Schmucker, 1970 (Scripps) - MTNet

2 Bulletin, Scripps Institution of Oceanography
At first sight the electrical conductivity 0" of subterranean matter may not
seem to be a very noteworthy parameter. There is, however, its close relation
to temperature, following the general theory of semiconduction, and
even a relatively small rise in ambient temperature may cause a substantial
increase of the conductivity (sec. 1.4). We have to bear in mind, however,
that semiconduction in nonmetallic solids is primarily an impurity effect.
Thus, minute changes of composition can have an equally strong effect upon
0", not counting the largely unknown influence of pressure.
This limits the effective use of the conductivity as an absolute thermometer
for the earth's interior. There remains, however, the important aspect
to use 0" as relative thermometer, namely, to infer deep-seated lateral gradients
of conductivity and possibly temperature from their distorting effect
upon the internal part of geomagnetic variations. Such thermal imbalances in
the upper mantle could be connected with ascending and descending branches
of convection cells or with local concentrations of radioactive heat sources,
which may be the underlying cause for the diversified tectonic and magmatic
history of the earth's outermost layers.
Two inherent limitations of geomagnetic depth sounding should be mentioned.
Since the magnetic observations are made within a small area, small
in comparison to the spatial extent of the primary inducing field from above,
a complete separation of internal and external parts of the transient variation
field is not possible. A separation of this kind would be necessary to determine
the overall change of conductivity with depth in the surveyed area (sec.
1. 3). Hence, induction anomalies of the internal part, which are recognized
usually without formal separation of internal and external parts, have to be
interpreted on the basis of a preconceived mean conductivity distribution
which has been inferred as function of depth from other sources of information
(sec. 1. 3).
Second, oceans and continental surface layers form a thin conducting cover
of great complexity. The flow of shallow eddy. currents is therefore highly
distorted, in particular near coastlines because of the outstanding conductivity
contrast of seawater and rock formations on land. This mayor may not
lead to an anomalous behavior of geomagnetic variations, depending on the
relative strength of those eddy currents and their contribution to the internal
part in the area under consideration. Surface effects of this kind have to be
taken into account before postulating a deep-seated cause for a given induction
anomaly.
1.2 Basic Equations
The incident variation field diffuses downward through the conducting layers
of the earth with amplitude reduction and phase rotation. The governing
equations are Maxwell's field equations of the transient electromagnetic vector
field E and F. We use here their quasi-stationary approximation (in
emu):
curl.F :; 417O"E and curl £= -MoFfat (1.1)
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