Modelling of airborne EM anomalies with magnetic and electric dipoles buried in a layered earth

Abstract

An automated algorithm is described that selects and models spatially discrete anomalies in airborne EM (AEM) data sets. For anomalies to be selected they have to be wide enough to have their origin in the subsurface and narrow enough to be caused by a discrete conductor. After determining background conductivity models with layered-earth inversions from the EM data, identified EM anomalies are modelled with magnetic and electric dipoles buried inside a layered earth. Magnetic dipoles are appropriate models for discrete, sheet-like conductors inside a resistive host, i.e., in scenarios where vortex currents dominate, whereas electric dipoles are expected to model well elongated structures excited by current channelling. The model parameters determined from each data segment include, for magnetic dipole solutions: the target conductor position, depth, dip, size, and conductance; and for electric dipoles: the position and depth. The method is fully automated with the dipole start models being determined by curve matching from digital look-up tables. Results from synthetic data indicate the efficiency and reliability of the method. The technique was applied to TEMPEST and GEOTEM data acquired across the Bull Creek prospect, Queensland and Harmony deposit, W.A., respectively. The algorithm provides a sensible description of both mineralisations. Other anomalies are interpreted as being caused by shallow structures channelling current, discrete conductors beneath the overburden, and the lateral heterogeneity of the overburden.