Pyramid Geophysical Services was contracted to perform an electrical resistivity (ER) survey on several current and proposed well drilling sites in Oklahoma. The purpose of the survey was to identify subsurface voids and depth to bedrock at the drilling sites, which are prone to karst formation. In karst conditions, the gradual dissolution of the underlying bedrock is a common driving force for the downward raveling of surface soils and the development of voids, in-filled channels and ravel zones. Several of the resistivity tests performed at the sites recorded evidence of extremely high resistivity, isolated anomalies that are characteristic of possible air-filled voids. Additionally, evidence of vertical “breaks” in the higher resistivity interpreted bedrock, represented by low resistivity features extending down into the rock unit, are typical of zones of dissolution that can result in the sagging and collapse of soil and weathered rock and the generation of voids.
Depths to bedrock were extracted from each transect to generate a contour map of the bedrock depths across the property, providing a plan view map of the site with the locations of voids spaces included. This method of comparatively quick and inexpensive site characterization provides guidance when selecting drilling locations, where subsurface hazards can be avoided.
