A field comparison of Fresnel zone and ray-based GPR attenuation-difference tomography for time-lapse imaging of electrically anomalous tracer or contaminant plumes

Ground-penetrating radar GPR attenuation-difference to- mographyisausefultoolforimagingthemigrationofelectrical- ly anomalous tracer or contaminant plumes. Attenuation-differ- ence tomography uses the difference in the trace amplitudes of tomographic data sets collected at different times to image the distribution of bulk-conductivity changes within the medium. Themostcommonapproachforcomputingthetomographicsen- sitivities uses ray theory, which is well understood and leads to efficient computations. However, ray theory requires the as- sumption that waves propagate at infinite frequency, and thus sensitivities are distributed along a line between the source and receiver. The infinite-frequency assumption in ray theory leads to a significant loss of resolution both spatially and in terms of amplitude of the recovered image. We use scattering theory to approximate the sensitivity of electromagnetic EM wave am- plitude to changes in bulk conductivity within the medium. These sensitivities occupy the first Fresnel zone, account for the finite frequency nature of propagating EM waves, and are valid whenvelocityvariationswithinthemediumdonotcausesignifi- cant ray bending. We evaluate the scattering theory sensitivities by imaging a bromide tracer plume as it migrates through a coarse alluvial aquifer over two successive days. The scattering theory tomograms display a significant improvement in resolu- tion over the ray-based counterparts, as shown by a direct com- parison of the tomograms and also by a comparison of the verti- calfluidconductivitydistributionmeasuredinamonitoringwell, located within the tomographic plane. By improving resolution, the scattering theory sensitivities increase the utility of GPR at- tenuation-difference tomography for monitoring the movement of electrically anomalous plumes. In addition, the improved ac- curacy of information gathered through attenuation-difference tomography using scattering theory is a positive step toward fu- turedevelopmentsinusingGPRdatatohelpcharacterizethedis- tributionofhydrogeologicproperties.