This project was initiated at the request of Dr. Joseph Calantoni, Naval Research Laboratory (NRL), and Dr. Raymond Lim, Naval Submarine Warfare Center (NSWC), in connection to their pilot study (MR20-5116) to assess the efficacy of operating a chirp sub bottom system and vibracorer to stratigraphically characterize the underwater unexploded ordnance (UXO) test bed location off Shell Island, Florida. The water depths in the test bed (shoreline to 5 m) and surf zone conditions represent a significant challenge for towing a chirp instrument. In May 2021, Dr. Calantoni ran a highly successful test survey operating a pole-mounted Edgetech 3400, 2-16 kHz chirp sub bottom sonar from a shallow-water capable boat, collecting a reconnaissance grid survey as well as a more focused grid in the UXO test bed. The University of Texas Institute for Geophysics' (UTIG) contribution to this effort is focused on processing and interpreting the chirp data, for the purpose of characterizing the sedimentary environment and shallow stratigraphy of the test bed.

The objective of this project was to provide high-resolution stratigraphic characterization of a UXO test site in the surf zone offshore of Shell Island, Florida. The test site is being set up for research into technologies and applications for UXO removal, and requires a detailed environmental characterization of the sedimentary properties in the shallow subsurface. The shallow depths and surf-zone setting make this objective challenging.

In collaboration with members of NRL and NSWC, this pilot project aimed to test the efficacy of utilizing chirp subbottom acoustic reflection data. To overcome challenges associated with shallow water depths, the project will manufacture and test a floating mount that keeps the towfish submerged near the sea surface, allowing operation in shallow water. The component of the project conducted here focused on processing and interpreting the chirp data to assess the usefulness of the data collected.

The chirp data were successfully processed and interpreted by UTIG personnel. The UXO test bed location off Shell Island, Florida consists of three principal sedimentary units in the shallow subsurface. The substrate consists of the Mississippi-Alabama-Florida marine sand sheet, which is organized into oblique-to-shore sand ridges, and which merges with the barrier island sands of Shell Island. Along the lower shoreface/inner shelf, this substrate is overlain by an ebb tide unit associated with sands exiting through Land’s End Canal, the 1930’s-era ship channel constructed by the United States Army Corps of Engineers for access to Saint Andrews Bay. The ebb tide delta is prograding along shore to the southeast, as evidenced by the internal dipping reflectors, and consistent with the measure sediment transport direction. Along the upper shoreface, the substrate is overlain by the nearshore sandbar, organized into crescentic morphology of alternating highs and lows with a spacing of ~750 m alongshore. These sediments are assumed to be highly mobile above the basal reflector imaged in the chirp data.

This project proved that a cost-effective, off-the-shelf technology (chirp) can be utilized in very shallow water environments.