Geology Reference
In-Depth Information
The fields produced by fixed sources can be observed at distances so large
that the effects of the ionosphere on signal propagation become significant,
and the Biot-Savart Law no longer applies. These
Controlled-Source Audio-
Magnetotelluric
(CSAMT) methods are discussed in Chapter 9.
8.4 Transient Electromagnetics
Transient electromagnetic (TEM) systems provide multi-frequency data by
repeatedly sampling the transient magnetic fields that persist after a transmit-
ter current has been terminated. A modified square wave of the type shown in
Figure 8.14 flows in the transmitter circuits, and transients are induced in the
ground on both the upgoing and downgoing
ramps
. Only currents induced
during the downgoing ramps are used, since only they can be observed in the
absence of the primary field. Ideally, the up-ramp transients should be small
and decay quickly, and the up-ramp is often
tapered
to reduce induction.
In contrast, on the down-ramp the current flow is terminated as quickly as
Tx current
Inducing e.m.f
Induced current
(poor conductor)
Induced current
(good conductor)
Usable
measurement
interval
Figure 8.14
Transient electromagnetic (TEM) transmitted and received sig-
nals.
