Outcrop-Based GPR Tomography Through Braided-Stream Deposits
Ginny L. Rust, Gary S. Weissman, Ulrike Werban, Jedediah D. Frechette, Timothy F. Wawrzyniec, 2011. "Outcrop-Based GPR Tomography Through Braided-Stream Deposits", Outcrops Revitalized: Tools, Techniques and Applications, Ole J. Martinsen, Andrew J. Pulham, Peter D.W. Haughton, Morgan D. Sullivan
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Cross-well or borehole ground-penetrating radar (GPR) tomography is commonly used to map subsurface aquifer heterogeneity; however, the influence of sedimentary architecture and anisotropy on GPR signal transmission is largely unknown. To address uncertainty in GPR tomography interpretation, we developed a method to combine GPR and LIDAR surveying to characterize lateral heterogeneity behind an outcrop of braided-stream deposits. Methods included using black paint to mark a regular grid of GPR shot points on opposing outcrop faces for the transmitter and receiver intervals. A 3D model of the outcrop was then constructed using LIDAR scans to determine distances between shot points. GPR tomography was conducted through the outcrop, and velocity calculations from these data were used for tomographic inversions to determine heterogeneities within the outcrop. We were able to successfully combine GPR shot-points with LIDAR data to calculate distances between transmitter and receiver points, and we were able to see first arrivals through the outcrop. Complications, however, were evident at both short offsets (∼ 3–5 m), when the air wave overprinted the first arrival, and longer offsets (> 8 m) when the signal dissipated before reaching the receiver. Air-time arrivals that were observed compared favorably to model predictions based on LIDAR-derived outcrop geometry, supporting the utility of LIDAR for constructing a digital, 3D outcrop. Furthermore, estimated values of radar velocity for this outcrop of 0.09 to 0.12 m/ns reasonably match known velocities for similar sediment. Had the GPR data acquisition been more successful at this outcrop site, we could have used detailed 3D facies architecture derived from the LIDAR data to assess the influence of sediment heterogeneity and anisotropy on GPR tomographic signals.