On the use of line current analogues in geomagnetic depth sounding
Alan G. Jones
Various workers have appealed to Biot-Savart's
law for interpreting their anomalous geomagnetic response
functions as an aid to describing the electromagnetic induction
process. This approach, of using such a line current
analogue, is shown herein to be erroneous if the conductivity
of the lower half-space is not taken into consideration.
A numerical solution, using an FFT, is derived from the
kernels of the electromagnetic field components involved,
and is compared with the simplistic solution offered by Biot-Savart's
static field approximation. It is shown that within
a close proximity of the surface position of the buried line
current, Biot-Savart's law is reasonable. However, outwith
this distance the anomalous magnetic field components, and
accordingly their ratio, cannot be described by Biot-Savart's
law, but closely resemble induction, at the inductive limit,
in an isolated well-conducting inhomogeneity.
Uses of a line current in a conducting half-space as a
suitable analogue of the true induction processes are illustrated
with comparison to three anomalies - a conducting
block, Alert, and the Great Glen fault.
Journal of Geophysics (Zeitschrift fuer Geophysik), 60, 56-62.
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Alan G Jones / 28 February 2010 /