Challenges and Validation of Cross Tomography Experimentation for Inverse Scattering Problems in Soils

Cross-Well Radar (CWR) is a a promising method for DNAPL (Dense Non-Aqueous Phase Liquid) detection in soils. An important issue for CWR implementation is to deploy the optimum number of antenna boreholes for best balancing the trade off between image accuracy and minimizing risk of drilling through contaminated zone. This requires knowledge about key positions in the scatterer signature on EM field. Using the appropriate 3D antenna pattern to achieve a 3D image is crucial. Computerized tomography is a powerful subsurface imaging technique used in both medical to environmental applications. Fundamental theoretical and experimental forward modeling knowledge of EM wave propagation and scattering are crucial to accomplish image reconstruction. Therefore, a pilot-scale (1/100) cross-well radar tomography setup was developed. Many challenges were encountered and resolved in achieving measurement reproducibility, validity and symmetry. The knowledge gained from these studies has established a specific methodology for cross-well tomography experimental setup and data collection. The required frequency response data was collected and classified, and used to validate the results of a Born Approximation forward model in the frequency domain. Due to the complexity of simulating actual antennas analytically, simplified parametric antenna models were used in the forward model. The experimental measurements are then used to determine the antenna model parameters with the best matching the radiation characteristics.