Monitoring the sea environment using acoustics the role of the acoustical observatories

The presentation deals with theoretical factors and technical specifications pertinent to the design of an acoustical observatory for the monitoring of the marine environment. Two types of observatories are mentioned, namely active and passive. Among the various cases of active observatories, the ones related to ocean acoustic tomography are presented in some detail and the inverse problem of retrieving information from measured acoustic data is explained. Some basic issues related to the type of measurements that should be made for optimal use of the acoustic field are also given with related references. Finally, the basic features of passive observatories are underlined without going into details

Time-angle ocean acoustic tomography using sensitivity kernels: Numerical and experimental inversion results

International audienceIn shallow water acoustic tomography, broadband mid-frequency acoustic waves (1 to 5 kHz) follow multiple ray-like paths to travel through the ocean. Travel-time (TT) variations associated to these raypaths are classically used to estimate sound speed perturbations of the water column using the ray theory. In this shallow water environment, source and receiver arrays, combined with adapted array processing, provide the measurement of directions-of-arrival (DOA) and directions-of-departure (DOD) of each acoustic path as new additional observables to perform ocean acoustic tomography. To this aim, the double-beamforming technique is used to extract the TT, DOA and DOD variations from the array-to-array acoustic records. Besides, based on the first order Born approximation, we introduce the time-angle sensitivity kernels to link sound speed perturbations to the three observable variations. This forward problem is then inverted with the maximum a posteriori method using both the extracted-observable variations and the proposed sensitivity kernels. Inversion results obtained on numerical data, simulated with a parabolic equation code, are presented. The inversion algorithm is performed with the three observables separately, namely TT, DOA and DOD. The three observables are then used jointly in the inversion process. The results are discussed in the context on ocean acoustic tomography

Inversions of statistical parameters of an acoustic signal in range-dependent environments with applications in ocean acoustic tomography

The paper presents an application of a method for the characterization of underwater acoustic signals based on the statistics of their wavelet transform sub-band coefficients in range-dependent environments. As it was illustrated in previous works, this statistical characterization scheme is a very efficient tool for obtaining observables to be exploited in problems of ocean acoustic tomography and geoacoustic inversion, when range-independent environments are considered. Now the scheme is applied in range-dependent environments for the estimation of range-dependent features in shallow water. A simple denoising strategy, also presented in the paper, is shown to enhance the quality of the inversion results, as it helps to keep the signal characterization to the energy significant part of it. The results presented for typical test cases are encouraging and indicative of the potential of the method for the treatment of inverse problems in acoustical oceanography

Challenges and methodologies in Passive Ocean Acoustic Tomography: An approach based on ICA

International audienceThe main idea of this manuscript consists on applying signal processing methods along with statistical approaches to achieve the characteriza- tion of a complex and hostile environment such as the oceanic underwater environment. In order to study, characterize (i.e. the measurement of the pressure, the salinity, the temperature, etc.) and extract the topographical features (depth, etc), Ocean acoustic tomography methods are widely used. For many reasons, Passive ocean Acoustic Tomography (PAT) configuration becomes more attractive than the classic active one. However the new passive configuration is more challenging. In our project, some of the facing problems have been addressed. In this manuscript, major problems are described and a new separation scheme based on modified ICA algorithms is proposed. Finally, our experiment results corroborate the effectiveness of our proposed approach. KEY WORD: Underwater acoustic applications, passive acoustic tomography, second order statistics in frequency domain, multipath channel, sparseness or non-stationary signals

Acoustic oceanographic buoy (version 1)

SiPLAB Report 06/05, FCT, University of Algarve,2005.One of the known impairements in the application of ocean acoustic tomography in operational scenarios has been the size, weight and difficulty of operation of actual ocean going equipment, such as hydrophone arrays and acoustic sources. The Acoustic Oceanographic Buoy - AOB is meant to be, at its final stage, an easy to deploy and easy to maintain autonomous vertical array that puts together in a single system acoustic and non-acoustic sensors, self-storing of geotime and GPS referred data, on the buoy preprocessing capabilities and network seamless integrability and data online transfer via a wireless lan high speed link. These capabilities make the AOB a versatile system for a variety of applications such as ocean tomography, underwater communications, rapid environmental assessment (REA), passive and multistatic sonar and underwater target tracking. The AOB version 1 described in this report was developed between 2003 and 2004 and tested at sea during the MREA’03 and MREA’04 sea trials

Oceanus.

v. 25, no. 2 (1982

Efficient and Stable Acoustic Tomography Using Sparse Reconstruction Methods

We study an acoustic tomography problem and propose a new inversion technique based on sparsity. Acoustic tomography observes the parameters of the medium that influence the speed of sound propagation. In the human body, the parameters that mostly influence the sound speed are temperature and density, in the ocean - temperature and current, in the atmosphere - temperature and wind. In this study, we focus on estimating temperature in the atmosphere using the information on the average sound speed along the propagation path. The latter is practically obtained from travel time measurements. We propose a reconstruction algorithm that exploits the concept of sparsity. Namely, the temperature is assumed to be a linear combination of some functions (e.g. bases or set of different bases) where many of the coefficients are known to be zero. The goal is to find the non-zero coefficients. To this end, we apply an algorithm based on linear programming that under some constrains finds the solution with minimum l0 norm. This is actually equivalent to the fact that many of the unknown coefficients are zeros. Finally, we perform numerical simulations to assess the effectiveness of our approach. The simulation results confirm the applicability of the method and demonstrate high reconstruction quality and robustness to noise

The value of remote sensing techniques in supporting effective extrapolation across multiple marine spatial scales

The reporting of ecological phenomena and environmental status routinely required point observations, collected with traditional sampling approaches to be extrapolated to larger reporting scales. This process encompasses difficulties that can quickly entrain significant errors. Remote sensing techniques offer insights and exceptional spatial coverage for observing the marine environment. This review provides guidance on (i) the structures and discontinuities inherent within the extrapolative process, (ii) how to extrapolate effectively across multiple spatial scales, and (iii) remote sensing techniques and data sets that can facilitate this process. This evaluation illustrates that remote sensing techniques are a critical component in extrapolation and likely to underpin the production of high-quality assessments of ecological phenomena and the regional reporting of environmental status. Ultimately, is it hoped that this guidance will aid the production of robust and consistent extrapolations that also make full use of the techniques and data sets that expedite this process

Oceanus.

v. 37, no. 2 (1994

Basin scale simulations of ocean acoustic tomography in the Portuguese exclusive economic zone

Rep. 03/02 - SiPLAB 16/October/2002This internal report describes some of the experiments developed in the eld of Ocean Acoustic Tomography, simulation results regarding the acoustic monitoring of the Portuguese Economic Exclussive Zone and a brief description of binary m-sequences