Sumedh M. Joshi

Analysis of wind-driven ambient noise in a shallow water environment with a sandy seabed

On the New Jersey continental shelf ambient sound levels were recorded during tropical storm Ernesto that produced wind speeds up to 40 knots in early September 2006. The seabed at the position of the acoustic measurements can be approximately described as coarse sand. Differences between the ambient noise levels for the New Jersey shelf measurements and deep water reference measurements are modeled using both normal mode and ray methods. The analysis is consistent with a nonlinear frequency dependent seabed attenuation for the New Jersey site.

Geoacoustic Inversions of Horizontal and Vertical Line Array Acoustic Data From a Surface Ship Source of Opportunity

The application of an inversion methodology produces the first demonstration of a simultaneous solution for geoacoustic and source track parameters from acoustic data collected in a shallow-water, sandy sediment environment. Inversion solutions from data collected in the 2006 Shallow Water Experiment (SW06) are extracted from noise measurements of a surface ship source on an L-array. The methodology includes a screening algorithm to determine a set of frequencies for the inversion data. In addition, the methodology assesses the accuracy of the inversion solution and incorporates an estimation of parameter value uncertainties. The solution from the inversion of the horizontal component of the L-array data from the surface ship source before its closest point of approach (CPA) is used to construct modeled propagation loss for comparison with observed received level (RL) structure as the source departs from CPA. Inversion of the data from a single element in the vertical component of the L-array produces a solution that agrees with the solution obtained from the inversion of horizontal subaperture data. Also, modeled transmission loss (TL) structure obtained from the single-element inversion solution reproduces the depth dependence of the RL structure observed at other elements of the vertical component of the L-array.

Applicability of two-dimensional boundary scattering models as a proxy for three-dimensional models

Three-dimensional numerical models offer unique insight into the nature of scattering from rough surfaces. However, use of these models is computationally prohibitive for any application more time-sensitive than basic research. This work seeks to determine a proxy for full threedimensional rough boundary scatting models using appropriate two-dimensional models. Specifically, a Monte Carlo Kirchhoff approximation model in 2D with a derived proxy relationship applied is compared to a similar model in 3D. The region of validity of the proxy will be explored. The usage of the proxy function when applied to an finite element method model will also be discussed. [Work supported by ONR Ocean Acoustics.]

Modeling sound propagation through internal waves using a spectral element method

Considered here is the problem of low-frequency sound propagation over shallow, shoaling bathymetry in the presence of perturbations to the background sound velocity profile due to internal waves (IW). The question we attempt to answer is: to what degree can heuristic models of IWs coupled to numerical sound propagation models capture the variability in sound propagation observed in the environment? A high-order finite element model is employed to compute the acoustic field as it propagates through these IWs. The generality of the finite element method allows for spatial and temporal sound speed variations, and its convergence properties yield arbitrarily small error as the grid is rened. Simulations in the presence and absence of IWs will demonstrate the degree to which IWs influence sound propagation. Different models of IWs will demonstrate the sensitivity of the sound propagation to the choice of heuristic used for the IWs. Results will be shown for shoaling waveguides of O(100 m) depth and O(10 km) r...

Energy conservation in the Kirchhoff approximation

The scattering of sound from rough interfaces is frequently modeled using the Kirchhoff approximation. As has been shown by Lynch and Wagner [J. Acoust. Soc. Am. 47(3)] and others, for the case of a pressure-release surface, the Kirchhoff approximation fails to conserve energy. In particular, Lynch and Wagner derive an analytical expression for the proportion of incident energy conserved for a surface with a Gaussian roughness spectrum. They demonstrate that energy is not conserved near normal incidence due to the failure of the Kirchhoff approximation to multiply scatter rays back into the upper half-space. In this work, a Monte Carlo technique is used to quantify the degree to which energy is not conserved in the three-dimensional Kirchhoff approximation; these results are compared with theoretical prediction of Lynch and Wagner for the Gaussian spectrum. A similar Monte Carlo analysis is undertaken for other roughness types. Finally, it is shown that the integral solution, a model that accounts for mul...

Backscattering from a pressure‐release rough surface.

The backscattering of an incident Gaussian‐tapered acoustic plane wave is modeled using finite elements, the Kirchhoff approximation, and perturbation theory, with the aim of quantifying the validity of the approximate models. von Karman‐type power spectra describing the bottom roughness are sampled from the literature with rms roughness from 3–15% of the acoustic wavelength. Realizations of each type of spectrum are made assuming a Gaussian surface height distribution, and an average backscattering cross section is obtained by ensemble averaging. The pressure‐release bottom is used instead of a penetrable sediment, as the focus in this study is to quantify the differences between finite elements and the approximate models due to roughness. [Work supported by ONR.]

Validity of first-order perturbation theory for scattering from one-dimensional and two-dimensional rough surfaces described by power-law spectra

First-order perturbation theory is a widely used model for estimating the backscatter of acoustic waves incident on a rough surface. The validity of perturbation theory for one-dimensional surfaces described by Gaussian spectra is well established. However, little has been done to confirm its range of validity when expanded to two-dimensional surfaces. Furthermore, the range of validity for surfaces described by power-law spectra has not been fully explored. This work seeks to benchmark first-order perturbation theory against a finite element method solution for scattering from one-dimensional and two-dimensional rough pressure-release surfaces described by power-law spectra. The relationship between ranges of validity of 1D and 2D surfaces will be considered. [Work sponsored by the Office of Naval Research, Ocean Acoustics.]

Three‐dimensional scattering from the ocean surface using finite elements.

Scattering from the ocean surface is a major parameter in propagation, reverberation, and coherence length models for shallow water waveguides. However, there are few models that quantify the effects of out‐of‐plane scattering at the ocean surface. In this work, a finite element model is used to quantify the effects of out of plane scattering by comparing 2‐D models to those in three dimensions for given realizations of the ocean surface. The ocean surface roughness is described by an ocean surface spatial spectrum and the scattering of a Gaussian tapered plane wave is considered. Results for several different ocean surfaces are compared to the Kirchhoff approximation to determine its range of validity. [Work sponsored by the Office of Naval Research, Ocean Acoustics.]

Three‐dimensional scattering from pressure‐release rough surfaces.

In order to compare a variety of three‐dimensional (3‐D) rough surface scattering theories, the scattering of a spherical wave incident on a pressure‐release rough surface is modeled. Random surface realizations are computed from a spatial roughness power spectrum measured as part of the EVA sea test conducted in 2006. Scattering from these surfaces is computed using boundary and finite element methods. A singularity removal technique is applied to solve the Helmholtz–Kirchhoff boundary integral equation in 3‐D. This integral solution is compared with 3‐D finite elements and the 3‐D Kirchhoff approximation, to determine the range of validity of the models.

Three‐dimensional rough surface scattering using finite elements.

In order to quantify the effects of three dimensional scattering, the scattering of a spherical incident wave from a rough, pressure release surface is modeled using a commercially available finite element (FE) code. The surface is generated by creating random realizations from a spatial power spectrum measured as part of the experimental validation of acoustic modeling techniques sea test conducted off the coast of Isola d’Elba in 2006. Since the FE model approaches an exact solution as the discretization density increases, it can be used as a benchmark for approximate methods. Therefore, the three‐dimensional (3‐D) FE model will be compared with a 3‐D Kirchhoff approximation solution as well as two‐dimensional solutions based on the integral equation, Kirchhoff, and FE models in order to determine the range of validity of the approximate methods.