Geoscience Reference
In-Depth Information
Input
signal
Output
signal
+
-
+
-
Transmit coil
Receive coil
Primary
signal
Time decay channels
+
+
Turn-on
(-)
Turn-off
Time
Time
Turn-on (+)
Turn-off
Turn-off
-
-
(a) Time Domain Input-output
+
Time
Time
-
+
Time
Time
-
(a) Frequency Domain Input-output Signals
fIGURe 6.6
The time and frequency domains transmit and receive signals: (a) time domain transmits
(input) and receives (output) signals, and (b) frequency domain transmits and receives signals.
Frequency domain systems utilize an input signal that is a sinusoidal wave with a fixed frequency
(a square wave time domain signal is composed of many frequencies), as shown in Figure 6.6b.
The signal recorded at the receive coil is delayed (called a phase shift) in time with a decreased
amplitude. The amplitude and phase of the received signals are used to interpret the propagation
path and induction of the signals that have traveled from the transmit coil, through the earth, to the
receive coil. Some frequency domain systems transmit and receive several different frequencies
in sequence, as illustrated by the low-frequency and high-frequency signals shown in the figure.
Lower-frequency signals penetrate more deeply into the earth than higher-frequency signals; hence,
the depth of investigation can be somewhat predicted by the chosen frequency. Typical frequencies
range from a few hundred Hz for intermediate-depth investigations (approximately 10 to 100 m) to
several hundred kilohertz for very shallow soil conductivity determinations.
So far, we only illustrated the use of coils for transmitting and receiving EM fields. However,
in many geophysical applications, EM signals are transmitted and received by straight wires (com-
monly called lines) or by loops (or coils) of wire. Most lines have electrodes in the ground at each
end of the line, as shown in Figure 6.7. If the grounded ends are very close together with respect
