First trial of multi-wavelength vector sensor: Sediment geoacoustic properties obtained from vessel noise off Senegal

Fisheries survey ECOAO 13, including the collection of surface sediment samples and acoustic propagation measurements, was carried out off the Senegalese coast in February 2013. The bottom acoustic properties were determined by geoacoustic inversion using the noise of R/V Antea as a noise sound source of opportunity. The vessel moved on a straight course with constant engine speed (1627 RPM) while the generated noise field was recorded by a vector sensor array deployed from a reinforced hull inflatable boat launched from the vessel. The multi-wavelength array, designed and built at EHL, is composed of four pressure sensors with different spacing combinations and two pressure gauges. The inversion approach estimates the bottom geoacoustic properties from observation of range-dependent features of vertical impedance at multiple frequencies and optimization based on a genetic algorithm. As compared to our previous two-element array, deployed offshore Amazon, the newly designed array provides more comprehensive observables and therefore more reliable geoacoustic inversion results. For this first trial, the vertical impedance data at the spectral lines of 140 Hz, 280 Hz and 321 Hz were extracted from pairs of adjacent array elements and jointly processed. The bottom compression speed (1640 m/s) and density (1.96 g/cm3) deduced from the posterior probability density functions suggest a sandy seabed. The acoustic inversion results are in general agreement with the grain size analysis of two sediment grab samples, which indicate that more than 80% of the material is composed of well-distributed sand particles of different sizes, the remaining part being shell fragments and silt/clay mixtures.