Biomedical subjects

W A Kuperman

Publications and source records attributed to W A Kuperman.

7 recordsLinked to original sources

PubMed2002

Time reversed reverberation focusing in a waveguide.

Time reversal mirrors have been applied to focus energy at probe source locations and point scatterers in inhomogeneous media. In this paper, we investigate the application of a time reversal mirror to rough interface reverberation processing in a waveguide. The method is based on the decomposition of the time reversal operator which is computed from the transfer matrix measured on a source-receiver array [Prada et al., J. Acoust. Soc. Am. 99, 2067-2076 (1996)]. In a similar manner, reverberation data collected on a source-receiver array can be filtered through an appropriate temporal window to form a time reversal operator. The most energetic eigenvector of the time reversal operator focuses along the interface at the range corresponding to the filter delay. It is also shown that improved signal-to-noise ratio measurement of the time reversal operator can be obtained by ensonifying the water column with a set of orthogonal array beams. Since these methods do not depend upon a priori environmental information, they are applicable to complex shallow water environments. Numerical simulations with a Pekeris waveguide demonstrate this method.

Journal Article↗

PubMed2002

Null-broadening in a waveguide.

Null-broadening, introduced in plane wave beamforming, is extended to an ocean waveguide in the context of matched field processing. The method is based on the minimum variance processor with white noise constraint and the distribution of fictitious sources using the theory of waveguide invariants. The proposed method is demonstrated in simulation as well as with data collected during the SWellEx-96 experiment. As another application, it is shown that the width of a null can be controlled in an adaptive time reversal mirror with a source-receive array.

Acoustics↗

PubMed2001

Long range source localization from single hydrophone spectrograms.

A source near the deep sound channel axis excites mode groups (or paths) that involve both deep sound channel and boundary interacting propagation. Dispersion from a broadband source as measured on a single hydrophone can be used to estimate source range. Furthermore, modal group speeds have a functional transition when passing through purely refractive to boundary reflecting phase speed regions which, under certain conditions, provides additional arrival structure to aid in source localization. This additional arrival structure is in the form of a focal region in a spectrogram. Indeed, different data sets from the Acoustic Thermometry of the Ocean Climate (ATOC) Program [ATOC Consortium, Science 281, 1327-1332 (1998)] show that localization can be accomplished using this focal region and/or the overall dispersion properties as originally suggested fifty years ago [M. Ewing and J. L. Worzel, Geo. Soc. Am., Memoir 27 (1948)].

Journal Article↗

PubMed2001

Matched field processing with data-derived modes.

The authors demonstrate MFP using data-derived modes and the sound speed profile, using no a priori bottom information. Mode shapes can be estimated directly from vertical line array data, without a priori knowledge of the environment and without using numerical wave field models. However, it is difficult to make much headway with data-derived modes alone, without wave numbers, since only a few modes at a few frequencies may be captured, and only at depths sampled by the array. Using a measured sound speed profile, the authors derive self-consistent, complete sets of modes, wave numbers, and bottom parameters from data-derived modes. Bottom parameters enable modes to be calculated at all frequencies, not just those at which modes were derived from data. This process is demonstrated on SWellEx-96 experiment data. Modes, wave numbers, and bottom parameters are derived from one track and MFP based on this information is demonstrated on another track.

Journal Article↗

PubMed2001

Adaptive time-reversal mirror.

The time-reversal mirror uses the received signal from a probe source to refocus the signal at the probe source location by backpropagating the time-reversed version of the received signal. In this study, an adaptive method is described to steer a null to an arbitrary position in a waveguide while maintaining a distortionless response at the probe source location. As an application, selective focusing in free space is demonstrated.

Journal Article↗

PubMed2000

Matched-field processing, geoacoustic inversion, and source signature recovery of blue whale vocalizations.

Matched-field processing (MFP) and global inversion techniques have been applied to vocalizations from four whales recorded on a 48-element tilted vertical array off the Channel Islands in 1996. Global inversions from selected whale calls using as few as eight elements extracted information about the surrounding ocean bottom composition, array shape, and the animal's position. These inversion results were then used to conduct straightforward MFP on other calls. The sediment sound-speed inversion estimates are consistent with those derived from sediment samples collected in the area. In general, most animals swam from the east to west, but one animal remained within approximately 500 m of its original position over 45 min. All whales vocalized between 10 and 40 m depth. Three acoustic sequences are discussed in detail: the first illustrating a match between an acoustic track and visual sighting, the second tracking two whales to ranges out to 8 km, and the final sequence demonstrating high-resolution dive profiles from an animal that changed its course to avoid the research platform FLIP (floating instrument platform). This last whale displayed an unusual diversity of signals that include three strong frequency-modulated (FM) downsweeps which contain possible signs of an internal resonance. The arrival of this same whale coincided with a sudden change in oceanographic conditions.

Acoustics↗

PubMed1991

Focalization: environmental focusing and source localization.

Conventional matched-field processing (MFP) requires accurate knowledge of the ocean-acoustic environment. Focalization, which simultaneously focuses and localizes, eliminates this stringent requirement by including the environment in the parameter search space. This generalization of MFP involves defining an appropriate high-resolution cost function, parametrizing the search space of the environment and source, constructing solutions of the wave equation, and utilizing a nonlinear optimization method to search the parameter landscape for the global minimum of the cost function. Focalization is implemented using cost functions based on ray theory and wave theory, empirical orthogonal functions for the environmental description, and simulated annealing for optimization. Numerical simulations are presented to demonstrate the feasibility of focalization.

Acoustics↗