Survey design for multicomponent electromagnetic systems

Abstract

A roving coil receiver is employed by several modern EM exploration systems to measure various spatial components of the time derivative of the magnetic field set up by a fixed transmitter. The absolute instantaneous secondary field measured above the surface of the earth is a potential field in this case, and, using well‐established potential field theory, it is known to be possible to predict the response of both horizontal components on any plane from a complete knowledge of the vertical component on the same plane. The required operation is simply performed in the wavenumber domain and is an amplitude invariant quadrature phase translation. In practice, errors are introduced in this transformation by spatial aliasing effects which depend upon the sampling density and by windowing effects which depend upon the areal extent of data coverage. The major source of noise in EM survey data collected in remote areas is sferic activity, which arises from the worldwide distribution of lightning discharges. Sferic noise, although time‐varying, is predominantly uniform in a spatial sense, with local spatially varying anomalies near conductive inhomogeneities. Since the horizontal component noise amplitude is typically an order of magnitude greater than the vertical component noise amplitude, a strong case can be made that it is advantageous in routine surveys to increase the spatial coverage of vertical component data rather than to read both horizontal components.