bluuebunny/crossref_metadata_2025 huggingface dataset:
TOMOGRAPHIC INVERSION FOR GEOACOUSTIC PARAMETERS IN SHALLOW WATER
Authors: TOLSTOY A.
Published: April 2003

This paper examines the linearized tomographic inversion of simulated data for a shallow water, multi-array, multi-source scenario. The environments represented include simulations of (1) highly idealized constant regions as well as (2) the Haro Strait Test of June 1996 which displays range, depth, and azimuthal variability, i.e., 3-D dependence on environmental parameters where these parameters can include water depths and multiple sediment sound-speed profiles, densities, depths, and attenuations. This tomographic inversion method is independent of the number of parameters to be determined. However, the method does assume that some inversion method (such as RIGS, simulated annealing, genetic algorithms, etc.) has already estimated range-independent average source-to-receiver environmental parameters. These average parameters are then input into the tomographic inversion which relies on a matrix of path-cell distances. The matrix condition number, Λ, is a determining feature for the inversion accuracy where Λ is a function of source and receiver distributions and their subsequent path distances through the region cells. Additionally, the accuracy of the input estimates for the average geoacoustic properties is also an important factor in the final 3-D tomographic inversion accuracy. Results using this (linearized) tomography inversion method show a potential for excellent error estimates (much less than 1%) for the environmental parameters assuming exact, idealized input values. Errors are still quite reasonable (well under 10%) if more realistic, i.e., erroneous, input values are assumed. This paper will conclude with a discussion of upcoming future directions.
NONLINEAR SOLITON INTERACTION WITH ACOUSTIC SIGNALS: FOCUSING EFFECTS
Authors: RODRÍGUEZ O. C. , JESUS S. , STEPHAN Y. , DEMOULIN X. , PORTER M. , COELHO E.
Published: April 2003

The problem of nonlinear interaction of solitary wave packets with acoustic signals has been intensively studied in recent years. A key goal is to explain the observed transmission loss of shallow-water propagating signals, which has been found to be strongly time-dependent, anisotropic, and sometimes exhibited unexpected attenuation versus frequency. Much of the existing literature considers the problem of signal attenuation in a static environment, without considering additional effects arising from groups of solitons evolving both in range and time. Hydrographic and acoustic data from the INTIMATE'96 experiment clearly exhibit the effects of soliton packets. However, in contrast with reported observations of signal attenuation, the observed transmission loss shows a pronounced signal enhancement that behaves like a focusing effect. This focusing is correlated with peaks in current, temperature, and surface tide. That correlation suggests that the nonlinear interaction of solitary wave packets with acoustic signals can lead to a focusing of the signal. To clarify this issue, hydrographic data was used to generate physically consistent distributions of "soliton-like" fields of temperature and sound velocity. These distributions were then used as input for a range-dependent normal-mode model. The results strongly support the hypothesis that the soliton field causes the observed signal enhancement.
COHERENT REFLECTION FROM A ROUGH INTERFACE OVER AN INHOMOGENEOUS TRANSITION FLUID LAYER
Authors: LIU JIN-YUAN , WANG CHAU-CHANG , HUANG CHEN-FEN
Published: April 2003

This paper considers plane-wave reflection from a rough interface over a transition layer within which the density and the sound speed vary simultaneously and continuously. By applying the Kuperman and Schmidt's formulation based upon the first-order boundary perturbation method, the coherent component of the reflection field may be conveniently solved by replacing the boundary conditions for the smooth interface with a set of effective boundary conditions. In contrast to the traditional analysis that the medium is generally discretized into a number of layers, the present study treats the transition layer as a whole by properly choosing the variations of the acoustic properties within the layer. The results for the coherent reflection coefficients in relation to various relevant parameters are generated and analyzed. It was found that the coherent reflection coefficient decreases as frequency increases. Also, it was shown that the root-mean-square roughness is the dominant factor in determining the coherent reflection coefficient, while the spatial correlation of the rough surface can have only a minor effect.
DEDUCTIVE MULTI-TONE INVERSION OF SEABED PARAMETERS
Authors: AINSLIE M. A. , HAMSON R. M. , HORSLEY G. D. , JAMES A. R. , LAKER R. A. , LEE M. A. , MILES D. A. , RICHARDS S. D.
Published: April 2003

An iterative matched field processing scheme is described for efficient inversion of geoacoustic parameters in shallow water using a vertical receiving array at three frequencies in the range 50–500 Hz. The method relies on the assumption that the acoustic data are sensitive to different geoacoustic parameters at different frequencies. First an exhaustive 2D search is carried out at high frequency to determine initial estimates for density and sound speed. A second 2D search follows at an intermediate frequency to determine sediment attenuation and sound speed gradient. An iteration is carried out over these first two phases until these four parameters converge. In a third phase, the low frequency data are used to search for the remaining unknown parameters (primarily sediment thickness, substrate density and substrate sound speed) with a differential evolution algorithm. Finally all three phases are repeated iteratively, in principle until a complete converged solution (a self-consistent set of all inverted parameters) is found, although for practical reasons the search is terminated before convergence is demonstrated. Tests on synthetic data are reported demonstrating the accuracy and stability of the method. Initial results for measured data are also presented.
APPLICATION OF A DOMAIN DECOMPOSITION METHOD WITH LAGRANGE MULTIPLIERS TO ACOUSTIC PROBLEMS ARISING FROM THE AUTOMOTIVE INDUSTRY
Authors: MAGOULÈS FRÉDÉRIC , MEERBERGEN KARL , COYETTE JEAN-PIERRE
Published: April 2003

The Finite Element Tearing and Interconnecting method for the Helmholtz equation is a recent nonoverlapping domain decomposition method for solving linear systems arising from the finite element discretization of Helmholtz problems in bounded domains. This method was validated on two-dimensional external problems with first-order absorbing boundary conditions. The purpose of this paper is to study the robustness and efficiency of iterative methods for the solution of the associated interface problem for three-dimensional interior problems arising from the automotive industry.
NEW SCHEMES OF OCEAN ACOUSTIC TOMOGRAPHY
Authors: SHANG E. C. , VORONOVICH A. G. , WANG Y. Y. , NAUGOLNYKH K. , OSTROVSKY L.
Published: April 2003

Ocean acoustic tomography based on modal horizontal refraction/phase difference measurement has been developed in recent years. In this paper, potential opportunities for remotely sensing various interesting ocean dynamic structures by using the modal-horizontal-refraction tomography and modal-phase tomography are discussed, including: (1) retrieval of the 3-D structure of a mesoscale eddy, (2) monitoring of the transverse component of the current, (3) retrieval of ocean frontal parameters, and (4) acoustically diagnosing internal solitary waves (ISW) in the coastal zone.
CONJUGATED INFINITE ELEMENTS FOR TWO-DIMENSIONAL TIME-HARMONIC ELASTODYNAMICS
Authors: PINTO TIAGO , COYETTE JEAN-PIERRE
Published: April 2003

Wave propagation problems in unbounded domains require the handling of appropriate radiation conditions (Sommerfeld). Various absorbing boundary conditions are available for that purpose. In a discrete finite element context, local and global Dirichlet-to-Neumann (DtN) and infinite element methods have shown their efficiency for the scalar wave equation.

The paper concentrates on the extension of an infinite element method to the elastodynamic vector wave equation. The extension is developed in the frequency domain for 2-D problems. The paper focuses on the development of a conjugated formulation using the Helmholtz decomposition theorem of smooth vector fields. The accuracy of the developed formulation is assessed through the study of benchmarks. The computed results are shown to be in good agreement with the analytical solution for a multi-pole field along a circular cavity and with the results produced by other numerical methods.
GEOACOUSTIC TOMOGRAPHY: RANGE DEPENDENT INVERSIONS ON A SINGLE SLICE
Authors: POTTY GOPU R. , MILLER JAMES H.
Published: April 2003

A hybrid inversion scheme is used for the inversion of bottom geoacoustic properties using synthetic data as well as broadband signals from explosive sources. This technique is shown to be effective for range dependent inversion on a single slice using synthetic and experimental data. The experimental data was acquired during the Shelf Break Primer Experiment. Time-frequency analysis was performed using wavelets. Group speeds obtained from the time-frequency scalograms were used as data for the inversion. The hybrid method consists of a Genetic Algorithm, which performs the global search, and a Levenberg-Marquardt method, which hastens the descent to the global minimum. Error estimates based on Hessians and a posteriori variances are also provided. Resolution is estimated using an a posteriori analysis.
NOISE SHIELDING BY SIMPLE BARRIERS: COMPARISON BETWEEN THE PERFORMANCE OF SPHERICAL AND LINE SOUND SOURCES
Authors: OUIS D.
Published: April 2003

This study is concerned with the theoretical solution to the problem of sound screening by simple hard barriers on the ground with special emphasis given to the type of wave incidence, namely a comparison between the use of either a spherical or a cylindrical sound source. For a receiver at the shadow of the noise source, the field may be assumed to be due to the edge wave and for this, exact solutions are used. Regarding the wave reflection on an impedance ground, exact formulations are also used, and finally, some calculations are made on the performance of a hard noise barrier on a two-impedance ground. As a conclusion, it is found that although the sound level at the receiver may show some small differences depending on the frequency and on the geometry of the problem, the overall insertion loss of the thin hard barrier is almost the same for the spherical and the line source, and the differences are found to amount to less than 1 dB for geometries of practical occurrence.
PARABOLIC EQUATION DEVELOPMENT IN THE TWENTIETH CENTURY
Authors: LEE DING , PIERCE ALLAN D. , SHANG ER-CHANG
Published: April 2003

In the 20th century, an important contribution to the modeling of wave propagation prediction is the Parabolic Equation (PE) approximation method. This did not happen until the last quarter of the century. During that time period, a number of contributions were made in the first 15 years, then, contributions were made continuously until the end of the 20th century. Contributions to the PE method are still going on. During the second half of the 1970 when the PE was introduced to the field of underwater acoustics, its main purpose was to predict long-range, low-frequency acoustic propagations under range-dependent environments in fluid medium; thus, there were a number of limitations. As time progresses these limitations were somewhat relaxed a great deal gradually due to many useful contributions. Up to this date, all contributions are aimed at the enhancement of the capability of the PE method. A few review papers of the PE have been presented in open archived literatures. This paper gives a complete survey of what has been done from the start until the end of 1999. Contributions up to the year 1994 were already reported in some detail in archived literatures; those contributions will be outlined. Surveying the contributions from 1995 to 1999 is a new addition to the already published survey papers. In recent years, the PE method has been used for many real applications; some results are included in this paper. A discussion will be given at the end of this paper on looking ahead how much more the PE can do in order to stimulate future research, development, and applications.
CONVERGENCE, STABILITY, AND VARIABILITY OF SHALLOW WATER ACOUSTIC PREDICTIONS USING A SPLIT-STEP FOURIER PARABOLIC EQUATION MODEL
Authors: SMITH KEVIN B.
Published: April 2003

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.
THREE-DIMENSIONAL EFFECTS ON SOUND PROPAGATION AND MATCHED-FIELD PROCESSOR PERFORMANCE
Authors: ARVELO JUAN I. , ROSENBERG ALLAN P.
Published: April 2003

The most recent 2-D split-step Padé parabolic-equation model was modified to include narrow-angle azimuthal coupling. This model was developed and applied to the Santa Barbara channel to assess the effect of 3-D propagation on the performance of a matched-field processor. These results, plus those from the Shallow Water Acoustic Modeling (SWAM) workshop's 3-D wedge and the Gaussian canyon cases, will demonstrate the importance of azimuthal coupling to sound propagation in littoral regions and its effect on the performance of a matched-field processor. Special emphasis is given to model limitations and suitable selection of model parameters that could be automated to achieve accurate predictions during production runs.
PECAN: A CANADIAN PARABOLIC EQUATION MODEL FOR UNDERWATER SOUND PROPAGATION
Authors: BROOKE G. H. , THOMSON D. J. , EBBESON G. R.
Published: April 2003

PECan is a Canadian N× 2D/3D parabolic equation (PE) underwater sound propagation model that was developed for matched-field processing applications. It is based on standard square-root operator and/or propagator approximations that lead to an alternating direction solution of the 3-D problem. A 2-D split-step Padé approximation is employed for propagation in range. The 3-D azimuthal corrections are computed using either a split-step Fourier method or a Crank–Nicolson finite-difference approximation. It features a heterogeneous formulation of the differential operators on an offset vertical grid, energy conservation, a choice of initial field including self-starter, and both absorbing and nonlocal boundary conditions. Losses due to shear wave conversion in an elastic bottom are handled in the context of a complex density approximation. In this paper, PECan is described and validated against some standard benchmark solutions to underwater acoustics problems. Subsequently, PECan is applied to several single-frequency test cases that were offered for numerical consideration at the SWAM'99 Shallow Water Acoustic Modeling workshop.
PRECISION CORRELATIONS BETWEEN THE GEOACOUSTIC PARAMETERS OF AN UNCONSOLIDATED, SANDY MARINE SEDIMENT
Authors: BUCKINGHAM MICHAEL J.
Published: April 2003

A recently introduced theoretical model of wave propagation in an unconsolidated, sandy marine sediment is discussed in this article. Essentially, the model consists of four analytical expressions, which specify four wave properties of the sediment, namely the speed and attenuation of the compressional and the shear wave. These expressions contain just three unknown constants, which must be evaluated from data. A technique is described in which these constants are determined from measured values of the compressional wave speed, the shear wave speed, and the shear wave attenuation. This technique is applied to preliminary data from O.N.R.s SAX'99 recent experiment on a sandy sediment in the Gulf of Mexico. From the fully specified equations, the fourth wave property, the attenuation of the compressional wave, is computed and found to lie within 0.5 dB/m of the measured value. Based on this limited evidence, it appears that the wave properties of an unconsolidated sediment are highly correlated and predictable. According to the theory, the causal connections between the wave properties originate in the sliding of one micro/asperity against another on the surfaces of contact between contiguous grains in the medium.
INTEGRAL EQUATION COUPLED MODE APPROACH APPLIED TO INTERNAL WAVE PROBLEMS
Authors: KNOBLES D. P. , STOTTS S. A. , KOCH R. A. , UDAGAWA T.
Published: April 2003

A two-way coupled mode approach based on an integral equation formalism is applied to sound propagation through internal wave fields defined at the 1999 Shallow Water Acoustics Modeling Workshop. Solutions of the coupled equations are obtained using a powerful approach originally introduced in nuclear theory and also used to solve simple nonseparable problems in underwater acoustics. The basic integral equations are slightly modified to permit a Lanczos expansion to form a solution. The solution of the original set of integral equations is then easily recovered from the solution of the modified equations. Two important aspects of the integral equation method are revealed. First, the Lanczos expansion converges faster than a Born expansion of the original integral equations. Second, even when the Born expansion diverges due to strong mode coupling, the Lanczos expansion converges. It is shown that the internal wave problems examined are essentially one-way propagation problems because one observes good agreement between the coupled mode solutions and those provided by an energy-conserving parabolic equation algorithm. In the Workshop examples, at both 25 and 250 Hz, significantly greater coupling between modes occurs in the linear internal wave field case than the nonlinear soliton case.
ENERGY-CONSERVING AND RECIPROCAL SOLUTIONS FOR HIGHER-ORDER PARABOLIC EQUATIONS
Authors: MIKHIN DMITRY
Published: April 2003

The energy conservation law and the flow reversal theorem are valid for underwater acoustic fields. In media at rest the theorem transforms into well-known reciprocity principle. The presented parabolic equation (PE) model strictly preserves these important physical properties in the numerical solution. The new PE is obtained from the one-way wave equation by Godin12 via Padé approximation of the square root operator and generalized to the case of moving media. The PE is range-dependent and explicitly includes range derivatives of the medium parameters. Implicit finite difference scheme solves the PE written in terms of energy flux. Such formalism inherently provides simple and exact energy-conserving boundary condition at vertical interfaces. The finite-difference operators, the discreet boundary conditions, and the self-starter are derived by discretization of the differential PE. Discreet energy conservation and flow reversal theorem are rigorously proved as mathematical properties of the finite-difference scheme and confirmed by numerical modeling. Numerical solution is shown to be reciprocal with accuracy of 10–12 decimal digits, which is the accuracy of round-off errors. Energy conservation and wide-angle capabilities of the model are illustrated by comparison with two-way normal mode solutions including the ASA benchmark wedge.
APPLICATION OF THE METHOD OF GENERALIZED RAYS TO ACOUSTIC WAVES IN A LIQUID WEDGE OVER ELASTIC BOTTOM
Authors: BOREJKO PIOTR , CHEN CHI-FANG , PAO YIH-HSING
Published: April 2003

A range-dependent problem of shallow water acoustics is modeled by a liquid wedge over fast-speed elastic substratum, and the solution is derived based on the method of generalized rays, where the total acoustic field is decomposed into a sum of partial waves, comprising the wave emitted from the point source and the multi-reflected waves. The contribution of each partial wave at a given time is represented by a single integral. The diffracted waves arising from scattering at the apex of the wedge are neglected in this otherwise exact solution. The pressure records are evaluated at the downslope receiver when the pressure pulse applied at a point situated within the wedge has impulsive or harmonic time history. It was found that the arrival of the source pulse is preceded by the group of head wave arrivals, and this early portion of the record (the ground wave) includes considerable pressure build-ups. For a Heaviside source-pulse, we have found that the pressure records of the partial waves exhibit a singular behavior such as the logarithmic singularity at the arrival time of the totally reflected spherical wave. This singular behavior was not found for less sharp input, that is, a triangular source-pulse. We have also found that for a causal harmonic input of lower frequency the ground wave stands out very clearly, whereas for that of higher frequency it is almost suppressed, and for both frequencies the response becomes more or less steady-state soon after the arrival of a few partial waves.
THE SWAM'99 WORKSHOP — AN OVERVIEW
Authors: TOLSTOY A. , SMITH K. , MALTSEV N.
Published: April 2003

The SWAM'99 (Shallow Water Acoustic Modeling, 1999) workshop on the topic of benchmarking shallow water range-dependent propagation modeling was held in Monterey CA in September No specific benchmarks were generated. Rather, each participant offered his (her) Transmission Loss curves for comparison with other results. This overview article will:

• discuss the six (6) test cases;

• discuss each individual's contribution to this benchmarking effort (see the associated articles in this Special issue for full details);

• conclude with suggestions for future efforts.
MULTIPLICATIVE PADÉ PE FORMULATION APPLIED TO SWAM'99 TEST CASES
Authors: SEONG WOOJAE , CHOI BYUNGHO
Published: April 2003

Accurate forward modeling of acoustic propagation is crucial in underwater sound applications that rely on coherent field predictions, such as source localization and geoacoustic inversion based on matched field processing concepts. As acoustic propagation in shallow water environments becomes important in recent years, range-dependent modeling due to environmental changes has to be considered of which parabolic equation (PE) method has received widespread use because they are accurate and relatively fast. In this paper, Seoul National University parabolic equation (SNUPE) employing a multiplicative Padé formulation is developed. Linearization of the depth direction operator is achieved via expansion into a multiplication form of Padé approximation. To approximate the depth directional equation, Galerkin's method is used with partial collocation to achieve computational efficiency. To approximate the range directional equation, Crank–Nicolson's method is used. Finally, numerical self-starter has been used to initiate the near-field solution.

The Shallow Water Acoustic Modeling (SWAM'99) Workshop provides an opportunity to test SNUPE's accuracy and compare its results with others for a variety of synthetic environments. In this paper, the numerical implementation and accuracy of SNUPE is tested by comparing with RAM12 results for the SWAM'99 test cases. Numerical experiments for SWAM'99 test cases give satisfactory results in accuracy for SNUPE and show the importance of the bottom information in the shallow water acoustic modeling.
LATTICE GAS TIME DOMAIN METHODS FOR ACOUSTICS WITH REDUCED COMPUTER MEMORY REQUIREMENTS
Authors: SUDO YASUSHI
Published: April 2003

A lattice gas system consists of many particles, which move in a discrete space-time system according to a set of simple motion rules. Lattice gas time domain methods are based on lattice gas systems and have been widely applied to the simulation of complex systems, such as a Navier–Stokes fluid, a dissipation system, and sound propagation. Applying this idea to sound propagation, an excellent simulation model can be obtained, which has no error for one-dimensional system and has small error for multi-dimensional cases. Despite these good characteristics, the amount of computer memories required to perform the simulation depends on the sound speed and that could be extremely large for some sound speed cases. In this article, a new formulation of the lattice gas sound propagation model is explained, in which the memory requirements are independent of the sound speed and can be much smaller than those of the original formulation.
VERY HIGH-ORDER SPATIALLY IMPLICIT SCHEMES FOR COMPUTATIONAL ACOUSTICS ON CURVILINEAR MESHES
Authors: VISBAL MIGUEL R. , GAITONDE DATTA V.
Published: April 2003

A high-order compact-differencing and filtering algorithm, coupled with the classical fourth-order Runge–Kutta scheme, is developed and implemented to simulate aeroacoustic phenomena on curvilinear geometries. Several issues pertinent to the use of such schemes are addressed. The impact of mesh stretching in the generation of high-frequency spurious modes is examined and the need for a discriminating higher-order filter procedure is established and resolved. The incorporation of these filtering techniques also permits a robust treatment of outflow radiation condition by taking advantage of energy transfer to high-frequencies caused by rapid mesh stretching. For conditions on the scatterer, higher-order one-sided filter treatments are shown to be superior in terms of accuracy and stability compared to standard explicit variations. Computations demonstrate that these algorithmic components are also crucial to the success of interface treatments created in multi-domain and domain-decomposition strategies. For three-dimensional computations, special metric relations are employed to assure the fidelity of the scheme in highly curvilinear meshes. A variety of problems, including several benchmark computations, demonstrate the success of the overall computational strategy.
ASYMPTOTIC EXPANSION OF THE BACKSCATTERING STRENGTH FOR THE PIERSON–MOSKOWITZ SEA SURFACE
Authors: PURCELL ANTHONY
Published: April 2003

The high-frequency behavior of the Kirchhoff approximation for the backscattering strength can be obtained by expanding the surface height correlation function W(r) in a power series in the displacement r. This approach leads to the well-known result that the incoherent backscattering strength is given by exp [- cot 2θ/(2σ2)]/(8πσ2 sin 4θ), where σ is the root-mean-square slope of the randomly rough surface and θ is the grazing angle of the incident plane wave. For the Pierson–Moskowitz model of the ocean surface, this formula is inapplicable due to the infinite root-mean-square slope associated with this surface. This paper modifies the usual method to apply to this special surface, finding that for high frequencies, the Kirchhoff approximation for the backscattering strength can be written as a sum of two terms. The first term is similar in structure to the standard formula, dominant near the normal direction and a weakly decreasing function of frequency. The second term, dominant near grazing incidence, is a slowly decreasing function of grazing angle and frequency independent in the limit of grazing incidence. The use of these asymptotic results allows the very rapid calculation of the incoherent backscattering strength for the Pierson–Moskowitz sea surface.
GAUSSIAN RAY BUNDLE (GRAB) MODEL SHALLOW WATER ACOUSTIC WORKSHOP IMPLEMENTATION
Authors: KEENAN RUTH ETA , WEINBERG HENRY
Published: April 2003

The Shallow Water Acoustic Workshop (SWAM) test cases stress low frequency propagation in shallow water environments dominated by energy refracted through the bottom. Ray based models are not normally considered applicable for such problems. The implementation and accuracy of the range dependent, Gaussian Ray Bundle eigenray model is examined for a subset of these test cases.
FRACTIONAL DIFFUSIVE WAVES
Authors: MAINARDI FRANCESCO , PARADISI PAOLO
Published: April 2003

By fractional diffusive waves we mean the solutions of the so-called time-fractional diffusion-wave equation. This equation is obtained from the classical D'Alembert wave equation by replacing the second-order time derivative with a fractional derivative of order β∈(0,2) and is expected to govern evolution processes intermediate between diffusion and wave propagation when β∈(1,2). Here it is shown to govern the propagation of stress waves in viscoelastic media which, by exhibiting a power law creep, are of relevance in acoustics and seismology since their quality factor turns out to be independent of frequency. The fundamental solutions for the Cauchy and signaling problems are expressed in terms of entire functions (of Wright type) in the similarity variable. Their behaviors turn out to be intermediate between those found in the limiting cases of a perfectly viscous fluid and a perfectly elastic solid. Furthermore, their scaling properties and the relations with some stable probability distributions are outlined.
ANALYSIS OF THE REFLECTION AND TRANSMISSION COEFFICIENTS IN THREE-PHASE SANDSTONE RESERVOIRS
Authors: RAVAZZOLI CLAUDIA L.
Published: April 2003

As it is well-known, seismic and acoustic data play a very important role in the characterization of hydrocarbon reservoirs. The correct interpretation of the anomalies observed in seismic wave velocities, quality factors and reflection coefficients in such environments makes necessary to use accurate models taking into account the main petrophysical features of the rocks. This also brings into play the importance of adequately describing the in situ properties of the reservoir fluids.

This work investigates the influence of gas saturation and excess pore fluid pressure on wave energy splitting at plane interfaces within a Biot-type porous medium saturated by a liquid–gas mixture. Using laboratory measurements made on a wide variety of sandstones, we incorporate in the model the effective pressure dependence of rock matrix properties such as bulk and shear modulus, porosity and pore space compressibility. Also, using empirical laws, we estimate the properties of real hydrocarbon gases, oils and brines of different compositions under variable pressure and temperature conditions. We present analytical computations of the amplitude reflection and transmission coefficients to study the combined effect of the forementioned variables, aiming at a further AVA trend analysis.
NUMERICAL SIMULATIONS OF STRONG GROUND MOTION DURING DESTRUCTIVE EARTHQUAKES IN HOKKAIDO, JAPAN
Authors: FURUMURA MITSUKO , FURUMURA TAKASHI
Published: April 2003

Seismic wave propagation in Hokkaido, Japan, for large destructive earthquakes are simulated by the 3-D finite-difference method. The simulation results indicate that the peak ground velocity distribution for an inland earthquake is nearly circle, but for a plate-boundary earthquake it is irregular, due to influence of lateral heterogeneity in the crust and upper mantle beneath the Hokkaido region.
PROPAGATION OF ULTRASONIC WAVES IN NONLINEAR MULTILAYERED MEDIA
Authors: CHIROIU V. , CHIROIU C. , RUGINA C. , DELSANTO P. P. , SCALERANDI M.
Published: April 2003

In the framework of the Local Interaction Simulation Approach (LISA), iteration equations are derived for the simulation of the propagation of ultrasonic waves in nonlinear multilayered 1-D media. The numerical results are compared with analytical solutions obtained in a perturbation treatment. The method is then applied to study several problems of nonlinear propagation, including the distortion of pulses, the formation of shock waves and, in the case of cw's, anharmonic effects and coupling between extended and localized vibration modes.
A GENERALIZED MODE MATCHING METHOD FOR SCATTERING PROBLEMS WITH UNBOUNDED OBSTACLES
Authors: BONNET-BENDHIA A.-S. , TILLEQUIN A.
Published: April 2003

The two-dimensional time-harmonic acoustic scattering by a semi-infinite waveguide composed of two parallel rigid plates is considered. An original mode matching method is developed to avoid the use of the Wiener–Hopf technique, generalizing usual mode matching methods to the case of unbounded media. A brief description of the method is given by means of Fourier decomposition leading to a well-posed variational problem with unknown being the trace of the solution on a well-chosen interface.

From the numerical point of view, a local approximation is first considered by using Lagrange P1 finite elements on a segment of the interface. This leads to the computation of oscillatory integrals involving Fourier transform and complex square root functions. As a matter of accuracy, a special function is added to the finite element space, in order to take into account the asymptotic behavior of the solution.

Finally, this method is extended to deal with local perturbations of the media by coupling the previous method to a classical integral one.
MODELED VELOCITY AND REFLECTIVITY PROPERTIES OF ANISOTROPIC HYDRATED SEDIMENTS
Authors: JAKOBSEN M. , JOHANSEN T. A. , RUUD B. O.
Published: April 2003

The potential of mapping the extent gas hydrate from seismic data relies on the micromechanical model linking the actual material properties to the relevant observational data. We here consider four-phase sediment models consisting of hydrate, fluid, quartz (grains) and clay (platelets). The hydrate may occur in two ways when the pore volume is partially saturated, either in the pore voids without grain contact (unconnected), or as a grain coating, i.e. acting as a cementation of the grains (connected). In this model, the spatial orientations of the clay platelets are taken into account. By considering a model with a dominant horizontal grain distribution, we find that the elastic stiffnesses and velocities increase with an increasing proportion of hydrate. Both P and S velocities are largest for connected hydrates. Furthermore, the P wave anisotropy is largest for connected hydrates, while the S wave anisotropy is largest for the unconnected hydrates. If we consider the hydrate model as unconnected for low saturation (less than 50%) and connected for higher saturation, the reflectivity properties of the bottom simulating reflector (BSR) are similar to those found by other investigators considering no preferred grain orientation.
MODE AND PE PREDICTIONS OF PROPAGATION IN RANGE-DEPENDENT ENVIRONMENTS: SWAM'99 WORKSHOP RESULTS
Authors: NIELSEN PETER L. , JENSEN FINN B.
Published: April 2003

Three numerical acoustic models, a coupled normal-mode model (C-SNAP), an adiabatic normal-mode model (PROSIM), and a parabolic equation model (RAM), are applied to test cases defined for the SWAM'99 workshop. The test cases consist of three shallow water (flat bottom) scenarios with range-dependent sound-speed profiles imitating internal wave fields and a shelf-break case, with range-dependent sound-speed profiles and bathymetry. The bottom properties in all the cases are range-independent and modeled as a homogeneous fluid half-space. The results from the modeling are presented as transmission loss for selected acoustic frequencies and source-receiver geometries, and as received time series. The results are compared in order to evaluate the effect of applying different propagation models to the same range-dependent underwater environment. It should be emphasized that the propagation analysis is not an attempt to benchmark the selected propagation models, but to demonstrate the performance of practical, range-dependent models based on different approximations in particular underwater scenarios.
BENCHMARK SOLUTIONS FOR 3-D SCATTERING FROM CYLINDRICAL INCLUSIONS
Authors: TADEU ANTÓNIO J. B. , GODINHO LUIS M. C. , ANTÓNIO JULIETA M. P.
Published: April 2003

The solutions for two-dimensional diffracting objects with simple geometry such as a cylindrical inclusion are frequently used as benchmark solutions to test the accuracy of numerical schemes, such as the Finite Elements Method and the Boundary Elements Method, and to better understand the wave propagation around inclusions. The importance of having benchmark solutions available is greater in the case of the evaluation of three-dimensional scattered fields, given its increase in complexity.

In this work, analytical-numerical solutions are used to evaluate the three-dimensional wave field elicited by monopole sources in the vicinity of a fluid-filled cylindrical cavity drilled through an unbounded homogeneous elastic medium. This model is used to assess the effects of the position of the source and receiver, on the propagation of both axisymmetric and nonaxisymmetric wave modes. Both frequency versus axial-wave number responses and time-domain transients are presented.
ACOUSTIC EMISSION FROM PLASTIC SOURCES
Authors: ZIEGLER FRANZ
Published: April 2003

Monitoring of structures which may be subjected to overloads can be based on observing signals emitted in the course of developing defects. Ductile structures react on overloads by the formation of (small scale) plastic zones. Within the linear elastic background concept such an event is considered by the formation of sources of eigenstress. Since a direct description of the source characteristics is rather cumbersome we choose the time convolution of the imposed plastic strains with the complementary Green's stress dyadic to describe the signal of the acoustice mission. Thus, the dynamic generalization of Maysel's formula of thermo-elasticity, to include all kinds of eigenstrains, enters the field of computational acoustics. In that context, the novel contribution of this paper to acoustic emission and monitoring of (layered) structures is the formulation of the full 3-D problems and the introduction of the generalized rays in the background considering an instantaneous oblique force point source at the transducer's location. That means, all the information on the wave guide is contained in the Green's stress dyadic. The expansion into plane waves of cylindrical or spherical waves propagating in a layered elastic half-space or plate proves to be quite efficient for short observation times. Even the divergence effects of dipping interfaces of wedge-type layers are perfectly included by proper coordinate rotations and the exact "seismograms" are observed at a point receiver (where the localized plastic source is assumed to develop, commonly buried and often localized at an interface) from any source located at the hypo center (the site of the transducer in receiving mode, commonly placed at the "outer" surface). This nontrivial technique relies on the invariance of the phase function (arrival time) and of the infinitesimal amplitude of the plane waves in the ray expansion. The concept of the elastic background is illustrated by elastic-viscoplastic waves propagating in thin rods and subsequently extended to the 3-D problem of spherical waves with point symmetry. In that context and in an incremental formulation, the notion of plastic sources is introduced, which emit elastic waves in the background. Finally, the full 3-D problem in a layered half space or layered plate is solved in terms of generalized rays to be received at a transducer in receiving mode. Taking into account the progress in symbolic manipulation with integrated numeric capabilities (e.g., of Mathematica), such a formulation seems timely and may prove to be competitive to the entirely computational Finite Element Method of analysis of signals received from plastic sources. Time signatures of Green's displacement components at the surface of a half-space are illustrated when produced by a vertical, horizontal and inclined line load with a triangular time source function, respectively.
INFORMATIONAL CAPACITY OF ACOUSTIC MEASUREMENTS
Authors: LICHMAN EUGENE
Published: April 2003

Presented is the theoretical model for extracting the system response from measurements of the acoustic wave propagating through the linear system. Based on the results of this analysis, measurements are described as a convolution of the impulse response of the system with the mixed-phase-lag nonstationary forward wavelet (or source-generated wavefield). The source-generated wavefield includes all multiple terms generated within the system as well as the energy source signature and the detector characteristics.

It is shown that the decay ratio of the source-generated wavefield can be used to separate the energy spectrum of the source-generated wavefield and the energy spectrum of the impulse response from the measurement function. The level of separability of energy spectrum of the source-generated wavefield and the impulse response reflects the amount of information about the measured system, which can be obtained from experimental data. In particular, if the source-generated wavefield does not decay during the propagation through the system, or, if the effective distance of the decay is comparable with the size of the measured system, the impulse response cannot be extracted from the result of measurements. Based on the theoretical conclusions, the computational procedure is proposed for one-dimensional deconvolution algorithm. The application of this algorithm is illustrated using seismic data as an example. The forward wavelet is extracted from seismic data itself. The deconvolution of data with the extracted wavelet provides surface-consistent scaling along with peg-leg and short-period multiples attenuation.
ACOUSTIC TIME SERIES VARIABILITY AND TIME REVERSAL MIRROR DEFOCUSING DUE TO CUMULATIVE EFFECTS OF WATER COLUMN VARIABILITY
Authors: LePAGE KEVIN D.
Published: April 2003

The variability of the arrival structure of narrow band waveforms is derived for adiabatic propagation in a fluctuating ocean channel. Expressions for the expected value of the signal intensity are obtained for sound speed fluctuations characterized in a second moment sense over range, depth and time. Similar expressions are obtained for the focus intensity of a time reversal mirror.
A COMPARISON OF NUMERICAL METHODS FOR ACTIVE SONAR ARRAY PERFORMANCE
Authors: MORGAN SUSAN , HARDIE DAVID J. W. , MACEY PATRICK C.
Published: April 2003

Low frequency active sonar (LFAS) arrays are complicated devices requiring careful design. Prototype LFAS arrays are expensive to construct and test. Accurate prediction of acoustic and electrical performance is therefore of great interest to LFAS designers. This generally involves solving a fully coupled problem relating the electrical drive to the resulting acoustic field. To derive results a numerical solution method is clearly the only recourse. This paper compares various numerical techniques in terms of accuracy, efficiency and overall applicability for the solution of LFAS problems. These are based around finite element (FE) and boundary element (BE) descriptions of the surrounding acoustic medium. Here we consider a pure FE approach based on wave envelope elements and a combined FE/BE scheme using an approximate BE formulation. These are contrasted with a pure BE approach that has been demonstrated to provide accurate predictions of LFAS array performance over a number of years. A piston stack transducer and a line array of free-flooding ring projectors are considered as example LFAS problems. The acoustic, structural and electrical responses are considered.
APPLICATION OF THE INTEGRAL LAGUERRE TRANSFORMS FOR FORWARD SEISMIC MODELING
Authors: KONYUKH G. V. , MIKHAILENKO B. G. , MIKHAILOV A. A.
Published: April 2003

The paper presents some efficient algorithms based on the application of the integral Laguerre transform for approximation of temporal derivatives. Some specific features of employing this algorithm for the first and the second order equations with respect to time are considered. A few examples of calculation of seismic fields for the layered medium model with drastically contrast elastic parameters and for the 2-D heterogeneous medium model are presented.
RECONSTRUCTION OF LAYERED ELASTIC BOTTOM CHARACTERISTICS BY THE FREQUENCY DEPENDENCE OF SOUND REFLECTIVITY
Authors: FOKIN VLADIMIR N. , FOKINA MARGARETE S.
Published: April 2003

The relation between the sound reflection losses measured at fixed grazing angles and the characteristics of the sediment layer and underlying half-space is considered. Based on this relation, a method of the reconstruction of the sea bottom characteristics is developed for a ocean bottom consisting of a single sediment layer overlaying a semi-infinite elastic half-space. Using this bottom model, the reconstruction of the characteristics of a layered elastic bottom is performed from the numerically simulated data with induced synthetic error.
A DIRECT DISCRETE FORMULATION FOR THE WAVE EQUATION
Authors: TONTI ENZO
Published: April 2003

The paper shows how to give a direct discrete formulation of the wave equation starting directly from physical laws, i.e. without passing through differential formulation. Using global variables instead of scalar and vector field functions, a close link between global variables and spatial and temporal elements immediately appears. A preliminary classification of physical variables into three classes: configuration, source and energy variables and the use of two cell complexes, one dual of the other, gives an unambiguous association of global variables to the spatial and temporal elements of the two complexes. Thus, one arrives at a discrete formulation of d'Alembert equation on an unstructured mesh.
THE KIRCHHOFF–HELMHOLTZ INTEGRAL PAIR
Authors: TYGEL MARTIN , SCHLEICHER JÖRG , SANTOS LÚCIO T. , HUBRAL PETER
Published: April 2003

The Kirchhoff–Helmholtz integral models the reflected acoustic wavefield by an integration along the reflector over the incident field multiplied by the specular plane-wave reflection coefficient. Based on the structural relationships between the reflector and the reflection-traveltime surface, we design an asymptotic inverse Kirchhoff–Helmholtz integral. Analogously to the forward integral, the proposed inverse consists of an integration along the reflection-traveltime surface over the recorded reflected field. We show that the new inverse integral asymptotically recovers the input to the standard Kirchhoff–Helmholtz integral, that is, the reflector position and the reflection coefficients along it. A simple numerical example demonstrates the inverse relationship between the proposed and the standard Kirchhoff–Helmholtz integrals. In this way, a new technique for kinematic (positioning) and dynamic (amplitude) wavefield inversion becomes available. This is realized by means of an integral operation that is most naturally related to its counterpart Kirchhoff–Helmholtz integral.