The Impact of the Spatial Variability of the Seafloor on Midfrequency Sound Propagation During the Target and Reverberation Experiment 2013

To support the modeling of reverberation data collected during the Target and Reverberation Experiment in 2013 (TREX13), transmission loss was measured in the 1.5-4.0 kHz band using a towed source and two moored vertical line arrays. The experiment site was located off the coast of Panama City Beach, FL, and the transmission loss measurements took place along a 7-km-long isobath, which ran parallel to the shore with a water depth of approximately 19 m. The seafloor at the TREX13 site consists of sand ridges, which run perpendicular to the track of the experiment, with narrow bands of softer sediments on the western sides of the ridges and in the ridge swales. Using data from a multibeam echosounder survey and direct measurements of the seafloor properties, a geoacoustic description of the seafloor is developed and used to model the transmission loss at the site. Although the soft-sediment bands only occur in 27% of the seafloor, they are found to have a significant impact on the transmission loss, increasing it by roughly 5 dB at 4 km over what would be expected from an entirely sand sediment. This is consistent with the previous work by Holland who showed that lossiest sediments play the largest role in propagation over range-dependent seabeds. Simulations also show that the exact locations of the soft sediments are less important for controlling propagation in the TREX13 environment than the proportions of the sediments. This suggests that a range-independent, effective media description of the sediment could be used to model propagation at the site. The limits of the use of an effective medium in describing both propagation and reverberation measurements made during TREX13 are considered.