Extending magnetic depths past 1000 m

Abstract

Doubling the number of points in the power spectrum and improving the method of fitting slopes to it at low frequencies, improves an existing method of obtaining depths to equivalent magnetic layers. The method assumes that a TMI anomaly is due to layers of random dipoles, so that the depth to each layer is obtained by inverting the slope of the power spectrum for depth, as if it were that of a single dipole. So far the method shows depths between 200 and 800 m. The improvement allows the tracking of equivalent layers down to at least 1500 m, doubling its useful range. By way of demonstration, a key indicator of some extensive gas-rich shales, the Derim Derim dolerite is tracked away from its intersection in boreholes. Using regional magnetic data alone, the dolerite is picked out at depths down to 1500 m.