Shallow Shear-Wave Seismic Analysis of Point Bar Deposits of False River, Louisiana

Abstract

Current point-bar complex models do not include subsurface unit bars as a normal feature. This study provides evidence for a potential buried unit bar amongst point-bar sediments of the large-scale, modern-day False River point-bar complex of the Mississippi River. We collect, process and interpret a two-dimensional, 150-m-long CMP seismic reflection profile that cuts perpendicularly across a major discontinuity surface in the False River point bar complex. The seismic source consists of a ground recoil device that fires a shotgun shell horizontally, producing shear waves. Multiple field experiments demonstrated which type of source and receiver provided the least amount of noise, with the most coherent incoming signal from reflections. LiDAR data allow the ridge-swale topography that exists above the point bar deposits to be readily mapped; this ridge-swale topography gives clues to the relative history of the meander bend. Seismic methods allow us to map the internal structure of the deposit, something that LiDAR cannot do. Gamma-ray and electrical conductivity data have previously been collected in a well located along our seismic line. These are correlated with the seismic data in order to assist with the interpretation. We find a seismic boundary dipping in the opposite direction that we anticipate in a point-bar complex. This may be a unit bar buried beneath many meters of point bar sediment, or may be the result of an erosive event. Unit bars add to the complexity of a point bar complex; they can lead to opposite-dipping boundaries than those caused by IHS layers. Two different models are created that could have resulted in the subsurface geometry seen in the seismic data. This study provides valuable insight into the evolution of fine-grained river systems, both modern and ancient.