Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

only adequate smoothing of the observed anomal.ous surface field.
Then a certain well defined depth dep'endence of the anomalous
current density is adopted as discussed below and the anomalous
current distribution can be derived in a straightforward manner.
Suppose an observed anomaly 51, at a given frequcncy has been ex-
plained in this way by a distribu-tion of anomalous internal.
currents. Its connection to the internal conductivity is esta-
blished by the normal and anomalous electric field vector accor-
ding to
Assuming the normal conductivity distribution un(z) to be known,
E as a function of depth 'is readily calculated. There is no
-I1
simple way, however, to derive the anomalous electric field of
the TE mode except by numerical models as discussed below. There
is in particular no justification to regard it as small. i.n com-
parison to the normal electric field and thus to drop the second
term in the above relation. Instead the following argumentati.on
has to be used:
The anomalous electric field in the TE mode can be thought to
contain two distinct components. The first component may be re-
garded as the result of local self-induction due to EaII. I -.
can be neglected a-t suffici.ently low frequencies, when the ;-lzlf-
width of the anomaly is small in comparison to the minimum skin-
depth value within the anomalous zone.
The second component arises from electric charges at boundari-es
and in zones of gradually changing conductivi-by. These charges
produce a quasi-static electric field norroal to bounclarics and
paral7.el -to in-ternal conduc-tivity gradients which ensures the
continuity of the current across boundaries and internal gradient
zones. Hence, this second component of the anomalous electric
field does not disappear, when the frequency becomes small.
It vanishes, however, if anomalous internal currents do not cross
boundaries of gradient zones, i.e. when the normal field is in
E-polarisa-tion with respect to the -tl>end of elongated s'iructures.