O. C. Rodríguez

Seabed geoacoustic characterization with a vector sensor array.

This paper proposes a vector sensor measurement model and the related Bartlett estimator based on particle velocity measurements for generic parameter estimation, illustrating the advantages of the Vector Sensor Array (VSA). A reliable estimate of the seabed properties such as sediment compressional speed, density and compressional attenuation based on matched-field inversion (MFI) techniques can be achieved using a small aperture VSA. It is shown that VSAs improve the resolution of seabed parameter estimation when compared with pressure sensor arrays with the same number of sensors. The data considered herein was acquired by a four-element VSA in the 8-14 kHz band, during the Makai Experiment in 2005. The results obtained with the MFI technique are compared with those obtained with a method proposed by C. Harrison, which determines the bottom reflection loss as the ratio between the upward and downward beam responses. The results show a good agreement and are in line with the historical information for the area. The particle velocity information provided by the VSA increases significantly the resolution of seabed parameter estimation and in some cases reliable results are obtained using only the vertical component of the particle velocity.

Physical limitations of travel-time-based shallow water tomography

Travel-time-based tomography is a classical method for inverting sound-speed perturbations in an arbitrary environment. A linearization procedure enables relating travel-time perturbations to sound-speed perturbations through a kernel matrix. Thus travel-time-based tomography essentially relies on the inversion of the kernel matrix and is commonly called “linear inversion.” In practice, its spatial resolution is limited by the number of resolved and independent arrivals, which is a basic linear algebra requirement for linear inversion performance. Physically, arrival independency is much more difficult to determine since it is closely related to the sound propagating channel characteristics. This paper presents a brief review of linear inversion and shows that, in deep water, the number of resolved arrivals is equal to the number of independent arrivals, while in shallow water the number of independent arrivals can be much smaller than the number of resolved arrivals. This implies that in shallow water th...

Seismo-acoustic ray model benchmarking against experimental tank data

Acoustic predictions of the recently developed traceo ray model, which accounts for bottom shear properties, are benchmarked against tank experimental data from the EPEE-1 and EPEE-2 (Elastic Parabolic Equation Experiment) experiments. Both experiments are representative of signal propagation in a Pekeris-like shallow-water waveguide over a non-flat isotropic elastic bottom, where significant interaction of the signal with the bottom can be expected. The benchmarks show, in particular, that the ray model can be as accurate as a parabolic approximation model benchmarked in similar conditions. The results of benchmarking are important, on one side, as a preliminary experimental validation of the model and, on the other side, demonstrates the reliability of the ray approach for seismo-acoustic applications.

Tidal Effects on MFP Via the Intimate96 Test

Examining Intimate96 field data we see clearly the effects of tidal changes, i.e., of changing water depths for a bottom-tethered vertical array at mid-frequencies (300 to 800 Hz). This work will examine the sensitivity of such data via Matched Field Processing (MFP) to tidal changes where the depth varies ±1.0 m from the nominal of 135 m. Is it possible to invert such data to estimate water depth as a function of time (tides)? Are the data dominated by source range estimates where water depths are known to shift in a predictable fashion as a function of source range errors? Results reported here will be for simulated data.

NONLINEAR SOLITON INTERACTION WITH ACOUSTIC SIGNALS: FOCUSING EFFECTS

The problem of nonlinear interaction of solitary wave packets with acoustic signals has been intensively studied in recent years. A key goal is to explain the observed transmission loss of shallowwater propagating signals, which has been found to be strongly time-dependent, anisotropic, and sometimes exhibited unexpected attenuation vs. frequency. Much of the existing literature considers the problem of signal attenuation in a static environment, without considering additional effects arising from groups of solitons evolving both in range and time. Hydrographic and acoustic data from the INTIMATE’96 experiment clearly exhibit the effects of soliton packets. However, in contrast with reported observations of signal attenuation, the observed transmission loss shows a pronounced signal enhancement that behaves like a focusing effect. This focusing is correlated with peaks in current, temperature, and surface tide. That correlation suggests that the nonlinear interaction of solitary wave packets with acoustic signals can lead to a focusing of the signal. To clarify this issue, hydrographic data was used to generate physically consistent distributions of “soliton-like” fields of temperature and sound velocity. These distributions were then used as input for a range-dependent normal-mode model. The results strongly support the hypothesis that the soliton field causes the observed signal enhancement.

Kernel-Function-Based Models for Acoustic Localization of Underwater Vehicles

This paper proposes a novel design for the localization system of autonomous underwater vehicles (AUVs) using acoustic signals. The solution presented exploits models based on kernel functions with two main purposes: 1) to reject outliers; and 2) to correct or improve accuracy of measurements. The localization system discussed is based on well-established techniques such as support vector data description (SVDD) and autoassociative kernel regression (AAKR) derived from machine learning theory that utilizes heuristic models for classification and regression tasks, respectively. By coupling the algorithm to the navigation system, we seek to reduce the sensitivity of the localization scheme to the reflected acoustic waves or fluctuations of underwater channel properties without modifying the solution used for data fusion or overloading the algorithm embedded in the vehicle. Data collected in the field with a light underwater vehicle (LAUV) were used to demonstrate the advantages of the proposed approach.

Vector sensor geoacoustic estimation with standard arrays

Vector Sensor Arrays (hereafter VSAs) are progressively becoming more and more attractive among the underwater acoustics community due to the advantages of VSAs over standard arrays of acoustic hydrophones. While the later record only acoustic pressure, VSAs record also particle velocity; such technical feature increases by a factor of four the amount of information that can be used for the processing ofacoustic data, leading to a substantial increase in performance. Since VSA sensor technology is relatively recent, and thus not yet fully available, one can consider the usage of closely located pairs of standard hydrophones, which can be used to estimate the vertical component of particle velocity as a difference of acoustic pressure, measured at each pair of hydrophones. The present discussion introduces a theoretical review of particle velocity calculations using different acoustic models, and tests the performance of estimators for geoacoustic inversion using acoustic pressure, particle velocity compon...

Using passive acoustics for monitoring seagrass beds

This paper discusses the ambient noise acquired during the period of one week, from May 8 to 15, 2013 over a Posidonia oceanica bed in the Bay of la Revellata, Calvi, Corsica. The acoustic receivers were moored at 3 locations with water depth ranging from 2 to 20 m. Simultaneously with acoustic measurements, the dissolved O2 was measured by an array of optodes. Preliminary results have shown that the acoustic noise power measured at various locations in the meadow is highly correlated with dissolved O2 (measured by other methods). Nevertheless, a close inspection of noise waveforms shows impulsive signals with shape similar to those produced by snapping shrimp and other shell fish. This work discusses the challenges faced on using these noise waveforms as ecosystem indicators and particularly to monitor the photosynthetic activity.

Variability of the ambient noise in a seagrass bed

This paper discusses the ambient noise acquired during the period of one week, from May 8 to 15, 2013 over a Posidonia oceanica bed in the Bay of la Revellata, Calvi, Corsica. The acoustic receivers were moored at 3 locations with water depth ranging from 2 to 20 m. Simultaneously with acoustic measurements, the dissolved O2 was measured by an array of optodes. The noise field in the band 2-7 kHz was dominant. The noise directionality and bottom reflectivity estimated using a short vertical array show a diurnal variability pattern. The mean noise power measured at all receivers shows also a diurnal variability pattern, which is negatively correlated with the O2 dissolved in the water. Changes in noise power between the night (highest noise power) and day period (lowest noise power) as high as 5 dB were observed at the various locations. These results suggest that the ambient noise power and directionality are correlated with the photosynthetic activity of the seagrass meadow and that a passive acoustic system can be used to monitor the O2 production of the meadow.

A time-reversal suboptimal detector for underwater acoustic barriers

This paper proposes an experimental setup composed of two interconnected vertical arrays: one transmit only array (TOA) and the other a receive only array (ROA). It is shown that using the time reversal principle, where the ocean is used as a spatial matched filter, signal energy can be simultaneously focused on each ROA hydrophone and thus obtain the detector output by simple summation of the received energy over the array. This setup effectively configures a multistatic system with several transmitters and several receivers coherently processed both in time and space. Simulations using a ray trace propagation model combined with a naive scattering formulation show that the obtained empirical detection probability is close to the theoretical optimal bound derived assuming space - time white Gaussian noise. This system was tested in a sea trial that took place during September 2007 in the Hopavagen Bay near Trondheim, Norway. The actual setup was composed of a 2 sources TOA on a shallow area of 8 m depth near the shore and a 4 receivers ROA approximately 100 m apart in a 8 to 25 m depth range dependent duct. The transmitted signals were 200 ms duration LFMs with 3 kHz bandwidths in two frequency bands centered on 5 and 10 kHz. The results obtained in realistic conditions show that a 1.7 m2 hard plate could be detected when placed across the barrier. Several results are presented and compared with the theoretical values. Although the system can be significantly improved, in particular by using more populated source and receiving arrays, it is believed that these results can be reproduced at sea in harbor like conditions.