The total field magnetometric resistivity (TFMMR) method: Part II: 2D resistivity inversion of data from the Flying Doctor Deposit, Broken Hill, Australia

Abstract

Total field magnetometric resistivity (TFMMR) is a high-resolution electrical technique that yields information on sub?surface resistivity. In a companion paper (Fathianpour et al., 2005, Part I) the basis of a 2.5D TFMMR finite-element modelling approach was developed for a point current source in an otherwise 2D resistivity structure. In this paper (Part II), we use the 2.5D forward modelling algorithm as the basis of a numerical inversion for 2D resistivity structure using a Marquardt-Levenberg algorithm. Application of a quasi-Newton updating formula for approximating the Fr?chet derivatives in the course of inversion results in a fast and reliable routine. To overcome the problems of the effect of the geomagnetic field direction and dependency of TFMMR data on all three vector-components, field data are initially reduced to the pole. By doing this, we require only Fr?chet derivatives for the vertical Bz anomalous magnetic field, and in the case of the 2D structures, Bz is the most sensitive component to vertical boundaries separating lateral changes in resistivity. TFMMR data collected across the Flying Doctor Deposit, near Broken Hill in New South Wales, Australia are inverted for 2D resistivity structure. The inverse models show the presence of a conductive zone in the central part of the surveying area corresponding to the known mineralisation and the Globe Vauxhall Shear Zone. Depth resolution is limited, but we demonstrate that the method can resolve lateral boundaries.