Jacques Marchal

Design and implementation of a multi-octave-band audio camera for realtime diagnosis

Noise pollution investigation takes advantage of two common methods of diagnosis: measurement using a Sound Level Meter and acoustical imaging. The former enables a detailed analysis of the surrounding noise spectrum whereas the latter is rather used for source localization. Both approaches complete each other, and merging them into a unique system, working in realtime, would offer new possibilities of dynamic diagnosis. This paper describes the :design of a complete system for this purpose: imaging in realtime the acoustic field at different octave bands, with a convenient device. The acoustic field is sampled in time and space using an array of MEMS microphones. This recent technology enables a compact and fully digital design of the system. However, performing realtime imaging with resource-intensive algorithm on a large amount of measured data confronts with a technical challenge. This is overcome by executing the whole process on a Graphic Processing Unit, which has recently become an attractive device for parallel computing.

Nonlinear Multi-Frequency Transmitter for Seafloor Characterization

To gather with a single surveying tool the backscatter frequency response of the seafloor would be a large asset to characterize the nature of the bottom. One proposes to take advantage of the saturation effect occurring with a high powered source: numerous harmonic waves are created by non linear interactions along the propagation. The feasibility of a multi-frequency source that is adapted to seafloor characterization is investigated. In a preliminary approach, a pseudo-1D model is used to estimate the on-axis levels of the first multiple frequencies. Experimental measurements performed with two typical geometries of transducers are compared with simulations.

Sparse underwater acoustic imaging: A case study

Underwater acoustic imaging is traditionally performed with beamforming: beams are formed at emission to insonify limited angular regions; beams are (synthetically) formed at reception to form the image. We propose to exploit a natural sparsity prior to perform 3D underwater imaging using a newly built flexible-configuration sonar device. The computational challenges raised by the high-dimensionality of the problem are highlighted, and we describe a strategy to overcome them. As a proof of concept, the proposed approach is used on real data acquired with the new sonar to obtain an image of an underwater target. We discuss the merits of the obtained image in comparison with standard beamforming, as well as the main challenges lying ahead, and the bottlenecks that will need to be solved before sparse methods can be fully exploited in the context of underwater compressed 3D sonar imaging.

Buried mines detection and classification: advanced technologies and signal processing

In order to improve the mine countermeasures capability, there is a need to investigate new techniques which would enable the detection, localisation and classification of buried mines. This paper deals with results obtained in France, under a sonar program involving GESMA and three academic laboratories. An experimental approach has been preferred. Two techniques are under evaluation: the low frequency synthetic aperture sonar (SAS) mounted on a platform as a side scan sonar dedicated to buried mines detection and a sonar mounted just below a platform, looking vertically at the seabed, dedicated to buried mines classification.

Nonlinear multifrequency transmitter for seafloor characterization

In the underwater context, it is known that the frequency diversity provides essential information to derive the nature of the seafloor. This presentation deals with a new concept based on a transmitter that generates simultaneously several harmonic frequencies. Our final objective is to assert the feasibility of a multi‐frequency tool whose desirable characteristics could be specified for applications such as detection of sunken oil slicks, sediment characterization, or surveys before cable or pipe laying. The acoustic beams are generated through the harmonic components of a shock wave radiated by an antenna driven at a high level. The source is unique in time and space so that the multi‐frequency responses are inherently perfectly matched. A numerical model based on a generalized KZK equation has been developed to estimate the saturated fields. Measurements of the first harmonic fields obtained in our outdoor tank facility are compared with simulations.

Two Underwater Examples of Imaging by Multi-Look Synthesis

We point out that images obtained with both systems don’t show any speckle patterns in the area where the redundancy is significant. Therefore, contrast and visibility of relatively small targets is enhanced. Non coherent imaging technique seems to be a promising technique for those who are interested in obtained high resolution imaging at low computational expense. It offers also good prospects in term of mapping rates. Our goal is now to implement this multi-look technique in a real time mapping tool.

A robust and passive method for geometric calibration of large arrays

This paper presents a complete strategy for the geometry estimation of large microphone arrays of arbitrary shape. Largeness is intended here in both number of microphones (hundreds) and size (few meters). Such arrays can be used for various applications in open or confined spaces like acoustical imaging, source identification, or speech processing. For so large array systems, measuring the geometry by hand is impractical. Therefore a blind passive method is proposed. It is based on the analysis of the background acoustic noise, supposed to be a diffuse field. The proposed strategy is a two-step process. First the pairwise microphone distances are identified by matching their measured coherence function to the one predicted by the diffuse field theory. Second, a robust multidimensional scaling (MDS) algorithm is adapted and implemented. It takes advantage of local characteristics to reduce the set of distances and infer the geometry of the array. This work is an extension of previous studies, and it overcomes unsolved drawbacks. In particular it deals efficiently with the outliers known to ruin standard MDS algorithms. Experimental proofs of this ability are presented by treating the case of two arrays. They show that the proposed improvements manage large spatial arrays.

A hybrid method for calibrating acoustic arrays

A method to calibrate the elements of large arrays devoted to underwater applications is presented. The goal is to measure the sensitivity and directivity of the elements over their full bandwidth. The main constraint comes from the bounded geometry of the experimental setups that limits the duration of the time windows available for analyzing the received signals. Using a short wideband pulse is detrimental to obtaining high signal-to-noise ratios. A classical method for handling this problem is time-delay spectrometry (TDS), which is based on the transmission of a linear frequency- modulated signal combined with a sliding frequency filter. An alternative, hybrid method based on the transmission of a sequence of time-frequency-limited signals is proposed. This hybrid method is shown to provide the same spectral density as TDS in the frequency scanning, but the filtering process is quite different. The transmitted signals are designed to take advantage of the coherent sums of the received signals to track the time of flight of the direct paths between the source and the elements. In addition, a fitting process based on the calibration geometry of data acquisition enables the boundaries of the interference-free time windows to be precisely delineated. An example of the application is described.

On Passive Symmetrical Two-Ports, Impedance Conversion and Power Transfer

The theory of linear passive symmetrical two-ports is reviewed. Many results of practical interest can be derived from the very compact analytical basis of this model. However, such derivations are scattered in the literature. In addition, the demonstrations do not always take advantage of the generality that can be obtained by avoiding implementations built on particular applications. A complete, self consistent analysis is presented here. Attention is focused on power transfer and the relations between the impedances at each port. The specific case of transmission lines is finally addressed.

Classification of angular backscattered responses obtained at sea with a forward looking sonar system

The prototype of a multibeam front‐scan sonar has been developed within the frame of a MAST contract (n° MAS3‐CT97‐0090 DG12‐ESCY, acronym COSMOS). A large amount of data has been collected at sea. With the forward looking geometry of acquisition, the foot‐prints of successive pings overlap largely, so that most parts of the surveyed areas are seen under a large range of incidence angles. It gives the capability to derive the local angular response of the bottom, which is indeed an important information for characterizing the nature of the seafloor. This presentation addresses the successive steps that were undergone to classify the backscattered responses: Data are conditioned to perform principal component analyzis; an original clustering identification process is also described; resulting maps are finally presented.