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

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 highvelocity 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.