Gaetano Canepa

Computing the far field scattered or radiated by objects inside layered fluid media using approximate Green’s functions

A numerically efficient technique is presented for computing the field radiated or scattered from three-dimensional objects embedded within layered acoustic media. The distance between the receivers and the object of interest is supposed to be large compared to the acoustic wavelength. The method requires the pressure and normal particle displacement on the surface of the object or on an arbitrary circumscribing surface, as an input, together with a knowledge of the layered medium Greens functions. The numerical integration of the full wave number spectral representation of the Greens functions is avoided by employing approximate formulas which are available in terms of elementary functions. The pressure and normal particle displacement on the surface of the object of interest, on the other hand, may be known by analytical or numerical means or from experiments. No restrictions are placed on the location of the object, which may lie above, below, or across the interface between the fluid media. The proposed technique is verified through numerical examples, for which the near field pressure and the particle displacement are computed via a finite-element method. The results are compared to validated reference models, which are based on the full wave number spectral integral Greens function.

Time-evolution modeling of seafloor scatter. II. Numerical and experimental evaluation

A time-evolution model of seafloor scatter is numerically implemented and experimentally evaluated. The model is based on analytically expressing the elementary time-backscattered response of every seafloor surface and every seafloor volume infinitesimal element. The implementation of the model is based on a statistical realization of the seabed interface and volume inhomogeneities, from which the time series are computed by coherent summation of the scatter from small elements over the insonified area and volume. The analytical expressions and the implementation are evaluated for the image solution case, for which an almost perfect agreement is found. Examples are shown of how the beam width and seabed roughness affect the time-series return from both the surface and from the volume. The results of the model are compared with data from two different bottom types recorded with a parametric sonar. Reasonable accordance is found between the model and the 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.

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 ...

Comparison of the magnitude and phase of the reflection coefficient from a smooth water/sand interface with elastic and poroelastic models

The reflection coefficient from a sand/water interface is an important parameter in modeling the acoustics of littoral environments. Many models have been advanced to describe the influence of the sediment parameters and interface roughness parameters on the reflection coefficient. In this study, the magnitude and phase of the reflection coefficient from 30 to 160 kHz is measured in a bistatic experiment on a smoothed water/sand interface at grazing angles from 5 to 75 degrees. The measured complex reflection coefficient is compared with the fluid model, the elastic model and poro‐elastic models. Effects of rough surface scattering are investigated using the Bottom Response from Inhomogeneities and Surface using Small Slope Approximation (BoRIS‐SSA). Spherical wave effects are modeled using plane wave decomposition. Models are considered for their ability to predict the measured results using realistic parameters. [Work supported by ONR, Ocean Acoustics.]

Time domain modeling of seafloor scatter at low grazing angle using the small slope approximation

Sea surface and bottom scattering components are usually quantified only in terms of scattering strength (SS). The SS corresponds to an ensemble averaged plane wave intensity scattered from a unit surface at a unit distance. However for non‐steady‐state cases when the acoustic footprint approaches the size of the acoustic wavelength, higher moment statistics and probability distribution functions (PDF) provide additional and essential information for detection and classification purposes. As a step toward higher moment scattering prediction the time domain model BORIS [Pouliquen et al., J. Acoust. Soc. Am. 105 (1999)] has been extended to low grazing angles. It uses the fourth‐order small slope approximation (SSA‐4) and the small perturbation theory for interface and volume scattering, respectively. This bistatic 3‐D model accounts for the full sensing geometry and sonar properties. It uses stochastic realizations of the boundaries and volumes with controlled statistics. BORIS‐SSA offers numerous possibil...

Incoherent sub-band averaging for improved target detection and Doppler estimation in linearly frequency modulated continuous active sonar

Linear frequency modulated (LFM) continuous active sonar (CAS) waveforms show promise for use in target tracking given that waveforms can be split into sub-waveforms (sub-bands), thereby increasing...

Towards Doppler estimation and false alarm rejection for Continuous Active Sonar

Linear frequency modulated (LFM) continuous active sonar (CAS) waveforms show promise for use in target tracking given that waveforms can be split into sub-waveforms (sub-bands), thereby increasing the target refresh rate. However, reducing the duration and bandwidth of the sub-band decreases the SNR/SRR, adversely affecting detection. We present a target detection technique in which sub-bands are averaged incoherently while exploiting the range bias error (commonly observed in large duration LFM CAS waveforms with significant Doppler) to significantly improve detection. Sub-band averaging, known to reduce the false alarm rate, also mitigates channel coherence losses while maintaining detectability in CAS. A method for performing incoherent averaging over many possible Doppler shifts and identifying contacts via clustering in the 3-dimensional range-bearing-Doppler parameter space will be described. Finally, the promising results obtained with this technique on data acquired by the 2016 Littoral CAS Multi-National Joint Research Project sea trial will be shown. [This work was funded by NATO Allied Command Transformation Future Solutions Branch under the Autonomous Security Network Programme and the LCAS MN-JRP.]

Efficient implementation of power-law attenuation of elastic waves in time-domain numerical simulations

Numerical simulation of wave propagation in the time domain is easily parallelizable on high performance computing systems due to the spatially local nature of the governing equations. The disadvantage of working in the time domain arises when lossy media must be modeled which generally gives rise to convolution-type loss terms in the governing time-domain equations. Computation of these convolutions usually requires the storage of several solution fields at thousands of previous time steps. This requirement can be memory prohibitive in three-dimensions. In this talk we present a recursive convolution approach to computing lossy (power-law) elastic wave propagation that is an extension of the one-way, one-dimensional acoustic wave equation work done by Liebler (M. Liebler et al., J. Acoust. Soc. Am., 116, 2004) in order to handle multiple dimensions and shear waves. Convolutions are computed recursively by first using a least-squares technique to fit the kernel of the convolution with a series of decaying...

A novel approach for a bistatic scattering calculation suitable for sonar performance model

One of the classical approaches to the calculation of the sound pressure scattered from a rough surface is to sum the contributions to the field from a set of small tiles that approximate the surface. If the tiles are smaller than the wavelength, or infinitesimal, the Kirchhoffs, the Small slope and small perturbation approximations can give good results. However, these approximations are computational intensive. In addition, if the tiles are larger than the wavelength these approximations cannot be applied. This work proposes a novel approach to accurately calculate the bistatic scattering from a rough surface approximated by tiles larger than the wavelength. The theory behind the calculations includes both specular and off-specular scattering components with their correct amplitude contributions. The results of the new approach are applied to a sonar performance model and compared with high frequency data. The model-data comparison demonstrates the validity of the theory.