Using the Loki 3D edge-finite-element program to model EM dipole-dipole drill-hole data

Abstract

Electromagnetic drill-hole measurements are an important tool for locating conductive orebodies in detailed exploration scale. Dipole-dipole systems have good spatial resolution, especially if all three magnetic components are measured over a wide frequency range. Interpretation of field data requires the use of sound 3D modelling tools capable of dealing with high conductivity contrasts and the close proximity of boundaries to the source or receiver. We modelled the response of the SlimBoris dipole-dipole drill hole system going through a block target using Loki, a 3D, full-domain, edge finite-element approach based on vector shape functions and scalar unknowns. This method achieves considerable speed advantage over conventional finite-element methods since only the one tangential component rather than three orthogonal components need be solved at each ``node'. Moreover, since no boundary conditions are violated in this approach, contrasts in excess of one million to one can be modelled accurately as verified through semi-analytical layered earth solutions. Computation time can be reduced by a further factor of five by solving initially for Schelkunoff potentials rather than electric or magnetic fields. This is due to the superior condition number of the resulting matrices. Accuracy is maintained using Green?s function projectors to obtain the fields at the receivers rather than differentiating the potentials. A 30,000 cell model takes 20 seconds on a 1.9GHz Intel chip per frequency per transmitter position. Control files for complex models can be set up rapidly using either the Encom EMGUI or Maxwell from EMIT. The results were in close agreement with scale model results at all contrast ranges. Another check was made using a 3D integral equation program. As expected, close agreement was obtained at contrasts of less than 300 but the integral equation results deteriorated at high contrast.