Roger C. Gauss

Small-slope scattering from rough elastic ocean floors: General theory and computational algorithm

In this article acoustic scattering by a random rough interface that separates a fluid incident medium from an underlying uniform scattering medium, either fluid or elastic solid, in cases for which the Bragg scale lies within the power-law tail of the roughness spectrum is dealt with. The physical foundation is an inherently reciprocity-preserving, local small-slope theory. A fully bistatic formulation is developed for the scattering strength, together with a robust numerical implementation that allows a wide range of spectral exponent values. The practical result for ocean acoustics is a significantly improved description of the interface component of sea floor scattering. Calculations are presented to demonstrate the advantage of this approach over perturbation theory, and to illustrate its dependence on frequency and environmental parameters as well as its operation in bistatic geometries.

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

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

Large thermal buckling of nonuniform beams and plates

Abstract This article furnishes detailed global descriptions of the properties of buckled states of nonlinearly thermoelastic beams and plates when heated at their ends and edges. Only the axisymmetric deformation of circular plates is considered. In contrast to the models previously studied in the literature, those used here furnish a geometrically exact description of the deformation and allow a very general material response. The beams and plates may be nonuniform. The analysis relies on the combination of classical results for ordinary differential equations and with new results from bifurcation theory. The presentation emphasizes the crucial role of constitutive assumptions in the analysis. The development exhibits a number of novel features of physical importance not observed in more primitive models.

Methods for Identifying and Controlling Sonar Clutter

Three methods for identifying, characterizing, and controlling active sonar clutter are described and demonstrated on broadband, low-frequency (<;1800 Hz) normalized data collected in two range-dependent, shallow-water environments: the Malta Plateau off Sicily and the Stanton Banks off Scotland. One method examines short time-scale clutter persistence to stabilize/minimize coherent propagation effects. A second method correlates strong clutter echoes with the depth-weighted topographic slope (relative to the sonar) to help predict the significant scatterers. A third method involves moment-based statistical measures [Poisson-Rayleigh-inspired method (PRIM), K-distribution shape parameter, kurtosis] to distinguish between spatially compact and spatially extended clutter objects. As illustrated with the isolation of several anthropogenic objects from widespread topographic clutter on the Malta Plateau, the new Poisson-Rayleigh (P-R)-based method appears to offer some promise in regards to reducing the false alarm rate.

Measurements and modeling of low-frequency near-surface scattering statistics

High-resolution acoustic measurements of low-frequency near-surface backscattering at low grazing angles have been made in the open ocean using vertical arrays of coherent sources. Over the range of wind speeds (4-18 m/s) encountered, the normalized data amplitudes exhibited variable non-Rayleigh behavior, from near Rayleigh in the highest sea states to near lognormal in low-to-moderate sea states. Seven probability density function (pdf) models were fit to the data, with the three-component Rayleigh mixture providing the most consistent fits and the least errors. One pdf model, the Poisson-Rayleigh, provided not only good fits to many data sets, but also physical insights into the scattering process. This models estimates of the expected number of discrete scatterers ranged from 200/km/sup 2/ at low wind speeds to 2000/km/sup 2/ at high wind speeds, consistent with the expected densities of fish and subsurface bubble clouds, respectively. These results are encouraging with regard to developing physical models capable of using local results (such as these) to accurately predict long-range reverberation and clutter statistics.

The Influence of the Sea Surface and Fish on Long‐Range Reverberation

Acoustic detection for active sonars involves identifying target signatures in the presence of environmental effects, such as acoustic scattering from the ocean boundaries and fish. The Naval Research Laboratory has recently developed 3D broadband models that provide physics‐based estimates of the dependence of scattering from the sea surface, bubble clouds and near‐boundary fish (including boundary‐interference effects) on the incident and scattered angles, and physical/biological descriptors of the environment. In this paper, these models and a surface‐loss model are used as kernels in reverberation models, which in turn are used to assess the sensitivity at 3.5 kHz of long‐range reverberation to environmental variables. It is shown that the acoustic field in shallow water waveguides could be quite sensitive to the values of sea surface (wind speed) and fish (density, size, depth) parameters, and that physics‐based models are needed for accurate field characterization.

Assessing the Variability of Near-Boundary Surface and Volume Reverberation Using Physics-Based Scattering Models

The increased importance of responding to regional conflicts has focused Navy attention on littoral waters, with active sonar expected to be a favored mode of operation. Major performance drivers of such systems are the acoustic interactions with the ocean boundaries and fish. The vicinity of the air-sea interface is in particular a complex mix of scattering by surface roughness and scattering from bubble clouds and fish, coupled with boundary-interference effects. The Naval Research Laboratory has recently developed broadband, physics-based scattering strength models that both unify and advance our understanding of boundary scattering at low frequencies (< 5 kHz) by providing a physical basis for isolating scattering mechanisms. In this paper, these models are used to assess both the sensitivity of scattering strength to environmental variables and their utility as tools for estimating these variables. These efforts are supported by a series of data-model comparisons that demonstrate both the environmental variability of acoustic response with frequency and scattering angle, and the importance of using physics-based tools to predict these responses.

Measurements of the spectral characteristics of low‐frequency, low‐grazing‐angle surface reverberation

Direct‐path measurements of low‐frequency (200–1000 Hz) and low‐grazing‐angle (≤10 deg) acoustic surface scattering were made in the Gulf of Alaska in April of 1990 and in March of 1992. Short‐duration (0.6–2.4 s) cw and PRN waveforms were used to quantify the spectral character of surface/near‐surface reverberation as a function of frequency and environmental conditions. Measures include spreading and peak‐Doppler‐shift statistics versus azimuth. Results over the range of wind speeds (10–35 kns) have revealed a dominant zero‐Doppler component and a weaker‐than‐expected dependence of spread on both sea conditions (wind speed and relative direction of the seas) and frequency. The results are consistent with sub‐surface bubbles as the primary mechanism for surface reverberation when white caps are present, and give insight as to the nature of the bubble clouds that could give rise to the observed acoustic scattering.

Investigation of bistatic invariance principles for active sonars.

Application of bistatic invariance principles to mid‐frequency active sonar systems is investigated using data collected during two recent experiments conducted at the Malta Plateau and East China Sea. Low‐frequency (350–650 Hz) and mid‐frequency (1.5–3.5 kHz) LFM pulses were transmitted and direct‐blast and return signals from an echo‐repeater and several strong scatterers simultaneously recorded on a towed array, a drifting volumetric array, and a moored vertical line array. At low frequencies, application of bistatic invariance principles to target detection is demonstrated. At mid‐frequencies, the spectrograms of the direct‐blast signals showed regular striation patterns that were used to estimate the waveguide invariant parameter beta. However, both measured and simulated spectrograms of the signals scattered from an oil rig indicated the complexity of the striation patterns. More complex patterns of striations in the measured spectrograms might be due to azimuthal dependency of the scattering kernel...