Mirjam Snellen

An evaluation of the accuracy of shallow water matched field inversion results

In this article the accuracy of geo-acoustic and geometric parameter estimates obtained through matched field inversion (MFI) was assessed. Multi-frequency MFI was applied to multi-tone data (200-600 Hz) received at a 2-km source/receiver range. The acoustic source was fixed and the signals were received at a vertical array. Simultaneously with the acoustic transmissions, a CTD (conductivity, temperature and depth)-chain was towed along the acoustic track. A genetic algorithm was used for the global optimization, whereas a normal mode model was applied for the forward acoustic calculations. Acoustic data received at consecutive times were inverted and the stability of the inverted parameters was determined. Also, the parameter estimates were compared with independent measurements, such as multi-channel seismic surveys (for geo-acoustic parameters). The obtained uncertainty in the inversion results was assumed to have two distinct origins. The first origin is the inversion method itself, since each optimization will come up with some solution close to the exact optimum. Parameter coupling and the fact that some parameters hardly influence the acoustic propagation further contribute to this uncertainty. The second is due to oceanographic variability. Both contributions were evaluated through simulation. The contribution of oceanographic variability was evaluated through synthetic inversions that account for the actual sound speed variations as measured by the towed CTD-chain.

Experimental study of geo-acoustic inversion uncertainty due to ocean sound-speed fluctuations.

Acoustic data measured in the ocean fluctuate due to the complex time-varying properties of the channel. When measured data are used for model-based, geo-acoustic inversion, how do acoustic fluctuations impact estimates for the seabed properties? In May 1999 SACLANT Undersea Research Center and TNO-Physics and Electronics Laboratory ~FEL!, conducted a shallow-water experiment on the Adventure Bank off the southwest coast of Sicily, Italy to assess the effects of a time-varying ocean on acoustic propagation and geo-acoustic inversion. A favorable area for acoustic propagation was identified which had slight internal wave activity and a weakly range-dependent bathymetry with sand-like bottom properties. Oceanographic and acoustic measurements were performed continuously over a 3-day period. Broadband ~0.2–3.8 kHz! acoustic signals from a bottom-moored source were transmitted over fixed paths and received on a moored vertical hydrophone array. During the transmissions extensive environmental measurements ~e.g., sound speed, current, sea-surface waveheight, etc.! were made to correlate the time-varying environmental and acoustic data. Modeled acoustic data show time variability which agrees with the measurements. Results illustrate severe problems when modeling shallow-water acoustic propagation at ranges beyond a few kilometers in the frequency band considered. Further, the acoustic fluctuations in time caused erroneous time variability in inverted seabed properties.

Principal Component Analysis of Single-Beam Echo-Sounder Signal Features for Seafloor Classification

Acoustic remote sensing is a useful tool for seafloor characterization. This contribution presents the results of seafloor sediment classification using single-beam echo-sounder (SBES) data based on a phenomenological method. Basic concepts of principal component analysis (PCA) and its applicability to the sediment classification using acoustical data are studied. This mathematical tool, which retains most of the variation of the data, is applied to the SBES echo shape parameters such as total energy, time-spread, skewness and flatness on three low (12 kHz), moderate (38 kHz), and high (200 kHz) frequencies, making 12 features in total. These parameters are dependent on sediment types and can therefore be used as attributes for seafloor classification. To decrease the statistical fluctuations of the extracted features, an averaging over a sufficiently large number of consecutive pings have been applied to the features. The SBES classification results based on the PCA and K-means clustering approach can clearly discriminate between different sediment classes. The signal at 12 kHz contains information on sediment layers (5 m depth). The performance of the method and the results obtained are assessed using the following independent criteria: 1) inspection of the track crossings indicates stable feature extraction and processing strategy; 2) comparison between the class numbers of the classification results and of the grab samples shows a significant correlation coefficient of 0.90; and 3) an error matrix verifies the stability and independence of the classification results from the features considered.

Riverbed sediment classification using multi-beam echo-sounder backscatter data

A method has recently been developed that employs multi-beam echo-sounder backscatter data to both obtain the number of sediment classes and discriminate between them by applying the Bayes decision rule to multiple hypotheses [Simons and Snellen, Appl. Acoust. 70, 1258-1268 (2009)]. In deep water, the number of scatter pixels within the beam footprint is large enough to ensure Gaussian distributions for the backscatter strengths and to increase the discriminative power between acoustic classes. In very shallow water (<10 m), however, this number is too small. This paper presents an extension of this high-frequency methodology for these environments, together with a demonstration of its performance using backscatter data from the river Waal, The Netherlands. The objective of this work is threefold. (i) Increasing the discriminating power of the classification method: high-resolution bathymetry data allow precise bottom slope corrections for obtaining the true incident angle, and the high-resolution backscatter data reduce the statistical fluctuations via an averaging procedure. (ii) Performing a correlation analysis: the dependence of acoustic backscatter classification on sediment physical properties is verified by observing a significant correlation of 0.75 (and a disattenuated correlation of 0.90) between the classification results and sediment mean grain size. (iii) Enhancing the statistical description of the backscatter intensities: angular evolution of the K-distribution shape parameter indicates that the riverbed is a rough surface, in agreement with the results of the core analysis.

Predicting Spatial Variability of Sediment Properties From Hydrographic Data for Geoacoustic Inversion

Seafloor classification using acoustic remote sensing techniques is an attractive approach due to its high coverage capabilities and limited costs compared to taking samples of the seafloor. This paper focuses on the characterization of sediments in a coastal environment by combining different hydrographic systems, which are a multibeam echosounder (MBES), a single-beam echosounder (SBES), and seismic systems. The area is located close to the west coast of Italy, southeast of Elba Island, which is known to be composed of very fine-grained material. Both MBES and SBES are, in general, high-frequency systems (100 kHz), providing bathymetry and backscatter information of the upper part of the sea bottom. MBES systems provide this information with a high resolution, due to the beam opening angle of typically 1 -3 , and high coverage. An SBES provides measurements directly underneath the ship only, but is widespread. For the classification by means of MBES data, we use the Bayesian approach, employing backscatter measurements per beam. For the SBES, echo shape parameters are determined and are combined in a principal component analysis (PCA). Both approaches give results that are in very good agreement with respect to the distribution of different surficial sediment types. Complementary, low-frequency seismic systems ( 20 kHz) give insight into the sediment layering. Combining the different acoustic approaches is shown to be an essential ingredient for establishing the environmental picture. This picture is of use for a large range of applications, such as habitat mapping, cable laying, or mine hunting. For the current research, it is aimed to act as a basis for selecting areas for subseafloor sediment classification by geoacoustic inversion techniques. Contrary to the hydrographic systems, geoacoustic inversion techniques provide the actual physical properties, i.e., densities, compression and shear wave speeds, and respective attenuations of the sediment body, and allow sediment characterization over large areas without the need to cover the complete area. A validation is given that the environmental picture, obtained by the hydrographic systems, indeed identifies regions with different acoustic properties.

Model-based sediment classification using single-beam echosounder signals

Acoustic remote sensing techniques for mapping sediment properties are of interest due to their low costs and high coverage. Model-based approaches directly couple the acoustic signals to sediment properties. Despite the limited coverage of the single-beam echosounder (SBES), it is widely used. Having available model-based SBES classification tools, therefore, is important. Here, two model-based approaches of different complexity are compared to investigate their practical applicability. The first approach is based on matching the echo envelope. It maximally exploits the information available in the signal but requires complex modeling and optimization. To minimize computational costs, the efficient differential evolution method is used. The second approach reduces the information of the signal to energy only and directly relates this to the reflection coefficient to obtain quantitative information about the sediment parameters. The first approach provides information over a variety of sediment types. In addition to sediment mean grain size, it also provides estimates for the spectral strength and volume scattering parameter. The need to account for all three parameters is demonstrated, justifying computational expenses. In the second approach, the lack of information on these parameters and the limited SBES beamwidth are demonstrated to hamper the conversion of echo energy to reflection coefficient.

An environmental assessment in the Strait of Sicily: measurement and analysis techniques for determining bottom and oceanographic properties

In October 1997, the EnVerse 97 shallow-water acoustic experiments were jointly conducted by SACLANT Centre, TNO-FEL, and DERA off the coast of Sicily, Italy. The primary goal of the experiments was to determine the sea-bed properties through inversion of acoustic data. Using a towed source, the inversion method is tested at different source/receiver separations in an area with a range-dependent bottom. The sources transmitted over a broadband of frequencies (90-600 Hz) and the signals were measured on a vertical array of hydrophones. The acoustic data were continuously collected as the range between the source and receiving array varied from 0.5 to 6 km. An extensive seismic survey was conducted along the track providing supporting information about the layered structure of the bottom as well as layer compressional sound speeds. The oceanic conditions were assessed using current meters, satellite remote sensing, wave height measurements, and casts for determining conductivity and temperature as a function of water depth. Geoacoustic inversion results taken at different source/receiver ranges show sea-bed properties consistent with the range-dependent features observed in the seismic survey data. These results indicate that shallow-water bottom properties may be estimated over large areas using a towed source fixed receiver configuration.

Functional Beamforming Applied to Imaging of Flyover Noise on Landing Aircraft

Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r...

AN ASSESSMENT OF THE PERFORMANCE OF GLOBAL OPTIMIZATION METHODS FOR GEO-ACOUSTIC INVERSION

Having available efficient global optimization methods is of high importance when going to a practical application of geo-acoustic inversion, where fast processing of the data is an essential requirement. A series of global optimization techniques are available and have been described in literature. In this paper three optimization techniques are considered, being a genetic algorithm (GA), differential evolution (DE), and the downhill simplex algorithm (DHS). The performance of these three methods is assessed using a test function, demonstrating superior performance of DE. Additionally, the DE optimal setting is determined. As a next step DE is applied for determining the geo-acoustic properties of the upper seabed sediments from simulated seabed reflection loss, indicating good DE performance also for real geo-acoustic inversion problems.

Evanescent wave coupling in a geophysical system: Airborne acoustic signals from the Mw 8.1 Macquarie Ridge earthquake

Atmospheric low-frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.