Abstract: Geophysical Techniques for Evaluating the Internal Structure of Cheniers, Southwestern Louisiana
Robbie R. Zenero, David L. Seng, Mark R Byrnes, Randolph A. McBride
Historically, Louisiana chenier plain studies have consisted of surficial mapping, sediment analyses, boring records 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 structures in non-marine environments. Because the chenier plain of southwestern Louisiana contains stranded clastic deposits within a prograding, fine-grained, marginal marine environment, three study sites were targeted for GPR profiling. Reconnaissance vertical electrical soundings (VES) performed at each site were used to determine the applicability of GPR in this setting. Vibracores obtained at each site to a known olocene/Pleistocene unconformity were used to ground truth VES and GPR data. GPR profiles were conducted using a Geophysical Survey Systems, Inc. Surface Interface Radar 10A unit with a 500MHz antenna on a grid designed to characterize the morphology of each chenier. Topographic data were later integrated into the processed profile to adjust for surface elevation differences.
Analogous to seismic reflection data, GPR records contrasts in electrical permittivity as a function of horizontal distance and time in nanoseconds. Soil conductivity and relative permittivity within the substrate limit the depth of penetration, as does the source antenna frequency. Assuming a relative permittivity of 25, maximum depth of penetration at the study sites was approximately 4 meters. At the selected chenier locations, good penetration was achieved revealing a complex internal structure within these clastic deposits. However, at the interface with adjacent or subsurface fine-grained deposits, a combination of clay and/or conductive pore water limited penetration. Electrical resistivity proved to be a valuable reconnaissance tool for determining applicability of GPR, and mo eled VES inversions correlated directly to cores.
AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana