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