Electric and magnetic dipoles for geometric interpretation of three-component electromagnetic data in geophysics

摘要

We have developed a number of simple inverse models for rapid geometric interpretation of the low-frequency (diffusive) electromagnetic field scattered by a bounded 3D heterogeneity in a conductive background. These models, based on the nature of the interaction of the source primary field with the anomalous body in its environment, consist mainly of magnetic and electric dipoles and their combinations, and of their respective finite-size equivalents. i.e. closed and open current circuits (filaments). These 'equivalent-source' models have been validated on synthetic data calculated with 'exact' 3D modelling codes for different anomalous targets of canonical shapes (parallelepipeds and spheres). In this paper we present the practical application of our dipolar model in mineral exploration and discuss the reliability of each kind of information derived from them. In particular it is seen that the 3D attitude of the target is always well retrieved, and that the 3D location of its centre is accurate to within a few per cent of its distance to the measuring profile. Though this specific inverse problem is generally ill posed due to limited spatial coverage of the observation grid-a common characteristic in geophysics-it is shown that in the near field of the target the cases of non-uniqueness are scarce and controllable, as opposed to what is expected in the far field.