(a) Parameters associated with ellipsoidal representation of the amplitude and vertical orientation of the field

FIGURE 6.5 The vector nature of the electromagnetic field: (a) the primary magnetic field vector, the secondary field vector, and their sum; and (b) the ellipsoid that represents the direction, amplitude, and tilt of the measured field.

If the amplitude and orientation of the field are measured, then the field can be viewed as an ellipsoid of revolution, as shown in Figure 6.5b. The parameters measured at the receiver are (a) the maximum amplitude corresponding to the major axis of the ellipsoid; (b) the minimum amplitude, which corresponds to the minor axis of the ellipsoid; and (c) the tilt angle of the major axis with respect to the horizontal ground surface. In some cases, the azimuthal (x-y position) orientation of the major axis may also be measured.

Field instruments are designed to separate (or normalize) the primary and secondary EM fields, because the secondary field is most important for detecting an object in the subsurface. In addition to the primary field and secondary field, some systems utilize the in-phase and out-of-phase components, or the real and imaginary components. The out-of-phase component is also sometimes referred to as the quadrature component. All of these definitions are related to the complex nature (oscillating, time-varying) of the EM field when it is referenced to the primary field.

One distinguishing characteristic of various EM methods is their operating domain and the frequency, or time period, of the signal transmitted and received. The operating domain refers to time or frequency domain designation, which in theory should provide equivalent results because the two domains are mathematically related by the Fourier Transform. The principles of the time and frequency domains are illustrated and contrasted in Figure 6.6. A time domain input signal is generally a square wave with a positive and negative polarity. The input signal is typically a few hundred milliseconds long, and may be as long as a second or more; the time period depends upon the application, with longer times used to investigate deeper into the earth. As shown in Figure 6.6a, the received signal is no longer a square wave. After the input signal is turned off, there is a decay and delay of the signal over time. This shape of the decay curve is a function of changes in the signal as it travels into the earth and encounters objects with different electrical conductivity values. The received signal (output decay) is recorded on a separate coil and the signal amplitude measured at different delay times. These amplitude decay signals are interpreted as a function of the subsurface distribution of electrical conductivity.

Transmit coil

Transmit coil

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