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Low frequency acoustic reverberation from highly porous seafloors under grazing incidence

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Author: Cristol, X. · Jespers, S. · Chalindar, B. · Juhel, B. · Dybedal, J. · Eidem, E.J. · Ivansson, S. · Vossen, R. van · Ainslie, M.A. · Andersson, B.L. · Colin, M.E.G.D. · Pihl, J.
Type:article
Date:2012
Source:Proceedings of the 11th European Conference on Underwater Acoustics - ECUA 2012, 2-6 July 2012, Edinburgh, UK
Identifier: 462561
Keywords: Defence · Low-Frequency Active Sonar · Sea Bed Parameters · Backscattering Strenght · Acoustic Reverberation · Inversion · Safety and Security · Defence, Safety and Security · Physics & Electronics · AS - Acoustics & Sonar · TS - Technical Sciences

Abstract

The European Defence Agency project RUMBLE-2 (ref.[1]) offered the opportunity for investigating experimentally acoustic reverberation at about 1.kHz, under grazing incidence (less than about 20°), from very porous clayey seafloors of a continental shelf (mean grain size spanning from about 7 to 10 in  units); the instrument was an operational, carefully calibrated LF towed triplet-array sonar. Assuming Lambert law, observed values for the normal backscattering strength μ0 spanned between –20 dB to –12 dB, i.e. 10-15 dB higher than usually recommended values for soft seabeds. The objective of this article is to understand such high values of inverted μ0’s during the RUMBLE2 experiments, which appear at first glance considerably high, particularly when considering the nature of the sedimentary cover. As displayed by the map on the left of Figure 1, the inverted μ0’s are ranging from about –20 to –12 dB, for a seabed covered by very thin clays with Mean Grain Sizes of 8 to 10  units (porosities more than 85%), as shown by the chart on the right of Figure 1. Usually recommended values for μ0 are 10 to 15 dB lower, or even more; we have to explain such a disturbing discrepancy.