Raymond J. Soukup

Backscatter from a limestone seafloor at 2–3.5 kHz: Measurements and modeling

Physics-based interface scattering models for the seafloor [H.-H. Essen, J. Acoust. Soc. Am. 95, 1299-1310 (1994); Gragg et al., ibid. 110, 2878-2901 (2001)] exhibit features in their predicted grazing angle dependence. These features have a strong dependence on the assumed composition and roughness of the bottom. Verifying such predictions requires data that cover a wide range of grazing angles and involve minimal sub-bottom penetration. Such measurements were performed in the frequency band 2-3.5 kHz over an exposed limestone bottom off the Carolina coast during the second Littoral Warfare Advanced Development Focused Technology Experiment of 1996 (LWAD FTE 96-2). Direct-path bottom scattering strengths were obtained in shallow water (198-310 m deep) for grazing angles from 8 degrees to 75 degrees using data fusion from multiple experimental geometries coupled with careful signal processing. The processing included corrections for the surface-reflected path, other multipaths, and characteristics of the reverberation decay observed over the pulse duration at higher grazing angles. The resulting frequency and grazing-angle dependences exhibit trends consistent with theoretical predictions, and geoacoustic parameters obtained by inversion are consistent with values expected for limestone.

Comparison of simulations and data from a seismo-acoustic tank experiment.

A tank experiment was carried out to investigate underwater sound propagation over an elastic bottom in flat and sloping configurations. The purpose of the experiment was to evaluate range-dependent propagation models with high-quality experimental data. The sea floor was modeled as an elastic medium by a polyvinyl chloride slab. The relatively high rigidity of the slab requires accounting for shear waves in this environment. Acoustic measurements were obtained along virtual arrays in the water column using a robotic apparatus. Elastic parabolic equation solutions are in excellent agreement with data.

Small-slope simulation of acoustic backscatter from a physical model of an elastic ocean bottom

An underwater acoustic experiment with a two-dimensional rough interface, milled from a slab of PVC, was performed at a tank facility. The purpose was to verify the predictions of numerical models of acoustic rough surface scattering, using a manufactured physical model of an ocean bottom that featured shear effects, nonhomogeneous roughness statistics, and root-mean-square roughness amplitude on the order of the acoustic wavelength. Predictions of the received time series and interface scattering strength in the 100-300 kHz band were obtained from the Bottom Reverberation from Inhomogeneities and Surfaces-Small-Slope Approximation (BORIS-SSA) numerical scattering model. The predictions were made using direct measurements of scattering model inputs-specifically, the geoacoustic properties from laboratory analysis of material samples and the grid of surface heights from a touch-trigger probe. BORIS-SSA predictions for the amplitude of the received time series were shown to be accurate with a root-mean-square residual error of about 1 dB, while errors for the scattering strength prediction were higher (2-3.5 dB). The work is part of an ongoing effort to use physical models to examine a variety of acoustic scattering and propagation phenomena involving the ocean bottom.

Mathematical Modeling and Computer-Aided Manufacturing of Rough Surfaces for Experimental Study of Seafloor Scattering

Diverse aspects of stochastic processes, time-series analysis, fractal geometry, and manufacturing technology are brought together to provide a unified theoretical framework for the mathematical characterization and physical fabrication of scale-model representations of the rough ocean bottom. These scale models can be used to validate the predictions of interface-scattering theories, particularly those relating to the dependence of scattering strength on roughness parameters. In acoustical oceanography, rough surfaces are conventionally described in terms of power-spectral density (PSD) or fractal dimension. Here, recently developed concepts describing modified power-law PSD and approximately self-affine stochastic fractals are used to account for issues of finite sample size and finite manufacturing resolution. The large- and small- bandwidth restrictions that these issues inherently impose are related to their effects on the properties of surfaces as characterized both mathematically and by visual observation. This development provides the groundwork for numerical generation of surfaces, using spectral methods, and their physical manufacture, using computer-aided manufacturing (CAM) techniques. Effects of the spectral method of numerical generation on the statistics of generated topography are detailed. A manufacturing technique using a computer numerically controlled (CNC) milling machine is discussed and topography-specific guidelines for accurate manufacture of rough surfaces are prescribed.

Experimental testing of the variable rotated elastic parabolic equation.

A series of laboratory experiments was conducted to obtain high-quality data for acoustic propagation in shallow water waveguides with sloping elastic bottoms. Accurate modeling of transmission loss in these waveguides can be performed with the variable rotated parabolic equation method. Results from an earlier experiment with a flat or sloped slab of polyvinyl chloride (PVC) demonstrated the necessity of accounting for elasticity in the bottom and the ability of the model to produce benchmark-quality agreement with experimental data [J. M. Collis et al., J. Acoust. Soc. Am. 122, 1987-1993 (2007)]. This paper presents results of a second experiment, using two PVC slabs joined at an angle to create a waveguide with variable bottom slope. Acoustic transmissions over the 100-300 kHz band were received on synthetic horizontal arrays for two source positions. The PVC slabs were oriented to produce three different simulated waveguides: flat bottom followed by downslope, upslope followed by flat bottom, and upslope followed by downslope. Parabolic equation solutions for treating variable slopes are benchmarked against the data.

Benchmarking of computational scattering models using underwater acoustic data from a corrugated wax slab

Measured time series for underwater acoustic scattering from a 30 cm x 30 cm x 5 cm wax slab with a two‐dimensional corrugated (rippled) surface are compared with simulation results. The experimental geometry and directionality of the sensors allowed for ensonification of the rippled surface and the appearance of shadowing effects at low grazing angles. The acoustic source transmitted impulses at 200‐800 kHz (wavelengths between 0.75‐0.19 cm). The height and spacing of the ripples were 0.3 cm and 3 cm, respectively, and the slab had negligible shear speed and a measured attenuation. We simulate the experiment with the following methods listed in increasing levels of physical accuracy and computational cost: Kirchhoff Approximation (KA), Second‐order Small‐Slope approximation (SSA2), the Wide‐angle On‐Surface Radiation Condition method (WOSRC), a Pseudo‐differential Impedance Operator method (PIO), a 2‐domain Integral Equation method (IE‐2DOM), and an Elastodynamic Finite Integration Technique (EFIT). The ...

Scattering measurements and interface model validation for a rocky bottom at 2–4 kHz

Validation of interface scattering models for the ocean bottom, such as the small slope approximation model of Gragg and Wurmser [J. Acoust. Soc. Am. 109 (2001)] requires acoustic scattering data covering a wide range of grazing angles with minimal sub‐bottom penetration. Such data measurements were performed in the frequency band 2–4 kHz over an exposed limestone bottom off the Carolina coast (120 to 310 m water depths) during the Littoral Warfare Advanced Development Focused Technology Experiment 96‐2 (LWAD FTE 96‐2). Direct‐path bottom scattering strengths were obtained at grazing angles ranging from 8 to 75 degrees using data fusion from multiple experimental geometries coupled with careful signal processing. This data processing includes corrections for the surface‐reflected and other hybrid paths, and the rapid reverberation decay observed over the length of the pulses at higher grazing angles. The resulting frequency and grazing‐angle dependencies exhibit trends consistent with those predicted by t...

Shallow‐water tank experiments and model comparisons over range‐dependent elastic bottoms

A series of tank experiments has been conducted in order to obtain high quality data for acoustic propagation in shallow-water environments with elastic bottoms. Such problems can now be solved accurately with the parabolic equation method, which is being used for comparisons with measured transmission loss. Results from the initial experiment with a flat or sloped slab of PVC demonstrated both benchmark quality agreement between computed solutions and data, and the necessity of accounting for elasticity in the bottom [J. M. Collis, et al., JASA 122]. This paper will present results of a second experiment, conducted in the same manner as the first, but modified to allow for variable bottom slopes. Time series were collected at 100-300 kHz on horizontal arrays for two source positions. Parabolic equation solutions for treating variable slopes, using coordinate transformation methods of mapping and axis rotations, will be benchmarked against the new data. [Work supported by the Oce of Naval Research.]

Topography measurement of scale-model representations of the rough ocean bottom by touch-trigger probe and its implications for spectral characterization

Scale models of the ocean bottom exhibiting multiscale roughness having power-law form power-spectral density are useful for validation of deterministic and stochastic rough-surface scattering theories. Such scattering theories require accurate knowledge of the topography of the scale-model surface, which, at acoustic scales, can be measured on a two-dimensional grid using a kinematic-resistive touch-trigger probe and represented by a digital elevation model. Both the discrete representation and the physical measurement process introduce spectral artifacts. While the theoretical relationship describing spectral effects of the discrete representation is well known, this relationship is more complex for the physical measurement process. In the later case, spectral effects result from a combination of random measurement errors and fundamental limitations of probe measurement. Here, a numerical model of the physical measurement process is presented, which is used to simulate spectral effects of probe measurement. While random measurement errors can be controlled for and tend to introduce only an additive white-noise component into the measured power-spectral density, limitations of probe measurement are due to the finite size of the probe stylus and result in a systematic error in the measured power-spectral density for spatial wavenumbers above a critical wavenumber

Rough surface scattering from an elastic scale model of an ocean bottom

Monostatic and bistatic scattering strength measurements with a rough PVC surface were collected during two experiments in an acoustic tank facility at the Allied Geophysical Laboratories in the University of Houston. The PVC surface was analogous to limestone ocean bottoms in its two‐dimensional power‐law roughness spectrum and its large dependence of scattering strength on the roughness parameters. The experiments represent an initial effort to use physical models with ground‐truth measurements of roughness and compressional/shear speeds and attenuations to verify the predicted effects of interface scattering models, e.g., the small‐slope model developed at the Naval Research Laboratory for elastic bottoms. Comparisons between the small‐slope model, perturbation theory, and the observed data are shown for the various geometries using acoustic transmissions in the 100–400 kHz band. The success in obtaining a good model‐data fit is shown to be directly related to the ensonification of an area that represents a sufficient statistical sample of the roughness. Plans for a series of tank experiments with physical models for verifying predictions of rough surface scattering theories and elastic PE are described. [Work supported by ONR.]