Volume 116, Issue 2, August 2004
Index of content:
- GENERAL LINEAR ACOUSTICS 
116(2004); http://dx.doi.org/10.1121/1.1765193View Description Hide Description
The reflection–transmission problem for a multilayer is examined in the time domain. The layers are linearly elastic, possibly anisotropic, but homogeneous. The multilayer is sandwiched between two homogeneous half-spaces and the incident wave propagates along the normal to the layers. By applying repeatedly the continuity of displacement and traction, the outgoing waves at each interface are evaluated in terms of the incoming ones. As a result, a system of equations is established which eventually determines the reflected and transmitted waves and the waves inside the layers, in terms of the incident wave. A few simple cases are examined in detail to show, e.g., the distortion of a rectangular incident pulse, originated by multiple reflections, and the occurrence of resonanceeffects.
On formulation of a transition matrix for poroelastic medium and application to analysis of scattering problem116(2004); http://dx.doi.org/10.1121/1.1755240View Description Hide Description
Based on the approach of Pao (1978) for elastic medium, we propose a set of the basis functions and an associated relationship of the material properties of the dilatational wave in the poroelastic medium. A transition matrix, which relates the coefficients of scatteredwaves to those of incident waves, is then derived through the application of Betti’s third identity and the associated orthogonality conditions for the poroelastic medium. To illustrate the application, we consider a simple case of the scattering problem of a spherical inclusion, either elastic or poroelastic, embedded within the surrounding poroelastic medium subjected to an incident plane compressional wave.
116(2004); http://dx.doi.org/10.1121/1.1766023View Description Hide Description
The impulsive sound reflection from a planar boundary with absorptive and dispersive properties is investigated. The acoustic properties of the boundary are modeled via a local impedance transfer function whose complex frequency domain representation is taken to be a Padé (2,2) expression. The coefficients in this representation are matched to frequency domain acoustic wave reflection measurements. With the aid of the Cagniard–De Hoop method, a closed-form space-time expression is derived for the acoustic pressure of the reflected wave arising from the incidence of a point-source monopole excited spherical pulse. Depending on the acoustic impedance properties of the boundary, large-amplitude oscillating surface effects can occur. These surface phenomena differ in nature from the true surface waves like the Rayleigh, Scholte, and Stoneley waves in elastodynamics. Illustrative numerical results are presented.
116(2004); http://dx.doi.org/10.1121/1.1771592View Description Hide Description
A modal theory is developed for investigating the acoustic scattering by elastic cylinders of arbitrary cross section immersed in a fluid. Numerical results are presented for a plane wave incidence normal to the axis of an elliptical cylinder but arbitrary with respect to the noncircular cross section. Experimental results are obtained for an aluminum elliptical cylinder with the use of an impulse method. Comparisons between theoretical and experimental data are performed in the broad frequency range (k is the wave number in the fluid and a the major axis radius of the elliptic cylinder). The experimental observations are in good agreement with the theoretical predictions.
116(2004); http://dx.doi.org/10.1121/1.1765197View Description Hide Description
The characteristics of the pseudo-Stoneley wave along boreholes in porous formations are studied in a broad band of frequencies (100 Hz–200 kHz). Experiments are performed using a shock tube technique to excite the pseudo-Stoneley wave in a water saturated confined reservoir. The formation is a natural Berea sandstone. Frequency-dependent phase velocities and damping coefficients are measured using this technique. Quantitative agreement between the experimental results and the theoretical predictions is found for the phase velocity in the frequency range from 10 to 50 kHz. Theoretically, the influence of the permeability on the phase velocity,attenuation, radial displacement, and pore pressure is studied on the basis of the Biot theory and the contribution of the different bulk modes to the average radial displacement is analyzed in the frequency domain. The numerical results indicate that the permeability dependence at low frequencies is caused by the Biot slow wave.