3-D Electrical Resistivity Tomography For Environmental Monitoring

Electrical Resistance Tomography (ERT) images the electrical properties of the subsurface using crosshole<br>resistivity measurements. Interpretation of ERT data requires multi-dimensional inverse modeling. We discuss a 3-<br>D Occam’s inversion routine. The routine uses a finite-element forward solution and a conjugate gradient based<br>inverse routine. The algorithm finds the smoothest possible model that fits the data to a given a-priori level. In<br>general, the 3-D algorithm takes 10 to 20 iterations to converge to a final solution. However, the 3-D algorithm<br>requires only a single forward solution per iteration and does not require direct solution of a large system of<br>equations. Instead, it requires only the multiplication of a vector times the sensitivity matrix or its transpose. This is<br>accomplished without creating or storing the full sensitivity matrix.<br>A field example is shown in which ERT was used to monitor the injection of air from a vertical well at a shallow<br>petroleum remediation site in California. Data was interpreted using both 3-D and 2-D methods. Air injection<br>caused large changes in resistivity. At early times these were confined to an area near the injection point. Later the<br>changes were along a dipping, tabular region. At the latest times there is evidence of mixing of brackish water at<br>the depth of the injection point with fresh water in a shallower aquifer on the site. This mixing would have<br>decreased the resistivity and thus the apparent the size and magnitude of the zone of influence of sparging.<br>Results of 3-D and 2-D inversion agreed well when the regions of interest were approximately 2-D.