Graham A. Warner

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.

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.

Estimation of subbottom geoacoustic properties from offshore airgun surveys in Atlantic Canada

This paper estimates seabed geoacoustic profiles at 14 sites offshore of the eastern Canadian coast using sound levels from a single airgun. A 210 in3 airgun was operated along two track lines at each site and a calibrated seabed-mounted Autonomous Multichannel Acoustic Recorder equipped with a single omnidirectional hydrophone recorded the airgun sounds. High-resolution bathymetry was acquired along the tracks using a ship-mounted multibeam system and conductivity-temperature-depth profiles were measured at each site. Seabed geoacoustic profiles were estimated in a non-linear trans-dimensional Bayesian inversion using energy spectral density levels at six frequencies between 10 and 320 Hz as a function of range. The inversion estimated the unknown number of subbottom layers, geoacoustic properties (including compressional and shear wave speeds and attenuations, density, and layer thicknesses), airgun source levels, a range-correction factor, and error statistics. Data were fit with error standard deviations of 0.6–5.6 dB. Convergence of the posterior probability density function was not reached within the time limits of the study so parameter uncertainty was estimated using a linearized formulation of the inverse problem centered around the model that minimized the Bayesian Information Criterion.This paper estimates seabed geoacoustic profiles at 14 sites offshore of the eastern Canadian coast using sound levels from a single airgun. A 210 in3 airgun was operated along two track lines at each site and a calibrated seabed-mounted Autonomous Multichannel Acoustic Recorder equipped with a single omnidirectional hydrophone recorded the airgun sounds. High-resolution bathymetry was acquired along the tracks using a ship-mounted multibeam system and conductivity-temperature-depth profiles were measured at each site. Seabed geoacoustic profiles were estimated in a non-linear trans-dimensional Bayesian inversion using energy spectral density levels at six frequencies between 10 and 320 Hz as a function of range. The inversion estimated the unknown number of subbottom layers, geoacoustic properties (including compressional and shear wave speeds and attenuations, density, and layer thicknesses), airgun source levels, a range-correction factor, and error statistics. Data were fit with error standard deviati...

Hydroacoustic measurements and modeling of pile driving operations in Ketchikan, Alaska

Underwater acoustic measurements of pile driving operations were made at the Ketchikan ferry terminal in July 2016. At the time of the measurements, marine mammal injury and disturbance criteria developed by the National Marine Fisheries Service (NMFS) were based on sound pressure level (SPL) thresholds. Shortly after the measurements, NMFS changed the injury thresholds to a dual criteria involving peak pressure and sound exposure levels specific to marine mammal functional hearing groups. This paper presents distances to both injury criteria and the (unchanged) SPL-based disturbance criteria for vibratory driving and impact hammering 30-inch diameter cylindrical piles. Threshold distances were obtained using empirical regressions of sound levels measured by seabed-mounted recorders at 10 and 1000 m nominal range. A finite-difference method pile driving source model was used with a parabolic equation propagation model to compare measurements with simulations and to estimate received levels at all ensonifi...

Using vessel noise from a single hydrophone to estimate environmental properties

This paper estimates seabed and water-column properties of a shallow-water site in the Chukchi Sea using vessel noise recorded on a single ocean-bottom hydrophone. A shallow-hazards seismic survey vessel transited with a fixed heading, passing within 200 m of the hydrophone. Sound pressure levels as a function of frequency and range are inverted using a trans-dimensional (trans-D) Bayesian approach to estimate range-independent environmental properties (sound-speed profile, water depth, and seabed geoacoustic properties). The trans-D inversion allows the data to determine the most appropriate environmental model parameterization in terms of the number of sound-speed profile nodes and subbottom layers. The inversion also estimates the vessel source levels, source depth, hydrophone height above the seabed, a range-correction factor, and error statistics, and provides uncertainty estimates for all model parameters and parameterizations. The sound-speed profile is found to be in good agreement with a measured...

Environmental characterization in the Chukchi Sea using Bayesian inversion of bowhead whale calls

This paper estimates seabed and water-column properties of a shallow-water site in the Chukchi Sea using bowhead whale calls recorded on asynchronous ocean-bottom hydrophones. Up- and down-swept bowhead whale calls were recorded on a cluster of seven hydrophones within a 5 km radius. The calls excited multiple propagating modes, with modal dispersion controlled by environmental properties and whale-recorder range. Frequency-dependent mode arrival times for nine whale calls are inverted using a trans-dimensional (trans-D) Bayesian approach that estimates the whale locations (easting and northing) and range-independent environmental properties (sound-speed profile, water depth, and seabed geoacoustic profiles). The trans-D inversion allows the data to determine the most appropriate environmental model parameterization in terms of the number of sound-speed profile nodes and subbottom layers. The inversion also estimates each whale-call instantaneous frequency function, relative recorder clock offsets, and re...

Bowhead whale localization using time-difference-of-arrival data from asynchronous recorders

This paper estimates bowhead whale locations and uncertainties using nonlinear Bayesian inversion of the time-difference-of-arrival (TDOA) of low-frequency whale calls recorded on onmi-directional asynchronous recorders in the shallow waters of the northeastern Chukchi Sea, Alaska. A Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by 0.5-9.2 km, was deployed for several months over which time their clocks drifted out of synchronization. Hundreds of recorded whale calls are manually associated between recorders. The TDOA between hydrophone pairs are calculated from filtered waveform cross correlations and depend on the whale locations, hydrophone locations, relative recorder clock offsets, and effective waveguide sound speed. A nonlinear Bayesian inversion estimates all of these parameters and their uncertainties as well as data error statistics. The problem is highly nonlinear and a linearized inversion did not produce physically realistic results. Whale location uncertainties from nonlinear inversion can be low enough to allow accurate tracking of migrating whales that vocalize repeatedly over several minutes. Estimates of clock drift rates are obtained from inversions of TDOA data over two weeks and agree with corresponding estimates obtained from long-time averaged ambient noise cross correlations. The inversion is suitable for application to large data sets of manually or automatically detected whale calls.

Time-difference-of-arrival localization of bowhead whales using asynchronous recorders

This paper estimates bowhead whale locations and uncertainties using linearized Bayesian inversion of the time-difference-of-arrival (TDOA) of whale calls recorded on omni-directional asynchronous recorders in the Chukchi Sea, Alaska. A Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by 0.5–9.2 km, was deployed for several months over which time their clocks drifted out of synchronization. Hundreds of recorded whale calls are manually annotated with time-frequency bounds and associated between recorders. The TDOA between all hydrophone pairs are calculated from filtered waveform cross-correlations and depend on the whale locations, hydrophone locations, relative recorder clock drifts, and an effective waveguide sound speed. The inversion estimates all of these parameters and their uncertainties as well as data error statistics using prior information to constrain the otherwise underdetermined problem. Whale location uncertainties are estimated to be approximately 100 m which allow...

Bowhead whale localization and environmental inversion using asynchronous hydrophones

This paper estimates bowhead whale locations and environmental properties using Bayesian inversion of the modal dispersion of whale calls recorded on asynchronous recorders in the shallow waters of the northeastern Chukchi Sea, Alaska. Bowhead calls were recorded on a Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by up to 9.2 km. We use a warping time-frequency analysis to obtain frequency-dependent relative mode arrival times for nine frequency-modulated whale calls. Trans-dimensional inversion is applied to invert mode arrival times for the whale location, water sound-speed profile, subbottom layering and geoacoustic parameters, source instantaneous frequency (IF), relative recorder clock drifts, and residual error standard deviation, all with estimated uncertainties. Joint inversion of multiple calls is found to substantially reduce uncertainties on whale location, source IF, and clock drifts. Estimated whale location uncertainties are 30–160 m and clock drift uncertainties a...

Airgun array sound measurements in the northeastern Chukchi Sea

Underwater acoustic measurements were made during eight 2D/3D seismic surveys with large airgun arrays and nine shallow hazards surveys using small arrays (2–4 airguns) in the shallow (30–50 m) waters of the northeastern Chukchi Sea between 2006 and 2013. The acoustic measurements were made with calibrated seabed-deployed recorders to assess potential noise exposures to marine fauna. Data were acquired from directly under the airgun arrays to over 150 km distance. The seismic pulses contain signals above ambient noise between approximately 5 Hz and 15 kHz. Modal structure develops by about 1 km distance from the sources at frequencies between approximately 30 Hz and 600 Hz, but it is strongest from 100 Hz to 300 Hz. This ocean environment therefore acts as a frequency band-pass filter, selectively passing the intermediate sound frequencies to long distances. The modes are strongly dispersed, with low frequency components arriving up to 2 s later than the higher frequency sounds of the same mode. This causes received signals to have a down-swept spectral shape. We present sound level measurement results from all of the surveys and we discuss the frequency-dependent acoustic transmission loss in context with the modal propagation characteristics of this ocean environment.