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 magneto-telluric 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 Dot-variations shows that the mantle resistivity below 600km depth is of spherical symmetry. The internal part of Sq-variations appears to be affected by lateral non-uniformities in the upper 400km. 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.Transient magnetic field pass through electrically conducting matter with amplitude reduction and phase rotation. We are concerned here with the inductive skin-effect of natural geomagnetic variations which they undergo within the Earth's interior. These variations are very slowly oscillating and can be regarded as quasi-stationary on a global scale. The pertinent resistivity (p)-frequency (f) parameter is the skin-depth valuea characteristic length for material of the magnetic permeability a (emu-cgs units). If the frequency is measured in cycles per hour (cph) and the resistivity in Qm, we obtain p=30.2/7j(km) with t=1 as free-space permeability in electromagnetic units. Geomagnetic induction studies involve frequencies from a few cycles per minute to fractions of a cycle per day. This yields in combination with internal resistivities between 1 and 104S2m skin-depth values in the kilometer range. The depth z* at which the main inductive attenuation of geomagnetic variations occurs can be inferred from Table 1.The inductive response of the Earth's interior to the spectrum of geomagnetic varia-

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Electromagnetic variations associated with the ionospheric currents have characteristic time scales from a few hours to a day. Those time-varying fields penetrate into the region called upper mantle (about 100-600 km below the surface) and induce electric currents therein (e.g., Schmucker 1970;Campbell 1987). The strength of the induced currents is roughly one third that of the ionospheric currents (e.g., Maeda 1965a, 1965b).…”

A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

“…These data were not used to investigate the question of induction in regions of dip equator latitudes. This question has since been addressed in a theoretical sense by many authors (see e.g., Srivastava, 1965;Hermance and Peltier, 1970;Peltier and Hermance, 1971;Schmucker, 1970), who derived impedance tensor estimates by introducing source ®eld geometry into the magnetotelluric equations. However, the lack of simultaneous observations of both electric and magnetic ®eld variations along a chain of equatorial stations has rendered impossible the validitation of theoretical results.…”

The geomagnetic coast effect

“…A number of reviews [Schmucker, 1970b;Gough, 1973b;Hutton, 1976] of various aspects of this subject have appeared. A review of theoretical models for electromagnetic induction in the ocean has been given by Ashour [1973], and a review of techniques and instrumentation for the study of electromagnetic induction itt sea by Filloux [1973].…”