Wavenumber domain interpretation of magnetic gradiometer data

Abstract

An interpretation scheme is discussed whereby the amplitude and phase spectra of airborne or ground gradiometer profile over a dipping tabular dike and contact are used to determine the width, depth to top and the horizontal position of the top center of the dike, and in the case of the contact, its depth, and location. It is shown that the amplitude spectra of the vertical and horizontal gradients are exactly the same but the phase spectra differ. The width and depth to top (and in the case of finite dikes, the depth extent) are determined from the amplitude spectrum or a semi-log plot of the amplitude spectrum. The phase spectra by themselves provide a unique determination of the horizontal position of the dike and contact and unlike the reduction to the equator or pole, the method does not require a knowledge of the directions of remanence or dip. When the dip of the body is known it is possible to evaluate the influence of remanence on the anomaly. Using theoretical models, the effect of depth, width, position, and remanence on the spectra and on its interpretability is analysed. In general, the interpretation method produces reasonable estimates of the parameters. Tests on field data show that the method provides a viable approach to the interpretation of total-field magnetic gradient data.