Using ocean ambient noise for array self-localization and self-synchronization

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.     

Null broadening with snapshot-deficient covariance matrices in passive sonar

Adaptive-array beamforming achieves high resolution and sidelobe suppression by producing sharp s in the adaptive beampattern. Large-aperture sonar arrays with many elements have small resolution cells; interferers may move through many resolution cells in the time required for accumulating a full-rank sample covariance matrix. This leads to "snapshot-deficient" processing. In this paper, the -broadening technique originally developed for an ideal stationary problem is extended to the snapshot-deficient problem combined with white-noise constraint (WNC) adaptive processing. Null broadening allows the strong interferers to move through resolution cells and increases the number of degrees of freedom, thereby improving the detection of weak stationary signals.     

An initial demonstration of underwater acoustic communication using time reversal

In July 1999, an at-sea experiment to measure the focus of a 3.5-kHz centered time-reversal mirror (TRM) was conducted in three different environments: an absorptive bottom, a reflective bottom, and a sloping bottom. The experiment included a preliminary exploration of using a TRM to generate binary-phase shift keying communication sequences in each of these environments. Broadside communication transmissions were also made, and single-source communications were simulated using the measured-channel response. A comparison of the results is made and time reversal is shown to be an effective approach for mitigating inter-symbol interference caused by channel multipath.     

An overview of matched field methods in ocean acoustics

A short historical overview of matched-field processing (MFP) is followed by background material in both ocean acoustics and array processing needed for MFP. Specific algorithms involving both quadratic and adaptive methods are then introduced. The results of mismatch studies and several algorithms designed to be relatively robust against mismatch are discussed. The use of simulated MFP for range, depth and bearing location is examined, using data from a towed array that has been tilted to produce an effective vertical aperture. Several experiments using MFP are reviewed. One successfully demonstrated MFP at megameter ranges; this has important consequences for experiments in global tomography. Some unique applications of MFP, including how it can exploit ocean inhomogeneities and make tomographic measurements of environmental parameters, are considered     

Beam forming on bottom-interacting tow-ship noise

Ship noise received on a horizontal array towed behind the ship is shown to be useful as a potentially diagnostic tool for estimating local acoustic bottom properties. In numerical simulations, tow-ship noise which bounces off the bottom is processed on a beamformer that shows the arrival angles; the beamformer output is readily interpreted by relating it to the Green's function of the acoustic wave equation. Simple signal processing is shown to be sufficient to extract the propagation angles of the "trapped" (i.e., propagating) modes of the acoustic waveguide. By relating the trapped modes to a basic geophysical model of the bottom, one can predict acoustic-propagation conditions for a particular bottom-interacting ocean acoustic environment.     

Improvement of Time-Reversal Communications Using Adaptive Channel Equalizers

The spatial and temporal focusing properties of time-reversal methods can be exploited for undersea acoustic communications. Spatial focusing mitigates channel fading and produces a high signal-to-noise ratio (SNR) at the intended receivers along with a low probability of interception elsewhere. While temporal focusing (compression) reduces significantly intersymbol interference (ISI), there always is some residual ISI depending upon the number of transmitters, their spatial distribution (spatial diversity), and the complexity of the channel. Moreover, a slight change in the environment over the two-way propagation interval introduces additional ISI. Using multilevel quadrature amplitude modulation (M-QAM) in shallow water, we demonstrate that the performance of time-reversal communications can be improved significantly by cascading the received time series with an adaptive channel equalizer to remove the residual ISI     

Demonstration of a high-frequency acoustic barrier with a time-reversal mirror

An acoustic barrier has been demonstrated using a time-reversal mirror. The experiments, at 3500 Hz, utilized a source-receiver array, a probe source collocated with a receive array, and an echo repeater to emulate a disturbance. The successful demonstration is based on the idea that a disturbance such as an object between a time-reversal mirror and its focus will significantly disturb the focal region and, in particular, the quiescent region.     

Using refracted shear waves for velocity estimation

The most difficult part of multicomponent processing is the estimation of the shear-wave velocity map for migration. We used refracted shear waves and a simple iterative method called wavefield continuation (WFC) to evaluate the shallow shear-wave velocity profile on a real data example. The WFC was developed in 1981 by Clayton and McMechan to determine compressional-wave velocity profiles from refracted compressional waves. The application to refracted shear waves is straightforward. The real data example shows that shear structure can be easily determined independently of the compressional structure.     

Multiuser Communications Using Passive Time Reversal

A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).     

Phenomenological and global optimization inversion

This paper discusses geoacoustic inversion results based on benchmark range-dependent data using SAGA, a global inversion package, and using phenomenological inversions. In phenomenological inversions, physical and signal-processing approaches are used to enhance the data to extract specific features. The global optimization approach is carried out on complex-valued vertical array data, transmission loss data, and reverberation data. The importance of checking the solution is emphasized by inspecting the match with the data and the error estimates and by checking the solution using data that has not been used in constructing the solution. The results show that we are able to estimate the geoacoustic parameters and that these parameters could be used to predict the field for different frequencies and/or source-receiver geometry than used in the inversion.