Standard Guide for Using the Frequency Domain Electromagnetic Method for Subsurface Site Investigations

Importancia y uso:

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.

Subcomité:

D18.01

Referida por:

D6820-20, D5092_D5092M-16R24, D6429-23

Volúmen:

04.09

Número ICS:

93.020 (Earthworks. Excavations. Foundation construction. Underground works)

Palabras clave:

electromagnetics; frequency domain electromagnetics; geophysics; ground conductivity; surface geophysics;