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.
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Outcrops are fundamental to everything we hope to achieve in geological understanding. They are gateways to geological processes, earth history and they help ground-truth remote sensing applications. With increasing resolution of subsurface tools and techniques, one could be forgiven in believing that outcrops have had their day and their utility is less than in the past great eras of field mapping and the development of facies models. This premise is far from the truth and this new SEPM volume illustrates how new analytical techniques are revitalizing outcrops and in the process creating a wealth of new data and fresh geological understandings. In this book you will find a compilation of the growing arsenal of outcrop tools and techniques and a consideration of future developments. This collection of papers, delivered at a SEPM Research Conference on the West coast of Ireland in the summer of 2008, is a smorgasbord of case studies, workflows, modeling, and applications which spans clastic and carbonate settings. Whatever your interest in outcrop geology and its application there is something in this volume for you. If you are seeking guidance for using new outcrop tools, looking for efficiencies in data collection or desiring new insights for old and favorite outcrops, this volume is a must have. This volume also makes an excellent reference or textbook for any group of professionals or students working or studying the new technologies that have allowed new insights from outcrops. We also consider this a superbly timed publication because many new outcrop tools are now becoming mainstream via reduced purchase and operating costs. Once you read this volume, and there are reduced prices for SEPM members and students, please share your new experiences with the authors and editors and help continue the revitalization of our shared and continually surprising outcrop library of the earth.