Garry J. Heard

Underwater light bulb implosions: a useful acoustic source

The implosion of sealed glass vessels-such as fishing floats, laboratory glassware, and various bottles-under the influence of hydrostatic pressure at depth in the ocean has long been known to produce moderately loud acoustic events. Common light bulbs have also been frequently used in practice, but it appears that the use of these particular vessels has not been described in the scientific journals. In fact, most users of light bulbs have no information on the crush depths, source level, and spectral content of the radiated signal. The aim of this paper is to remedy this shortcoming, to describe the use of common light bulbs as acoustic sources, and to provide guidance to researchers on the source level, spectrum, and usage of common sizes of light bulbs and fluorescent lighting tubes. With the current focus on shallow-water operations, bearing in mind the prohibition against the use of all but the smallest explosives, imploding light bulbs may be the most cost-effective acoustic source at depths under 300 m that minimizes environmental impact.

Genetic Algorithm Inversion of the 1997 Geoacoustic Inversion Workshop Test Case Data

An analysis of the 1997 Geoacoustic Inversion Workshop test case data was carried out to benchmark the performance of a Genetic Algorithm (GA) inversion code called SAGA_INV.1 The inversion program made use of Westwoods ORCA propagation model,2 FORTRAN subroutines, and Interactive Data Language (Research Systems Inc. IDL). SAGA_INV is capable of performing inversions with either Simulated Annealing (SA) or GA optimization schemes; however, only the GA portion of the code has been benchmarked with the workshop test cases at the present time. Not all of the workshop test cases were processed: this study was concerned only with the CAL, SD, SO, AT, and WA data sets. The CAL data was processed using three different cost functions: (i) standard Bartlett processor, (ii) a broadband coherent processor, and (iii) a transmission loss mismatch function. These processors were applied to three frequency bands: (i) 76 frequencies between 25 Hz and 100 Hz, (ii) nine frequencies between 28 Hz and 36 Hz, and (iii) 13 frequencies between 44 Hz and 56 Hz. The latter two frequency regimes were intended to simulate 1/3-octave bands centered at 32 Hz and 50 Hz, respectively. Four different receiving arrays were simulated: (i) a 1550 m aperture horizontal, bottom mounted array at approximately 1-km range, (ii) a similar array at approximately 4.2-km range, (iii) a 55-m aperture 12-element vertical array located at 1-km range, and (iv) a similar vertical array at 5-km range. In addition to processing the CAL data set, all three subcases of the SD, SO, AT, and WA data sets were also processed; however, only the transmission loss cost function and the two simulated 1/3-octave bands were considered for these test cases.

Experimental validation of regularized array element localization

This paper examines and validates regularized inversion for array element localization (AEL) by quantitative comparison of inversion results to direct measurements of receiver positions for a full-scale AEL survey. Regularized AEL treats both receiver and source positions as unknown parameters in a ray-based inversion; prior information on source/receiver positions, inter-receiver spacing in depth, and/or a smooth array shape can be included, subject to statistically fitting the acoustic data. Uncertainties in the recovered receiver positions are estimated via Monte Carlo appraisal. To study this approach, a specially stabilized, two-dimensional receiver array and a series of impulsive sources (imploding glass light bulbs) were deployed from shore-fast (motionless) Arctic sea ice. Sources and recordings were not synchronized in time, so AEL inversions are based on relative arrival times. Receiver positions were measured to an uncertainty of ∼5 cm in each dimension [9 cm in three dimensions (3D)] using nonacoustic (optical) methods. Average AEL errors (difference between measured receiver positions and inversion results) of 13 cm in depth, 27 cm in the horizontal, and 30 cm in 3D, as well as good agreement between the measured errors and estimated AEL uncertainties validate the regularized approach and provide benchmarks for acoustic AEL. Receiver-position errors are quantitatively investigated as a function of the number of sources, source-position errors, and different regularizations.

Source bearing estimation in the Arctic Ocean using ice-mounted geophones

This paper investigates the use of geophones mounted on the surface of Arctic sea ice for estimating the bearing to acoustic sources in the water column. The approach is based on measuring ice seismic waves for which the direction of particle motion is oriented radially outward from the source. However, the analysis is complicated by the fact that sea ice supports several types of seismic waves, producing complex particle motion that includes significant nonradial components. To suppress seismic waves with transverse particle motion, seismic polarization filters are applied in conjunction with a straightforward rotational analysis (computation of particle-motion power as a function of angle). The polarization filters require three-dimensional (3D) measurements of particle motion, and apply theoretical phase relationships between vertical and horizontal components for the various waves types. In addition, the 180 degrees ambiguity inherent in the rotational analysis can be resolved with 3D measurements by considering particle motion in the vertical-radial plane. Arctic field trials were carried out involving two components. First, a hammer source was used to selectively excite the various ice seismic waves to investigate their propagation properties and relative importance in bearing estimation. Second, impulsive acoustic sources were deployed in the water column at a variety of bearings and ranges from 200-1000 m. For frequencies up to 250 Hz, source bearings are typically estimated to within an average absolute error of approximately 100.

Environmental inversion and matched-field tracking with a surface ship and an L-shaped receiver array

Acoustic data from the natural broadband signature of a quiet surface ship, recorded on the vertical leg of an L-shaped array, is used to invert for the local geo-acoustic parameters and the resulting effective environment is used for subsequent tracking of the surface ship using a matched-field tracking technique applied to the full array. The matched-field analysis includes a comparison of the incoherent product of the processed data from the horizontal and vertical subapertures with coherent processing of the data from the full L-shaped array. Subaperture processing is of interest since there is a (loose) requirement that the number of data snapshots be greater than or equal to the number of array elements. This presents averaging difficulties for large arrays when the source being observed is moving. Analyzing each array leg separately allows the use of a smaller number of snapshots from which averaged quantities are constructed. Taken separately, the vertical leg of the array provides range-depth inf...

Bottom reflection coefficient measurement and geoacoustic inversion at the continental margin near Vancouver Island with the aid of spiking filters

This paper presents the results of an experiment to measure the reflection coefficient versus grazing angle at the continental margin west of Vancouver Island. The measured reflection coefficients are inverted and estimates of the surficial sediment geoacoustic properties obtained. The receiver used in this experiment was a long towed array with the source in aft-endfire aspect. The acoustic sources for this experiment were explosives that produced direct-path arrivals approximately 0.1 s in length. Bottom and sub-bottom arrivals continued for several seconds duration. Explosives were detonated at intervals such that bottom reflected energy received at the array sampled grazing angles in the interval 10°–80°. Source–receiver ranges were computed by measuring arrival time differences at each of the array elements, comparing these time differences with those computed using a ray-tracing propagation model, and globally minimizing the accumulated squared time difference residuals. To aid in the measurement of...

Arctic field trials of source bearing estimation using ice-mounted geophones (L)

Results are presented from Arctic field trials carried out to estimate the bearing to acoustic sources in the water column using seismic particle motion measured at a tri-axial geophone mounted on the sea ice surface. Source bearings are estimated by applying polarization filters to suppress seismic waves with transverse particle motion and computing the incident power rotated into radial look angles from 0° to 360°; the inherent 180° ambiguity is resolved by requiring out-going (prograde) particle motion in the vertical-radial plane. An earlier study considered impulsive sources at ranges of 200–1000 m at a site characterized by mixed annual ice. The present work considers two studies of similar scale carried out at sites with uniformly smooth annual ice and rough, ridged annual ice, and a third study on multi-year ice involving sources at 2–50-km range. The results indicate good bearing estimation to long range with little dependence on ice type.

Estimating geoacoustic parameters using matched-field inversion methods

In this paper, we use matched-field inversion methods to estimate the geoacoustic parameters for three synthetic test cases from the Geoacoustic Inversion Techniques Workshop held in May 2001 in Gulfport, MS. The objective of this work is to use a sparse acoustic data set to obtain estimates of the parameters as well as an indication of their uncertainties. The unknown parameters include the geoacoustic properties of the sea bed (i.e., number of layers, layer thickness, density, compressional speed, and attenuation) and the bathymetry for simplified range-dependent acoustic environments. The acoustic data used to solve the problems are restricted to five frequencies for a single vertical line array of receivers located at one range from the source. Matched-field inversion using simplex simulated annealing optimization is initially used to find a maximum-likelihood (ML) estimate. However, the ML estimate provides no information on the uncertainties or covariance associated with the model parameters. To estimate uncertainties, a Bayesian formulation of matched-field inversion is used to generate posterior probability density distributions for the parameters. The mean, covariance, and marginal distributions are determined using a Gibbs importance sampler based on the cascaded Metropolis algorithm. In most cases, excellent results were obtained for relatively sensitive parameters such as wave speed, layer thickness, and water depth. The variance of the estimates increase for relatively insensitive parameters such as density and wave attenuation, especially when noise is added to the data.

Heard Island Feasibility Test: Analysis of Pacific path data obtained with a horizontal line array

Preliminary results from the Heard Island Feasibility Test are presented for data obtained with a towed horizontal line array off the coast of southern California. Signal characteristics examined include signal‐to‐noise ratios, sound‐pressure levels, transmission loss, and arrival bearing angles. For the transmissions analyzed to date the mean sound‐pressure level was found to be (63.8±2) dB//1 μPa. The mean transmission loss was found to be (150±3) dB which is significantly greater than that found for receivers in the Atlantic at similar ranges. The mean arrival angle was found to be (214±3)°T and a travel time of approximately 3 h 17 m was measured. Measurements compare favorably with model predictions which indicate that the relatively large transmission loss is due to bottom interactions in the south western Pacific.

Removal of time‐varying Doppler using phase tracking with application to ocean warming measurements

Coded signals received by a moving receiver or from a moving source exhibit a time‐scaling Doppler effect that can be detrimental to their ability to be correctly decoded. The effect is especially critical for the highly Doppler sensitive pseudo‐random noise M sequences frequently used to measure propagation travel times. This paper develops a technique to measure dynamic Doppler effects empirically by tracking the phase of the transmitted carrier, and to remove such effects through interpolation and resampling of the signal. These techniques were very effective for simulated signals and will in the future be employed to help resolve the propagation times of the Heard Island feasibility test data collected by the Canadian receivers.