Digital analysis of aeromagnetic maps: Detection of a fault

A fault in rock strata would invariably disrupt the pattern of the magnetic field contours from which an experienced geophysicist is able to detect a fault, that is, find its location and orientation. In this paper digital signal processing tools, namely, directional filtering and angular spectrum are developed for the detection of a fault from the digital aeromagnetic maps and their performance is compared. Digital filter design tools are used to design a filter with a wedge shaped passband. Both the angle and direction of the axis of the wedge are user specified. The filter is then applied to a model where there are two faulted magnetic layers, each of which is free of any other directionality. It is shown that the filter is able to detect the fault in the deeper layer even in the presence of another faulted layer at a shallow depth. In the second approach the angular spectrum is used to measure the orientation of a known fault. Some care need to be exercised, as a peak in the angular spectrum may also be caused by other factors, such as, the direction of polarization, or by tapered spectral window, etc. We have computed the angular spectrum of the eastern part of the aeromagnetic map over the south Indian shield. It is found that the angular spectrum in the low frequency band shows a peak corresponding to the east-northeast-west-southwest fault suggested in Reddy et al. (1988) [Reddy, A.G.B., Mathew, M.P., Singh, B., Naidu, P.S., 1988. Aeromagnetic evidence of crustal structure in the granitic terrain of Tamilnadu-Kerala. J. Geol. Sec. India 32, 368-381]. Also there are peaks corresponding to the direction of polarization, the tapered window used in the analysis, and a few shallow lineations. (C) 1998 Elsevier Science B.V.