Time Domain and Multifrequency Electromagnetic Response in Mineral Prospecting

Extensive, wide band EM surveys were made over three previously known bedrock conductors in the Canadian Shield. The conductors varied in size from small to very large, having strike lengths of approximately 100 m., 300 m. and 2 kms. All the conductors are located in crystalline host rocks which appear to be quite resistive. They are mostly covered by overburden whose thickness and conductivity are not well known, but which probably do not exceed 15 m. and 10-2 mhos/m respectively. The characteristics of the conductors (geometry and conductance) are only partially known from drilling. The surveys were carried out with UTEM Mark II system built at the University of Toronto by Y. Lamontagne and G. F. West. The observed data were compared with the theoretical data computed by using a free-space rectangular thin plate model. The parameters of the models were adjusted, within the known geological constraints, to achieve a best fit between the model and field responses. It was apparent in each case that some aspects of the observed responses cannot be explained on the basis of the simple models. However, in all cases the response at intermediate or later delay times could be matched reasonably well with the responses computed for models whose parameters are consistent with the known geology. The present research was devoted to investigating the validity of EM interpretation methods based on the use of simple uniform models in free space. It is well known that such simple models are not valid for the geological situations that exist in some parts of the world. However, it is widely believed that such simple models are indeed valid in areas of relatively resistive host rock and overburden such as the Canadian and Baltic Shields. The three studies reported in this thesis give some qualified support to this belief in simple models. In all three cases the induced current system (at least over most of the time range studied) is believed to be basically a vortex current in the main conductor and does not involve the host rock and overburden. Considerable deviations of the observed response from that of simple model response are found at early delay times. These deviations are believed to be due to the effects of the finite thickness of the conductive zone and of a halo of poorly conducting material situated around the main core. Whatever its cause, this effect has an important bearing on the interpretation techniques. It appears that natural conductors do not have a clear inductive limit to their response as do simple models, and this makes the interpretation of size and geometry more difficult. It also means that conductivity thickness (conductance) estimates based on simple models can be erroneous (too low) unless the late stage of the time delay (or the low frequency, resistive range of the frequency response) is observed. Computer programmes were developed by extending Annan's (74) eigen function technique to compute responses for airborne, ground and drill hole EM systems in the time or frequency domain for a free space, rectangular, finite, thin sheet model. This model was then utilised to compare the responses of various recently developed time domain and multifrequency EM systems. UTEM, Crone PEM and INPUT systems were studied in the time domain. Turam loop and horizontal loop EM systems were studied in the frequency domain.

Keywords: Geophysics
Advisor: West, G. F.