UBC Theses and Dissertations
Broad-band geomagnetic depth-sounding along an anomalous profile in the Canadian Cordillera Dragert, Herbert
A comprehensive, practical investigation of broad-band geomagnetic depth-sounding for profiles located in anomalous regions has been carried out in three stages. First, instrumentation has been developed to record geomagnetic induction data over a wide range of frequencies (0.01 to 100 mHz). Both geomagnetic depth-sounding (GDS) and magnetotelluric (MT) data can be obtained; frequency response is linear or closely matched, permitting data analysis without frequency-dependent corrections. By recording the data in two overlapping frequency ranges, a wide dynamic range (~80 db) can be obtained, thus making this system particularly well suited to 'low-ΔZ' regions such as the Canadian Cordillera, where attenuated short-period and normal-amplitude long-period vertical-field variations demand good sensitivity as well as an extensive dynamic range. Secondly, analysis techniques for the evaluation of data from anomalous GDS profiles have been reviewed. For spectral estimation, an adaptation of the periodogram technique intended for large quantities of data, and an adaptation of the maximum entropy technique intended for limited data are discussed. The formulations of the single-station vertical transfer function and the paired-station transfer function matrix are reviewed and simple interrelations are derived. These techniques are then closely examined in the light of implicit assumptions and practical limitations. Finally, a field study has been carried out investigating the detailed structure of the lateral conductivity discontinuity in the Cordillera geomagnetic 'transition' zone between Revelstoke and Calgary. Analysis of data has revealed the presence of three conductive structures: 1) a near-surface conductor associated with sediments of the Rocky Mountain Trench; 2) a lower-crust/upper-mantle conductivity heterogeniety 40 to 50 km beneath the trench area, associated with possible hydration and partial melting; 3) a second deep conductivity structure orthogonal to the previous structure, associated with a buried Precambrian rift. The interaction of the latter two conductors indicates that local deflection of current patterns induced over a larger region takes place.
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