Gerald L. D’Spain

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 animals 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.

Vector sensors and vector sensor line arrays: Comments on optimal array gain and detection

This paper examines array gain and detection performance of single vector sensors and vector sensor line arrays, with focus on the impact of nonacoustic self noise and finite spatial coherence of the noise between the vector sensor components. Analytical results based on maximizing the directivity index show that the particle motion channels should always be included in the processing for optimal detection, regardless of self noise level, as long as the self noise levels are taken into account. The vector properties of acoustic intensity can be used to estimate the levels of nonacoustic noise in ocean measurements. Application of conventional, minimum variance distortionless response, and white-noise-constrained adaptive beamforming methods with ocean acoustic data collected by a single vector sensor illustrate an increase in spatial resolution but a corresponding decrease in beamformer output with increasing beamformer adaptivity. Expressions for the spatial coherence of all pairs of vector sensor compon...

Localization using Bartlett matched-field processor sidelobes

Ambiguity surface sidelobes generated by the Bartlett matched-field processor (MFP) shift location with frequency. This sidelobe shift can be viewed as a continuous trajectory in a range-frequency plane at a fixed depth, where the trajectories converge to the correct source range for a perfectly matched surface. In isovelocity or bottom-interacting environments the sidelobe trajectories are straight lines that converge to the true range at zero frequency, while environments with upward-refracting sound-speed profiles have trajectories that asymptotically converge as the frequency approaches infinity. This behavior can be explained by the theory of waveguide invariants, which predict the local behavior of interference maxima/minima of acoustic intensity in the frequency-range plane. As the ambiguity surface of the Bartlett matched-field processor has a physical interpretation in terms of a time-reversed acoustic field, with the sidelobes analogous to local interference maxima, these invariant concepts can be reformulated for application to MFP. These interference trajectories are demonstrated to exist in simulations, broadband source tows, and a type A blue whale vocalization. Sidelobe trajectories also exist in the range-depth plane, but they contain no information about the correct source depth. An appendix demonstrates how these sidelobe properties can be exploited when combining ambiguity surfaces through use of gradient and Radon transform information. The resulting range estimators demonstrate better peak-to-sidelobe ratios than a simple incoherent average.

A generalized power-law detection algorithm for humpback whale vocalizations

Conventional detection of humpback vocalizations is often based on frequency summation of band-limited spectrograms under the assumption that energy (square of the Fourier amplitude) is the appropriate metric. Power-law detectors allow for a higher power of the Fourier amplitude, appropriate when the signal occupies a limited but unknown subset of these frequencies. Shipping noise is non-stationary and colored and problematic for many marine mammal detection algorithms. Modifications to the standard power-law form are introduced to minimize the effects of this noise. These same modifications also allow for a fixed detection threshold, applicable to broadly varying ocean acoustic environments. The detection algorithm is general enough to detect all types of humpback vocalizations. Tests presented in this paper show this algorithm matches human detection performance with an acceptably small probability of false alarms (P(FA) < 6%) for even the noisiest environments. The detector outperforms energy detection techniques, providing a probability of detection P(D) = 95% for P(FA) < 5% for three acoustic deployments, compared to P(FA) > 40% for two energy-based techniques. The generalized power-law detector also can be used for basic parameter estimation and can be adapted for other types of transient sounds.

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)].

Near‐grazing, low‐frequency propagation over randomly rough, rigid surfaces

When a low‐frequency point source radiates sound close to a low‐roughness, steep‐sloped, rigid surface, multiple coherent forward scatter creates a boundary wave which propagates with cylindrical divergence and dispersion in the fluid region near the surface. Measurements using laboratory models of surfaces with two‐ or three‐dimensional roughness elements, and a generalization of periodic roughness theory, have been used to develop empirical formulas which describe the amplitude and phase velocity of the coherent boundary wave over natural, randomly rough, rigid surfaces. The formulas are stated in terms of the average height and the rms slope of the steep‐sloped roughness elements.

Autonomous underwater glider based embedded real‐time marine mammal detection and classification.

Autonomous marine vehicles offer the potential to provide low‐cost data suitable for passive acoustic monitoring applications of marine mammals. Due to their extremely low‐power consumption and long range, gliders are an attractive option for long‐term deployments. Challenges related to power availability, payload size, and weight have previously restricted the viability of marine mammal monitoring. As an example, the wide bandwidth of odontocete echolocation clicks requires a high sampling rate and poses challenges with respect to limitations in power, size, and weight of the deployed system. Recent developments in commercial off‐the‐shelf hardware driven by the mobile phone industry’s need for multimedia‐rich smart phones have resulted in low‐power architectures capable of performing computationally demanding signal processing and stochastic recognition tasks in real time. We describe our work on a small form‐factor, light‐weight package used to perform real‐time passive acoustic detection and classific...

Reliable acoustic path and convergence zone bottom interaction in the Philippine Sea 09 Experiment.

One of the objectives of the Philippine Sea 09 (PhilSea09) was to examine the multipath structure of RAP propagation at the seafloor. For a source near the surface, a receiver at the seafloor, and ranges approaching one‐half a CZ, one expects a direct path, seafloor reflected and refracted paths plus water column multiples. These were measured using the bottom elements of the SIO deep vertical line array. Similarly, for both a source and a receiver at the surface and at CZ ranges, these same paths propagate back to the surface. These were also measured with the PSU FORA (five octave research array). The experiment was part of an NPAL Group effort. [Work supported by ONR Ocean Acoustics.]

Simultaneous source panging and bottom geoacoustic inversion using shallow water, broadband dispersion of fin whale calls

Beamformed time series of fin whale calls from the North Atlantic recently have been obtained. Spectrograms of these recordings have an unusual appearance—three to four downsweeping clicks occur in rapid succession, almost as if the whale was stuttering. These arrivals can be explained in terms of the differences in group velocity between the lowest‐order modes that propagate in the shallow water (about 350 m) waveguide. By matching predicted spectrograms calculated from a shallow water waveguide model with actual spectrograms, estimates of both the range of the whale and the compressional wave velocity in the ocean bottom are obtained. The approach makes various assumptions, including that the whale emits a signal with a downsweep rate of 25 Hz per second. However, known deviations from the assumptions can be incorporated into the modeling. The results have several implications such as that whale signals may provide loud, broadband, tomographic sources of opportunity for determining oceanographic/geophys...

Low‐frequency grazing propagation over periodic steep‐sloped rigid roughness elements

Extensive experimental studies of the coherently forward scattered sound at grazing incidence to low roughness rigid surfaces with periodic steep‐sloped elements have confirmed several of the theoretical predictions of large boundary wave amplitude and subsonic dispersion [Tolstoy, J. Acoust. Soc. Am. 75, 1–22 (1984)]. For example, at short ranges the boundary wave amplitude diverges cylindrically and is proportional to ek3/2r−1/2, where e is the scattering parameter, k is the wavenumber, and r is the range; thereby, at sufficient ranges and frequencies it exceeds the amplitude of the spherically diverging direct wave. The dispersion is subsonic and goes as Ak2e2, where A depends on the roughness element. The experiments have also revealed an attenuation factor exp (−δr), where δ=αk6 due to incoherent scatter up to critical range kr=π/(Ak2e2) beyond which an attenuation of form exp (− 1/2 A2e4k6r2) becomes dominant. This leads to frequency‐independent peak amplitudes two to six times the direct wave ampli...