Variability in the acoustic response of shallow-water marine sediments determined by normal-incident 30-kHz and 50-kHz sound

Abstract

To quantify the variability in the acoustic response of typical shallow-water marine sediments, a series of high-frequency (30 and 50 kHz), normal-incident measurements were made at six sites within the Ship Island Test Bed off Gulfport, MS, USA. These measurements were made using the Naval Research Laboratory’s (NRL) Acoustic Seafloor Classification System (ASCS) in sediments ranging from silty clay to medium sand, and with sediment structures ranging from layered, to unlayered, and methane gas-charged. A broadband, narrow-beamwidth transducer was mounted on a remotely movable trolley suspended from a horizontal I-beam connected to a pair of legs at each end of the beam. This ‘swingset’ was lowered to the seafloor with the transducer mounted at normal incidence and at an altitude of 3.0 m above the sediment surface. Measurements were made at both 30 and 50 kHz at each of up to eight positions 0.3 m apart along the beam. A coefficient of variation was calculated for each horizontally corresponding sample of the 16-bit, 125-kHz sample rate A/D at each of the positions along the transect. Variability was determined at ∼6-mm depth intervals in the sediment to the limit of significant signal return. These measurements indicate that the greatest variability in acoustic response occurs in the upper 0.5 m of the sediment column with muds ranging from 18 to 31%, whereas sands ranged from 23 to 26%. Acoustic response variability was found to decrease with increased depth for both muds and sands. Below 1.0 m, the sediment depth limit of bioturbation variability, decreases to 6–13% for muds and to 18% for sands. Methane gas-charged sediment located approximately 3.0 m below the sediment–water interface produced a variability of 20%. For all sediment types, the 50-kHz acoustic response was consistently more variable than the 30-kHz acoustic response.