f Vertical electrical imaging using focused current and its application

Since electric logging techniques use only in-hole electrodes, they usually provide no information about the inclination or orientation of structures. Although electrical crosshole imaging techniques have been recently used (e.g., Shima,1992a), they require complex analyses and much computation. To provide a practical solution to these problems I proposed "Vertical Electrical Imaging" (Shima, 1992b). In the VEI measurement, the electric field generated by a surface source is measured by three borehole-electrodes, and electric reflectivity or the second derivative of apparent resistivity is calculated (see Fig .l). These values reflect only the structure near the potential electrodes and the directionality of sensitivity can be controlled by changing the distance between the borehole and the surface source. Therefore, the VEI provides the information about the inclination and orientation of the structure near a borehole.