Attenuation-difference Radar Tomography: Results of a Multiple- Plane Experiment at the U.s. Geological Survey Fractured-rock Research Site, Mirror Lake, New Hampshire

Attenuation-difference, borehole-radar tomography was used to monitor a series of sodium chloride tracer injection tests conducted within the FSE wellfield at the U.S. Geological Survey Fractured-Rock Hydrology Research Site in Grafton County, New Hampshire, USA. Borehole-radar tomography surveys were conducted by using the sequential-scanning and injection method in three boreholes that form a triangular prism of adjoining tomographic image planes. Attenuation-difference data were inverted by using a weighted damped least-squares (WDLS) inversion method and several different solution simplicity schemes to suppress tomogram artifacts induced by the acquisition geometry, the location and magnitude of the anomaly, and noisy data. Qualitatively, flat tomograms generated by minimizing the norm of the first spatial derivative were most effective at suppressing artifacts. Although artifact suppression measures can be somewhat effective, negative consequences of artifact suppression include (1) reduction in the magnitude of the attenuation differences in the vicinity of the target anomaly and (2) blurring of the anomalies. These effects distort estimates of the location and magnitude of attenuation anomalies. In order to estimate robustly the location and magnitude of attenuation differences, areal constraints were imposed on the WDLS inversions to confine changes in attenuation to regions that are intersected by rays with large attenuation differences and bounded by rays with insignificant attenuation differences. Results from forward modeling support the application of these constraints. The resolution matrix was used to model the effects of acquisition geometry, target anomaly shape, location, and magnitude. For this study, the method of forward modeling indicates that estimates of pixel attenuation are improved by applying areal constraints to the WDLS inversions. Analysis of time-lapse attenuation-difference tomograms from the FSE wellfield indicates the network of fractures that connect the injection well and the pumped wells traverses the tomography-image planes in several different locations. Assuming the inverted magnitudes of pixel attenuation differences were sufficiently robust, changes in attenuation were interpreted to estimate tracer concentration in the image planes. Secondary-porosity estimates were made by assuming that the tracer concentration in the pixels adjacent to the injection zones was equivalent to the injected concentration. By using this method, the secondary porosity estimates range from 7.5x10 to 8.5x10, which is consistent with estimates reported from previous tracer tests in the FSE wellfield. The experimental results indicate that attenuation-difference radar tomography can provide high-resolution time-lapse images of the movement of a saline tracer, yielding insight into the three-dimensional spatial distribution of permeable fractures at the site. Planned study efforts will use resolution matrix modeling to optimize acquisition geometry and modified tracer-injection procedures to maximize the resolution of spatial, temporal, and physical property changes that accompany saline-tracer tests.