BOREHOLE-TO-SURFACE ELECTRICAL-RESISTIVITY MONITORING OF A SALT-WATER INJECTION EXPERIMENT

A field experiment was conducted at the University of California Richmond Field Station to demonstrate the sensitivity of borehole-to-surface resistivity measurements in groundwater investigations. A quantity of saline water was injected into a fresh water aquifer while the resistivity was monitored using a multichannel borehole-to-surface system. Two experiments were conducted using pole-pole and pole-dipole receiver electrode arrays. The data from the pole-pole experiment were superimposed to simulate a dipole-pole array and the data from the pole-dipole array were superimposed to simulate a dipole-dipole array. This superposition of the data was done to enhance the anomaly and facilitate interpretation. A numerical modeling study was performed in conjunction with the field experiment in order to interprete the results. A three-dimensional modeling program was used to simulate the geological setting of the field experiment and the salt water injection. This modeling revealed that an asymmetric displacement of the salt water slug results in asymmetric current channeling which is observable as a 25 to 40 percent difference between preinjection and postinjection borehole-to-surface resistivity. In addition to demonstrating the sensitivity of subsurface arrays, this experiment demonstrated that the measurement of bulk resistivity can identify a groundwater flow pattern not detected by hydrological measurements.