Adrian D. Jones

Single parameter description of seafloors for shallow oceans

For a shallow ocean, sound transmission beyond short range is dominated by seafloor interactions at small grazing angles, for which the loss in dB on each bottom reflection may be approximated by a function F dB/radian which is linear with grazing angle. Acoustic inversion techniques have been shown to be able to obtain the value of F for a particular frequency. The suitability of this single parameter F as a seafloor descriptor has now been studied for an extensive range of seafloor types. It is shown that the phase‐incoherent transmission loss (TL) obtained using the parameter F is not greatly different to TL predicted using complete knowledge of a particular seafloor, for a shallow ocean. Further, if the phase angle for a seafloor reflection is linked to the parameter F via a simple approximation, the phase‐coherent properties of the shallow water interference field may be estimated to an accuracy which is surprising. This paper reviews the relevant theory and presents comparisons between TL predicted ...

Approximation of a physics-based model of surface reflection loss

A model of coherent acoustic reflection loss at the ocean surface had been prepared by the authors by combining a model of surface roughness loss with a description of surface incidence angle which accounted for the refractive effects of a uniformly stratified distribution of wind-driven bubbles. Here, surface roughness loss was based on a second-order small-slope approximation, and the surface incidence angle was obtained using a formulation for stratified media from Brekhovskikh applied to the sound speed variation resulting from the bubble distribution used by Ainslie [JASA 118, (2005)]. More recent work by the authors has shown that the analyses for each of surface roughness loss, and surface incidence angle, may be approximated adequately by relatively simple expressions, and that the complete model of surface reflection loss inclusive of the refractive effects of bubbles may be approximated in expressions suitable for hand calculations. Results from the use of this approximated model with a Gaussian...

Estimating rates of coherent leakage from a variety of ocean surface duct types

In prior work, we obtained a semi-empirical algorithm for the rate of leakage from a mixed layer surface duct with linear sound speed gradients both within and below the duct. Realistic surface ducts are, however, likely to include zones of nonlinear sound speed gradient. In this paper, we made an initial test of whether the leakage rate is strongly dependent on the shape of the sound speed profile or whether it may be adequately approximated based on consideration of the trapping frequency alone. We used a normal mode model to obtain the rate of leakage for each of several nonlinear surface ducts for which the sound speed profiles can be expressed analytically. In each case, the leakage has been compared with that for a duct with a linear gradient, obtained using the algorithm above. In this comparison, the different ducts have been designed so that they all have the same trapping frequency, the same gradient at the top of the respective profiles, and the same sound speed gradient below the duct.

Practical estimates of the coherent leakage of sound from a mixed layer surface duct

In a mixed-layer surface duct, acoustic transmission at frequencies below that required for duct trapping, as well as at frequencies moderately higher, is subject to coherent leakage of energy to the thermocline. The rate of coherent leakage with range is related to the imaginary part of the horizontal wavenumber of a mode, for which an iterative technique is commonly used for evaluations. In order to obtain alternative direct analytic expressions of the leakage rate, the analysis of Furry, described, for example, by Pederson and Gordon (JASA 47, 304–326, 1970), has been revisited. Consideration has been given to both very large and very small ratios between sound speed gradients in the duct and below the duct, and ratios of sound speed gradients typical of those encountered as sea. The leakage rate for the first mode was found to well approximate the total signal leakage, hence the proposed analytical expressions relate to the first mode. Leakage values based on these expressions are compared against res...

Comparison of sonar transmission models at mid‐frequency for synthetic and real environments

The suitability of sonar transmission loss models for application to the mid‐range of acoustic frequencies has been studied by comparing the outputs from a number of models. Emphasis has been placed upon the requirements for models for circumstances relevant to both continental shelf and deep ocean zones within the Australian region. In particular, the ability to describe transmission in surface ducted environments for a deep ocean, and in a shallow ocean with both ducted and downward refracting environments has been considered. For the deep ocean case, a synthetic environment was selected, and the ability to describe duct trapping was examined. For the shallow ocean case, scenarios selected for comparison correspond with ocean tracks for which environmental parameters had been collected and received sound pressure time series measured. The subject transmission loss models are from a variety of types. To enable the most valid comparison, input parameters have been matched across models to the fullest exte...