Physics
Scientific paper
Sep 1973
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1973pepi....7..365s&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 7, Issue 3, p. 365-378.
Physics
19
Scientific paper
The geomagnetic skin-effect is specified by setting three length scales in relation to each other: L1 for the overhead source. L2 for the lateral non-uniformity of the subsurface conductor, L3 for the depth of penetration of a quasi-uniform transient field into this conductor. Relations for the skin-effect of a quasi-uniform source in layered conductors are generalized to include sources of any given geometry by introducing response kernels as functions of frequency and distance. They show that only those non-uniformities of the source which occur within a distance comparable to L3 from the point of observation are significant. The skin-effect of a quasi-uniform source in a laterally non-uniform earth is expressed by linear transfer functions for the surface impedance and the surface ratio of vertical/horizontal magnetic variations. In the case of elongated structures and E-polarisation of the source, a modified apparent resistivity is defined which as a function of depth and distance gives a first orientation about the internal distribution of conductivity. The skin-effect of a non-uniform source in a non-uniform earth is considered for stationary and ``running'' sources. Recent observations on the sea floor and on islands indicate a deep-seated change of conductivity at the continent-ocean transition, bringing high conductivity close to the surface, a feature which may not prevail, however, over the full width of the ocean. There is increasingly reliable evidence for high conductivities (0.02 to 0.1 micro -1 m-1) at subcrustal or even at crustal depth beneath certain parts of the continents, in some cases without obvious correlation to geological structure.
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