Volume 116, Issue 5, November 2004
Index of content:
- UNDERWATER SOUND 
116(2004); http://dx.doi.org/10.1121/1.1802811View Description Hide Description
The nonlocal boundary conditions (NLBCs) for high-order finite-difference parabolic equations (PEs) are obtained by Z transformation of the discrete PE in a homogeneous medium. The considered NLBCs include the free-space radiation condition, possibly with a density jump at the NLBC interface, the NLBC at an arbitrary impedance interface, and the NLBCs for sources and the starting field beyond the NLBC interface. The derivation is presented for the multiterm Padé PE model OWWE (one-way wave equation), but the developed technique is applicable to a broad class of finite-difference PEs. The obtained NLBCs are exact for the given finite-difference scheme. They are not limited by the order of Padé approximation or by the PE steps in range and depth. The NLBC convolution coefficients are calculated by the numerical inverse-Z transformation. The accuracy and performance of the algorithm are analyzed for several benchmark problems. The solution is robust for the range steps over 25 wavelengths and/or the approximations up to the tenth order. The NLBCs are faster and more accurate for large steps because fewer previous range steps contribute to the convolution. Precomputation of the NLBC coefficients may be required in time-demanding applications. The results are compared with earlier proposed NLBCs for high-order PEs.
Measurements of scattering by suspensions of irregularly shaped sand particles and comparison with a single parameter modified sphere model116(2004); http://dx.doi.org/10.1121/1.1808458View Description Hide Description
Measurements are presented of multi-frequency underwater acousticbackscattering from suspensions of glass spheres and sands. The data were collected in a sediment tower, specifically designed for such measurements and capable of generating a homogeneous suspension over a distance of approximately 1 m. The glass sphere data were collected to assess the capability of the system and for calibration. The measurements on suspensions of sands were obtained as part of on-going studies into the measurement of nearbed sediment transport processes using acoustics. Utilizing the backscatteredsound from sand suspensions, both the form function and total scattering cross section of the sediments have been measured for a range of sediments and particle sizes. Interpretation of the observations has been carried out within a framework of sphere scattering. The results show enhanced scattering for suspensions of sand grains, relative to that of similar size spherical scatterers and the enhancement can be described by a function dependent on the particle size and the wave number of the insonifying sound, with one free parameter.
Environmental inversion and matched-field tracking with a surface ship and an L-shaped receiver array116(2004); http://dx.doi.org/10.1121/1.1802755View Description Hide Description
Acoustic data from the natural broadband signature of a quiet surface ship, recorded on the vertical leg of an L-shaped array, is used to invert for the local geo-acoustic parameters and the resulting effective environment is used for subsequent tracking of the surface ship using a matched-field tracking technique applied to the full array. The matched-field analysis includes a comparison of the incoherent product of the processed data from the horizontal and vertical subapertures with coherent processing of the data from the full L-shaped array. Subaperture processing is of interest since there is a (loose) requirement that the number of data snapshots be greater than or equal to the number of array elements. This presents averaging difficulties for large arrays when the source being observed is moving. Analyzing each array leg separately allows the use of a smaller number of snapshots from which averaged quantities are constructed. Taken separately, the vertical leg of the array provides range-depth information, while the horizontal leg provides bearing information. The incoherent product of each leg is compared to processing the full array coherently illustrating that the incoherent product generally worked as well, or better than, processing the full array, producing compact maxima at the ship location, and producing fewer false source locations.