Electromagnetic
Electromagnetic methods detect anomalies in the inductive properties of the earth`s subsurface rocks. An alternating voltage is introduced into the earth by induction from transmitting coils either on the surface or in the air, and the amplitude and phase shift of the induced potential generated in the subsurface are measured by detecting coils and recorded. Metal based can often be detected by this technique.
Electromagnetic (EM) surveys provide an accurate and cost effective means of characterizing subsurface conditions at a site. A number of EM methods are available for use, each with its own advantages and limitations.
In electromagnetic (EM) surveying, the electrical conductivity of the ground is measured as a function of depth and/or horizontal distance. Different rocks, buried structures and objects exhibit different values of electrical conductivity. Mapping variations in electrical conductivity can identify anomalous areas worthy of further geophysical or intrusive investigation.
Electromagnetic methods fall in two categories, frequency domain and time domain.
In the frequency domain method, the transmitter emits a sinusoidally varying current at a specific frequency. Because the mutual inductance between the transmitter and conductor is a complex quantity, the electromagnetic force induced in the conductor will be shifted in phase with respect to the primary field. At the receiver, the secondary field generated by the currents in the conductor will also be shifted in phase by the same amount. Frequency domain measures the amplitude and phase of an induced electromagnetic field.
Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. EM surveys will measure variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal).
The term "metal detector" (MD) generally refers to some type of electromagnetic induction instrument, although traditional magnetometers are often used to find buried metal. The disadvantage of magnetometers is that they can be used only for locating ferrous metals. MD instruments in geotechnical and hazardous-waste site investigations have several uses.
The main advantages of MD instruments are that both ferrous and nonferrous metals may be detected; the surface area of the target is more important that its mass; and surveys are rapid, detailed, and inexpensive. The main disadvantages are that the depth of investigation is very limited with most instruments, and metallic litter and urban noise can severely disrupt MD at some sites.
This method has a relatively restricted applicability, resuming itself to tracking buried metallic bodies or lines. Its principle is based on the response of metallic bodies when radio waves are deployed. The investigation depth is of a few meters, the method being very good in surveying for cables, pipes, drums and other buried metal objects.
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The VLF method uses powerful remote radio transmitters set up in different parts of the world for military communications (Klein and Lajoie, 1980). In radio communications terminology, VLF means very low frequency, about 15 to 25 kHz. Relative to frequencies generally used in geophysical exploration, these are actually very high frequencies. The radiated field from a remote VLF transmitter, propagating over a uniform or horizontally layered earth and measured on the earth's surface, consists of a vertical electric field component and a horizontal magnetic field component each perpendicular to the direction of propagation.
The VLF method uses relatively simple instruments and can be a useful reconnaissance tool. Potential targets include tabular conductors in a resistive host rock such as faults in limestone or igneous terrain. The depth of exploration is limited to about 60% to 70% of the skin depth of the surrounding rock or soil. Therefore, the high frequency of the VLF transmitters means that in more conductive environments, the exploration depth is quite shallow; for example, the depth of exploration might be 10 to 12 m in 25-Ωm material. Additionally, the presence of conductive overburden seriously suppresses response from basement conductors, and relatively small variations in overburden conductivity or thickness can themselves generate significant VLF anomalies. For this reason, VLF is more effective in areas where the host rock is resistive and the overburden is thin.
Ground-penetrating radar (GPR) uses a high-frequency (80 to 1,500 MHz) EM pulse transmitted from a radar antenna to probe the earth. The transmitted radar pulses are reflected from various interfaces within the ground, and this return is detected by the radar receiver. Reflecting interfaces may be soil horizons, the groundwater surface, soil/rock interfaces, man-made objects, or any other interface possessing a contrast in dielectric properties. The dielectric properties of materials correlate with many of the mechanical and geologic parameters of materials.
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The radar signal is imparted to the ground by an antenna that is in close proximity to the ground. The reflected signals can be detected by the transmitting antenna or by a second, separate receiving antenna. The received signals are processed and displayed on a graphic recorder. As the antenna (or antenna pair) is moved along the surface, the graphic recorder displays results in a cross-section record or radar image of the earth. As GPR has short wavelengths in most earth materials, resolution of interfaces and discrete objects is very good. However, the attenuation of the signals in earth materials is high, and depths of penetration seldom exceed 10 m. Water and clay soils increase the attenuation, decreasing penetration.
The objective of GPR surveys is to map near-surface interfaces. For many surveys, the location of objects such as tanks or pipes in the subsurface is the objective. Dielectric properties of materials are not measured directly. The method is most useful for detecting changes in the geometry of subsurface interfaces.
Geologic and geophysical objectives determine the specific field parameters and techniques. Delineation of the objectives and the envelope of acceptable parameters are specified in advance. However, as the results cannot be foreseen from the office, considerable latitude is given to the field geophysicist to incorporate changes in methods and techniques.
