Mapping a 3-D conductivity anomaly using a vertical electric source: Field results

A method for monitoring an enhanced oil recovery (EOR) process is the mapping of electrical conductivity changes due to injection of steam or other fluids. Since the conductivity change will be three-dimensional (3-D) in character, it is necessary to develop techniques to map 3-D conductivity distributions. At the Richmond test site, near Berkeley California, we conducted a field experiment to map a 3-D salt water plume using a vertical electric source (VES) emplaced in the earth. We selected this source because when emplaced in a horizontally layered host it produces no vertical magnetic field nor horizontal magnetic field at or above the earth's surface. Because there is no layered host response in these instances, the detectability of 3-D electrical structures is greatly increased since only secondary magnetic fields are measured for this case. Data consisting of the horizontal magnetic field were collected at the surface before and after injection of the salt water to create a plume. These data were spatially filtered to eliminate near surface geologic variations. The post-injection data were then normalized with respect to the pre-injection data and interpreted. Our preliminary interpretation of the data is based on 3-D forward modeling. We interpret a 3-D conducting, elongatedmore » target centered approximately 10 m away from the injection well. At the present time our analysis has not provided depth information; however, we expect the target to be contained within the aquifer at a depth of 30 m. The results of this field test illustrate the utility of this technique. 10 refs., 6 figs.« less