Geology Reference
In-Depth Information
long-wire sources are to generate sufficiently strong signals at the distances
required by the far-field approximation, and it is seldom easy to find sites
where wires carrying many amperes of current can be laid out on the ground
safely (or even at all) over distances of kilometres. Even where this can
be done, topographic irregularities may create significant distortions in the
signal. Closed loop sources can be considerably smaller but require currents
that are even larger (by factors of as much as ten) than those needed for line
sources.
In the far-field, magnetic and electric field strengths decrease roughly as
the inverse cube of distance from the transmitter. The dimensions of the re-
ceiver 'dipoles' affect target resolution as well as signal strength but, because
signal strength is normally low at the distances at which AMT equations
can be applied to CSAMT data, it may be impractical to use dipoles less
than 20 m long. Even so, noise may exceed signal by a factor of ten or more,
and observations must be extended over periods long enough to permit very
high folds of stacking.
With every CSAMT system there is a distance, which is to a first approx-
imation independent of both frequency and resistivity, beyond which the
signals generated for a given input power become too weak to be usable.
For every system there is also a distance, dependent on both frequency and
resistivity, below which the far-field approximation can no longer be used
(Figure 9.10). Inevitably there will, for any value of ground resistivity, be a
frequency at which the two distances are identical, and this is the CSAMT
limit for that system in that area. Signals can, of course, be transmitted at
Figure 9.10
Investigation constraints in controlled-source audio-magneto-
telluric (CSAMT) surveys. The area within which survey work can be carried
out lies between a maximum distance determined by signal strength and
the minimum determined by the limit below which the Cagniard equation
ceases to apply. The latter is frequency-dependent, and the operational zone
therefore varies in width according to frequency
(f )
.
ρ
=
resistivity.
