Extending The Range Of Investigation Of Borehole Electrical Measurements

Increased cost of drilling for both minerals and fossil fuels provides strong motivation for the development of techniques which can determine the physical properties of rock units a long distance from the borehole. This paper presents theoretical and field results from hole-to-hole measurements (where source and receiver are placed in separate, widely spaced holes) and theoretical results for direct-current measurements from widely space delectrodes placed in a single hole. Theoretical hole-to-hole studies give an insight into the complex nature of the signal received in an inhomogeneous half space. Anomalies can be misinterpreted if the contribution to the signal from the earth-air interface is not considered. Theoretical studies of layered models show that the vertical resolving limits of the normal and bipole-bipole single hole arrays are essentially the same. A recent investigation in Wyoming illustrates the potential usefulness of hole-to-hole measurements in helping to reduce the number of drill holes needed to delineate a uranium ore body. This study also shows the importance of using conventional well logs and theoretical models to interpret data from widely spaced electrodes.