5.1 Concepts: 

5.1.1 This guide summarizes the equipment, field procedures and interpretation methods used for the investigation of subsurface materials and geological structure as based on their properties to conduct, enhance or obstruct the flow of electrical currents induced in the ground by an alternating electromagnetic field.

5.1.2 The frequency domain method requires a transmitter or energy source, a transmitter coil, receiver electronics, a receiver coil, and interconnect cables (Fig. 5).

5.4.3 Natural and Cultural Sources of Noise (Interferences): 

5.4.3.1 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.

5.4.3.2 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 is 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.

5.4.3.3 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.

5.4.3.4 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, and 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). Frequency domain instruments with small intercoil spacings are generally unaffected.

5.4.3.5 Surveys should not be made in proximity to buildings, metal fences, or buried metal pipelines that can be detected by the instrument, 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 interference. The survey design should incorporate this information (see 6.3.2.2).

5.4.4 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.

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 site 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 instruments typically measure 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, the in-phase response is more sensitive to metallic items (either above or below the ground surface) while the quadrature response is more sensitive to geologic variations in the subsurface. However, both components are, to some degree, affected by both metallic and geologic features. 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 investigated 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 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) and 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 (for example, through the use of drones). 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 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this test method.

1.4 Precautions: 

1.4.1 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.

1.5 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.