The purpose of this project was to test the capability of three-dimensional (3-D) electrical<br>resistance tomography (ERT) to monitor changes in ground-water conductivity in a<br>saturated zone. This project is Phase I of a project funded by BHP Copper to monitor<br>injection of acid for in-situ leaching. Phase I of the BHP project was to inject the target<br>formation with a safe fluid consisting of natural ground water with a conductivity of about<br>double that of the target formation ground-water conductivity. This project was carried<br>out at a test site near Florence, Arizona. ERT was used to monitor the movement of the<br>injected fluid. Background ERT data was collected before injection began. The<br>conductive ground water was then injected into a well in the center of the test area. At<br>the same time, fluid was extracted from four wells, one at each corner of the test area.<br>This injection was continuous for about two months. Once injection started, ERT data<br>was collected bi-weekly for the first two weeks, weekly for the next two weeks, and every<br>two weeks for the next month. Data sets for a block were inverted to get a 3-D image of<br>resistivities. The 3-D ERT inversion technique was capable of detecting small changes in<br>electrical conductivity of the target formation due to the injected fluid, and provided a<br>realistic image of how the injected fluid moved through the formation with time.


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