Now you see it, now you don't: Chirp imaging of the intermittently shelly shoreface ravinement surface on the inner shelf of Panama City, Florida

Abstract

Reconnaissance CHIRP data and vibracores were collected on the inner shelf off Panama City, Florida, in April, 2011, for the purpose of providing seabed characterization for an upcoming ONR acoustic reverberation experiment. The seafloor in this region is part of the MAFLA sand sheet: Holocene shelfal marine sands, 0–5 m thick, extending from Mississippi to the Florida panhandle. Coring often samples a thin shelly layer, associated with the shoreface ravinement, at the base of the sand sheet, followed by finer-grained estuarine sediments. Prior CHIRP data collected by Steve Schock off nearby Fort Walton Beach revealed a highly intermittent reflector that could be correlated to the base of the sand sheet; whether the reflector is caused by the estuarine sediments or the shells was uncertain. The new data also reveal an intermittent basal reflector. Estuarine layering can also be identified, and in parts of our survey area the basal and estuarine horizons are distinct. A core at one of these locations sampled a 0.5-m thick shell layer corresponding to the basal reflector with sand both above and beneath. Shells are therefore likely responsible for this intermittent reflector, and thus themselves likely very heterogeneous in concentration at the ravinement surface.