Schmucker, 1970 (Scripps) - MTNet

3. DATA REDUCTION
3. 1 Basic' Concepts
Already a visual inspection of magnetograms from a closely spaced net of
stations can reveal characteristic differences of the traces from site to site,
which mayor may not be connected with local induction anomalies. It has to
be left to a more refined analysis, to recognize anomalies of truly internal
origin, and to present them as function of frequency and location in a condensed
statistical form.
The theory for the internal induction process shows that inducing and induced
fields above a conductive substratum supplement each other in the
horizontal components, but oppose each other in the vertical components.
Thus, we obtain a nearly tangential variation field under "normal" conditions,
i. e., in the absence of lateral conductivity gradients, provided that the overall
depth of the eddy currents is small in comparison to the dimensions of the
primary field (cf. eq. 3.2).
Consequently, internal conductivity anomalies, which disturb locally this
sensitive balance between external and internal Z -variations, are more obvious
in Z than in Hand D, where the anomalous parts are superimposed
upon substantial normal parts. Nevertheless, we shall study the anomalous
behavior in all three components, since this provides us with an effective
control for the presumed internal origin of the anomalies.
The analysis of the magnetograms is directed toward a statistical correlation
between anomalous and normal parts of outstanding magnetic disturbances,
using events of the same general type (e. g., bays), but of different
form and intensity. This postulated correlation is necessarily linear as is
readily seen from the governing equations in section 1.2 which establish
linear relations for the transient magnetic field vector F.
We obtain in this way for each survey station a 3 x 3 matrix of transfer
functions, connecting the components of the anomalous (Fa) and no:r:mal CF')
disturbance vector at that site in the frequency domain (eq. 3.11). These
transfer functions give the proper statistical basis for the subsequent interpretation
of the induction anomaly in terms of internal conductivity structures.
Strictly speaking, some regard should be given to the nature of the primary
inducing field from above, 1. e. , . to its spatial configuration. We may
disregard, however, this complication when dealing with the smooth midlatitude
field of bays and similar events. Their depth of penetration into the
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