil contact. 1.2 Limitations : 1.2.1 This standard guide provides an overview of the FDEM method using coplanar coils at or near ground level and has been referred to by other names including Slingram, HLEM (horizontal loop electromagnetic) and Ground Conductivity methods. This guide does not address the details of the electromagnetic theory, field procedures or interpretation of the data. References are included that cover these aspects in greater detail and are considered an essential part of this guide (Grant and West, 1965; Wait, 1982; Kearey and Brook, 1991; Milsom, 1996; Ward, 1990). It is recommended that the user of the FDEM method review the relevant material pertaining to their particular application. ASTM standards that should also be consulted include Guide D420 , Terminology D653 , Guide D5730 , Guide D5753 , Practice D6235 , Guide D6429 , and Guide D6431 . 1.2.2 This guide is limited to frequency domain instruments using a coplanar orientation of the transmitting and receiving coils in either the horizontal dipole (HD) mode with coils vertical, or the vertical dipole (VD) mode with coils horizontal (Fig. 2). It does not include coaxial or asymmetrical coil orientations, which are sometimes used for special applications (Grant and West 1965). 1.2.3 This guide is limited to the use of frequency domain instruments in which the ratio of the induced secondary magnetic field to the primary magnetic field is directly proportional to the ground's bulk or apparent conductivity (see 5.1.4). Instruments that give a direct measurement of the apparent ground conductivity are commonly referred to as Ground Conductivity Meters (GCMs) that are designed to operate within the “ low induction number approximation. ” Multi-frequency instruments operating within and outside the low induction number approximation provide the ratio of the secondary to primary magnetic field, which can be used to calculate the ground conductivity. 1.2.4 The FDEM (inductive) method has been adapted for a number of special uses within a borehole, on water, or airborne. Discussions of these adaptations or methods are not included in this guide. 1.2.5 The approaches suggested in this guide for the frequency domain method are the most commonly used, widely accepted and proven; however other lesser-known or specialized techniques may be substituted if technically sound and documented. 1.2.6 Technical limitations and cultural interferences that restrict or limit the use of the frequency domain method are discussed in section 5.4. 1.2.7 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 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 without consideration of a project's many unique aspects. The word standard in the title of this document means that the document has been approved through the ASTM consensus process. 1.3 Precautions : 1.3.1 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 regulations prior to use. 1.3.2 This standard guide does not purport to address all of the safety concerns that may be associated with its use. It is the responsibility of the user of this standard guide to determine the applicability of regulations prior to use. FIG. 1 Principles of Electromagnetic Induction in Ground Conductivity Measurements (Sheriff, 1989) FIG. 2 Relative Response of Horizontal and Vertical Dipole Coil Orientations (McNeill, 1980) ====== Significance And Use ====== Concepts : This guide summarizes the equipment, field procedures and interpretation methods used for the characterization of subsurface materials and geological structure as based on their properties to conduct, enhance or obstruct the flow of electrical currents as induced in the ground by an alternating electromagnetic field. The frequency domain method requires a transmitter or energy source, a transmitter coil, receiver electronics, a receiver coil, and interconnect cables (Fig. 5). The transmitter coil, when placed on or near the earth's surface and energized with an alternating current, induces small currents in the near earth material proportional to the conductivity of the material. These induced alternating currents generate a secondary magnetic field (H s ), which is sensed with the primary field (H p ) by the receiver coil. Under a constraint known as the “ low induction number approximation ” (McNeill, 1980) and when the subsurface is nonmagnetic, the secondary magnetic field is fully out-of-phase with the primary field and is given by a function of these variables. where: σ a = apparent conductivity in siemens/meter, S/m, ω = 2 π f in radians/sec; f = frequency in Hz, µ o = permeability of free space in henrys/meter 4 π × 10 – 7 , /m, s = intercoil spacing in meters, m, and H s /H p = the ratio of the out-of-phase component of the secondary magnetic field to the primary magnetic field, both measured by the receiver coil. Perhaps the most important constraint is that the depth of penetration (skin depth, see section 6.5.3.1) of the electromagnetic wave generated by the transmitter be much greater than the intercoil spacing of the instrument. The depth of penetration is inversely proportional to the ground conductivity and instrument frequency. For example, an instrument with an intercoil spacing of 10 m (33 ft) and a frequency of 6400 Hz, using the vertical dipole, meets the low induction number assumption for earth conductivities less than 200 mS/m. Multi-frequency domain instruments usually measure the two components of the secondary magnetic field: a component in-phase with the primary field and a component 90 ° out-of-phase (quadrature component) with the primary field (Kearey and Brook 1991). Generally, instruments do not display either the in-phase or out-of-phase (quadrature) components but do show either the apparent conductivity or the ratio of the secondary to primary magnetic fields. When ground conditions are such that the low induction number approximation is valid, the in-phase component is much less than the quadrature phase component. If there is a relatively large in-phase component, the low induction number approximation is not valid and there is likely a very conductive buried body or layer, that is, ore body or man-made metal object. The transmitter and receiver coils are almost always aligned in a plane either parallel to the earth's surface (axis of the coils vertical) and generally called the vertical dipole (VD) mode or aligned in a plane perpendicular to the earth surface (axis of the coils horizontal) generally called the horizontal dipole (HD) mode (Fig. 3). The vertical and horizontal dipole orientations measure distinctly different responses to the subsurface material (Fig. 2). When these vertical and horizontal dipole mode measurements are made with several intercoil spacings or appropriate frequencies, they can be combined to resolve multiple earth layers of varying conductivities and thicknesses. This FDEM method is generally limited to only 2 or 3 layers with good resolution of depth and conductivity and only if there is a strong conductivity contrast between layers that are relatively thick and relatively shallow (in terms of the intercoil spacing). The conductivity value obtained in 5.1.4 is referred to as the apparent conductivity σ a . For a homogeneous and isotropic earth or half space (in which no layering is present), the apparent conductivity will be the same for both the measurements. Since the horizontal dipole (HD) is more sensitive to the near surface material than the vertical dipole (VD), these two measurements can be used together to tell whether the conductivity is increasing or decreasing with depth. For instruments referred to as Ground Conductivity Meters (GCMs), the system parameters and constants in 5.1.4 are included in the measurement process, giving a calculated reading of σ a , usually in mS/m. In some instruments, the ratio of the in-phase components of the secondary to primary magnetic fields (H s /H p p ) is displayed in ppt (parts per thousand). For other frequency domain instruments, the measurements for both the in-phase and quadrature phase of the secondary magnetic field are given as ratios. For a homogeneous horizontally layered earth, the measured apparent conductivity calculated by the instrument is the sum of each layer's conductivity weighted by the appropriate HD or VD response function (Fig. 2). When the subsurface is not homogeneous or horizontally layered (such as when there is a geologic anomaly or man-made object present), the apparent conductivity may not be representative of the bulk conductivity of the subsurface material. Some anomalous features can, because of their orientation relative to the instrument coils, produce a negative apparent conductivity. While this negative value is not valid as a conductivity measurement, it is an indication of the presence of a geologic anomaly or buried object. Many common geologic features such as fracture zones, buried channels, dikes and faults, and man-made buried objects, can be detected and identified by relatively well-known anomalous survey signatures (Fig. 3). Parameters Measured and Representative Values : The FDEM method provides a measure of the apparent electrical conductivity of the subsurface materials. For ground conductivity meters (GCMs), this apparent conductivity is read or recorded directly. For instruments not using the “ low induction number approximation ” the measurement is given by the ratio of the secondary magnetic field to the primary magnetic field (H s /H p ). Some GCMs also give an in-phase measurement corresponding to the in-phase component of the secondary magnetic field in parts per thousand (ppt) of the primary field. The in-phase component is especially useful for mineral exploration, detecting buried man-made metallic objects, or for measuring the soil or rock magnetic susceptibility and verifying the assumption that the subsurface is nonmagnetic (McNeill, 1983). Fig. 6 shows the electrical conductivities for typical earth materials varying over five decades from 0.01 mS/m to a few thousand mS/m. Even a specific earth material (Fig. 6) can have a large variation in conductivity, which is related to its temperature, particle size, porosity, pore fluid saturation, and pore fluid conductivity. Some of these variations, such as a conductive contaminant pore fluid, may be detected by the FDEM method. Equipment : The FDEM equipment consists of a transmitter electronics and transmitter coil, a receiver electronics and receiver coil, and interconnect cables. Generally these vary only from one instrument to another in transmitter power, coil size, intercoil separation and transmitter frequency. Some instruments are designed with a rigid, fixed intercoil separation usually less than about 4 meters (13 ft) and are used for relatively shallow measurements of less than 6 meters (20 ft). For deeper measurements of up to 100 meters (330 ft), depending on the instrument, the instrument consists of separate coils interconnected by cable, (Fig. 5) and generally operates at several intercoil spacings. Instruments using the “ low induction number approximation ” usually have a single frequency for each intercoil spacing and are generally referred to as Ground Conductivity Meters (GCMs). Measurements of apparent conductivity, σ a , are calculated and displayed in millisiemens per meter (mS/m). FDEM instruments taking multiple frequency measurements at a fixed intercoil separation usually give their results as a ratio of the secondary to primary magnetic fields (H s /H p ). These instruments usually have some frequencies that satisfy the low induction number approximation from which the apparent conductivity is calculated. The larger multiple coil separation, multiple frequency instruments are mainly used for mineral exploration, whereas the smaller multiple frequency instruments are used for much the same applications as the GCMs. Limitations and Interferences : General Limitations Inherent to Geophysical Methods : A fundamental limitation inherent to all geophysical methods lies in the fact 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 some additional information, such as borehole data, is required. Because of this inherent limitation in geophysical methods, a frequency domain or ground conductivity survey alone can never be considered a complete assessment of subsurface conditions. It should be noted that multiple methods of measuring electrical conductivity in the earth (that is, FDEM, TDEM, DC Resistivity) will only produce the same answers for the ideal conditions of a nonmagnetic, frequency-independent, isotropic homogeneous half-space. The presence of heterogeneities (for example, layering, objects), anisotropy, magnetic materials, and frequency dependent mechanisms will result in varying geometric patterns of electrical current flow in the ground and consequent different values of measured apparent conductivity between the methods. Properly integrated with other information, conductivity surveying can be an effective method of obtaining subsurface information. In addition, all surface geophysical methods are inherently limited by decreasing resolution with depth. Limitations Specific to the FDEM Method : The interpretation of subsurface conditions from frequency domain measurements assumes a nonmagnetic homogeneous horizontally layered earth. Any variation from this ideal results in variations in the interpretation from the actual subsurface. There are areas with soils that contain significant quantities of ferromagnetic or superparamagnetic minerals or metal fragments in which this assumption is no longer valid. This can be tested with electromagnetic instruments (see 5.2.2). If the assumption is incorrect, then the apparent conductivity will be higher than it should be. Ground conductivity meters using a single frequency and one intercoil spacing are limited to detecting lateral variations. With two coil orientations, (horizontal and vertical dipole modes), a qualitative interpretation of whether the conductivity is increasing or decreasing with depth is available. Information as to the layering or vertical distribution of the subsurface conductivity can be derived from measurements at different heights above the surface. For soundings, using both coil orientations and multiple intercoil separations, only two or three layers can be reasonably interpreted. There must still be a significant conductivity contrast between layers and layer thicknesses. Equivalence problems occur when more than one layered model fits the data because combinations of layer conductivities and thicknesses produce the same sounding responses. For example, a thin highly conductive layer will look much like a thicker, less conductive layer of approximately the same conductivity thickness product. These problems are sometimes resolved by using borehole conductivity or resistivity data, knowing the general geology of the area, or by knowing what is being looked for and what response is expected. FDEM systems give the best results when searching for a conductive layer in a resistive medium. It is difficult to resolve resistive thin layers in a conductive medium even if the layers have a significant electrical contrast. Frequency domain instruments are best used under relatively high electrical conductivity conditions (greater than 1 mS/m). For low conductivity materials (less than 1 mS/m), useful measurements are better obtained with resistivity methods (Guide D6431 ). Ground conductivity meters (GCMs) have a straight-line (linear) relationship between the true bulk conductivity of a homogeneous half space and the apparent conductivity read by the instrument, provided that the true conductivity is within the region controlled by the low induction number approximation for the physical parameters of the particular instrument-intercoil separation and frequency. As the conductivity of the half space increases, making the approximation less and less valid, the apparent conductivity measured by the GCM or calculated using the low induction number approximation (5.1.4) deviates more and more from the true ground conductivity. Fig. 7 shows this nonlinearity for a short one-meter (3.3 ft) intercoil spaced instrument operating at 13 kHz, and shows that, for this spacing, nonlinearity of response is not a problem for most earth materials. The deviation from linearity, however, can be quite significant for instruments with large intercoil spacings (upwards of 20 m [66 ft]) and relatively high frequency of operation. Here the nonlinearity can start at relatively low values of conductivity and can result in negative values at high values of the true conductivity (Fig. 8). Natural and Cultural Sources of Noise (Interferences) : Sources of noise referred to here do not include those of a physical nature such as difficult terrain or man-made obstructions but rather those of a geologic, ambient, or cultural nature that adversely affect the measurements and hence the interpretation. The project's objectives in many cases determine what is characterized as noise. If the survey is attempting to characterize geologic conditions, responses due to buried pipelines and man-made objects are considered noise. However, if the survey were attempting to locate such objects, variations in the measurements due to varying geologic conditions would be considered noise. In general, noise is any variation in the measured values not attributable to the object of the survey. Natural Sources of Noise — The major natural source of noise in FDEM measurements is naturally occurring atmospheric electricity (spherics). This interference is caused by solar activity or electrical storms. Information about solar activity can be obtained on the Internet at the National Oceanic and Atmospheric Administration web site (http://www.noaa.gov). Electrical storms many miles away can still cause large variations in measurements. When these conditions exist, it is best to abandon the survey until a better time. Increasing the transmitter power can significantly reduce the effect of spherics. This increases the secondary field strength and hence the signal to noise ratio. Unfortunately such a process is at the expense of a larger and heavier transmitter coil. Cultural Sources of Noise — Cultural sources of noise include interference from electrical power lines, communications equipment, nearby buildings, metal fences, surface or near surface metal, pipes, underground storage tanks, landfills and conductive leachates. Interference from power lines is directly proportional to the intercoil spacing and mainly only affects large intercoil spacings (greater than 15 or 20 m [50 or 66 ft]). Frequency domain instruments with small intercoil spacings are generally unaffected. Surveys should not be made in close proximity to buildings, metal fences or buried metal pipelines that can be detected by frequency domain, unless detection of the buried pipeline, for example, is the object of the survey. It is sometimes difficult to predict the appropriate distance from potential noise sources. Measurements made on-site can quickly identify the magnitude of the problem and the survey design should incorporate this information (see 6.3.2.2). Alternate Methods — In some instances, the preceding factors may prevent the effective use of the FDEM method. Other surface geophysical (see Guide D6429 ) or non-geophysical methods may be required to investigate the subsurface conditions. Alternate methods, such as DC Resistivity (Guide D6431 ) or TDEM, which may not be affected by the specific source of interference affecting the frequency domain method may be used to show an electrical contrast. FIG. 5 Schematic of Frequency Domain Electromagnetic Instrument FIG. 6 Earth Material Conductivity Ranges (Sheriff, 1991) FIG. 7 Non-linearity for a Short-spaced Instrument FIG. 8 Non-linearity for a Long-spaced Instrument