In the first experiment, air was injected by a compressor at a depth of 50 m in the GSK-1 well. The injected point was in an unsaturated zone, which was about 5 m above the water table. Thirty-seven electrodes were placed at 3-m intervals along the 108-m long line that crosses immediately near the well. Using dipole-dipole, Wenner and Schlumberger electrode arrays, resistivity data were collected before, during and after the injection. Although the water level went down by only 1.5 m during the air injection, resistivity increases at the unsaturated zone near the well. In contrast, resistivity decreases at the further areas from the well. It seems that moisture in the unsaturated zone was moved outside by the injected air.
In the second experiment, CO2 gas was injected with the gas cylinders from the top of the SCW-1 well. The CO2 gas was diffused into underground at a depth of 43.76 m that was the depth of the well casing end. Multi-electrode resistivity monitoring were done in the same way as in the air injection experiment, with 49 electrodes placed at 3-m intervals along the 144-m long lines that crosses immediately near the SCW-1 well. The depth to water level in the well was about 10 m before the injection and went down by 40 m during the injection. Although the intermittent rainfall affected the resistivity near the surface, a large resistivity increase was observed above the water table near the well during the injection. The resistivity increase zone spread outside and remained two days after the injection. This result suggests that multi-electrode resistivity monitoring is effective to monitor the behavior of CO2 gas in saturated and unsaturated zones.