Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

This law can be used to i-nterpret an observed surface anomaly
of geomagnetic variations in terms of a subsurface distribution
of anomalous induction currents, which in turn are related to
an internal conductivity anomaly a i,e. to local deviations
a'
of a from a normal. layered distribution o (z),
n
It has been pointed out in Sec.8.3 that the anomalous part of
the geomagnetic and geoelectric variation field can be split
into TE and TM modes (= tangential electric and tangential
magnetic modes) and that only -the TE modes produce an observable
magnetic variation anomaly at the Earth's surface. Consequently,
3
only a TE anomalous current density .distribution c~~~ can be
related to the anomalous magnetic surface field Ha, the sub-
script "11" refering to the solutioll I1 of -the diffusion
equation.
Denoting the tangential components of simply with i and
c
-a11 ax
J. the components of the anomalous surface field are given
ay'
A 0
with r2 = (x-x)' + (Y-~ + G2. The inverse problem, namely to
find from a given surface anomaly the internal anomalous current
distribution, has no unique solution. Within certain constrai.nts,
however, it can be made unique, at least in principle. For in-
stance, it is assumed that the anomalous current flow is limited
to ;I certain depth range which implies that the matter between
this depth range and the surface is regarded as non-conducting
at the considered frequency.
The surface anomaly can be extended no:,? dowr~trard t o the top of
.the anomalous depth range with standard metliods which require
A