David E. Hannay

High‐resolution, deep‐towed, multichannel seismic survey of deep‐sea gas hydrates off western Canada

A multichannel seismic survey was carried out using the high‐resolution deep‐towed acoustics/geophysics system (DTAGS) to image the structure of deep‐sea gas hydrates on the continental slope off Vancouver Island and to determine the velocity profile of the hydrated sediments. The high‐frequency DTAGS data provide the means to estimate the frequency response of the bottom simulating reflector (BSR) that defines the base of the hydrate stability field in these sediments, over a broad frequency band from 15 to 650 Hz. The DTAGS sections resolved fine‐scale layering as thin as a few meters within the hydrated zone and below the BSR, and they revealed small‐scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas. Interval velocities determined from the DTAGS data indicate uniformly low values of about 1500 m/s to depths of 100 m below sea floor (mbsf), increasing to about 1850 m/s at the BSR (250 mbsf). The reflection from ...

New seismic study of deep sea gas hydrates results in greatly improved resolution

A multichannel seismic survey has resulted in greatly improved resolution of structural details of deep sea gas hydrates off the west coast of Canada, revealing numerous geological features not before evident. The survey using the Naval Research Laboratory deep-towed acoustic/geophysics system (DTAGS), provided high-resolution images and layer velocities more than 10 times better than those obtained in the past using conventional systems. Vertical resolution within 2 m and horizontal resolution within 20 m were achieved. The work demonstrates that high-resolution seismic surveys with deep towed multichannel systems can provide important new information about pathways of fluid and gas migration that control the formation of gas hydrates. Conventional surface-towed seismic systems are unable to do this.

Head wave data inversion for geoacoustic parameters of the ocean bottom off Vancouver Island

Broad-band acoustic signals received by a vertical line array show a rich structure of precursors arriving prior to the water wave. A combination of beamforming, spectral analysis, and travel-time considerations is applied to separate and identify individual head wave arrivals in the sequence of precursor signals. The travel times for individual head waves are inverted to determine compressional wave speeds in two distinct subbottom layers. The frequencyand range-dependence of the head waves amplitudes are analyzed to estimate the compressional wave attenuation. INTRODUCTION As a part of the Pacific Shelf-93 experiment, a 16-element vertical line array has been used to receive acoustic signals generated by small explosive charges (Fig. l) on the continental shelf off Vancouver Island. The signals contain precursors propagating faster than the direct water wave and consisting of a number of arrivals [I]. In this paper, the precursors are analyzed to retrieve information on geoacoustic parameters of the ocean bottom. DATA ANALYSIS With increasing range, the number of identifiable individual precursors increases. For each individual precursor, travel time is a linear function of receiver range (Fig. 2) and depth. Polarity and the kinematic properties of the precursors are consistent with the environmental model (Fig. I) where head waves [2] are generated at two interfaces within the ocean bottom. Trajectories of the four identified head wave arrivals generated at the interface B differ by the number of reflections at the interface and ocean surface. From the derivatives of the travel times with respect to range, one finds 1730*10 m/s and 2159*10 m/s, respectively, for sound velocities in the sediment layers below the interfaces A and B.

Tracking of Pacific walruses in the Chukchi Sea using a single hydrophone

The vocal repertoire of Pacific walruses includes underwater sound pulses referred to as knocks and bell-like calls. An extended acoustic monitoring program was performed in summer 2007 over a large region of the eastern Chukchi Sea using autonomous seabed-mounted acoustic recorders. Walrus knocks were identified in many of the recordings and most of these sounds included multiple bottom and surface reflected signals. This paper investigates the use of a localization technique based on relative multipath arrival times (RMATs) for potential behavior studies. First, knocks are detected using a semi-automated kurtosis-based algorithm. Then RMATs are matched to values predicted by a ray-tracing model. Walrus tracks with vertical and horizontal movements were obtained. The tracks included repeated dives between 4.0 m and 15.5 m depth and a deep dive to the sea bottom (53 m). Depths at which bell-like sounds are produced, average knock production rate and source levels estimates of the knocks were determined. Bell sounds were produced at all depths throughout the dives. Average knock production rates varied from 59 to 75 knocks/min. Average source level of the knocks was estimated to 177.6 ± 7.5 dB re 1 μPa peak @ 1 m.

Bayesian environmental inversion of airgun modal dispersion using a single hydrophone in the Chukchi Sea

This paper presents estimated water-column and seabed parameters and uncertainties for a shallow-water site in the Chukchi Sea, Alaska, from trans-dimensional Bayesian inversion of the dispersion of water-column acoustic modes. Pulse waveforms were recorded at a single ocean-bottom hydrophone from a small, ship-towed airgun array during a seismic survey. A warping dispersion time-frequency analysis is used to extract relative mode arrival times as a function of frequency for source-receiver ranges of 3 and 4 km which are inverted for the water sound-speed profile (SSP) and subbottom geoacoustic properties. The SSP is modeled using an unknown number of sound-speed/depth nodes. The subbottom is modeled using an unknown number of homogeneous layers with unknown thickness, sound speed, and density, overlying a halfspace. A reversible-jump Markov-chain Monte Carlo algorithm samples the model parameterization in terms of the number of water-column nodes and subbottom interfaces that can be resolved by the data. The estimated SSP agrees well with a measured profile, and seafloor sound speed is consistent with an independent headwave arrival-time analysis. Environmental properties are required to model sound propagation in the Chukchi Sea for estimating sound exposure levels and environmental research associated with marine mammal localization.

Estimation of seismic velocities of upper oceanic crust from ocean bottom reflection loss data

This paper describes a Bayesian inversion of acoustic reflection loss versus angle measurements to estimate the compressional and shear wave velocities in young uppermost oceanic crust, Layer 2A. The data were obtained in an experiment on the thinly sedimented western flank of the Endeavor segment of the Juan de Fuca Ridge, using a towed horizontal hydrophone array and small explosive charges as sound sources. Measurements were made at three sites at increasing distance from the ridge spreading center to determine the effect of age of the crust on seismic velocities. The inversion used reflection loss data in a 1/3-octave band centered at 16 Hz. The compressional and shear wave velocities of the basalt were highly sensitive parameters in the inversion. The compressional wave velocity increased from 2547 + or - 30 to 2710 + or - 18 m/s over an age span of 1.4 million years (Ma) from the spreading center, an increase of 4.5 + or - 1.0%/Ma. The basalt shear wave velocity increased by nearly a factor of 2, from approximately 725 to 1320 m/s over the same age span. These results show a decreasing trend of Poissons ratio with age, from a value of 0.46 at the youngest site closest to the ridge axis.

Geoacoustic profile estimation by inversion of head wave data

An approach is described for estimation of geoacoustic model parameters in shallow water based on inversion of head wave data obtained in experiments with vertical line arrays (VLA). Analysis of head waves obtained in seismic refraction surveys with horizontal arrays is a widespread practice in exploration seismology. However, in shallow water, there can be practical advantages in the use of VLAs. Inversion of head wave data obtained with a VLA provides estimates of the layer depths, and compressional wave speeds and attenuations. Three inversion techniques are compared: inversion of travel time versus range data for a single sensor, direct measurement of the critical angle using the VLA, and inversion of travel time versus hydrophone depth data for a specific range. The techniques are applied to data from an experiment on the continental shelf off Vancouver Island. Shallow explosive charges were used as sound sources out to ranges of 5 km. Strong head wave signals were recorded from two distinct layers, ...

Bowhead whale localization using asynchronous hydrophones in the Chukchi Sea

This paper estimates bowhead whale locations and uncertainties using non-linear Bayesian inversion of their modally-dispersed calls recorded on asynchronous recorders in the Chukchi Sea, Alaska. Bowhead calls were recorded on a cluster of 7 asynchronous ocean-bottom hydrophones that were separated by 0.5-9.2 km. A warping time-frequency analysis is used to extract relative mode arrival times as a function of frequency for nine frequency-modulated whale calls that dispersed in the shallow water environment. Each call was recorded on multiple hydrophones and the mode arrival times are inverted for: the whale location in the horizontal plane, source instantaneous frequency (IF), water sound-speed profile, seabed geoacoustic parameters, relative recorder clock drifts, and residual error standard deviations, all with estimated uncertainties. A simulation study shows that accurate prior environmental knowledge is not required for accurate localization as long as the inversion treats the environment as unknown. Joint inversion of multiple recorded calls is shown to substantially reduce uncertainties in location, source IF, and relative clock drift. Whale location uncertainties are estimated to be 30-160 m and relative clock drift uncertainties are 3-26 ms.

Underwater passive acoustic localization of Pacific walruses in the northeastern Chukchi Sea

This paper develops and applies a linearized Bayesian localization algorithm based on acoustic arrival times of marine mammal vocalizations at spatially-separated receivers which provides three-dimensional (3D) location estimates with rigorous uncertainty analysis. To properly account for uncertainty in receiver parameters (3D hydrophone locations and synchronization times) and environmental parameters (water depth and sound-speed correction), these quantities are treated as unknowns constrained by prior estimates and prior uncertainties. Unknown scaling factors on both the prior and arrival-time uncertainties are estimated by minimizing Akaikes Bayesian information criterion (a maximum entropy condition). Maximum a posteriori estimates for sound source locations and times, receiver parameters, and environmental parameters are calculated simultaneously using measurements of arrival times for direct and interface-reflected acoustic paths. Posterior uncertainties for all unknowns incorporate both arrival time and prior uncertainties. Monte Carlo simulation results demonstrate that, for the cases considered here, linearization errors are small and the lack of an accurate sound-speed profile does not cause significant biases in the estimated locations. A sequence of Pacific walrus vocalizations, recorded in the Chukchi Sea northwest of Alaska, is localized using this technique, yielding a track estimate and uncertainties with an estimated speed comparable to normal walrus swim speeds.

Pacific walrus vocal repertoire in the northeastern Chukchi Sea: Call type description and relative proportion in time and space

Pacific walrus are present in the northeastern Chukchi Sea (NCS) from June to October. The study of their sounds has been largely restricted to the knock and bell sounds produced by males during the breeding season and in-air grunts and barks from mother and pups. A passive acoustic monitoring program conducted in the NCS since July 2006 has brought strong evidence that the underwater vocal repertoire of walrus is more diverse. Nine call types (including knocks and bells) and their variants identified over four years of acoustic monitoring will be described. Spectral measurements along with estimates of variability for high signal-to-noise ratio calls will be provided. The relative proportion of each call type across the study area and throughout the season is currently analyzed based on the identification of all calls in samples recorded multiple times per day in 2009 and 2010. Preliminary results suggest that the vocal repertoire of walrus is dominated by grunt-type calls, which is consistent with the N...