Equipment

There are many implementations of EM methods that have been developed for geophysical applications over the years. In addition to the type and orientation of transmitter and receiver, EM methods may be classified in a fundamental way as those that use an artificial source of EM energy, and those that use the earth's natural EM field (telluric, magnetotelluric [MT], audio magnetotelluric [AMT] methods). Artificial source methods are often classified by the type of source and detector (ground electric field line, or magnetic field coils or magnetometers), and the nature of the transmitter and receiver (e.g., long line or dipole electric field, small loop or large loop). Artificial source methods may be further subdivided by the type of wave that is transmitted and received: a system that transmits and receives a wave of a single frequency is called a frequency domain system, and a system that transmits and receives a multiple-frequency pulse of energy is called a time-domain system. Artificial and natural systems are further classified by the range of frequencies (e.g., low frequency, very low frequency [VLF], extremely low frequency [ELF], audio frequency, etc.), and the characteristics of the EM field vector that is measured (e.g., ellipticity, tilt, amplitude, or phase). To further complicate things, methods have come to be known by their commercial names, and EM techniques have been developed for use on the ground, in the air, and in boreholes.

The EM field at any point in space is a vector, with a magnitude and a direction. The magnitude of the field at a given point in space is a function of the orientation of the transmitted field, the modification of the direction of the field by materials between the transmitter and receiver (the object of the measurements), and the direction of the field measured by the receiver (receiver orientation). EM measurement instruments and field surveys are designed to exploit the vector nature of the EM field, and to measure attributes of the EM field that indicate the size, depth, and orientation of the objects in the subsurface. The attributes measured relate to either the spatial or time relationships of the measured secondary EM field. The field attributes include the amplitude, time delay (phase), and orientation of the received field with respect to the primary transmitted field. Specifically, the following parameters can be measured: (1) the phase of the spatial components with respect to the source, (2) the orientation of the field (tilt, and the axis of the field ellipsoid), (3) the relative amplitudes of measurements at different frequencies, (4) the amplitude ratio and phase differences between different spatial components, or (5) the phase and amplitude of individual spatial components with respect to the source. Methods are designed to "normalize" the effect of the primary field. The separation of most transmitter-receiver pairs is fixed to eliminate geometric effects from the primary field caused by varying the relative positions of the transmitter and receiver.

If the transmitter and receiver are located above the surface of the ground, then the field measured at the receiver is a combination of the field that propagates directly through the air (the primary field), the EM field that is influenced by the background material in the subsurface (sometimes referred to as the terrain), and the induced secondary field from the object of a contrasting conductivity located in the subsurface. The field that propagates directly from the transmitter is called the primary field, and the field caused by the eddy currents induced on the subsurface terrain and any buried object in the subsurface is called the secondary field, as shown in Figure 6.4a. The direction (sign) of the secondary field induced by the conductive object in the subsurface is opposite to the primary field, in accordance with Lenz's law.

The secondary field is also "shifted" in time, and this shift is called a phase shift. The terminology describing the phase of a wave is not discussed very clearly in most of the literature. However, popular usage of the term "phase" in describing induction EM methods usually refers to the fact that the secondary field has a certain phase relationship to the primary field. The phase shift is

Transmit loop

Handbook of Agricultural Geophysics Receive loop

Primary signal

Primary signal

Object causing secondary field

Object causing secondary field

(a) Primary field from transmit loop, and secondary field caused by conductive object

Secondary signal

Secondary eddy currents eddy currents

(a) Primary field from transmit loop, and secondary field caused by conductive object

Primary field

Secondary field

Primary field

Secondary field

Phase

(a) Primary and secondary fields arriving at the receive antenna

Phase

(a) Primary and secondary fields arriving at the receive antenna

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