Hassan B. Ali

Oceanographic variability in shallow-water acoustics and the dual role of the sea bottom

Acoustic propagation in shallow water is examined. Multipath propagation and extensive boundary interactions, which along with a host of other phenomena produce a highly variable and often unpredictable acoustic field, are discussed. The responsible mechanisms, and hence the acoustic effects, cover a wide range of temporal and spatial scales and are classified as either deterministic or random, although the two types often act in concert. Because of extensive interactions with the sound field, the bottom can severely degrade waterborne propagation, although the sea bottom (and subbottom) can provide a seismic path that not only is relatively stable, but exists even under environmental conditions that preclude an effective waterborne path. Propagation in the bottom is particularly significant at very low frequencies. These various aspects of shallow-water acoustics are illustrated using the results of experiments conducted in diverse geographic areas. >

Dissipative shallow water internal waves and their acoustical properties

Zhou and colleagues, J. Acoust. Soc. Am., vol.44, p.2042-54 (1991), showed that nonlinear shallow water internal waves (IWs) have the frequency dependent property of enhancing bottom interaction of sound. At preferred frequencies, internal waves can induce the coupling of lower-order waterborne modes to higher order modes, which, in turn, can penetrate more deeply into lossy ocean bottom sediments. In a follow up study Broadhead, J. Acoust. Soc. Am., Submitted (1995), analyzed a simplified example of a shallow water waveguide in which internal waves were incorporated through the use of KdV solitons. A frequency-dependent enhancement of acoustic energy transfer from below the thermocline into the mixed layer was observed from the simulations. Analysis revealed that mode coupling induced by the presence of the internal wave was responsible for the energy transfer. This paper extends that work by including dissipative effects on the IWs, and their concomitant effects on acoustic propagation. Simulations of the effect of dissipation on IWs was accomplished by numerical solution of the KdVB equation. Especially of interest is the effect of IW pulse broadening on the mixed layer resonance. This led to an increase in the resonant acoustic frequency.

The influence of sediment layering and geoacoustics on the propagation of Scholte interface waves

Results of recent Scholte wave measurements at two diverse test sites showed considerable differences in the dispersive behavior of these waves. For the southern California site, the Scholte wavers show strong, normal dispersion with group velocities ranging from approximately 30-75 m/s. On the other hand, at the Oregon Margin site the dispersion was less clearly defined, with group velocities ranging from approximately 30-205 m/s. Using a full-wave numerical model (SAFARI/OASES) and a seismic normal mode method, forward modelling (iterative inversion) was successful in matching the dispersion curves obtained from the measured Gabor matrix. A thin soft sediment; overlying a harder subbottom at the Oregon site is shown to be the crucial factor in the differences; moreover, the precise thickness of the sediment is significant. For the thick sediment site off southern California, the higher frequency fundamental Scholte mode is controlled largely by the upper few (2-10) meters of sediment, whereas the lower frequency dispersion is dominated by deeper layers, which also strongly influence the higher-order Scholte modes. The conclusions are believed to be of more general applicability than just the two sites examined.<<ETX>>

Scholte wave attenuation estimates from two diverse test sites

Scholte wave arrivals at two diverse test sites were analyzed with the spectral ratio method to obtain estimates of the shear attenuation properties of deep sea bottom sediments. The quantities obtained are interpreted as depth averaged estimates of the quality factor Q. The Scholte waves were generated by bottom located explosive charges and recorded on ocean bottom seismometers. The source-to-receiver ranges were approximately 0.6 to 2.0 km, and the bandwidth of the Scholte waves used m the analyse was approximately 0.3 to 6.0 Hz. The results obtained are internally consistent and fall within the range of values previously published in the literature. The results from one site are compared to Q estimates reported by another investigator for an experiment site in the same area.<<ETX>>

Shear Wave Properties From Inversion of Scholte Wave Data

Scholte interface wave data from a deep-water site are inverted to estimate the ocean bottom sediment shear wave parameters. Techniques used include dispersion analysis, the spectral ratio method, and elastic modelling to produce shear velocity and attenuation estimates for the first tens of meters of a sediment bottom comprised largely of hemipelagic clays. The methodologies presented help provide insight into the first order shear properties of the sediments, and can be valuable in constructing initial guess models and bounds for automated inversion schemes.

A Study Of Pulse Scattering In A Waveguide

Abstract : The problem of the scattering of a continuous acoustic wave(CW)from a submerged object in a waveguide has received extensive treatment in the frequency domain, where phenomena associated with the resonant behavior of the object have been studied in detail. This paper concentrates on the time-domain interpretation of the scattering problem. Computation of the scattered field from an incident pulse, which requires calculations for many frequencies, is performed using a new perturbation technique (computed to all orders that allows rapid calculation of the waveguide modes. An implementation of the inverse Fourier transform is presented that is more appropriate to resonance-type spectra. The computational tools developed are used to elucidate various physical aspects of pulse scattering in a waveguide, including: group velocity behavior, direct modal contributions, and resonance contributions from individually excited modes. Acoustic scattering, shallow water, waveguide propagation.

An Assessment Of The Effects Of Sound Speed Fluctuations On Sound Propagation In Shallow Water Using A Perturbation Method

Abstract : Scintillations in the intensity of an acoustic signal are a common feature of propagation of sound in the sea, manifesting temporal variability in the index of refraction (sound velocity) of the medium. In this paper, a recently developed high-order perturbation method is described and applied to the problem of sound propagation in the sea. The method uses a canonical solution (sound speed profile) to form a set of basis functions that span the solution space and adequately represent the exact eigenvalue problem. The basis functions used in the calculations are derived from sound speed profiles obtained in an acoustic propagation experiment conducted in a shallow-water region of the Mediterranean. At particular source frequencies, calculations of modal functions and acoustic transmission loss were compared for the mean and several perturbed profiles. The results confirm the significant effects on acoustic transmission of seemingly minor variations in sound speed and, moreover, demonstrate the efficacy of the new perturbation method in handling such problems.

Critical Frequencies In Scattering From Submerged Elastic Shells

Abstract : In the process of scattering from submerged elastic shells, it is possible to excite many types of resonances. Among these are the lowest order symmetric and antisymmetric Lamb modes, and the waterborne waves, such as the pseudo-Stoneley resonances and the higher order symmetric and antisymmetric Lamb modes Si and Ai (i = 1, 2, 3, ...). the frequency at which these originate are referred to as critical frequencies. We establish simple rules to determine the frequencies at which the resonances originate as a function of shell thickness and material properties.

Importance of the S1 symmetric Lamb wave in high-frequency scattering from spherical shells

In the investigation of the scattering of high frequency waves from spherical shells it has been observed that it is possible to excite very large return signals at frequencies for which the integral/half integral wavelengths (due to the shear/dilatational wave of the material) are equal to the shell thickness. The large returns are shown to be attributable to resonance effects associated with the S1 symmetric Lamb wave, the upper limit of the resonance being predicted by the flat plate limit of the shell thickness. Following a brief sketch of the pertinent types of elastic waves, the role of the S1 symmetric Lamb wave in producing these large return signals is demonstrated using partial wave analysis.

The influence of seabed properties on the generation and propagation of Scholte interface waves.

Scholte seismic interface waves can be an important component of very low‐frequency (VLF) acoustic propagation, particularly in shallow‐water environments. Under certain conditions, these waves may provide the only effective mechanism for the propagation of VLF energy. Moreover, they are often a significant, if not dominant, contributor to the VLF/ULF ambient noise field, both seismic and waterborne components. This work examines the generation and propagation mechanisms of Scholte waves using the results of recent NOARL measurements and new numerical predictions. It is shown that the propagation characteristics of these seismic waves, including their presence in the water column, are dependent not only on the bottom geoacoustics (particularly shear speeds) but also on the types and thicknesses of layers overlying the basement level. The measurement of Scholte waves even in the apparent absence of near‐bottom sound sources suggests the excitation of secondary Scholte waves by the roughness of the seabed i...