3-D Ert Inversion Used To Monitor An Infiltration Experiment

Abstract

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