Rapid acoustic field computations within environmental uncertainty bounds

The complex nature of the ocean environment requires advanced computational acoustic models to gain insights into the dominating physical factors controlling the underwater sound propagation and scattering in the ocean. In this study, we explore the “ab-initio” approach to solve wave equations with numerical algorithms that can be implemented in distributed memory parallel computers. The goal is to improve the calculation speed so long-range global scale hydroacoustic wave propagation can be studied more efficiently and effectively. Two major algorithms: the finite difference time domain (FDTD) method and the Parabolic Equation (PE) method are investigated. Two PE-based numerical models are considered. One is the Split-Step-Fourier Parabolic Equation (SSFPE) model using split-step Fourier schemes in three-dimensional (3D) environments. The second PE model is a multi-frequency implementation of the Range-dependent Acoustic Model (RAM) that computes two-dimensional (2D) sound pressure fields. One of the primary challenges in global-scale hydroacoustic propagation modeling with the “ab-initio” approach is the demand for significant computational resources on both memories and computational speeds. To address this, we employed parallel computing techniques using directive-based programming languages, such as OpenACC and XscalableACC, to leverage the power of multiple Graphics Processing Unit (GPU) systems and Central Processing Unit (CPU) cluster computers. Our performance evaluation revealed substantial speedup gains. For the FDTD method, we achieved approximately 3.5-fold and 4-fold speedups with four GPUs and four CPU cluster nodes, respectively. With the 3D SSFPE model, we obtained a speedup of 3.7-fold using four CPU cluster nodes. A significant result was observed with the 2D broadband multi-frequency RAM model, where we achieved a 110-fold speedup compared to one core original implementation. These results demonstrate the promising potential of massively parallel computers for ocean acoustic propagation modeling and highlight the significant performance benefits of utilizing parallel computing techniques. Our findings emphasize the importance of efficient computational strategies.

Great strides have been made in the ability to model and predict oceanography (temperature, salinity, currents, etc.) accurately and in a timely manner. There exists a need to characterize the variability of the ocean based on its acoustic propagation characteristics. That is, how and where does the evolution or variability of the environment significantly impact the acoustic propagation characteristics of an oceanographic waveguide? Due to the complexity of the acoustic propagation in a waveguide, variability in the oceanography is not always indicative of the variability in the acoustic propagation. For example, a significant change in temperature in an area may not significantly impact the acoustic propagation in the area. There is also a limit on the ability to sense the oceanography. Sensor availability and coverage, as well as time put constraints on efforts to measure a large ocean area. The work presented here shows that analysis of acoustic variability computed using predicted oceanography over an area provides a better insight into the oceanographic variability for the purposes of sensor placement. Acoustic coverage integrated over many source depths is a representation of how energy travels in a waveguide and can therefore provide a good estimate of the propagation properties of the environment over a large area. A method of estimating the acoustic variability over a period of time using this integrated acoustic coverage (IAC) computation, which is derived from estimated transmission loss is presented here. Two and three dimensional oceanographic model predictions of temperature and salinity (converted to sound velocity) over a time period are used as inputs to the acoustic model. The parabolic equation Range-dependent Acoustic Model (RAM, [Collins, M. D., "Applications and time-domain solution of higher-order parabolic equations in underwater acoustics," J. Acoust. Soc. Am, 86 (3), 1097-1102, 1989]) is used to compute the complex pressure and transmission loss (TL) for calculation of a range independent IAC. This quantity is computed for each time period and variability over time is computed for the ocean volume (longitude, latitude and depth). The RAM is then run for range dependent (RD) environments across user-selected, highly variable tracks for estimation of (range dependent) IAC variance. The acoustic computation is currently done over the whole area in a range independent mode to save computation time. The capability to compute along selected tracks adds a necessary RD evaluation of the areas with the most variability. Comparisons of the range independent (RI) IAC over the volume and RD over the highly variable track show that the use of the range independent IAC parameter is valid for acoustic variance estimation over an area. This work shows that the integrated range independent acoustic variability provides a better estimate of variability than does the oceanographic variability, for the purposes of sensor placement. As work continues, more capability can be added to include functionality such as range dependence.

Broadband acoustic propagation models vary greatly in their computation times. To define the speed-versus-accuracy tradeoffs, two questions are addressed: (1) For ducted environments, how does the parabolic equation (PE) model compare with the faster Labianca virtual-mode model (VM) and the FAME eigenray part of the generic sonar model (GSM)? (2) In nonducted environments, must a GSM complex transmission loss (TL) be computed for all the signal frequencies, or is a single frequency of computation sufficient (i.e., is it fair to assume that the only frequency dependence in the complex-TL phase is in the ray time delay)? To answer these questions, the effects of the computed complex-TL on sine-wave-burst and HFM-burst signals are presented. The multipath arrival structures of the environments enforced a 4-s received-signal duration, and the transmit signals enforced a bandwidth of 200 Hz. The results are as follows: (1) A weak duct shows favorable comparison between GSM, VM, and PE models; however, a strong duct does not, and in fact reveals numerical instability in the PE computations. (2) Single-frequency GSM captures the basic arrival structure of multiple-frequency GSM, but the multipath interference is different and sensitive to the dominant signal frequency. It is concluded that GSM (but not single-frequency GSM) may in most practical cases yield adequate predictions for the acoustic filtering of broadband signals.

When leakage occurs for natural gas pipelines, acoustic waves generated at the leakage point will propagate to both ends of the pipe, which will be measured and processed to detect and locate the leakage. When acoustic waves propagate in the gas, the amplitude will attenuate and the waveform will spread, which decides the installation distance of acoustic sensors. Therefore, computational fluid dynamic (CFD) simulation research on the acoustic wave propagation model is accomplished and verified by experiments to provide the foundation for the acoustic leak location method. The propagation model includes two parts: amplitude attenuation model and waveform spreading model. Both can be obtained by the established CFD simulation model. Additionally, the amplitude attenuation model can be verified by the experiments. Then, the simulation method is applied to conclude the propagation model under variable conditions, including different flow directions, Reynolds numbers, and diameters. Finally, the experimental demonstration of the leak location based on the propagation model is given. The results indicate that not only the gas viscosity but also the gas flow can influence the propagation model, and the leak location method based on the propagation model is effective. Conclusions can be drawn that CFD simulation on the propagation model for natural gas pipelines is an efficient way to carry out research and provide the theoretical basis for acoustic leak location method application.

The Shallow Water Acoustic Modeling (SWAM'99) Workshop was organized to examine the ability of various acoustic propagation models to accurately predict sound transmission in a variety of shallow water environments designed with realistic perturbations. In order to quantify this, tests of reciprocity, convergence, and stability must be considered. This paper presents the results of an established parabolic equation model based on the split-step Fourier algorithm. The test cases examined in this paper include a simple isospeed water column over a flat bottom with geoacoustic parameter variations, a randomly sloping bottom with geoacoustic parameter variations, and a canonical shallow water profile perturbed by internal waves over a flat, homogeneous bottom. Source configurations were generally held constant but numerous single frequency and broadband runs were performed. Model testing is emphasized with specific criteria for accurate solutions being specified. Random perturbations are added to one test case to examine the influence of environmental uncertainty on the details of the propagation. The results indicate that point-wise accurate solutions to the acoustic field in shallow water cannot be achieved beyond a few kilometers. This is partly due to the inaccuracies of the split-step Fourier algorithm employed in these shallow water scenarios and the treatment of the bottom interface boundary conditions, but also due to the inherent variability caused by uncertain environmental specification. Thus, more general features of the acoustic field should be emphasized at longer ranges.

This talk presents results from an acoustic propagation experiment conducted in the Lower Laguna Madre to characterize the acoustical properties of a seagrass meadow. At the location of the experiment, the water was one meter deep, and the seabed was covered by a dense growth of Thalassia testudinum, a type of seagrass that grows from a long, jointed rhizome buried 5 cm to 10 cm below the seafloor. The biological processes and physical characteristics associated with seagrass are known to affect acoustic propagation due to bubble production, which results in dispersion, absorption, and scattering of sound. During the experiment, a Combustive Sound Source was used to produce broadband signals at ranges of 20 m to 1000 m from the receiver location. Three sensors were positioned at the receiver location: two hydrophones located within and above the seagrass canopy, and a single-axis geophone. The data were analyzed for the purposes of predicting acoustic propagation in seagrass meadows and for estimating environmental parameters in very shallow, biologically active environments. Additionally, seagrasses are a vital part of the global carbon cycle and this work explores the feasibility of using of acoustic signals to estimate carbon stores. [Work sponsored by ARL:UT IR&D and ONR.]

Section 1 Physical properties and laboratory measurements.- Shear waves in marine sediments-bridging the gap from theory to field applications (Opening paper).- Surface waves in poro-viscoelastic marine sediments.- An investigation of causality for Biot models by using Kramers-Kronig relations.- Soil properties for shear wave propagation (Invited).- The relevance of shear waves for structural subsurface investigations.- Experimental investigation of seismic surface waves in the seafloor.- Laboratory studies on pulsed leaky Rayleigh wave components in a water layer over a solid bottom.- Assessment of shear strength of the sea bottom from shear wave velocity measurements on box cores and in-situ.- Numerical modelling and laboratory experiments on underwater sound propagation over a shear supporting bottom.- A review of laboratory shear wave techniques and attenuation measurements with particular reference to the resonant column (Invited).- Relationship between acoustic and mechanical properties of two marine clays.- A laboratory method to investigate shear waves in a soft soil consolidating under self weight.- Laboratory measurements of acoustic properties of periplatform carbonate sediments.- Comparison of measured compressional and shear wave velocity values with predictions from Biot theory.- Wave velocities in sediments.- Shear wave attenuation in unconsolidated laboratory sediments.- Shear wave velocities of glacio-marine sediments: Barents Sea.- Rock acoustics: relevance of the porous viscoelastic model.- Influence of stress state and stress history on acoustic wave propagation in sedimentary rocks.- Section 2 Field experiments and interpretation.- A summary of DREA observations of interface waves at the seabed.- Observations of the relative contributions of waterborne and sediment paths to the received acoustic signal.- Shear-wave anisotropy in marine sediments around Britain from surface sources.- Refraction measurement of shear wave anisotropy in shallow marine sediments and implications for reflection processing.- Spatial variability in ground motion: effects of material heterogeneity in seafloor sediments.- Implications of deep-water seismometer array measurements for Scholte wave propagation.- Shear wave velocity structure from interface waves at two deep water sites in the Pacific ocean.- The effects of shear velocity structure on seafloor noise.- Concurrent observations of directional spectra of ocean surface waves and microseisms from an ocean subbottom seismometer (OSS) array.- Wave propagation in a borehole (Invited).- Application of shear wave measurements in boreholes.- Experience with the seismic cone penetrometer in offshore site Investigations.- Comparison of techniques for shear wave velocity and attenuation measurements.- Shear wave velocity gradients in near-surface marine sediment.- ISSAMS: an in situ sediment acoustic measurement system.- Small-scale in situ measurements of S-H velocity in surficial sedimentary deposits: localised textural and biological controls.- In situ measurements of shear-wave velocity in ocean sediments.- Seafloor shear wave velocity data acquisition: procedures and pitfalls.- Mapping of the sea bed via in situ shear wave (SH) velocities.- Observations of split shear-waves from young ocean crust.- Estimation of shear-wave speed in ocean-bottom sediments by comparing oblique-angle reflectivity measurements with normal incidence data.- SedimentQpfrom spectral ratios of converted shear reflections.- Constraints on shear velocities in deep-ocean sediments as determined from deep-tow multichannel seismic data.- Changes inPandSvelocities caused by subduction related sediment accretion off Washington/Oregon.- Compressional and shear wave velocities in the upper crust.- Sedimentary shear modulus and shear speed profiles from a gravity wave inversion.- Measurements of compressional wave and shear wave speeds, attenuation, permeability, and porosity in marine sediments.- Sea-bed shear moduli from measurements of tidally induced pore pressures.- Section 3 Modelling and inversion techniques.- Excess attenuation in low-frequency shallow-water acoustics: a shear wave effect? (Invited).- Sensitivity of bottom loss to attenuation and shear conversion.- The effect of shear wave attenuation on acoustic bottom loss resonance in marine sediments.- The influence of sediment rigidity on the plane-wave reflection coefficient.- Shear wave conversion due to subbottom facets.- A seismo-acoustic finite element model for underwater acoustic propagation.- Finite difference modelling of shear waves.- A stable higher-order elastic parabolic equation with application to Scholte wave propagation.- ELASTIC PE: a parabolic approximation for propagation modelling of shear wave effects in sediment layers.- Computation of shear waves by integral equations methods in stratified media.- Normal modes identification in shallow water using spectral analysis: theory and experiments.- Estimation of geoacoustic properties by inversion of acoustic field data (Invited).- A fast simulated annealing algorithm for the inversion of marine sediment seismo-acoustic parameters.- Determination of the sediment shear speed profiles from phase and group velocity dispersion data of SH wave.- Measurement of the elastic properties of the ocean bottom by inversion of reflection amplitude data.- Surface wave inversion for shear wave velocity (Invited).- Determination of shear velocity profiles by inversion of interface wave data.- Using synthetic seismograms to determine shear wave velocities in sediments from ocean bottom seismometer refraction observations.- Analytical investigation of seismic surface waves in the seafloor.- List of Participants.- Author Index.

: Three computer models have been configured to give predictions under a range of atmospheric and terrain conditions which have allowed comparisons with measurements. A New Parabolic Equation (PE) model, (HIPEDEM) which uses the robust Crank Nicholson (CN) algorithm and which includes novel procedures for treating the ground and upper atmospheric boundaries has been developed together with a new turbulent atmosphere front end processor which can be coupled to the high atmosphere PE to give turbulent atmosphere predictions. A new generalised terrain PE model (GT-PE) has been developed which is capable of predictions over any smooth two dimensional terrain profile in the presence of a moving atmosphere. A feasibility study on the Split Step high speed PE has also been completed and a tutorial code for testing it has been written. Some interesting insights into the operational requirements of the Greens Function PE (GF-PE) have been gained.

In this paper, the Chebyshev spectral method is used to solve the normal mode and parabolic equation models of underwater acoustic propagation, and the results of the Chebyshev spectral method and the traditional finite difference method are compared for an ideal fluid waveguide with a constant sound velocity and an ideal fluid waveguide with a deep-sea Munk speed profile. The research shows that, compared with the finite difference method, the Chebyshev spectral method has the advantages of a high computational accuracy and short computational time in underwater acoustic propagation.

The coastal environment is characterized by variability on small spatial scales and short temporal scales. These environmental uncertainties translate—in a highly non-linear fashion—into uncertain acoustic propagation properties, often severely affecting the sonar performance prediction capabilities. Conventional oceanographic measurement systems cannot capture these environmental uncertainties due to their limited predictability and the lack of mobility of the traditional measurement platforms. Taking advantage of the mobility of modern autonomous underwater vehicle technology, the Adaptive Rapid Environmental Assessment (AREA) concept has been developed to optimally use the available resources to capture the acoustic uncertainty by adaptive and rapid in situ measurement of the environmental properties most significant to the actual sonar system. The ocean area of interest is usually fairly large, and the in situ measurement resources are limited, and the adaptive sampling strategy in AREA can therefore make a significant difference in capturing and mitigating the sonar performance uncertainties. To determine an optimal or sub-optimal sampling strategy and test the optimization effect before doing costly on-site experiments, an Adaptive Rapid Environmental Assessment Simulation Framework (AREASF) has been developed, based on state-of-the-art forecasting frameworks and acoustic propagation models. The simulation framework has been applied to investigate the performance of AREA under different environmental conditions, and based on these results, the feasibility of performing real-time adaptive environmental assesment using AREA under realistic ocean conditions will be discussed. [Work supported by ONR.]

A one-dimensional model of acoustic wave propagation in a multilayered structure of a spot weld is developed. The inhomogeneity of the material properties due to the thermal inhomogeneity is included in the equation of motion. The model enables us to deal with arbitrary spatial distributions of Lamé constants and density. The model allows analysis of travel time, multiple reflections, and interference in a given geometry. Use of this model could provide information to help predict behavior of the waves in the transmission (reflection) mode at different plate thicknesses and welding settings.

A field experiment was carried out in the Connecticut River in June 2017, one goal of which was to investigate the low-to-mid-frequency acoustic propagation characteristics of the fresh water plume front outside the river mouth during ebb. Linear frequency-modulated (LFM) acoustic signals in the 500-2000 Hz band were collected during several tidal cycles. Transmission loss in excess of that which is expected by sound speed gradient-driven refraction is explored via acoustic propagation modeling. The excess attenuation at the ebb plume front is attributed to large clouds of subducted bubbles by downwelling at the ebb plume front.

<div class="htmlview paragraph">The high-frequency performance of automotive sound packages may be predicted by combining transmission loss and absorption calculations with statistical energy analysis (SEA) models. Vehicle SEA modeling therefore requires analytical tools that accurately describe the acoustic properties of layered structures such as trim panels. While widely accepted prediction models for fibrous materials exist, less work has been done on foam layers. This paper presents a numerical calculation of sound transmission through layered structures using a direct global matrix (DGM) solution. This numerical technique is used to compare several models for acoustic propagation in foams. Predictions are compared to transmission loss measurements for several panel configurations. A rigid porous model compares well with data up to 1 kHz for the foams studied, while foam elasticity appears to be important at higher frequencies.</div>

Solving an acoustic wave equation using a parabolic approximation is a popular approach for many existing ocean acoustic models. Commonly used parabolic equation (PE) model programs, such as the range-dependent acoustic model (RAM), are discretized by the finite difference method (FDM). Considering the idea and theory of the wide-angle rational approximation, a discrete PE model using the Chebyshev spectral method (CSM) is derived, and the code is developed. This method is currently suitable only for range-independent waveguides. Taking three ideal fluid waveguides as examples, the correctness of using the CSM discrete PE model in solving the underwater acoustic propagation problem is verified. The test results show that compared with the RAM, the method proposed in this paper can achieve higher accuracy in computational underwater acoustics and requires fewer discrete grid points. After optimization, this method is more advantageous than the FDM in terms of speed. Thus, the CSM provides high-precision reference standards for benchmark examples of the range-independent PE model.

Influence of internal wave induced sound speed variability on acoustic propagation in shallow waters of North West Bay of Bengal