Schmucker, 1970 (Scripps) - MTNet

1. INTRODUOTION
1.1 Induction Anomalies of Geomagnetic Variations
Superposed upon the permanent magnetic field of the earth are those transient
fluctuations of small amplitude and short duration which are commonly known
as geomagnetic variations. They consist of two parts, one of external and
the other of internal origin. The external and primary part arises from
transient current systems in the ionosphere or beyond. The internal part is
from secondary eddy currents which are induced electromagnetically within
the conductive layers of the earth's interior. These currents limit the depth
of penetration of the incident variation field which varies from a few kilometers
for fast pulsations (60 cph = cycles per hour) to hundreds of kilometers
for the slow diurnal variations (1/24 cph). Hence, the subterranean eddy
currents flow during sufficiently slow variations well within the earth's upper
mantle, thereby revealing the internal distribution of electrical conductivity
down to a depth of, say, 600 km.
Except for a complex outermost shell of oceans and continental surface
layers we may assume that the conductivity changes primarily with depth in
accordance with the radial changes in composition, temperature, and pressure
(sec. 1.4). Currents which are induced from the outside in such a
stratified conductor flow in concentric shells or sheets and produce a secondary
surface field which resembles in its spatial configuration the primary
source field from above.
Suppose pronounced and consistent differences occur among simultaneously
recorded Variations at adjacent sites, say, less than 100 km apart, and suppose
that they cannot be related to some ionospheric current concentration
such as the equatorial or auroral jet. We may conclude then that these differences
are anomalous in the sense that they are of internal origin and due
to an unequal distribution of conductivity. The resulting perturbation of the
otherwise stratified flow of internal eddy currents is responsible for the observed
induction anomaly of the transient variation field at the earth's surface.
It is the objective of geomagnetic depth sounding to find such anomalies
with a closely spaced net of temporary magnetic recording stations.
This presentation deals with various anomalies which were discovered in
the southwestern United States between 1959 and 1962. A preliminary evaluation
of the material presented here has been published in the proceedings of
the Berkeley symposium on geomagnetic induction problems (Schmucker,
1964). A comprehensive review of the entire subject can be found inRikitake's
treatise entitled "Electromagnetism and the earth's interior" (1966).
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