Geology Reference
In-Depth Information
the 'imaginary' square root of 1 (conventionally indicated by either 'i' or
'j', according to taste). The in-phase and quadrature components are then
described as being respectively real and imaginary . A knob labelled 'I' on
an EM instrument may therefore control either the 'In-phase' or the 'Imag-
inary' signal, according to the manufacturer's whim. Yet another reason for
reading the handbook.
Since electromagnetic waves travel through air at the speed of light and not
instantaneously, phase alters with distance from the transmitter. The small
distances between transmitters and receivers in most geophysical surveys
ensure that these shifts are negligible and can be ignored.
5.2.4 Transients
As an alternative to sinusoidal signals, currents circulating in transmitter
coils or wires can be terminated abruptly. Transient electromagnetic ( TEM )
methods are effectively multi-frequency, because a square wave contains
elements of all the odd harmonics of the fundamental up to theoretically
infinite frequency (Figure 5.4). Most of the many advantages of TEM over
continuous wave (CW) methods derive from the fact that TEM measure-
ments are made after the termination of the primary current. There is thus
Figure 5.4 The square wave (A) as a multi-frequency sinusoid. A good
approximation (B) to the wave can be obtained by summing the first five
odd harmonics (integer multiples 3, 5, 7, 9 and 11) and the fundamental
frequency (1). The amplitudes required for each of these component waves
can be determined using the techniques of Fourier analysis. The addition of
higher odd harmonics with appropriate amplitudes would further improve
the approximation.
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