Index of content:
Volume 119, Issue 6, June 2006
- ACOUSTIC SIGNAL PROCESSING 
119(2006); http://dx.doi.org/10.1121/1.2197606View Description Hide Description
Frequency-invariant beamforming aims to parameterize array filter coefficients such that the spectral and spatial response profiles of the array can be adjusted independently. Solutions to this problem have been presented for specific sensor configurations often requiring a larger number of sensors. However, in practical applications, the number and location of sensors are often restricted. This paper proposes to find an optimal linear basis transformation that decouples the frequency response from the spatial response. A least-squares optimal basis transform can be computed numerically for arbitrary sensor configurations, for which typically no exact analytical solutions are available. This transform can be further combined with a spherical harmonics basis resulting in readily steerable broadband beams. This solution to broadband beamforming effectively decouples the array geometry from the steering geometry. Furthermore, for frequency-invariant beams, this approach results in a significant reduction in the number of beam-design parameters. Here, the method is demonstrated for an optimal design of far-field response for an irregular linear array with as few as three sensors.
Time reversal operator decomposition with focused transmission and robustness to speckle noise: Application to microcalcification detection119(2006); http://dx.doi.org/10.1121/1.2190163View Description Hide Description
The decomposition of the time reversal operator (DORT) is a detection and focusing technique using an array of transmit-receive transducers. In the absence of noise and under certain conditions, the eigenvectors of the time reversal operator contain the focal laws to focus ideally on well-resolved scatterers even in the presence of strong aberration. This paper describes a new algorithm, FDORT, which uses focused transmission schemes to acquire the operator. It can be performed from medical scanner data. A mathematical derivation of this algorithm is given and it is compared with the conventional algorithm, both theoretically and with numerical experiments. In the presence of strong speckle signals, the DORT method usually fails. The influence of the specklenoise is explained and a solution based on FDORT is presented, that enables detection of targets in complex media. Finally, an algorithm for microcalcification detection is proposed. In-vivo results show the potential of these techniques.
119(2006); http://dx.doi.org/10.1121/1.2197790View Description Hide Description
Recently the concept of adjoint modeling has been introduced in shallow water acoustics for solving inverse problems. Analytical adjoints have been derived for normal modes and for both the standard parabolic equation and Claerbout’s wide-angle approximation (WAPE). This paper proposes the application of a semiautomatic adjoint approach that has been successfully applied in the past for multidimensional variational data assimilation in meteorological and climate modeling. Starting from a modular graph representation of the underlying forward model, a programming tool facilitates the generation and coding of both the tangent linear and the adjoint models. The potential of this numerical adjoint approach for the physical characterization of a shallow water environment is illustrated with two applications for geoacoustic inversion and ocean acoustictomography using Claerbout’s WAPE in combination with nonlocal boundary conditions. Furthermore, the adjoint optimization is extended to multiple frequencies and it is shown how a broadband approach can enhance the performance of the inversion process. For a sparse array geometry in particular, the generalization of the adjoint-based approach to a joint optimization across multiple frequencies is necessary to compensate for the lack of vertical sampling of the propagation modes. Results with test data synthesized from geoacoustic inversion experiments in the Mediterranean show that with the numerical adjoint approach the acoustic field, the sound speed profile in the water column and the bottom properties can be efficiently retrieved.