Simon E. Freeman

Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea

Conventional and adaptive plane-wave beamforming with simultaneous recordings by large-aperture horizontal and vertical line arrays during the 2009 Philippine Sea Engineering Test (PhilSea09) reveal the rate of occurrence and the two-dimensional arrival structure of seismic phases that couple into the deep ocean. A ship-deployed, controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels. Ninety T-phases from unique azimuths were recorded between Yeardays 107 to 119. T-phase azimuth and S-minus-P-phase time-of-arrival range estimates were validated using United States Geological Survey seismic monitoring network data. Analysis of phases from a seismic event that occurred on Yearday 112 near the east coast of Taiwan approximately 450 km from the arrays revealed a 22° clockwise evolution of T-phase azimuth over 90 s. Two hypotheses to explain such evolution-body wave excitation of multiple sources or in-water scattering-are presented based on T-phase origin sites at the intersection of azimuthal great circle paths and ridge/coastal bathymetry. Propagation timing between the source, scattering region, and array position suggests the mechanism behind the evolution involved scattering of the T-phase from the Ryukyu Ridge and a T-phase formation/scattering location estimation error of approximately 3.2 km.

Cross-correlation, triangulation, and curved-wavefront focusing of coral reef sound using a bi-linear hydrophone array

A seven element, bi-linear hydrophone array was deployed over a coral reef in the Papahãnaumokuãkea Marine National Monument, Northwest Hawaiian Islands, in order to investigate the spatial, temporal, and spectral properties of biological sound in an environment free of anthropogenic influences. Local biological sound sources, including snapping shrimp and other organisms, produced curved-wavefront acoustic arrivals at the array, allowing source location via focusing to be performed over an area of 1600 m(2). Initially, however, a rough estimate of source location was obtained from triangulation of pair-wise cross-correlations of the sound. Refinements to these initial source locations, and source frequency information, were then obtained using two techniques, conventional and adaptive focusing. It was found that most of the sources were situated on or inside the reef structure itself, rather than over adjacent sandy areas. Snapping-shrimp-like sounds, all with similar spectral characteristics, originated from individual sources predominantly in one area to the east of the array. To the west, the spectral and spatial distributions of the sources were more varied, suggesting the presence of a multitude of heterogeneous biological processes. In addition to the biological sounds, some low-frequency noise due to distant breaking waves was received from end-fire north of the array.

On the origins of ambient biological sounds in shallow water tropical ecosystems

Although discovered more than 60 years ago, the origins of much ambient underwater biological noise remain unclear. Snapping shrimp sounds dominate some environments but elsewhere the shallow-water biological sound field is often heterogeneous. Here we show that dominant components of underwater ambient noise recorded on coral reefs around the Line Islands archipelago in the central Pacific are spectrally consistent with the interaction of hard-shelled benthic macro-organisms with the coral substrate. Acoustic recordings taken from shallow coral reef environments show a consistent, nightly 4.7 to 6.9 dB increase in estimated pressure spectral density level in the 11 to 17 kHz band with a spectral peak centered between 14 to 15 kHz. Intensity-filtered recordings of an example species, the hermit crab Clibanarius diugeti, in quiet aquarium conditions reveal that transient sounds produced by the interaction between the crustaceans carapace, shell, and coral substrate are structurally consistent in spectra with the dominant components of the recordings from the central Pacific. Passive acoustic monitoring of such ambient noise may be useful as a complimentary ecological survey technique to SCUBA-based visual observations, which are typically poor in estimating the abundance and diversity of cryptobenthic organisms.

Locating invertebrate sound sources, including hermit crabs, on shallow water reefs in the Northwestern Hawaiian Islands using an L-shaped array of hydrophones

Using ambient noise for extracting ecological information from coastal waters is a tantalizing idea that has gained momentum with the increasing use of long-term passive recorders. Single-element hydrophone recordings from different reef locations reveal substantial variation in the biologically produced sound field, the spatial scales and sources of which are poorly understood. A seven-element L-shaped array was deployed in a spur-and-groove coral reef environment at Kure Atoll in the Papahanaumokuakea Marine National Monument, Northwestern Hawaiian Islands, in an effort to resolve small-scale spatial variability in reef-generated ambient noise. For each given time step, a fast, pair-wise coherence technique was initially used to estimate correlation in the ambient noise field. Curved-wavefront adaptive beamforming was then performed within range and azimuth windows that encompassed each estimated source location (areas of high correlation). Array processing performance was evaluated and frequency-depend...

Photosynthesis by marine algae produces sound, contributing to the daytime soundscape on coral reefs

We have observed that marine macroalgae produce sound during photosynthesis. The resultant soundscapes correlate with benthic macroalgal cover across shallow Hawaiian coral reefs during the day, despite the presence of other biological noise. Likely ubiquitous but previously overlooked, this source of ambient biological noise in the coastal ocean is driven by local supersaturation of oxygen near the surface of macroalgal filaments, and the resultant formation and release of oxygen-containing bubbles into the water column. During release, relaxation of the bubble to a spherical shape creates a monopole sound source that ‘rings’ at the Minnaert frequency. Many such bubbles create a large, distributed sound source over the sea floor. Reef soundscapes contain vast quantities of biological information, making passive acoustic ecosystem evaluation a tantalizing prospect if the sources are known. Our observations introduce the possibility of a general, volumetrically integrative, noninvasive, rapid and remote technique for evaluating algal abundance and rates of primary productivity in littoral aquatic communities. Increased algal cover is one of the strongest indicators for coral reef ecosystem stress. Visually determining variations in algal abundance is a time-consuming and expensive process. This technique could therefore provide a valuable tool for ecosystem management but also for industrial monitoring of primary production, such as in algae-based biofuel synthesis.

Beamforming using chip-scale atomic clocks in a controlled environment

Recently developed low-power Chip-Scale Atomic Clocks (CSACs) hold promise for underwater acoustics applications because they enable time-coherent processing, critical for estimating the directionality of the sound field, when acoustic array elements cannot share a timing reference. Controlled, tank-based experiments with a small acoustic array (N = 4) featuring CSAC-equipped elements show that optimal disciplining is important for continued array coherence. Clock drift equivalent to a 10% wavelength error at 0.3, 1, and 10 kHz was reached at approximately 25, 10, and 3 days, respectively. Within application-specific limits, this technology brings enhanced capabilities to acoustic thermometry, geoacoustic, biological, and under-ice acoustic oceanography.

Small-scale soundscapes over coral reef ecosystems: Latitudinal variation in the Northwestern Hawaiian Islands

Two-dimensional seafloor “maps” of near-field ambient sound produced by biological sources in a coral reef environment were obtained from four spur-and-groove shallow water reef environments along a latitudinal gradient in the Papahanaumokuakea Marine National Monument, Northwestern Hawaiian Islands. Acoustic data were collected in conjunction with SCUBA based ecological surveys in an effort to establish correlation between components of the acoustic field and the ecological state of each field site. Simultaneous acoustic measurements and remote underwater photographs taken during the day and at night allowed for comparisons of biological activity with recordings over time. Using a bottom-mounted L-shaped array of hydrophones, the spatial distribution, frequency, and temporal characteristics of sounds produced by small-scale biological processes were estimated within a 40 m by 40 m region around the array. A fast cross-correlation guidance technique lessened the computational burden imposed by conventiona...

Spatial variation of the underwater soundscape over coral reefs in the Northwestern Hawaiian Islands

Coral reefs create a complex acoustic environment, dominated by sounds produced by benthic creatures such as crustaceans and echinoderms. While there is growing interest in the use of ambient underwater biological sound as a gauge of ecological state, extracting meaningful information from recordings is a challenging task. Single hydrophone (omnidirectional) recorders can provide summary time and frequency information, but as the spatial distribution of reef creatures is heterogeneous, the properties of reef sound arriving at the receiver vary with position and arrival angle. Consequently, the locations and acoustic characteristics of individual sound producers remain unknown. An L-shaped hydrophone array, providing direction-and-range sensing capability, can be used to reveal the spatial variability of reef sounds. Comparisons can then be made between sound sources and other spatially referenced information such as photographic data. During the summer of 2012, such an array was deployed near four differe...

Bearing estimation and 2D localization of bowhead whale calls using directional autonomous seafloor acoustic recorders

Passive acoustic monitoring of the summer/fall westerly bowhead whale migration in the Beaufort Sea has been conducted by Greeneridge Sciences, sponsored by SEPCO, every year since 2006. The directional autonomous seafloor acoustic recorder (DASAR) packages used in this effort each contain three acoustic sensors that simultaneously measure the two horizontal components of acoustic particle motion and acoustic pressure. A variety of data-adaptive beamforming methods have been applied to a selected subset of these data to examine direction-of-arrival estimation performance, including quantifying bearing bias and variance. A principal component analysis (PCA)-type eigenanalysis of the sensor data cross spectral matrix is used to decompose the received field into orthogonal components having different particle motion polarization and energy flux properties. Appropriate manipulation of these components provides high resolution directional estimates of the primary arriving energy while maintaining robustness to...

Time-evolving T-phase arrival structure using simultaneous recordings by large-aperture horizontal and vertical line arrays in PhilSea09

A number of models have been proposed to explain the mechanisms by which seismic phases couple to the deep ocean sound channel in order to create water-borne acoustic tertiary (T) phases. Beamforming conducted on simultaneous recordings by large-aperture horizontal towed and vertical moored line arrays during PhilSea09 shows the temporal evolution of a T-phase arrival consistent with the down-slope modal conversion/propagation model. Towed array calibration is conducted using ship-deployed, controlled multi-tone acoustic sources. Conventional, minimum variance distortionless response, white noise constrained, and dominant mode rejection beamformers are compared in their ability to minimize bias and variance in estimating the azimuthal arrival directions of signals from both the controlled source and the seismic phases recorded by the horizontal array. Horizontal array beamformer-derived azimuth and time-of-arrival range estimates from P, S, and T-phase arrivals at towed and moored receivers indicate the e...