Qunyan Ren

Acoustic interferometry for geoacoustic characterization in a soft-layered sediment environment

The broadband spectrogram of a moving surface ship usually exhibits striations. Their structure is determined by bottom conditions of the shallow water waveguide and can therefore be used for environmental characterization. A two-step acoustic interferometry technique is proposed to estimate main geoacoustic properties of unconsolidated sediment by exploiting local features of the striations. Their positions at low frequencies are first used to detect the changes in sediment properties with respect to a reference sediment and provide a reliable estimation of the changes through the determination of a frequency shift. Then toward higher frequencies, local frequency-range areas with salient striations are selected to refine the solution with their structure features. The technique is tested with passive acoustic ship run data collected southeast of the island of Elba in the Mediterranean Sea in 2007. Data from the four receivers of a shallow sparse vertical array are processed to estimate the thickness and compression wave speed of a soft clay layer overlying a harder bottom. The results from individual receivers are close and agree well with active inversion results and seismic profiles in the same area. Moreover, a better resolution is obtained by combining these results. This method is demonstrated to be robust to source range uncertainties due to the striation stability to its small variation. The good experimental results suggest the technique is an effective tool for mapping the geoacoustic properties of wide coastal areas with easily deployed receiver systems or even one single receiver.

Passive geoacoustic inversion using waveguide characteristic impedance: a sensitivity study

It is difficult to apply existing active geoacoustic inversion techniques to passive acoustics in the sense of dealing unknown source signature, track, appearance and duration for traditional measurement systems. The capacity of a vector in simultaneously measuring the pressure and vector fields may lead to a novel method for passive geoacoustic inversion. This paper discusses the feasibility of using the vertical waveguide characteristic impedance, i.e., the ratio of pressure over vertical particle velocity for passive geoacoustic inversion. Such impedance is independent of source spectrogram but highly correlated with source range due to the transmission loss and phase difference between the pressure and vertical particle velocity during propagation. Through numerical studies, the vertical waveguide characteristic impedance is demonstrated to be high sensitive to bottom geoacoustic parameters, e.g., sound speeds, attenuation coefficients and densities of the seabed. Promising results suggest the verti...

Modelling flint acoustics for detection of submerged Stone Age sites

In accordance with the increasing industrial interests in the sea areas a two-pronged development can be observed: an appearance of new methodologies for up-to-date and costeffective management and protection of these cultural areas and environmental resources which is underpinned by a parallel development of a legislative framework. For submerged cultural heritage UNESCOs Convention on the Protection of the Underwater Cultural Heritage (UNESCO 2001) forms an important part of this administrative development. The method for acoustic detection of submerged Stone Age sites outlined in this paper will, if it can be demonstrated to work in practice, contribute to the spectrum of ongoing new technologies facilitating access to large amounts of environmental data, useful for the understanding of environmental changes such as rising sea levels and their impact on human cultural systems in prehistory. This paper mainly addresses the acoustical characterization of buried flints for Stone Age underwater archaeology. A finite element time domain method is used to simulate acoustic remote sensing in a realistic environment. The method is capable of accurately representing the complex interplay between the acoustic waves, the sediments and the flints embedded in the cultural layer. The predicted signals, once compared with in-situ measurements, provide the basis for the solution of inverse problems that can pinpoint the presence of worked flint over large areas.

Passive vector geoacoustic inversion in coastal areas using a sequential unscented Kalman filter

An unscented Kalman filter (UKF) for geoacoustic inversion using scalar and vector sound fields created by a passing ship is discussed in this paper. The continuous sound field emitted by a ship of opportunity is processed by the sequential filtering technique to estimate slowly changing environmental properties along the source range. The inversion problem is solved by the UKF with a random-walk parameter model, which is expected to perform well when dealing with highly nonlinear problems. Synthetic geoacoustic inversions are performed using multi-frequency pressure, vertical particle velocity and waveguide impedance (a ratio between pressure and vertical particle velocity) data for the geoacoustic model of a mud environment offshore at the mouth of the Amazon River in Brazil (CANOGA 12). For the preliminary tests, the sound source is composed of a flat spectrum. Numerical results demonstrate that the sequential filtering technique is capable of estimating the evolution of environmental properties along the source range. In practice, ship data have complex time-varying spectral characteristics that can greatly limit the accuracy of broadband or multi-frequency passive applications. Since the vertical waveguide impedance is independent of the source spectral level, it is preferred for environmental characterization by the sound field generated from a ship of opportunity. Because of this independence property, the vertical waveguide impedance is expected to yield a more reliable inversion than that of pressure or vertical particle velocity field.

Remote nautical bottom estimation for the safety of navigation offshore Amazon River mouth

In muddy areas, the fluid sediment layer can reach a level of “nautical bottom” that can contact with a ships keel causing either damage or unacceptable effects on controllability and maneuverability. Consequently, the minimum depth and allowed draught need to be determined for the safety of navigation. An acoustic remote sensing technique is proposed to facilitate navigation safety applications through the determination of sediment layer properties. It uses range and frequency-dependent features of the vertical waveguide characteristic impedance, as defined by the ratio of pressure and vertical particle velocity (or pressure gradient) at a given frequency. Such ratio can circumvent inversion uncertainty due to insufficient knowledge of complex time-varying ship noise spectrum because of its source spectral level independent. Real ship noise data recorded on a compact array offshore at the mouth of the Amazon River in Brazil, 2012, are processed by a global optimization based inversion scheme. The inverte...

A robust passive interferometry technique for sediment geoacoustic characterization

Due to the dispersive characteristics of acoustic propagation in shallow water, the broad-band sound field excited by a passing ship usually exhibits an interference structure with the form of striations in the space-frequency plane. Waveguide invariant theory is derived to interpret the striation slope and has been used for underwater inverse problems including sediment geoacoustic characterization, source localization, target recognition and others. Owing to the interference structure processing methods previously used, most applications only use the overall striation slope as acoustic observable, however, the local striation structure that is also closely related to the environmental properties was not considered. In this paper, a passive acoustic interferometry technique is proposed for sediment geoacoustic characterization using local striation features extracted by a multi-scale line filter. Based on the Yellow Shark environmental model, a synthetic study using the proposed method is presented for sediment geoacoustic characterization. The robustness of the passive acoustic interferometry technique to source depth and range uncertainties are also studied by theory analysis and numerical simulation. Finally, the acoustic data due to passing ships collected in Mediterranean Sea in 2007 are processed to test the feasibility of the proposed method.

Striation processing for sediment geoacoustic characterization.

Broadband spectrogram of sound radiated by a moving source, i.e., surface ship, exhibits striations. According to normal mode theory, the slope and position of the striations are related to the modal group and phase speeds, which are determined by the propagation medium and source movement. Waveguide invariant theory provides an interpretation of the phenomenon and has been applied in underwater inverse problems including source localization, sediment geoacoustic characterization, etc. The successful extraction and identification of the striations is a critical step for these applications. In this paper, a multiscale line enhancement filter based on Hessian matrix eigenvalue analysis is adopted for the features’ extraction. Typical shallow‐water environment models are selected for numerical simulations. The interference structure is effectively enhanced, the striations are extracted, and their position and slope are estimated. The effects of varying the respective parameters of a sediment layer on the str...

Bayesian tracking of time or space varying environment from ship noise recorded on a drifting vector sensor

In this paper, a Bayes filter is adapted to determine environmental variation using ship noise data measured on a vector sensor. As compared to batch processing approaches, the Bayesian framework can monitor ocean processes or environments that substantially vary in time or space. The use of vertical impedance (a ratio of pressure and vertical particle velocity) is emphasized here, which is shown to be source spectral independent but highly sensitive to environmental properties. The scenario tested model is inspired from environmental and acoustic data collected at the Amazon River mouth in June 2012. Results show that Bayesian approach can effectively resolve environmental properties variations along range, suggesting the feasibility of using this approach for complex range-dependent environmental characterization.

Particle filtering for geoacoustic characterization of the soft sediment using ship noise

In coastal areas, the properties of the soft sediment are crucial for sound propagation prediction and relevant sonar applications. The noise of intense ship traffic is a convenient acoustic source in these areas for geoacoustic characterization. This paper introduces a sequential particle filter (PF) technique that can take advantage of the continuous ship sound field to predict the soft sediment properties along ship track. The PF is based a point mass (or “particle”) representation of the probability density function associated to different parameters. The particle set evolves recursively according to newly input data, which allows updating the estimation in real time and suitable for dynamical system monitoring if used properly. The approach is tested on synthesized vertical waveguide impedance data to characterize the sediment properties for the obtained bottom geoacoustic model offshore the Amazon River mouth. The vertical waveguide impedance is proven to be source spectrum independent and valuable for passive geoacoustic inversion. Simulation results demonstrate that the PF provides a statistical estimation of the geoacoustic parameters close to true values versus range, in addition, the intrinsic features of the PF filtering technique in successively updating the estimate suggest our approach is capable of characterizing more complex environments with varying geometries.

Non-invasive characterization of fluid mud from scalar and vector noise fields due to a small boat

The passive geoacoustic characterization of sediment using the ratio between pressure and vector field that are measured by an easily deployable system is discussed. The ratio is very sensitive to environmental properties but independent of unknown source spectrogram, e.g., boat noise that exhibits complex spectral shape. Noise data sets due to different runs with a small boat were recorded on two closely adjacent hydrophones offshore the Amazon Rio mouth and processed by a nonlinear inversion scheme. Global optimization based on genetic algorithms provides the geoacoustic parameters of the fluid mud and underlying mud sediment, and the marginal posterior probabilities. Good consistency among the respective inversion results demonstrates the feasibility of the method under far from ideal conditions for environmental characterization, in term of unknown range, source ship navigation data, unknown source spectra, uncertain receiver depth, tilt, etc.