154th ASA Meeting, New Orleans, LA

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Influence of roughness on coherence of signals reflected by the seafloor

Laurent Guillon laurent.guillon@ecole-navale.fr
French Naval Academy
BP 600
29240 Brest Armes, France

Charles W. Holland
Applied Research Laboratory
The Pennsylvania State University
State College, PA 16804

Popular version of paper 2aUWa4
Presented Wednesday morning, November 28, 2007
154th ASA Meeting, New Orleans, LA

For many applications such as habour protection, mine hunting or biological studies, there is nowadays an increase in the use of low-frequency  sonar (f < 10 kHz) in shallow water areas. Consequently, there are a lot of interactions between acoustic signals and the seafloor and these acoustic signals can have a large penetration into the seafloor.  Therefore, it is necessary to develop tools that can find the geoacoustic nature of the seafloor on a large vertical scale in order to predict the acoustic propagation and scattering.

Our work is based on experiments lead offshore of Italy in 1997 by NURC (Nato Undersea Research Center) and sketched on this figure : a broad-band low frequency (200-6000 Hz) acoustic source is towed by a research ship and the acoustic signals are recorded on a 15-hydrophones vertical array moored on the seafloor. These experiments have been used to invert the geoacoustic properties of the seafloor by studying the reflection coefficient in time and frequency domain.

We re-read these data with new "glasses", focusing on the "coherence" of these signals in temporal domain (with the maximum of the cross-correlation coefficients) and in frequency domain (with the phase of the cross-spectra). In previous work, it was shown that these coherence parameters are very sensitive to the geoacoustic nature of the seafloor which may open the door for a new method of probing the seabed properties on both large scales (i.e., deterministic) and small scales (i.e., statistical). The aspect examined in this present work is the influence of the seafloor roughness on the coherence. To do so, the time series are simulated by a three-step process: 1) computation of the plane-wave reflection coefficient, 2) computation of the spherical wave reflection coefficient by a numerical integration of the Sommerfeld integral, and 3) Inverse Fourier Transform. Using these simulations, with the measured data as a guide, the potential for detecting and extracting roughness estimates from the coherence in complex multi-layered seabed environments is explored.

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