Geology Reference
In-Depth Information
9.1.3 Magnetic field effects
Advances in instrumentation and in the power of computers to solve com-
plex equations have also led to increased use of AMT in mineral exploration.
Almost by definition, mineral deposits are associated with lateral inhomo-
geneities, and it is no longer sufficient simply to measure the horizontal field
components. The eddy currents induced by alternating magnetic fields in
subsurface conductors produce secondary magnetic fields at the same fre-
quency as the primary but with different phases, and generally are directed
so as to oppose the changes in the primary field. The basis of the technique
is most easily demonstrated in terms of the (now virtually obsolete) methods
that used single-frequency VLF signals from military transmitters.
Figure 9.4 shows the effect of a thin, sheet-like vertical conductor on a
VLF wave. Immediately over the conductor the secondary magnetic field is
horizontal and reinforces the primary field, but on either side it has oppositely
Figure 9.4
Very low frequency (VLF) magnetic component anomaly over a
vertical conducting sheet striking towards the transmitter. An arbitrary sign
convention is used to distinguish the maximum from the minimum.
