Shallow Water Tomography Research Articles

Passive ocean acoustic tomography in shallow water.

It has been demonstrated that an estimate of an empirical Green's function (EGF) can be extracted from the ocean ambient noise cross-correlation functions, which can provide an alternative method for ocean acoustic tomography. However, the requirement for a long recording time to obtain EGFs with a high signal-to-noise ratio limits the application. This article focuses on using array signal processing to accelerate the convergence rate of EGFs between two horizontally separated arrays. With the extracted EGFs and data assimilation, ocean sound speed profiles (SSPs) can be inverted every 2 h in shallow water. The experimental results indicate that the variation in ocean SSPs can be reconstructed with reasonable agreement using an average variance of 1.14 m/s over three months.

Feasibility of geoacoustic tomography in shallow water

In shallow water, spatial and temporal variability of the water column often restricts accurate estimations of bottom properties from long-range acoustic propagation data. Two recent shallow-water experiments, conducted at a sandy site on the New Jersey Shelf and a muddy site on the New England Shelf, showed 5-10 dB acoustic transmission-loss differences between two sites. The New Jersey Shelf experiment was conducted during May 2015 and The New England Shelf experiment was conducted during November 2015, representing summer and winter propagation conditions, respectively. Acoustic modal analysis and pulse-decay tomography methods were used to map the geoacoustic variability of each site covering a 40 km by 40 km area. Both numerical simulations and the New England Shelf experimental results showed that geoacustic variability can be measured by geoacoustic tomography. Tomographic results from the New Jersey Shelf sites showed less geoacoustic variability that may be due to the presence of a harder bottom....

Acoustic tomography of shallow water with unknown relief of hard bottom

The possibility of implementing tomography in shallow water with unknown relief of bottom is considered. It is shown that the use of time delays of different modes at different frequencies allows for reconstruction of bottom relief and the sound speed profile in the water layer without any additional activities for separating the effects of reconstructed characteristics of shallow water on the received data.

Arrival time fluctuation of normal modes caused by solitary internal waves

Internal waves (IW) can significantly affect sound propagation in shallow water. When arrival times of normal modes are used for ocean tomography, IW-caused variations of the modal arrival time should be taken into account. The sensitivities of the normal mode arrival time to solitary IWs are analyzed by using the SWARM95 environments. It is shown through theoretical analysis that acoustic modes with more energy distribution in the thermocline are more sensitive to the internal waves. The arrival times of these normal modes fluctuate larger with the thermocline depth variation. Moreover, the sensitive mode and insensitive mode can be the same mode, but with different frequencies. By contrast, the modes with smaller grazing angles and propagating mainly below the thermocline have stable arrival time. The results are meaningful for source depth location and frequency selection in shallow water tomography.

Effect of source depth on shallow water tomography

Effect of source depth on shallow water tomography

Travel-time tomography in shallow water: Experimental demonstration at an ultrasonic scale

Acoustic tomography in a shallow ultrasonic waveguide is demonstrated at the laboratory scale between two source-receiver arrays. At a 1/1,000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. Two coplanar arrays record the transfer matrix in the time domain of the waveguide between each pair of source-receiver transducers. A time-domain, double-beamforming algorithm is simultaneously performed on the source and receiver arrays that projects the multi-reflected acoustic echoes into an equivalent set of eigenrays, which are characterized by their travel times and their launch and arrival angles. Travel-time differences are measured for each eigenray every 0.1 s when a thermal plume is generated at a given location in the waveguide. Travel-time tomography inversion is then performed using two forward models based either on ray theory or on the diffraction-based sensitivity kernel. The spatially resolved range and depth inversion data confirm the feasibility of acoustic tomography in shallow water. Comparisons are made between inversion results at 1 and 3 MHz with the inversion procedure using ray theory or the finite-frequency approach. The influence of surface fluctuations at the air-water interface is shown and discussed in the framework of shallow-water ocean tomography.

Shallow water tomography: Ray theory vs travel-time sensitivity kernels.

In underwater acoustics, ray tomography is the classical method used to estimate velocity variations, but recently. Travel-Time Sensitivity Kernels (TSK) approaches have been developed. In this paper, we deal with TSK for two source-receive arrays in an acoustic waveguide for shallow water tomography. As a first step, we show that separation of the different raypaths is improved by using a recently proposed new array processing [time-delay double beamforming (DBF) algorithm]. DBF consists of changing the 3-D data space from source depth, receiver depth and time into a new 3-D space related to ray propagation expressed by the beamformed variables, source angle, receive angle and time. As a consequence, each eigenray of the multipath propagation for a source-receiver couple can be identified and separated through DBF. In this context of DBF, the TSK is no longer point-to-point as usual, but relies on all source-receiver time series. Kernels are computed using the fact that the processed signal is a linear combination of time-delayed signals between all sources and receivers. Results in simulated data and in real datasets recorded in an ultrasonic tank prove that combination of TSK and DBF increases the resolution and robustness performance of shallow-water acoustic tomography.

High-frequency broadband acoustic current tomography in shallow water

To study current tomography in very shallow water regions a simultaneous oceanographic and broadband (1-25 kHz) acoustic experiment was conducted in the Delaware Bay. The mean water depth was 15 m and the source-receiver range was 760 m. In this paper, we discuss the feasibility of using reciprocal acoustic transmission for current tomography applications. A beamforming technique is used to resolve the arrival time of direct and surface-bounced rays since in shallow water the received acoustic signals are more complicated due to multiple interactions with bottom and sea surface. Using the experimental data, the accuracy of travel time measurements for variable environmental conditions is examined for different center frequencies and bandwidths. The current velocity prediction results are compared with ADCP measurements to determine the feasibility of current tomography in shallow water.

A simulation study of shallow water tomography for coastal monitoring

Developing operational oceanographic models for coastal environment is an exciting challenge for the next decades. The typical sparsity of assimilated in‐situ observations often creates biases in the model predictions reducing the overall accuracy of the forecasting. In such a highly dynamic environment, acoustic tomography can be a good candidate to provide synoptic measurements over wide areas while a range‐dependent inversion scheme allows to achieve a reasonable spatial resolution. In this work, we present simulation results of a Kalman‐based assimilation of ocean‐acoustic data for a basic model of the Ushant front west off Britanny. In a first part, a single vertical slice tomography experiment is simulated for a static front model to study in which way the modal propagation of a multifrequency acoustic signal is affected by the characteristics of the front (position, intensity). In a second part, the problem of assimilating full‐field acoustic data into a dynamic model and tracking of the range‐depe...

High-resolution shallow water tomography using an array of sources and receivers

During the FAF05 experiment, two vertical arrays of 29 sources and 32 receivers both covering a large part of the shallow water ocean were located 5 km from each other. A complete acquisition of the waveguide impulse responses between each source and each receiver was performed every 20 s during several hours using broadband pulses at 3.5 kHz. Using time-delay beamforming on both the source and receiver arrays, each ray path was isolated and matched to a ray code based on an a priori model of the environment. Following this approach, the impulse responses data matrices were then inverted over time to obtain a movie of the range- and depth-dependent fluctuations of the sound speed between the two arrays. Spatial resolution is discussed as a function of the number of rays used in the inversion. [Work supported by ONR.]

Time-reversal arrays in underwater acoustics

Since 1996, several time-reversal experiments have been performed in shallow water acoustics involving source and receive arrays made of many elements. This equipment allowed us to reach different goals among which were long-range time-reversal focusing, passive and active multiple input multiple output communications, bottom refocusing from reverberation, acoustic barrier detection, comparison between reciprocal and nonreciprocal time-reversal and shallow water tomography. This presentation will review this work with emphasis on array signal processing.

Modal wave number tomography and bottom parameter dependence

A shallow water tomography scheme based on the modal wave number inversion technique is considered in this paper. The scheme is based on the assumption that modal wave number for transpped mode can be measured in a suitable way. The tomographic inversion is accomplished into steps; firstly, the bottom parameter are inverted by using the bottom reflection phase shift with the known sound speed profile; secondly, the variation of sound speed profile at different time is inverted provided the bottom parameters are known. A numerical simulation shows that the proposed scheme works well, and the sensitivity analysis of sound speed profile inversion is performed, for shallow water environmental parameters: sound speed, density and attenuation coefficient of the bottom.

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 there are physical limitations to the number of independent travel times. Furthermore, those limitations are explained through the analysis of an equivalent environment with a constant sound speed. The results of this paper are of central importance for the understanding of travel-time-based shallow water tomography.

Optimal modal beamforming of bandpass signals using an undersized sparse vertical hydrophone array: theory and a shallow-water experiment

Conventional methods for modal beamforming of underwater acoustic signals using a vertical-line hydrophone array (VLA) can suffer significant degradation in resolution when the array is geometrically deficient, i.e., consists of sparsely spaced elements and spans the water column partially or is poorly navigated. Designed for estimating the coefficients of the normal modes, these conventional methods include the direct projection (DP) of the data on the calculated mode shapes and least-squares (LS) fitting of the mode sum to the data. The degradation, in the form of modal cross talk or sidelobes, is a result of an undersampling in depth. This cross talk may be mitigated with the application of proper space-time filter constraints in the case of a pulse transmission. In this paper, a generalized least-squares (GLS) mode beamformer, capable of incorporating physical space-time constraints on the propagation of sound, is presented. The formulation is based on the well-known theorem of Gauss and Markov. Initialized by a model prediction of the basic arrival structure of the normal modes and incorporating, iteratively, refined estimates of the statistics of the modal fluctuations, this GLS technique strives to boost the resolution of a geometrically deficient VLA. The improvement is demonstrated using the VLA data collected during a shallow-water tomography experiment in the Barents Sea. The superiority of the GLS method over the conventional DP and LS methods is evident, providing a high-quality time series of modal arrivals as a function of geophysical time, which, in turn, reveals the dominant time scales of the oceanic processes associated with the Barents Sea Polar Front.

Intimate ’96: Shallow water tomography in the Sea of the Condemned

As is well-known, the tidal force of the moon and the sun can cause notable changes in the sea level. Besides this so-called barotropic effect, the tidal force also drives internal waves in a daily rhythm. Thus, the internal wave spectrum is often dominated by a single component with perhaps 10 km from crest to crest. This ‘‘internal tide’’ tends to propagate toward shore and has its greatest height near the shelfbreak. As this tide propagates it modulates the surface duct and its acoustic signature is often seen in data. The Intimate ’96 experiment (conducted off the coast of Portugal) was specifically designed to acoustically image the internal tide with an eye toward a more precise understanding of its structure and acoustic impact. A towed source emitted chirps every 8 s for several days and the chirps were received on the SACLANTCEN portable array. The data show a textbook multipath structure with early refracted paths followed by some 30 distinct bottom and surface echoes which shift with the internal tide. The acoustic and oceanographic interpretation of this data will be discussed.

Experimental investigation of features of low‐frequency broadband acoustical signal propagation in shallow water

In shallow water, the combination of downward refracting sound‐speed profiles and strong sound absorption by the bottom limits the range of (tomographic) transmissions and causes one to use lower‐frequency broadband signals. In this paper, investigations of low‐frequency broadband (25–95 Hz) acoustic signal propagation in a slightly variable shallow waveguide with a negative water column sound‐speed gradient are presented. An experiment was carried out in the Barents Sea along a 14‐km stationary track. In travel time tomography in shallow water, as in deep water, the resolvability and identifiability of the acoustic arrivals is a necessary condition. In this experiment, seven stable arrival peaks are noted. Over 2 hr, these peaks fluctuate in rms travel time by several milliseconds, which is consistent with the measurement accuracy of the system used. Identification of the arrivals using both mode theory and rays with beam displacement, combined with the ambiguity diagram approach of Munk and Wunsch, has shown that some of the arrivals are resolvable modes and some are resolvable rays.

On ray/mode approaches to the broadband shallow water tomography problem in the coastal Barents Sea acoustic test experiments

In shallow water tomography, use of acoustic sources with 100-Hz bandwidth causes great problems with resolution and identification of the arrivals due to diffraction and dispersion at low frequency. These problems can be resolved in terms of ray theory with beam displacement and mode theory in different ways. This paper discusses the results of numerical calculations of ray/mode resolution and estimates dispersion in shallow water waveguides. Numerical calculations were carried out for an ideal model waveguide with linear and isovelocity sound-speed profiles as well as for the coastal Barents Sea acoustic test experiment conditions. The experiment took place during October 1990 in the coastal region of the Barents Sea (to be published in Sov. Phys. Acoust. in 1992). Presented here are also plans for future coastal Barents tomography experiments.