Abstract
The authors used resistivity tomography, a kind of geotomography, to investigate ground water flow in rock. Ground water flow through rock often is complex because it is affected by faults and fractures. The authors used resistivity tomography with a low resistivity salt tracer to detect and estimate faults and fractures, which show up as thin sheets.
Prior to field experimentation, the authors conducted numerical experiments to examine the characteristics of a reconstruction algorithm and evaluate its applicability to rock investigations. These experiments indicated the successful reconstruction of the model structure and the possibility to detect the resistivity change caused by injection of a salt tracer.
Next, the authors applied these techniques to a rock investigation at a dam site. It had been known that the site contains three faults that would affect the flow of ground water. The electrodes used in measurement were placed in a vertical borehole, a horizontal adit and on the ground surface in such a way as to surround the objective area. Resistivity tomography was conducted three times, once before injection of the salt tracer, one day and six days after the injection. During this period, the conductivity of ground water was also measured in the adit.
The results show a complex resistivity distribution, and provide images of a fracture zone. The resistivity changes caused by the salt tracer clearly reveal the motion of the tracer. The tomographic images of the area agreed with previously available geological infomation.
These numerical and field experiments showed that resistivity tomography is effective not only for the investigation of structures in rock, but also for estimating the ground water flow when used with a salt tracer.
