Index of content:
Volume 106, Issue 3, September 1999
- UNDERWATER SOUND 
Full-wave simulation of the forward scattering of sound in a structured ocean: A comparison with observations106(1999); http://dx.doi.org/10.1121/1.427164View Description Hide Description
Between 1983 and 1989, acoustic pulse-like signals at 133-Hz, 60-ms resolution, were transmitted from Oahu to Northern California. Analysis of the data indicates that the early arriving, steep paths are stable over basin scales, whereas the late, near-axial paths are sensitive to oceanstructure. The late paths undergo vertical scattering on the order of the acoustic waveguide, i.e., 1 km [J. Acoust. Soc. Am. 99, 173–184 (1996)]. The parabolic approximation is used to simulate pulse propagation over the vertical plane connecting the source and receiver. Several prescriptions are used for the speed of sound: (1) Climatologically averaged sound speed with and without a realization of internal waves superposed; (2) Measured mesoscale structure with and without a realization of internal waves superposed. The spectrum of the internal waves is given by Garrett and Munk. Modeled internal waves and the measured mesoscale structure are sufficient to explain the vertical scattering of sound by 1 km. The mesoscale structure contributes a travel time bias of 0.6 s for the late multipath. This bias is seen to be a relevant contribution in accounting for the travel times of the last arrival.
106(1999); http://dx.doi.org/10.1121/1.428034View Description Hide Description
Knowledge of the background reverberation environment is a prerequisite for the design of any target detection scheme. While the problem of understanding and predicting high-frequency background seafloor reverberation level or mean energy scattered per unit area of the seabed has received considerable attention, studies of high-frequency reverberation amplitude statistics are relatively scarce. Of these studies, many have dealt with scattering from more or less homogeneous seafloors in terms of bottom type, whereas most shallow-water areas will not be homogeneous but will have patchiness in space and time which will have a strong influence on scattered amplitude statistics. In this work, a comparison is presented between 80-kHz seafloor backscatter statistics obtained at shallow-water sites around Sardinia and Sicily. The data include measurements from several distinct bottom provinces, including sites with Posidonia Oceanica sea grass and sites covered with live shellfish. Results of the analysis are cast both in terms of mean power level or backscattering coefficient as well as of the amplitude statistics. The reverberation statistics did not always exhibit a Rayleigh probability distribution function (PDF), but exhibited statistical distributions with heavier tails. Several more appropriate models of reverberation PDF were examined in order to better describe the measured amplitude distributions. The Rayleigh mixture and the K models were found to be the most robust in describing the observed data.
106(1999); http://dx.doi.org/10.1121/1.427165View Description Hide Description
A method is introduced for using a single underwater sensor and knowledge of a shallow water environment to track a moving, multi-frequency, acoustic source. The motion of the source creates a synthetic aperture that is adaptively matched against an acoustic propagation model in a manner similar to matched field processing. However, in this case, many more unknowns must simultaneously be determined. Results are presented for tracking using experimental data subject to unknown source speed, heading, level, and initial position. A new parallel adaptive method manages these simultaneous unknowns. Perturbations in the sound speed based on empirical orthogonal functions are used to extend the approach to operating when there also is uncertainty in this parameter. The performance of the tracking process with hydrophone data for various signal-to-noise cases is considered.