Geology Reference
In-Depth Information
Figure 9.5
Very low frequency (VLF) magnetic field anomalies at the mar-
gins of an extended conductor. Sign convention as for Figure 9.4.
directed vertical components. Because the field strengths at the military
transmitters were, inevitably, unknown, the results were usually presented
in the form of profiles of either the dip-angle or the dip-angle tangent
(ratio of the vertical to the horizontal field) of the magnetic component.
For a vertical sheet, the anomaly would be anti-symmetric, but a dipping
sheet would produce an anomaly in which maximum and minimum had
different amplitudes. A wider conductor would be characterised by a greater
separation between the maximum and minimum and, in the limiting case, a
steeply dipping interface would be marked by either a simple maximum or
a simple minimum (Figure 9.5).
9.1.4 Coupling
The magnetic-component response shown in Figure 9.4 depends critically
on conductor orientation. This is also true in conventional EM surveys, but
EM traverses can be laid out at right angles to the probable geological strike,
automatically optimising responses. In VLF work the traverse direction was
almost irrelevant, the critical parameter being the relationship between the
strike of the conductor and the bearing of the transmitting station. A body
striking towards the transmitter was said to be
well-coupled
, since it was
at right angles to the magnetic vector and induction was maximised. In
other cases less current would flow, reducing the strength of the secondary
field. If the probable strikes of the conductors in a given area were either
variable or unknown, two transmitters, on bearings roughly at right angles
and distinguished by slightly different operating frequencies, were used to
produce separate VLF maps.
Coupling is more problematic in AMT surveys, because the source loca-
tion is not only unknown but also variable. The only (partial and not always
satisfactory) solution is to record at each station for an extended period and
