An Introduction to Induction Anomalies

Abstract

The inductive response of the electrically conducting Earth's interior to geomagnetic variations can be inferred from the magnetic Z-H or the magnetotelluric E-H relations. In the absence of lateral non-uniformities of the electrical resistivity within a distance range which is comparable to the depth of penetration these relations are expressible by a single transfer function in in the frequency-wave number domain or by convolutions in the frequency-space domain. Lateral non-uniformities within the mentioned range produce anomalous Z-H and E-H relations. The resulting induction anomaly is dependent on the direction of source-field polarization with respect to subterranean resistivity gradients. The interpretation proceeds from a known or assumed normal inductive response for a layered Earth which is defined on a regional scale and serves as normal reference for lateral non-uniformities to be investigated. -The uniform internal part of Dst-variations shows that the mantle resistivity below 600 km depth is of spherical symmetry. The internal part of Sq-variations appears to be affected by lateral non-uniformities in the upper 400 km. For fast variations (e. g. bays) the inductive shielding by oceans and interspersed sedimentary basins on land becomes significant. In addition to this surface effect there exist regional and local variation in the depth of penetration of fast variations which originate from non-uniformities in the uppermost mantle. © 1970, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.