Making Euler deconvolution applicable to small ground magnetic surveys

Abstract

Euler deconvolution is a commonly employed magnetic interpretation method because it requires only a little a priori knowledge about the magnetic source geometry, and, more importantly, because it requires no information about the magnetization vector. As a result, it may be successfully applied in areas where the geology is poorly known. However, it requires a priori knowledge about the nature of an equivalent source producing a magnetic anomaly with the same falloff rate of the observed anomaly. This is a crucial limitation of the method, requiring that a parameter known as structural index ( η) be determined. The customary application of the Euler method and the process of estimating η are benefited by having a large number of data and solutions preventing its application to ground magnetic surveys which may consist of a limited number of observations. We show that if the structural index is estimated by a new criterion, Euler deconvolution becomes a feasible technique to interpret anomalies defined by just a few observations. This new criterion is based on the correlation between the total-field anomaly h o and the estimates of the base level b. These estimates are obtained for each position of the moving data window along the observed profile and for several tentative values for the structural index. However, differently from the customary method, instead of estimating the structural index as the tentative value producing the smallest solution dispersion, the best estimate of η is taken as the tentative value leading to the smallest correlation between h o and the estimates of b. This criterion is deduced from the Euler's equation, so it does not depend on the inclination and declination of the geomagnetic field. The good results obtained with this new criterion in determining the correct structural index is illustrated in tests using synthetic data from different latitudes and the feasibility in using this criterion in applying Euler deconvolution to ground surveys is illustrated with a real magnetic anomaly defined by just 12 observations and produced by a basic/ultrabasic body at the emerald deposit of Socotó, Bahia, Brazil. The results of Euler deconvolution, combined with the geological information that the basic/ultrabasic body outcrops, show that the intrusive body may be approximated by an outcropping horizontal cylinder with a diameter of 68 m and center at a depth of 34 m, which is consistent with the geologic knowledge of the deposit.