Borehole Radar Attenuation-Difference Tomography During The Tracer/Time-Lapse Test At The Boise Hydrogeophysical Research Site

A tracer test and time-lapse radar imaging experiment was conducted at the Boise<br>Hydrogeophysical Research Site to investigate the utility of crosswell radar in imaging an electrically<br>conductive tracer plume. A multilevel water sampling system down gradient from the tracer injection<br>well and in the radar imaging plane was used to collect detailed, 1-dimensional, fluid electrical<br>conductivity data during the tracer test. We compare the spatial and temporal position and concentration<br>variations of the plume as indicated by the fluid conductivity data to those suggested by radar level run<br>attenuation differences, shot-receiver attenuation difference crossplots, and an attenuation-difference<br>tomogram. We find that attenuation differences generally correlate well with changes in fluid<br>conductivity. Where correlations are not so strong, the discrepancies can be explained by the difference<br>in support volumes for the radar and chemistry measurements, and also by regularization of the radar<br>tomogram. Our results indicate that crosswell radar imaging coupled with hydrologic tracer testing can<br>provide useful information about subsurface fluid flow and mass transport in complex fluvial aquifers.