Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

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7. Basic concepts of geomagnetic and magnetotelluric depth sounding
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. 7.1. . General characteristics of the method
Two types of geophysical surface data can be distinguished to in-
vestigate the distribution of some physical property m(g) of matter
beneath the Earth's surface. The first type is connected with
static or quasi-static phenomena (gravity and magnetic fields), the
second type with time-dependent phenomena (seismic wave propagation)
or with controlled experiments under vartable experimental con-
ditions (DC-geoelectric soundings). Geomagnetic and magnetotellur7ic
soundings utilize the skin-effect of transient electromagnetic
fields. Their penetration into the Earth represents a time-depen-
dent diffusion process, thus the observation of these fields at
the surface produces data of the second type.
The interpretation of static data y = y(R) ...- is non-unique alld an
arbitrary choice can be made among an unlimited number of possible
distributions m(r), - explaining y(R) - equally well. The interpretation
of transient data is with certain constraints unique in the
sense that oniy - one distribution m(r) - can explain the surface data
y = y(R,t), basically because opeadditional variable t (time,
variable parameter of controlled experiment) is involve?,
If the observation of the transient process or the perforrfiance of
the experiment is made at .a single site, the data y '= .~(t) permit
a vertical sounding of the property m = 'm(z), assumed to be a sole
function of depth z beneath that site, If the observations or ex-