Volume 109, Issue 2, February 2001
Index of content:
- UNDERWATER SOUND 
109(2001); http://dx.doi.org/10.1121/1.1339827View Description Hide Description
In this article a sphere is taken to be partially filled with fluid so that its interior is part fluid and part air. A set of basis of functions, based upon an origin at the fluid/air interface, is used for the interior and a set of basis functions based upon the center of the sphere is used for the shell and exterior of the sphere. These sets of basis functions are coupled at the shell/interior interface and the resulting coupled system of equations solved to yield the scattered field. Numerical computations using this approach are presented for varying amounts of fluid-fill and for varying incident plane waves.
109(2001); http://dx.doi.org/10.1121/1.1339828View Description Hide Description
In this article the accuracy of geo-acoustic and geometric parameter estimates obtained through matched field inversion (MFI) was assessed. Multi-frequency MFI was applied to multi-tone data (200–600 Hz) received at a 2-km source/receiver range. The acoustic source was fixed and the signals were received at a vertical array. Simultaneously with the acoustic transmissions, a CTD (conductivity, temperature and depth)-chain was towed along the acoustic track. A genetic algorithm was used for the global optimization, whereas a normal modemodel was applied for the forward acoustic calculations. Acoustic data received at consecutive times were inverted and the stability of the inverted parameters was determined. Also, the parameter estimates were compared with independent measurements, such as multi-channel seismic surveys (for geo-acoustic parameters). The obtained uncertainty in the inversion results was assumed to have two distinct origins. The first origin is the inversion method itself, since each optimization will come up with some solution close to the exact optimum. Parameter coupling and the fact that some parameters hardly influence the acoustic propagation further contribute to this uncertainty. The second is due to oceanographic variability. Both contributions were evaluated through simulation. The contribution of oceanographic variability was evaluated through synthetic inversions that account for the actual sound speed variations as measured by the towed CTD-chain.
The intensity coherence function of time for partially saturated acoustic propagation through ocean internal waves109(2001); http://dx.doi.org/10.1121/1.1322020View Description Hide Description
The intensity coherence function of time for partially saturated acoustic propagation through internal waves is calculated with a method that is improved over previous treatments. Two specific improvements are introduced: the usual expansion in is carried out to a higher order, and then the terms of that expansion are calculated with a new perturbative method. The method is applied to propagation without a sound channel, for both phase-screen and continuous-medium cases. The validity of the new perturbative method is estimated by calculating the next order error terms. Accuracies at the few-percent level are found. The new analytic formulas are also corroborated with numerical integration. Finally, the method is applied to a specific ocean-acoustic experiment [Azores Fixed Acoustic Range (AFAR)]. In order to achieve good agreement with experiment it will be necessary to add an accurate treatment of the sound channel to the present perturbation method.
109(2001); http://dx.doi.org/10.1121/1.1338560View Description Hide Description
Acoustic time reversal is a robust means of retrofocusing acoustic energy, in both time and space, to the original sound-source location. However, noise may limit the performance of a time-reversing array (TRA) at long source–array ranges, or when the original-source or TRA-element power levels are low. The operation of a TRA requires two steps (reception and transmission) so both TRA-broadcast noise and ambient noise must be taken into account. In this paper, predictions are made for how a simple omnidirectional noise field influences the probability that the signal amplitude from a narrow-band TRA will exceed the noise at the TRA’s retrofocus. A general formulation for the probability of TRA retrofocusing, which can be used for TRA design, is developed that includes: the variance of the noise field, the original source strength, the TRA’s element output power, the number of TRA elements (N), and the propagation characteristics of the environment. This formulation predicts that a TRA’s array gain (in dB) at the retrofocus may be as high as to depending on the relative strengths of the original source and the TRA’s elements. Monte Carlo simulations in both a free-space environment and a shallow-ocean sound-channel environment compare well to this probability formulation even when simple approximate parametric relationships for the appropriate Green’s functions are used. The dominant deviation between theory and simulation in the sound channel is caused by acoustic absorption.