High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Environmental Characterization of Proposed Experiment Site
Off Panama City Beach, Florida

Peter Fleischer and William B. Sawyer
Naval Research Laboratory: Marine Geosciences Division, Code 7430
Stennis Space Center, MS 39529
Peter.fleischer@nrlssc.navy.mil, bill.sawyer@nrlssc.navy.mil
(228) 688-4399, -5752 (fax)

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Preliminary site characterization surveys were conducted for the DRI SAX99 Experiment site area during the August/September 1998 portion of the field deployment aboard the R/V Seward Johnson. Two locations, both in 20-m water depths, were investigated. The first (site 9), selected from older side-scan and diver surveys, is off Panama City, FL, east of the inlet. After extensive surveys and measurements and observations, it was determined that this location is too muddy to serve as the experiment site. The second location (site 10) is west of the inlet, off Panama City Beach, near the now destroyed "Texas tower" Stage II of the Coastal Systems Station.

The surveys at the second site resulted in the acquisition of two sets of side scan sonar imagery (~18 km2) from the Seward Johnson. Bottom-characterization surveys of the potential SAX99 experiment site (a 3.5 km x 5 km rectangle), at a depth of ~20 m, were conducted two days prior to and two days after Hurricane Earl passed directly over the site. Initial results from the pre-Hurricane Earl survey show a level, uniformly bioturbed bottom (~10 cm small-scale relief) having a uniform acoustic backscatter strength field. Bottom samples were predominantly fine to medium grain sized sands of terrigenous origin. Bottom conditions following the passing of Earl were modified by the storm’s passing influence. Post-hurricane morphology within the immediate site consisted of long-crested small dunes or oscillation ripples having 0.5–1 m crestal spacings. Near the site boundaries, corresponding to bathymetric lows, sand dunes were supplanted by incipient short-crested medium dunes with 3–7 m spacings. A discontinuous layer of soft mud covered most bedforms, predominantly in local lows and in bedform troughs. Consequently, the backscatter strength following the storm was non-uniform and somewhat reduced.

The Panama City Beach experiment site was vibracored by the USGS Center for Coastal Geology (Nancy DeWitt, POC) in March 1999. Ten cores, having lengths of 0.5 to 6 m, were successfully collected. Core tops within the site were clean sand, but muddy sand was found at the northwestern boundary and in shallower depths (30’, 40’, and 50’ locations). Analysis and results from these cores will be available from the USGS by early summer 1999. Additional ground truth of the area is available from historical vibracore data, which were acquired by the US Army Corps of Engineers (COE) in 1984 and 1994 field coring expeditions. These data, in the form of core logs and select down-core grain size analyses, will be used in concert with the more recent vibracore data to evaluate bottom and subbottom characteristics for the SAX99 experiment site.

Further data were acquired through the engineering office of Coastal Planning & Engineering, Inc., Boca Raton, FL (Mr. Stephen Keene, PE). These data and the COE vibracore data were acquired as part of the beach replenishment project for Panama City Beach. Nine borrow areas were selected for the beach sand source material, and approximately 1 million cubic yards of material have been removed from each of these borrow pits for the beach replenishment project which is due to be completed by May 1999. The tentative site for the SAX99 experiment is located seaward of borrow pits 1 and 2.

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