The role of simple computer models in interpretations of wide‐band, drill‐hole electromagnetic surveys in mineral exploration

Abstract

Drill‐hole geophysical surveys are a means of extending the search for massive‐sulfide deposits to depths which are inaccessible to conventional surface techniques. The present investigation combines field and model studies of an electromagnetic (EM) prospecting method which utilizes a large, fixed transmitter loop with a downhole, axial‐component magnetic field sensor (solenoid). The system is shown to be well‐suited for detection of deeply buried massive‐sulfide conductors located in resistive host rock at appreciable distances from the drill hole. We propose that drill‐hole survey data collected with a wide‐band large‐loop EM system can be routinely used for estimating target parameters by forward modeling with two simple conductor shapes: a plate and a sphere in free space. Analysis of confined conductors is facilitated by “eigencurrent decomposition” of the induced current vortex into a set of noninteracting loops with simple RL‐circuit behavior. Solutions have been implemented in interactive computer programs which are fast, inexpensive, and sufficiently versatile to accommodate configurations and waveforms used in many practical EM systems. Both stationary and dynamic aspects of the induction process are exploited for diagnosis of three‐dimensional targets. Field studies at test sites in Sudbury and Noranda base‐metal mining areas with a commercial pulse EM system indicate that many important effects predicted by the model studies are, indeed, observable in survey data.