The application of the Signum transform to the interpretation of magnetic anomalies due to prismatic bodies

Abstract

The Signum transform is a simple derivative-based method for qualitative and quantitative interpretation of magnetic anomalies from discrete sources. The methodology is based on the normalization of a filtering function, which is a derivative of the anomalous field or function of this, by its absolute value. The filtered anomalies have only two values (+1 or -1) and the causative sources are represented by the positive values. The transform has been applied to three different functions, namely the first order vertical derivative of the magnetic anomaly, the first-order vertical derivative minus total horizontal derivative and second-order vertical derivative For a vertical magnetisation the edges of the sources can be recognised from the locations where one or more of the spatial derivatives change its sign: the zero crossover point. The zero cross over point and actual source edge are separated by an amount which depends on the dykes depth and the type of data being transformed. Thus, actual edge locations are easily computed from the Signum transformed data. The method performs well when closely spaced sources cause anomalies to overlap. Imagery based on the Signum transformation of first and second-order derivative based transforms of the magnetic data combines the advantages of the resolution of the second order transform with the greater stability of the first-order transform.