Geomagnetic induction studies in Scandinavia. I. Determination of the inductive response function from the magnetometer array data

Data frm the Munster IMS Magnetometer Array (Kuppers et al. 1979) have been analysed in the frequency domain to drive the inductive response function, c(w,o), from the ratio of the vertical magnetic field to the spatial gradient of the horizontal magnetic field. The response function was best determined by statistical frequency analysis techniques after the spatial gradients had been derived by least-squares fitting of two-dimensional second-order polynomials to the observations, with the constraint imposed that the solutions be curl-free. The derived response function was found to obey two different causality requirements, and most of the inequality constraints imposed on it (Weidelt 1972). A preliminary model, in which conductivity is a function of depth only and which explains the major details of the observed response, is presented. It has a highly resistive uppermost layer of some 10 ⁴Omega m and of the order of 30 km thick, underlain by a layer of about 125Omega m to a depth of around 140 km, where a transition takes place to a highly conducting (3Omega m) half space. The effect of various non- uniform souces on the observations of c(w,k) for this 1D model is illustrated. -Author