Use Of Borehole-Radar Methods To Monitor The Movement Of A Saline Tracer In Carbonate Rock At Belvidere, Illinois

Common-depth (CD) radar surveys and cross-hole radar tomography methods were used to<br>monitor the movement of a saline tracer in a dual-porosity dolomite aquifer at Belvidere, Illinois. The<br>tracer test was conducted using an array of six open-hole bedrock wells at the Parson’s Casket Hardware<br>Superfund site. The injection and recovery boreholes were about 20 m (meters) apart, and the imaging<br>boreholes were arranged to provide planar coverage across and along the anticipated tracer path. A<br>hydraulically conductive zone identified during previous investigations was isolated using straddle<br>packers and pumped to establish a hydraulic gradient between the injection and recovery wells. A sodium<br>chloride (NaCl) solution was continuously injected into this zone to move the tracer across the<br>tomographic image plane.<br>CD cross-hole radar surveys and cross-hole tomography surveys were conducted before and<br>periodically during the tracer injection. Background tomograms contain similar radar velocity and<br>attenuation changes with depth, consistent with a layered dolomite that has variable porosity and<br>electrical conductivity. Slow changes in attenuation associated with low tracer velocity permitted the<br>acquisition of multiple CD surveys and two cross-hole tomography surveys during injection. CD surveys<br>were used to rapidly identify the presence of tracer between wells. Attenuation-difference tomograms<br>contain attenuation increases that delineate the spatial distribution with time of the saline tracer and show<br>the progressive movement of the tracer within the tomographic image plane. Formation porosity and<br>resistivities calculated from radar velocity and attenuation tomograms were used to estimate changes in<br>fluid resistivity and tracer concentration in the tomographic image plane.