A low-frequency magnetotelluric study in southern Saskatchewan
Maidens, John Michael
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A low-frequency magnetotelluric sounding survey was conducted in the Bengough area of southern Saskatchewan during the fall of 1981. The objectives of this investigation were twofold: to locate the western edge of the North American Central Plains conductivity anomaly in this region, together with its depth, and to compare the results given by the customary cross-spectral method of magnetotelluric data processing to those obtained with a recently proposed technique which incorporates the complex singular value decomposition (CSVD) of the data matrix. The Bengough area is of particular interest due to a history of unusual seismic activity. The CSVD algorithm does not require the assumption of a pair of independent noise-free signals, and the procedure readily separates the data into noise and signal components from which the geophysical transfer functions can be extracted. A brief history and mathematical summary of the magnetotelluric method are provided, and the data acquisition equipment and procedure are described. The data processing steps are outlined and some noise sources are discussed. The results are interpreted with the aid of digitized well-log data, apparent resistivity pseudosections, and one-dimensional modeling based upon the apparent resistivity and phase curves obtained at each of the five recording sites. These results reveal a deep zone (at 10 to 15 km depth) with a resistivity in the 50-75 Ωm range. The resistivity of this conductive zone is significantly lower than the values (1000-1500 Ωm) acquired from well-log resistivity readings taken at the top of the Precambrian. There is a general trend of decreasing resistivity in this deepest zone from site to site, traveling west to east along the survey line. Measurements taken at the west end of the survey line reveal this zone has a greater depth (20 km) and higher resistivity (250 Ωm) there than at sites to the east. Without further observations to the west of the present survey it cannot be concluded whether these measurements mark the western edge of the NACP conductor or a more gradual lateral change in its depth and resistivity. It was felt that the CSVD method did not offer any significant advantage over the cross-spectral computation method, especially in light of recently introduced improvements in data acquisition and processing techniques.