A magnetotelluric (MT) survey using naturally occurring ultra low frequency (ULF) and extremely low frequency (ELF) sources was conducted in the frequency range of 0.5 mHz to 20 Hz in order to locate the western edge and the depth of the North American Central Plains (NACP) conductivity anomaly in the Bengough area of southeastern Saskatchewan. The data base was also used to evaluate the complex singular-value decomposition (CSVD) method of MT processing and to corroborate certain geologic interpretations in this part of the Williston Basin.Modelling of the resulting impedance tensors revealed a deep (10–15 km) zone with resistivity (35–85 Ω∙m) significantly lower than typical values (1000–1500 Ω∙m) obtained from a borehole resistivity log of the top 30 m of the Precambrian at a depth of 2.3 km. An increase in depth (to 20 km) and resistivity (150–275 Ω∙m) of this deep zone measured at the survey's west end was interpreted as indicating the anomaly's western edge. The CSVD processing of the data did not show any particular advantages over the conventional cross-spectral method.

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