Variability in the acoustic response from natural shallow-water marine sediments

Abstract

The large variation in sediment acoustic response observed between soft clays and sands provides a satisfactory means to estimate sediment properties remotely using acoustic techniques. However, the accuracy of these predictions is limited by variations in sediment properties, both horizontally and vertically on a scale of centimeters to meters, within a given sediment volume, and sets the accuracy limits or ‘‘error bars’’ for prediction of sediment properties. In order to quantify these limits, a series of measurements of acoustic response were made at six sites off Gulfport, MS in sediments ranging from sands to muds. A broadband transducer with an 8° constant beamwidth was mounted on a trolley suspended from an 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. Measurements were made at both 30 and 50 kHz at each of eight positions 0.3-m apart along the beam. Results suggest greater variability occurs in the vertical direction (6–24 dB) than in the horizontal direction (6–18 dB).