Dale D. Ellis

A shallow‐water normal‐mode reverberation model

A practical model to compute shallow‐water boundary reverberation is described. Normal modes are used to calculate the acoustic energy propagating from the source to the scattering area, and from the scattering area to the receiver. At the scattering patch each mode is decomposed into up‐ and down‐going waves, then ray‐mode analogies and empirical scattering functions can be used to compute the scattered energy. The method was first described by Bucker and Morris [J. Acoust. Soc. Am. 44, 827–828 (1968)], and papers which first appeared in the Chinese literature. Their work is extended here by using group velocities to obtain the travel times for each mode pair, and by further developing the ray‐mode analogy. The effects of summing the modes coherently or incoherently and of including the time spreading due to the modal group velocities are examined. This paper deals with the range‐independent monostatic case, although the technique is extendible to bistatic geometries and range‐dependent environments. Cal...

BISTATIC REVERBERATION CALCULATIONS USING A THREE-DIMENSIONAL SCATTERING FUNCTION

The ocean bottom scattering function depends, in general, on the grazing angles and the azimuthal angles of the incident and scattered energy. However, most measurements are for backscatter only. The few general measurements that are available indicate strong forward scattering near the angle of the specularly reflected ray and weaker, azimuthally isotropic, diffuse scattering away from the specular angle. By combining Lambert’s law scattering with a surface scattering function based on the Kirchhoff approximation, Ellis and Haller [J. Acoust. Soc. Am. Suppl. 1 82, S124 (1987)] proposed a function that incorporated these features. The function is quite simple, and depends on three parameters that can be fitted to backscatter measurements. The functional form thus allows a reasonable extension from backscatter to the general three-dimensional scattering function, which can then be used in bistatic reverberation calculations. It is an improvement over two commonly used methods (which do not include azimutha...

Low‐frequency acoustic propagation loss in shallow water over hard‐rock seabeds covered by a thin layer of elastic–solid sediment

Shallow‐water seabeds are often varied and complex, and are known to have a strong effect on acoustic propagation. Some of these seabeds can be modeled successfully as fluid or solid half‐spaces. However, unexpectedly high propagation loss with respect to these models has been measured in several regions with rough, partially exposed, hard‐rock seabeds. It is shown that the high propagation loss in these areas can be modeled successfully by introducing a thin layer of elastic–solid sediment over the hard‐rock substrate. Propagation loss predictions using the safari fast‐field program exhibit bands of high loss at regularly spaced frequencies. Normal‐mode calculations show resonance phenomena, with large peaks in the modal attenuation coefficients at these same frequencies, and with rapid changes in the mode wavenumbers. Bottom reflection loss calculations indicate that the high propagation loss is due to absorption of shear waves in the sediment layer.

The group velocity of normal modes

A simple, general formula for normal mode group velocities provides an intuitive grasp of the factors influencing group velocity, especially for shallow water environments associated with strong seabed interaction. This is demonstrated using some hypothetical shallow water environments, and by comparing mode shapes at different frequencies. The formula is especially convenient to implement in normal mode computer codes which already perform similar calculations for mode normalization and attenuation coefficients. Using the WKB approximation to the normal mode functions, the formula is identified with the average horizontal speed of the ray equivalent to the mode. For a shallow water environment with bottom interaction, the group velocity formula is shown to include the beam and the time displacement effects of modified ray theory. This result strenghthens the ray/mode analogy which is used in analyzing ocean acoustics problems.

Shallow water reverberation: normal-mode model predictions compared with bistatic towed-array measurements

A normal-mode model for calculating reverberation in shallow water is presented. Some illustrative calculations are given for the bistatic case and for vertical and horizontal line-array receivers. Emphasis is on comparison with measurements of bistatic reverberation obtained at a shallow-water area in the Mediterranean. The data are from explosive sources received by a towed array, analyzed in one-tenth-decade frequency bands at subkilohertz frequencies. Model calculations for a flat-bottomed environment indicate a strong dependence on propagation conditions and a weak dependence on beam steering direction. Preliminary comparisons give quite good agreement between measured reverberation and model predictions, but point to the need for extending modeling efforts to handle range-dependent environments. >

Geoacoustic parameter extraction using reverberation data from the 2000 boundary characterization experiment on the Malta plateau

This paper presents some new results from measurements of seafloor reverberation and pulse spreading using horizontal and vertical line arrays. The principal objective of this paper is to extract useful geoacoustic and bottom-scattering parameters that apply over a large ocean area. Analysis is presented on reverberation data from the 2000 Boundary Characterization Experiment performed jointly with North Atlantic Treaty Organization (NATO) Undersea Research Center (NURC), Applied Research Laboratory (ARL) of Pennsylvania State University, Defence Research and Development Canada (DRDC), and Naval Research Laboratory (NRL). Sources were SUS charges and coherent pulses. The receivers were horizontal arrays used monostatically. Data were analyzed in bands from 80 to 4000 Hz. Highlights of the reverberant returns are discussed. The experiment site is the Malta Plateau area south of Sicily, a relatively flat heavily sedimented area, but with a rocky ridge to the east. An original aspect of this paper is the design and implementation of a new automated inverse method using towed-array data to accomplish that goal. For each data set, a multiple-step simulated annealing (SA) algorithm is used together with the Generic Sonar Model (GSM). After automatically adjusting bottom loss and scattering strength, good agreement is achieved between the diffuse reverberation data and model predictions in relatively flat areas. Model/data differences are generally correlated with bottom-scattering features. Since reverberation from SUS charges typically lasts 10-40 s or more, extracted parameters apply over wide areas. Independent acoustic measurements provided a basis for a comparison with extracted values. Local bottom-loss and backscattering measurements were made by Holland in these areas. Additionally, chirp-sonar measurements were analyzed by Turgut. A comparison of geoacoustic models obtained with their methods and with this one was quite good. Comparing transmission loss (TL) predicted with Turguts local inverse method and TL predicted with the method presented here gave answers that were usually within 3 dB of each other. Typical two-way time spreads of 0.25 s were seen at a range of 7.5 km, with normalized peak correlations of 0.5, and which were fairly consistent with predictions made using the inverse results

The effective depth of a Pekeris ocean waveguide, including shear wave effects

Weston’s [J. Acoust. Soc. Am. 32, 647–654 (1960)] concept of the effective depth of a Pekeris‐type shallow water waveguide has been extended to admit seabeds that support shear waves. The amended effective depth formula leads to estimates of normal mode phase speeds that are surprisingly accurate, without numerical iteration. Results for six hypothetical, though realistic, bottom types are compared with an ‘‘exact’’ normal mode model that includes shear wave effects and another normal mode model that does not.

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

A simple shallow water propagation model including shear wave effects

The Perkeris model has proved to be very useful in describing some features of acoustic propagation in shallow water, and as a simple test of ideas in normal mode theory. The basic model consists of a homogeneous layer of fluid overlying an infinite homogeneous fluid half‐space of greater sound speed. Here we extend the Pekeris model to handle the case of a fluid overlying an elastic basement in which the shear speed is less than the (compressional) speed of sound in the fluid. This gives rise to leaky modes in which both the mode eigenfunctions and eigenvalues are complex. The model predictions are compared to some measured propagation losses for a shallow water site overlying a chalk bottom, where shear‐wave conversion at the water‐chalk interface causes large losses. The predictions of the simple model explain the very high losses measured at frequencies less than 200 Hz. At higher frequencies the sound speed profile and a thin sediment layer become important, but then an all‐fluid normal mode model is...

Data-model comparisons of reverberation at three shallow-water sites

Reverberation measurements made by the SACLANT Undersea Research Centre at three shallow-water sites (130-190-m depth) are compared with each other and with estimates from the DREA normal-mode reverberation model OGOPOGO. The experiments over silt-clay and sand seabeds were conducted at slightly bistatic geometries (0.7-6.0-km source-receiver separation), using explosive sources detonated at mid-water depths. The signals were received on hydrophones of either a vertical or horizontal array and analyzed in one-tenth-decade frequency bands from 25 to 1000 Hz. The data are compared with each other to investigate the site differences and frequency dependencies, and with the estimates from the reverberation model OGOPOGO to interpret the data and to obtain a qualitative measure of the scattering. For modeling purposes, geoacoustic models of the seabed were assumed, and the reverberation data were fitted by adjusting the Lambert bottom scattering coefficients. Good model agreement was obtained with both individual hydrophone and data. Though somewhat sensitive to the geoacoustic the Lambert coefficients give a measure of the frequency dependence of the scattering. For the silt-clay bottom, the scattering is weak but is independent of frequency; for the sand bottoms, the scattering is stronger and increases with frequency. These results are compared with estimates from other experiments.