Shallow Water Experiment Research Articles

Inversion for Range-Dependent Sediment Compressional-Wave-Speed Profiles From Modal Dispersion Data

A perturbative inversion method for estimating sediment compressional-wave-speed profiles from modal travel-time data is extended to include range-dependent environments. The procedure entails dividing a region into range-independent sections and obtaining estimates of the sediment properties for each region. Inversion results obtained using synthetic data show that range-dependent properties can be obtained if an experiment is designed to include multiple source/receiver combinations. This approach is applied to field data collected during the 2006 Shallow Water Experiment (SW06). The sediment compressional-wave-speed profiles resulting from analysis of the field data are evaluated by comparing acoustic fields predicted based on the inversion to acoustic fields measured during a different experiment conducted in the same region. The model is also compared to seismic reflection survey data collected during SW06. Resolution and variance estimated for the inversion results are also presented.

Accounting for spatiotemporal variability of shallow water waveguides in the geoacoustic inverse problem.

Reliable acoustic field predictions are essential for sonar performance modeling and matched field processing, particularly related to the geoacoustic inverse problem. On continental shelves and slopes, properties of the water column can vary widely in both time and space, while the seabed can be heterogeneous over scales of only a few kilometers and sometimes less. This combined spatiotemporal variability of the acoustic waveguide was a hallmark of the Shallow Water Experiment 2006 (SW06). This is illustrated using data collected during SW06 for an acoustic field measured on a fixed receiver array for a source towed along a radial track and repeated several times. In these experiments, the large degree of variability in the observed acoustic fields was attributed to large-scale changes in the water column that occurred over the course of a day. Properly accounting for this variability proved to be essential for obtaining robust geoacoustic inversion results for the seabed. Specifically, the spatiotemporal properties of the water column had to be accounted for in the forward models from which objective functions were derived. Methods for capturing the observed water-column variability are discussed as used in the inverse problem as well as for the more general problem of model-data comparison.

Absolute seafloor vertical positioning using combined pressure gauge and kinematic GPS data

Knowledge of the position and motion of points on the seafloor can be critically important in both fundamental research (for example, global geodesy and plate tectonics) and for more practical applications such as seismic risk evaluation, off-shore construction and pipeline monitoring. In the Vanuatu subduction zone, for example, measuring deformation underwater could provide valuable information for modeling deformation and understanding the seismic cycle. We report a shallow water experiment in Vanuatu to measure the relative and absolute depth of seafloor points. The experiment differs from previous efforts mainly in that it uses the height of the sea surface determined by kinematic GPS, allowing us to locate the points in a global reference frame. The ITRF2005 ellipsoidal height of a seafloor benchmark was determined with a 1-sigma uncertainty of 0.7–2.1 cm. The estimated ellipsoidal height differs only by a few tenths of a centimeter between measurements made in 2004 and another set made in 2006. These results are encouraging and open new perspectives for vertical underwater deformation monitoring in shallow water areas. Sea-surface GPS measurements can also help to reduce the uncertainty in depth difference determination for relative measurements.

Coherence-Time of Matched-Field Processing in Shallow Water in the Presence of Linear Internal Waves

Relationships among the signal coherence-time of matched-field processing (MFP), the acoustic frequency, the source-receiver range, and the sound speed standard deviation (STD) caused by internal waves in shallow water, are numerically investigated based on oceanographic data from two shallow water experiments. It is found that the coherence-time can be fitted with an inverse square-root power of range, a near inverse 1 power of frequency, and inverse 1.3 power of sound speed STD.

Laboratory model of the Obukhov geostrophic vortex

The results of laboratory modeling of geostrophic adjustment in a shallow-water layer in rotating paraboloid are presented. According to the Rossby-Obukhov theory, this process excites nonstationary wave and stationary vortex (geostrophic) components of motion in a rotating fluid. In our experiments, the wave and vortex components were excited by extracting a preliminarily imbedded hemisphere (which made the initial distribution of the depth of the fluid inhomogeneous) from the central area of a rotating vessel with a parabolic base. Under this excitation technique, a prominent cyclonic eddy is formed in the central portion; the structure of this eddy is satisfactorily described within the linear theory of adjustment. Along with the shallow-water experiments, the published experimental data on modeling geostrophic adjustment in a two-layer medium are analyzed. A simple analytic solution to the corresponding problem of the adjustment theory is obtained, and this solution agrees with the experiment.

The impact of ocean sound speed variability on the uncertainty of geoacoustic parameter estimates

This paper investigates the influence of water column variability on the estimates of geoacoustic model parameters obtained from matched field inversions. The acoustic data were collected on the New Jersey continental shelf during shallow water experiments in August 2006. The oceanographic variability was evident when the data were recorded. To quantify the uncertainties of the geoacoustic parameter estimates in this environment, Bayesian matched field geoacoustic inversion was applied to multi-tonal continuous wave data. The spatially and temporally varying water column sound speed is parametrized in terms of empirical orthogonal functions and included in the inversion. Its impact on the geometric and geoacoustic parameter estimates is then analyzed by the inter-parameter correlations. Two different approaches were used to obtain information about the variation of the water sound speed. One used only the profiles collected along the experimental track during the experiment, and the other also included observations collected over a larger area. The geoacoustic estimates from both the large and small sample sets are consistent. However, due to the diversity of the oceanic sound speed, more empirical orthogonal functions are needed in the inversion when more sound speed profile samples are used.

Shallow-Water Acoustic Tomography Performed From a Double-Beamforming Algorithm: Simulation Results

Recent shallow-water experiments in sea channels have been performed using two vertical coplanar densely sampled source and receive arrays. Applying a double-beamforming algorithm on the two arrays both on synthetic numerical simulations and on experimental data sets, we extract efficiently source and receive angles as well as travel times for a large number of acoustic rays that propagate and bounce in the shallow-water waveguide. We then investigate how well sound-speed variations in the waveguide are reconstructed using a ray time-delay tomography based on a Bayesian inversion formulation. We introduce both data and model covariance matrices and we discuss on the synthetic numerical example how to choose the a priori information on the sound-speed covariance matrix. We attribute the partial sound-speed reconstruction to the ray-based tomography and we suggest that finite-frequency effects should be considered as the vertical and horizontal size of the Fresnel zone significantly spreads in the waveguide. Finally, the contribution of a different set of ray angles for tomography goal is also presented.

Numerical investigation of out-of-plane sound propagation in a shallow water experiment

In an experiment in the Florida Straits, broadband pulses were transmitted over a range of 10 km and received by a vertical hydrophone array. For pulses with center frequency below 400 Hz, the received signal consisted of a dominant arrival followed by a secondary one delayed by about 0.4 s. A hypothesis that the secondary arrival was caused by 3D out-of-plane propagation is investigated here numerically with a 3D parabolic equation model (3DWAPE) and a 3D ray model (MOC3D). Both models clearly predict a secondary arrival caused by 3D horizontal refraction from the sloping bottom in the shoreward direction.

Low- to mid-frequency model of attenuation and dispersion.

A model of attenuation and sound speed in ocean sediments in the low- to mid-frequency range is presented. It is based on a combination of the Biot–Stoll and contact squirt and shear drag model [Chotiros and Isakson, J. Acoust. Soc. Am. 116, 2011–2022, (2004)] and the frame virtual mass model [Chotiros and Isakson, J. Acoust. Soc. Am. 121, EL70 (2007)]. This combination can match measured sound speeds below Wood’s equation lower bound and it addresses issues related to the Kramers–Kronig relationship. It is able to match both the sound speed dispersion and attenuation measurements from a large number of sites, including the sediment acoustics experiments (SAX99 and SAX04) in the Gulf of Mexico, the shallow water experiment on the Atlantic coast, and the Yellow Sea [Zhou, J. Acoust. Soc. Am. 78, 1003–1009 (1985)]. [Work supported by ONR, Ocean Acoustics.]

Spatial extension of the acoustic time reversal region

According to the data of a full-scale shallow-water experiment (in the Barents Sea, at sea depths of about 120 m), a considerable gain in the signal-to-noise ratio is obtained for an acoustic signal received from a source at a distance of 12 km when matching with the medium is performed by the signal from the same source at a distance of 10.5 km. To interpret this experimental fact, a numerical simulation is performed to determine the size of the region of signal focusing due to the time reversal of waves in an ideal waveguide with a soft bottom. It is shown that, for narrowband signals, within a distance of ±5 km along the path from the point of emission of the reversed signal, a regular interference pattern whose maxima are comparable with the principal maximum is observed throughout the whole waveguide depth. For a spectrum width from 100 to 300 Hz, only the principle maximum with an extension of about 100 m is observed at a single depth.

Modal processing for acoustic communications in shallow water experiment

Acoustical array data from the Shallow Water Acoustics experiment was processed to show the feasibility of broadband mode decomposition as a preprocessing method to reduce the effective channel delay spread and concentrate received signal energy in a small number of independent channels. The data were collected by a vertical array designed at the Woods Hole Oceanographic Institution. Phase-shift Keying (PSK) m-sequence modulated signals with different carrier frequencies were transmitted at a distance 19.2 km from the array. Even during a strong internal waves activity a low bit error rate was achieved.

Transmission loss measurements and geoacoustic sensitivity modeling at 1.2kHz

The Shallow Water Experiment 2006 was conducted off the coast of New Jersey in the summer of 2006. Defence Research and Development Canada-Atlantic performed a series of experiments designed to validate the use of rapid environmental assessment tools and methods to improve active sonar performance predictions. The sensitivity of acoustic propagation to a varying or uncertain environment is determined by examining the relative change of acoustic pressure caused by environmental variability, using the method described recently [Dosso et al., J. Acoust. Soc. Am. 121, 42 (2007)]. The variability of the modeled environmental parameters is based on measured and estimated oceanographic and geoacoustic properties. The resulting sensitivity is compared to measured transmission loss data at 1.2 kHz.

Geoacoustic inversion on the New Jersey Margin: Along and across the shelf

This paper presents results of a range-independent perturbative inverse approach applied to data from the Shallow Water Experiment 2006. The inversion technique is based on a linearized relationship between sound speed in the sediment and modal eigenvalues. Horizontal wave numbers were estimated from data collected from two distinct source/receiver tracks oriented along and across the shelf. The specific inversion algorithm used is based on qualitative regularization and uses known information about the environment to constrain the solution. Locations of the R reflector and other layering information are used as a priori information for the inversion to emphasize the layered structure of the sediment.

Frequency Dependence of Longitudinal Correlation Length in the Yellow Sea

Spatial correlation coefficient is one of the most important parameters for the description of sound propagation in shallow water. Frequency dependence of the longitudinal correlation length is still an open topic. We observe in a shallow water experiment that the longitudinal correlation length in units of wavelength increases with the increase of frequency. This phenomenon has not been seen in the published papers. The theoretical analysis and numerical simulations indicate that the non-linear frequency relationship of the bottom attenuation is the main cause of this phenomenon.

Spatial and temporal sound field fluctuations due to propagating internal waves in shallow water

Fluctuating three dimensional acoustic wave propagation in shallow water presents a challenge due to the anisotropic nature of the environment. Temporal and spatial changes in the water column caused by the passage of internal waves are among the the primary causes of the anisotropy. Three main mechanisms for the acoustic field variations have been determined: 1) adiabatic propagation,. 2) horizontal refraction, and 3) mode coupling. All these mechanisms provide different characteristics of the sound fluctuations, and depend on the angle between the internal wave direction and the source receiver propagation path, as well as frequency and mode number. In a recent multi‐institutional shallow water experiment (SW06) a series of source‐receiver positions were created to examine the effects of 3D environmental fluctuations on the propagation of low to mid frequency (0.05‐3 kHz) broadband acoustic signals while extensive environmental observations were made. These recent observational results confirm the above theoretical hypothesis. Work supported by ONR‐321OA and RFBR.

Geoacoustic inversion using combustive sound source signals

In the summer of 2006, Combustive Sound Sources (CSS) were deployed in shallow waters off New Jersey during the Shallow Water experiment (SW 06). The depth of these sources were 26 m in water depths of the order of 100 m. CSS are low frequency broadband sound sources and suitable for modal dispersion based inversion. CSS data collected on various receivers (Single Hydrophone Receiving Units (SHRU), Vertical Line Array, etc.) will be used for the inversions. The inversions are based on an iterative scheme using a new time‐frequency analysis technique (dispersion based short‐time Fourier transform) in which each time‐frequency tiling is adaptively rotated in the time‐frequency plane, depending on the local wave dispersion. Data from various receivers will be used to investigate the spatial variation in compressional wave speed. Compressional wave attenuation will also be estimated using modal amplitude ratios. Inversion results will be validated by comparing the data with model predictions. [Work supported by the Office of Naval Research]

Three dimensional geoacoustic inversion on the New Jersey shelf

Perturbative inversion, based on a linearized relationship between sound speed in the sediment and modal eigenvalues, is applied to data from the Shallow Water Experiment 2006. Data were collected by towing a low‐frequency sound source out and back along radials, spanning a 90 degree angular sector, from a common receiver location. Range‐dependent estimates of horizontal wave numbers are obtained along each of the radials using high‐resolution signal processing techniques capable of detecting and localizing changes in sub‐bottom properties, and that are particularly sensitive to changes in layer structure. Wave number estimates at each range are used in a linearized inversion algorithm to estimate local sediment properties. Locations of the R‐reflector and other layering information are used as a priori information in the inversion algorithm. The additional information both constrains the solution of an otherwise ill‐posed problem and emphasizes the layered structure of the sediment. These methods have been shown to yield accurate estimates of the sound speed profile deep into the sediment using very few perturbations to the forward model. Combining the local inversion results, a three‐dimensional map of sediment sound speed structure is obtained for a 25 km2 region of the seafloor. [Supported by NDSEG and ONR]

Experimental verification of range‐dependent inverse method for geoacoustic parameters from modal dispersion data

The procedure for determining the sediment compressional wave speed profile from modal dispersion data in a range‐independent case is well documented in the literature. In this paper, the procedure developed for the range‐independent case is extended to cover the range‐dependent case. In this procedure the region under investigation is divided into discrete range intervals and the sediment characteristics at each range interval estimated assuming that the sediment properties of each range interval are range independent. The approach has been demonstrated to yield range‐dependent sediment properties when applied to synthetic data generated for an experiment consisting of multiple source/receiver combinations. During the Shallow Water Experiment 2006, broadband data transmitted by a single source were measured on multiple receivers. These data are used here to estimate range‐dependent properties of the sediment. The results obtained in this manner are compared with inversion results obtained for the same region by other investigators who employed different approaches for estimating the geoacoustic properties. [Work supported by ONR, Ocean Acoustics].

Modal inversion using the acoustic field emitted by a moving source and measured on a vertical line array of hydrophones

As part of the Shallow Water Experiment 2006 (SW06), a low‐frequency continuous wave acoustic source was towed out and back along radials from a fixed vertical line array of hydrophones. The resulting modal pressure field, measured on synthetic range apertures created by the relative source motion, is used to derive input data to inversion algorithms for estimating bottom properties. The inversion algorithms considered are based on well established relationship between modal eigenvalues and geoacoustic properties of the sediment. In this talk, linear inversion methods based on modal input data will be reviewed for their application to the SW06 data. The first step in these methods is the accurate estimation of modal eigenvalues from the pressure field data. For a moving source, theory predicts a Doppler shift along with frequency spreading proportional to both the transmitted frequency and tow speed. Propagating modes and their corresponding modal eigenvalues are predicted to correspond with the Doppler shifted frequencies. An emphasis of this work is on Doppler shift as an observable in the data and properly accounting for it in inversion. Sediment sound speeds estimated from data are consistent with sandy sediments found in the experimental area. [Work supported by ONR and NDSEG]

Reconstruction of temporal invariants of the time‐reversal operator for communication in shallow water

Time‐reversal techniques allow focusing in waveguides with application for detection or telecommunication (Kupperman et al., J. Acoust. Soc. Am. 103(1), 25‐40, (1998)). It has been shown that the DORT method (French acronym for Decomposition of the Time Reversal Operator) can be used to achieve detection and selective focusing on pointlike scatterers in a waveguide (Mordant et al., J. Acoust. Soc. Am. 105, 2634‐2642 (1999)). However, the decomposition is done in frequency domain providing eigenvectors V(ω) of undetermined phase. In order to achieve temporal focusing, it is necessary to built 'temporal eigenvectors' V(t) as an appropriate combination of the eigenvectors V(ω). Due to dispersion, the reconstruction of the temporal eigenvectors is difficult in waveguides. One solution was proposed by Mordant et al., using the symmetry of the array response matrix and assuming continuous frequency dependence of the invariants. However this method does not work if eigenvalues crossings occur. Furthermore, it cannot be applied to distinct arrays of transmitters and receivers. We propose a simple method to reconstruct the temporal invariants by using the back propagation of eigenvector in free space. Application to data from a laboratory experiment and from the DOREV 2005 shallow water experiment are presented.