Journal of Computational Acoustics

New way for quantitative evaluation of any value of formation permeability based on numerical experiments with Pride's model of electrokinetic phenomena is presented. This approach is based on simultaneous measurement of pressure field and axial component of electric field excited by an acoustic source in fluid-filled borehole with help a set of receivers in borehole. Frequency dependence of ratio of the complex-valued amplitudes of the electric Stoneley wave to the pressure Stoneley wave obtained as a result of plane-wave decomposition of pressure field and mentioned component of electric field carries important information about permeability. In case of open and sealed pores on borehole wall were obtained enough simple analytical expressions for this ratio.

Numerical solutions are given for a parabolic approximation to the acoustic wave equation at 100 and 150 Hz in two and three spatial dimensions to determine if azimuthal coupling in the horizontal coordinate significantly affects horizontal correlation in the presence of internal gravity waves in the sea. Coupling is a small effect at 4000 km. This implies that accurate solutions are possible using computations from uncoupled vertical slices. Shape of horizontal correlation is inconsistent with shapes given by two theories. Estimates of horizontal correlation at 4000 km and 100 and 150 Hz are about 1 km and 0.5 km respectively.

A multi-time-step strategy for overset grids is proposed based on an optimized time interpolation scheme. Time interpolation is adopted in the vicinity of the interface between adjacent blocks, which are marching in time with different time steps satisfying the local numerical stability. There is no strict constraint on the ratio of mesh sizes between neighboring blocks, and it can alleviate the burden of the grid generation for multi-time-step marching methods. The optimized time interpolation scheme can be simply combined with the existing typical time marching schemes to achieve multi-time-step marching for overset grids. Some numerical examples are presented to demonstrate the feasibility and efficiency of the proposed strategy.

A combination of a commercially available finite element software package and additional user-written programs is used to modify the shape of a square plate made of steel. The structure's local geometry modification values at the selected surface key-points are considered as design variables. The objective of the optimization includes the minimization of the root mean square level of structure borne sound. The optimization process continues automatically until the predefined maximum number of function evaluations is reached. A statistical approach is followed for the comparison of seven different optimization methods. It will be shown that the method of moving asymptotes and the mid-range multi-point method produce the best results to find a substantial improvement of the objective function within a limited number of function evaluations.

In order to understand guided wave propagation through real structures containing flaws, a parallel processing, 3D elastic wave simulation using the elastodynamic finite integration technique (EFIT) has been developed. This full field, numeric simulation technique easily examines models too complex for analytical solutions, and is developed to handle built up 3D structures as well as layers with different material properties and complicated surface detail. The simulations produce informative visualizations of the guided wave modes in the structures as well as the output from sensors placed in the simulation space to mimic experiment.

As part of the Windy Island Soliton Experiment (WISE), a sound source and a receiver were moored on two deep-water moorings, 167-km apart, in the northeastern South China Sea basin. The source transmitted phase-modulated acoustic signals with a carrier frequency of 400 Hz for approximately one year. The receptions thus captured the multi-scale variability in the signal transmissions induced by the ocean mesoscale variability and the progression of the transbasin ocean internal tides and waves. A numerical two-dimensional acoustic propagation model based on Hamiltonian ray tracing is utilized to replicate the observed basic arrival structure and transmission loss. Being able to accurately model the basic arrival structure is a necessary first step before modeling the observed variability can be attempted. The comparison of the modeled arrival structure with the actual data is utilized to refine the angular resolution of the ray fan in the model, to estimate the geo-acoustic properties of the bottom, and to develop transmission loss estimates. Transmission loss measurements obtained from several sonobuoys deployed along the transmission path are used as an independent metric to evaluate the model.