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Geophysical Techniques for Evaluating the Internal Structure of Cheniers, Southwestern Louisiana
Robbie R. Zenero (1), David L. Seng (2), Mark R. Byrnes (2), Randolph A. McBride (2)
Historically, Louisiana chenier plain studies have consisted of surficial mapping, sediment analyses, cores, and direct observation of limited exposures from borrow pits. Ground penetrating radar (GPR), a shallow geophysical technique, provides high resolution profiles of contrasts in electrical permittivity that can be correlated with sedimentary layers in sand and gravel depositional environments not effected by salt water. Reconnaissance vertical electrical soundings (VES) were performed at several sites to determine the applicability of GPR on the chenier plain. Based on resistivity data, three field sites were targeted for GPR profiling.
GPR profiles were collected using a Geophysical Survey Systems, Inc. Subsurface Interface Radar 10A unit with a 500MHz antenna on a grid designed to assess three-dimensional variability in subsurface characteristics of each chenier. Topographic corrections were applied for each processed profile to adjust for surface elevation differences that effect reflection orientation. At selected chenier locations, subsurface reflections reveal a complex internal structure within these clastic deposits. However, at the interface with adjacent or subsurface fine-grained deposits, clay and/or conductive pore water limit penetration. Electrical resistivity proved to be a valuable reconnaissance tool for determining applicability of GPR, and VES models correlated directly with vibracore data. Preliminary interpretations of radar profiles suggest cheniers evolve through multiple processes in different environments. Radar facies observed in dip-oriented profiles at two sites are described as landward stepping foresets and may be interpreted as transgressive washover deposits. Bedset orientation of strike-oriented profiles indicates a westward component of migration. At a third site landward and seaward dipping reflections, interpreted as upper shoreface and washover deposits, indicate shifting shoreline position related to transgression and regression associated with variations in sediment supply.
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