Azimuthal resistivity to characterize fractures in the Battleford formation, Birsay, Saskatchewan

Abstract

Azimuthal resistivity was performed at the King site, near Birsay, Saskatchewan to characterize the orientation and extent of fracturing in a glacial till. The target consisted of shallow (less than 4m deep) fractures in the upper oxidized portion of the Battleford Formation. The fractures were visible in soil cores, but their orientation and extent were not known at the time of investigation. It was hoped that if the azimuthal resistivity method could be successfully applied at this site it could be used as an in situ fracture mapping tool at other sites. Preliminary azimuthal resistivity surveys failed to detect a consistent anisotropic pattern that could be attributed to fracturing. A refined method of azimuthal resistivity was developed that built a 3D resistivity model of the site assuming a heterogeneous, isotropic earth. This model was used to predict and remove the effect of heterogeneity on the azimuthal resitivity observations. The results from the refined method also failed to detect a consistent anisotropic pattern. Any single azimuthal resistivity observation from either the preliminary or refined surveys would have provided data that could have been interpreted as anisotropy due to fractures. It was only by comparing many azimuthal observations across the site that the lack of consistent azimuthal pattern became apparent. It is recomended that an analysis of several observations be made before any interpretation of anisotropy is made for azimuthal resistivity sureveys in general.