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Magnetotelluric investigation of Pemberton area, British Columbia Sule, Peter O


The magnetotelluric method has been used in the determination of the electrical conductivity structure of the Pemberton area of British Columbia: Meager Mountain, which is at the junction of the Garibaldi volcanic belt and the Pemberton volcanic belt, is the focus of a detailed geothermal resource evaluation. The aim of this project is to determine whether there is a resistivity structural change across the Pemberton volcanic belt. A knowledge of this will give some information about the structural control. This will aid in the investigation of any correlation between the electrical conductivity structure and the hot springs which almost circumscribe Meager Mountain. The temporal variations in the electric and magnetic components of the earth's field were recorded at Alta Lake (ALT), Pemberton (PEM) arid D'Arcy (DAR) from July to September 1975. This profile runs across the Pemberton volcanic belt. Selected sections of the analogue data were converted to digital form and power spectral analyses were made on the latter. The computed apparent resistivity curves show a discrepancy between the EX/D and EY/H which indicates the presence of a resistivity anisotropy/inhomogeneity in the region. Since there is some power in the vertical magnetic component (Z) in the region, it can be concluded that there is inhomogeneity in the conductivity structure. Also the computed Z power attenuation ratios between stations infer that any lateral conductivity change does not persist to large depths. It is also deduced that Z power increases slightly from ALT towards DAR. Some of the difference between the EX/D and EY/H apparent resistivity curves may be due to near surface inhomogeneities and the physical topography of the region. However, the bulk of this difference can be explained by considering a vertical fault zone near PEM, with ALT and PEM on the up-fault and DAR on the down-fault structure. The wider displacement between the EX/D and EY/H curves at DAR as compared to ALT can then be due to the fact that ALT is nearer the fault zone than DAR. On the basis of this interpretation one would expect a more pronounced change in the vertical magnetic component than observed. Apparent resistivity type curves for several theoretical layered earth models were computed and matched with the experimental curves. The results thus obtained indicate that the electrical resistivity structure in the Pemberton area fits the following layered earth model. The upper crustal layer has a resistivity of the order of 300 ohm-m and a thickness of about 40 km under ALT and PEM and about 60 km under DAR. At ALT and PEM, this layer is underlain by a more conductive material of resistivity about 30 ohm-m and a thickness of approximately 20 km. There is no trace of this layer at DAR. The resistivity value of this second layer is of the same order of magnitude as those usually reported from regions of geothermal investigations. The next layer at all the stations is highly resistive (greater than 2000 ohm-m) and has a thickness of about 500 km. This is underlain by a highly conductive basement having a resistivity of about 10 ohm-m or less.

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