Pierre Cervenka

Differential phase estimation with the SeaMARCII bathymetric sidescan sonar system

A maximum-likelihood estimator is used to extract differential phase measurements from noisy seafloor echoes received at pairs of transducers mounted on either side of the SeaMARC II bathymetric sidescan sonar system. Carrier frequencies for each side are about 1 kHz apart, and echoes from a transmitted pulse 2 ms long are analyzed. For each side, phase difference sequences are derived from the full complex data consisting of base-banded and digitized quadrature components of the received echoes. With less bias and a lower variance, this method is shown to be more efficient than a uniform mean estimator. It also does not exhibit the angular or time ambiguities commonly found in the histogram method used in the SeaMARC II system. A figure for the estimation uncertainty of the phase difference is presented, and results are obtained for both real and simulated data. Based on this error estimate and an empirical verification derived through coherent ping stacking, a single filter length of 100 ms is chosen for data processing applications. >

Sidescan sonar image processing techniques

A four-step processing sequence is described to produce image mosaics from the various segments of a sidescanned acoustic imaging survey of a given seafloor area. Starting with data consisting for each ping of acoustic backscatter levels versus horizontal range across-track, median prefiltering is used first to reduce the influence of outliers on subsequent linear processes. Artifacts that are clearly unrelated to the backscattering properties of the seafloor are then isolated on a ping by ping basis through a spectral analysis that relies on a decomposition using Chebyshev polynomials to filter the low spatial frequency components of the image. Contrast enhancement is then achieved through an original implementation of the classical gray level histogram equalization technique by balancing local versus global histogram contributions. Pixels are mapped on a geographic grid taking due account of the geometry of the measurement and of the spacing between pings to minimize along-track smearing of features. Examples of results obtained with these processing techniques are given for SeaMARC II data recorded during a complete survey of Fieberling Guyot (32 degrees .5 N, 128 degrees W). >

A new efficient algorithm to compute the exact reflection and transmission factors for plane waves in layered absorbing media (liquids and solids)

This paper describes a matrix method for computing the exact reflection and transmission coefficients for harmonic plane waves within a stratified medium of homogeneous, isotropic, and absorbing plane layers. The new feature is that each layer can be either liquid or solid, whatever their successive order. Furthermore, this algorithm takes into account evanescent waves, but also applies whatever the thickness of each layer. A numerical example is shown.

Geometric Corrections on Sidescan Sonar Images based on Bathymetry. Application with SeaMARC II and Sea Beam Data

Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.

Fourier formalism for describing nonlinear self-demodulation of a primary narrow ultrasonic beam

Presented here is the derivation of nonlinear interactions that occur within a primary narrow beam for which the temporal spectrum is continuous and narrow. This follows the bases of the Fourier formalism. Acoustics levels are presumed weak enough so that second‐order equations may be used. In the quasilinear case, the exact theoretical expression of the created parametric farfield, formed from a transient modulated primary signal, is established, by using weakly restrictive assumptions. The case of high primary levels is discussed. Some experimental results are presented.

Non intrusive measurements of the acoustic pressure and velocity fluctuations of fluids flowing in pipes

A nonintrusive flowmeter based on nonlinear acoustical interactions, has been developed. This tool measures simultaneously the pressure and velocity fluctuations of fluids flowing in pipes. A transmitter produces a CW ultrasonic beam which crosses the tube wall, propagates through the opposite side, and is collected by a receiving transducer. Both transducers are just put against the tube wall so that it is not necessary to drill holes in order to install the probes. The ultrasonic propagation across the tube is modelled using an exact matrix numerical method which allows one to specify the geometry of the device. The high frequency beam and the low frequency flow fluctuation interact nonlinearly so that the ultrasonic signal is phase modulated. The amplitude of the modulation (about a hundredth radian) is a linear combination of the pressure and velocity fluctuations. Specific electronics, based on a PLL technique, has been developed for demodulating the signal. We present measurements obtained after generating flow fluctuations in a cavity excited over a broad spectrum

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.