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Structural imaging in the Rocky Mountain Foothills (Alberta) using magnetotelluric exploration

¹ Institute of Geophysical Research, Department of Physics, University of Alberta, Edmonton, Alberta, Canada, T6G 2J1; wxiao{at}phys.ualberta.ca
² Institute of Geophysical Research, Department of Physics, University of Alberta, Edmonton, Alberta, Canada, T6G 2J1; unsworth{at}phys.ualberta.ca

Wen Xiao received his B.Sc. degree in 1993 in geology from the Northwest University at Xian in the People's Republic of China. From 1993 to 2000, he worked as a hydrocarbon exploration researcher for Sichuan Petroleum Research Institute. He moved to Canada in 2001 and completed his M.Sc. degree in geophysics in the Department of Physics at the University of Alberta in 2004.Martyn Unsworth is currently a professor at the University of Alberta. He received both his B.A. degree in natural sciences (1986) and his Ph.D. in earth sciences (1991) from Cambridge University. Following 2 years of postdoctoral research at the University of British Columbia, he was a research professor at the University of Washington in Seattle from 1993 to 2000. Since 2000, he has been on the faculty of the University of Alberta, and his research is focused on electromagnetic exploration methods. This includes applications in continental tectonics, with recent studies of the San Andreas fault, the Tibetan Plateau, and the Canadian Cordillera. He has also applied electromagnetic methods to environmental and exploration studies.

The magnetotelluric method has improved significantly in recent years and is being used in hydrocarbon exploration in regions where seismic exploration is difficult. This includes areas where high-velocity carbonates and volcanic rocks are present in the near surface, overthrust belts, and in subsalt imaging. Magnetotelluric exploration was used in the Rocky Mountain Foothills in 2002 to determine if thrust-related structures could be imaged through the subsurface resistivity structure. Broadband magnetotelluric data were collected at 26 stations on a profile that extended southwest from Rocky Mountain House to the Front Ranges. Two-dimensional inversion was used to derive a resistivity model that was a compromise between fitting the magnetotelluric data and being spatially smooth. The resistivity model imaged the Alberta basin as a thick package of low-resistivity units that could be traced to the southwest where they form the footwall of the Brazeau thrust fault. The subsurface geometry in the magnetotelluric-derived resistivity model is in good agreement with well-log data and a coincident seismic section. Zones of low resistivity in the underthrust rocks may be caused by fracture-enhanced porosity associated with anticlines and fault-bend folds.