Chris H. Harrison

A passive fathometer technique for imaging seabed layering using ambient noise

A passive acoustics method is presented that uses the ocean ambient noise field to determine water depth and seabed sub-bottom layering. Correlating the noise field measured by two sensors one can recover a function that closely resembles the two-point Green’s function representing the impulse response between the two sensors. Here, a technique is described that is based on noise correlations and produces what is effectively a passive fathometer that can also be used to identify sub-bottom layers. In principle, just one or two hydrophones are needed—given enough averaging time. However, by combining the cross correlations of all hydrophone pairs in a vertical array a stronger signature can be obtained and this greatly reduces averaging time. With a moving (e.g., drifting) vertical array, the resulting algorithm yields both a map of the bottom depth (passive fathometer) and the locations of significant reflectors in the ocean sub-bottom. In this paper, the technique is described and illustrated using numer...

Formulas for ambient noise level and coherence

This paper investigates the various approximations commonly made in noise and noise coherence models and shows that in many cases a very simple ray approach can produce the same answers as full wave treatments such as RANDI‐2. The solution presented here takes the form of a single angle integral which is valid for range‐independent environments. Some closed‐form solutions are presented, and the approach makes it very easy to understand such phenomena as the ‘‘noise notch.’’ The method can be extended to range dependence, and demonstrations are given of performance near a boundary (inhomogeneous field) and in the presence of nonuniform horizontal distributions of noise sources.

Adaptive passive fathometer processing

Recently, a technique has been developed to image seabed layers using the ocean ambient noise field as the sound source. This so called passive fathometer technique exploits the naturally occurring acoustic sounds generated on the sea-surface, primarily from breaking waves. The method is based on the cross-correlation of noise from the ocean surface with its echo from the seabed, which recovers travel times to significant seabed reflectors. To limit averaging time and make this practical, beamforming is used with a vertical array of hydrophones to reduce interference from horizontally propagating noise. The initial development used conventional beamforming, but significant improvements have been realized using adaptive techniques. In this paper, adaptive methods for this process are described and applied to several data sets to demonstrate improvements possible as compared to conventional processing.

Passive fathometer processing

Ocean acoustic noise can be processed efficiently to extract Greens function information between two receivers. By using noise array-processing techniques, it has been demonstrated that a passive array can be used as a fathometer [Siderius, et al., J. Acoust. Soc. Am. 120, 1315-1323 (2006)]. Here, this approach is derived in both frequency and time domains and the output corresponds to the reflection sequence. From this reflection sequence, it is possible to extract seabed layering. In the ocean waveguide, most of the energy is horizontally propagating, whereas the bottom information is contained in the vertically propagating noise. Extracting the seabed information requires a dense array, since the resolution of the bottom layer is about half the array spacing. If velocity sensors are used instead of pressure sensors, the array spacing requirement can be relaxed and simulations show that just one vertical velocity sensor is sufficient.

Sub-bottom profiling using ocean ambient noise

Spectral factorization is shown to restore the phase of an incoherent layered sediment reflection coefficient so that its Fourier transform is the minimum phase impulse response at each angle. The method requires the reflection coefficient to be known over a range of frequencies and the grazing angles in question to be above critical. It is developed here in the context of another recently established technique for extracting the seabed’s plane wave reflection coefficient from ambient noise data measured on a moored or drifting vertical array (VLA). Thus it offers the possibility of sub-bottom profiling from a single platform with no sound source. Limitations of the phase restoration method are discussed and, using modeled data, comparisons are made between the “true” impulse response derived from the known complex reflection coefficient and the result of applying spectral factorization to the absolute value of the reflection coefficient. The method is also demonstrated on experimental reflection loss inf...

Bayesian geoacoustic inversion using wind-driven ambient noise.

This paper applies Bayesian inversion to bottom-loss data derived from wind-driven ambient noise measurements from a vertical line array to quantify the information content constraining seabed geoacoustic parameters. The inversion utilizes a previously proposed ray-based representation of the ambient noise field as a forward model for fast computations of bottom loss data for a layered seabed. This model considers the effect of the arrays finite aperture in the estimation of bottom loss and is extended to include the wind speed as the driving mechanism for the ambient noise field. The strength of this field relative to other unwanted noise mechanisms defines a signal-to-noise ratio, which is included in the inversion as a frequency-dependent parameter. The wind speed is found to have a strong impact on the resolution of seabed geoacoustic parameters as quantified by marginal probability distributions from Bayesian inversion of simulated data. The inversion method is also applied to experimental data collected at a moored vertical array during the MAPEX 2000 experiment, and the results are compared to those from previous active-source inversions and to core measurements at a nearby site.

Boundary characterization experiment series overview

Ocean acoustic propagation and reverberation in continental shelf regions is often controlled by the seabed and sea surface boundaries. A series of three multi-national and multi-disciplinary experiments was conducted between 2000-2002 to identify and measure key ocean boundary characteristics. The frequency range of interest was nominally 500-5000 Hz with the main focus on the seabed, which is generally considered as the boundary of greatest importance and least understood. Two of the experiments were conducted in the Mediterranean in the Strait of Sicily and one experiment in the North Atlantic with sites on the outer New Jersey Shelf (STRATAFORM area) and on the Scotian Shelf. Measurements included seabed reflection, seabed, surface, and biologic scattering, propagation, reverberation, and ambient noise along with supporting oceanographic, geologic, and geophysical data. This paper is primarily intended to provide an overview of the experiments and the strategies that linked the various measurements together, with detailed experiment results contained in various papers in this volume and other sources

Multipath pulse shapes in shallow water: theory and simulation.

In shallow water propagation the steeper ray angles are weakened most by boundary losses. Regarding the sound intensity as a continuous function of angle it can be converted into a function of travel time to reveal the multipath pulse shape received from a remote source (one-way path) or a target (two-way path). The closed-form isovelocity pulse shape is extended here to the case of upward or downward refraction. The envelope of the earliest arrivals is roughly trapezoidal with a delayed peak corresponding to the slowest, near horizontal refracted paths. The tail of the pulse falls off exponentially (linearly in decibels) with a decay constant that depends only on the bottom reflection properties and water depth, irrespective of travel time, a useful property for geoacoustic inversion and for sonar design. The nontrivial analytical problem of inverting explicit functions of angle into explicit functions of time is solved by numerical interpolation. Thus exact solutions can be calculated numerically. Explicit closed-form approximations are given for one-way paths. Two-way paths are calculated by numerical convolution. Using the wave model C-SNAP in several broadband cases of interest it is demonstrated that these solutions correspond roughly to a depth average of multipath arrivals.

Performance and limitations of spectral factorization for ambient noise sub-bottom profiling

Using a drifting vertical array as a means to measure noise directionality one can infer reflection coefficient vs angle and frequency. This can be converted to a time-varying impulse response, i.e., a sub-bottom profile, by using spectral factorization to recover the phase. Limitations of the technique are discussed with simulations and experiment. First, spectral factorization provides a minimum phase realization of the impulse response for the given spectrum, whereas the true reflection coefficient may not actually be minimum phase. Second, the beam width, determined by array length, slightly smudges the interference fringes that are characteristic of the reflection coefficient. This reduction in frequency resolution reduces the maximum depth to which layers can be detected. Consideration of the detailed mechanism leads to a way of improving frequency resolution and hence maximum depth. Spectral factorization tends to function well unless the impedance contrast between water and upper layers is very sm...

Bistatic reverberation benchmarking exercise: BiStaR versus analytic formulas

The newly developed BiStaR range‐dependent bistatic reverberation model is benchmarked against analytic formulas for bistatic reverberation intensity in range‐dependent environments. The BiStaR model is based on the C‐SNAP range‐dependent normal mode propagation model with a narrow‐band approximation time domain extension and a coherent patch interaction model. The code is also capable of producing incoherent reverberation predictions that here are directly compared to the analytic formulas. The analytic formulas have been derived using ray‐based intensity formulas for propagation in range‐dependent waveguides and a generalized class of Lambert law type scattering kernels. Comparison between the BiStaR and the analytic predictions show good agreement for a variety of range‐dependent bistatic scenarios. Such agreement generates confidence in the closed form formulas, which are particularly valuable for generating insight into the simple mechanisms which control the dominant characteristics of range‐depende...