A study of some effects of a conducting host rock with a new modelling apparatus

Abstract

An instrument which uses a new sampling technique has been constructed and used to study the electromagnetic induction effects in a laboratory scale model representing conductors in the presence of host rock. It measures, in a dipole-dipole setup, the in-phase and out-of-phase components of the anomalous field directly, and has provision for recording the same components on a chart recorder. Idealized ore bodies can be represented by solid conductors and the host rock, by an aqueous salt solution. The operating frequency can be smoothly varied and set at any value between 10 khz and 100 khz. The operating separation between the source and the receiver in the coplanar coil setup may be up to 21 cm at 10 khz and up to 29 cm at 30 khz or above. Measurements may be made to within ±0.1 percent of the free-space field, though absolute accuracy is less. It is found that the presence of a conducting host rock may substantially enhance the observed EM anomaly if the introduction of the target in the host rock alters the geometry of induced current paths. If this geometry is unaltered, a simple screening effect involving a reduction and phase rotation of the anomalous field occurs.