Geology Reference
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However, such slow progress would be acceptable only in exceptional
circumstances.
9.2 Controlled-Source Audio-Magnetotellurics (CSAMT)
The problems associated with the use of natural fields have led to the devel-
opment of methods whereby similar but more stable signals are generated
artificially. One application of controlled sources has been to 'fill-in' the
dead-band gaps in the natural spectrum (Figure 9.1), which can be critical
in some applications.
9.2.1 CSAMT principles
The source for a CSAMT survey is usually a long (2-km or more) grounded
wire in which current is 'swept' through a range of frequencies, which may
extend from as little as 0.1 Hz to as much as 100 kHz. Such signals are
now routinely available from multi-purpose transmitters that can equally
well be used for CWEM and TEM surveys, and often also for frequency-
domain and time-domain IP. High powers are needed, because CSAMT
measurements are made at greater distances from the sources than in other
types of electrical surveys. Close to a source, the Biot-Savart equations
(see Figure 8.12) apply, but transmission effects dominate at the far-field
distances of, generally, a few kilometres and the wavefront in the subsurface
can then be treated as planar and horizontal. Because the source direction
is known, only the horizontal electrical field parallel to the source wire ( E x )
and the horizontal magnetic field at right angles to it ( H y )needbemeasured
to determine the Cagniard resistivity (see Section 9.1.2). However, the tradi-
tional AMT orthogonal detector arrays are needed if, as recommended with
the Geometrics EH4 Stratagem, a dual-loop antenna is used to obtain full
tensor resistivity information.
The sensors used in CSAMT surveys are similar to those used for MT and
AMT. Since the magnetic field from a long transmitter wire laid out parallel
to the regional strike usually varies only slowly over any area, reconnaissance
surveys are often made using measurements at up to ten electric dipoles for
every magnetic measurement.
The far field for CSAMT measurements is commonly considered to begin
at a distance of three skin depths from a long-wire source, and is therefore
frequency dependent. On a single sounding plot, the onset of intermediate-
field conditions can usually be recognised by an implausibly steep gradient
in the sounding curve (Figure 9.8). The effective depth of investigation is
then usually equal to somewhere between a quarter and a fifth of the distance
from the source. This simple relationship can be used in planning surveys,
but plans may have to be modified in the light of actual field conditions.
In principle, the Cagniard equation should not be used in intermediate and
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