Geomagnetic deep sounding with arrays of magnetometers

Abstract

Arrays of about 25 portable magnetic variometers have recently been used to record the total varying geomagnetic field over an area 1000–2000 km square. The total varying field consists of a portion generated outside the solid earth (in the ionosphere and the magnetosphere) and a portion induced by this external field (in the material of varying conductivity in the crust and upper mantle of the earth). By plotting sets of magnetograms of the observed varying fields, Fourier spectra over the full range of periods available, or contours of Fourier amplitudes at specified periods, localized anomalies in the total field are identified. Careful examination of these various plots yields considerable qualitative information about the subsurface variations in conductivity that produce these anomalies. Formal methods of separating the observed field into components of internal and external origin exist and can be applied to this problem, but with questionable accuracy. Semiquantitative estimates of the separate components may be just as accurate. Quantitative interpretation of the anomalous internal fields in terms of inhomogeneities in conductivity is at present almost entirely based on numerical modeling techniques. Three such methods have been used so far, the one using finite differences being the most successful. Results of such modeling are neither unique nor very close to the observed field patterns. Another method of analysis is based on transfer functions and cross‐correlation spectra, and this method may turn out to be more useful.