Publications

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2015
Helble, TA, Ierley GR, D'Spain GL, Martin SW.  2015.  Automated acoustic localization and call association for vocalizing humpback whales on the Navy's Pacific Missile Range Facility. Journal of the Acoustical Society of America. 137:11-21.   10.1121/1.4904505   AbstractWebsite

Time difference of arrival (TDOA) methods for acoustically localizing multiple marine mammals have been applied to recorded data from the Navy's Pacific Missile Range Facility in order to localize and track humpback whales. Modifications to established methods were necessary in order to simultaneously track multiple animals on the range faster than real-time and in a fully automated way, while minimizing the number of incorrect localizations. The resulting algorithms were run with no human intervention at computational speeds faster than the data recording speed on over forty days of acoustic recordings from the range, spanning multiple years. Spatial localizations based on correlating sequences of units originating from within the range produce estimates having a standard deviation typically 10 m or less (due primarily to TDOA measurement errors), and a bias of 20 m or less (due primarily to sound speed mismatch). An automated method for associating units to individual whales is presented, enabling automated humpback song analyses to be performed.

Freeman, SE, Buckingham MJ, Freeman LA, Lammers MO, D'Spain GL.  2015.  Cross-correlation, triangulation, and curved-wavefront focusing of coral reef sound using a bi-linear hydrophone array. Journal of the Acoustical Society of America. 137:30-41.   10.1121/1.4904523   AbstractWebsite

A seven element, bi-linear hydrophone array was deployed over a coral reef in the Papahanaumokuakea 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. (C) 2015 Acoustical Society of America.

2014
Freeman, SE, Rohwer FL, D'Spain GL, Friedlander AM, Gregg AK, Sandin SA, Buckingham MJ.  2014.  The origins of ambient biological sound from coral reef ecosystems in the Line Islands archipelago. Journal of the Acoustical Society of America. 135:1775-1788.   10.1121/1.4865922   AbstractWebsite

Although ambient biological underwater sound was first characterized more than 60 years ago, attributing specific components of ambient sound to their creators remains a challenge. Noise produced by snapping shrimp typically dominates the ambient spectra near tropical coasts, but significant unexplained spectral variation exists. Here, evidence is presented indicating that a discernible contribution to the ambient sound field over coral reef ecosystems in the Line Islands archipelago originates from the interaction of hard-shelled benthic macro-organisms with the coral substrate. Recordings show a broad spectral peak centered between 14.30 and 14.63 kHz, incoherently added to a noise floor typically associated with relatively "white" snapping shrimp sounds. A 4.6 to 6.2 dB increase of pressure spectral density level in the 11 to 17 kHz band occurs simultaneously with an increase in benthic invertebrate activity at night, quantified through time-lapse underwater photography. Spectral-level-filtered recordings of hermit crabs Clibanarius diugeti in quiet aquarium conditions reveal that transient sounds produced by the interaction between the crustaceans' carapace, shell, and coral substrate are spectrally consistent with Line Islands recordings. Coral reef ecosystems are highly interconnected and subtle yet important ecological changes may be detected quantitatively through passive monitoring that utilizes the acoustic byproducts of biological activity. (C) 2014 Acoustical Society of America.

2013
Helble, TA, D'Spain GL, Campbell GS, Hildebrand JA.  2013.  Calibrating passive acoustic monitoring: Correcting humpback whale call detections for site-specific and time-dependent environmental characteristics. Journal of the Acoustical Society of America. 134:EL400-EL406.   10.1121/1.4822319   AbstractWebsite

This paper demonstrates the importance of accounting for environmental effects on passive underwater acoustic monitoring results. The situation considered is the reduction in shipping off the California coast between 2008-2010 due to the recession and environmental legislation. The resulting variations in ocean noise change the probability of detecting marine mammal vocalizations. An acoustic model was used to calculate the time-varying probability of detecting humpback whale vocalizations under best-guess environmental conditions and varying noise. The uncorrected call counts suggest a diel pattern and an increase in calling over a two-year period; the corrected call counts show minimal evidence of these features. (C) 2013 Acoustical Society of America

Heaney, KD, Campbell RL, Murray JJ, Baggeroer AB, Scheer EK, Stephen RA, D'Spain GL, Mercer JA.  2013.  Deep water towed array measurements at close range. Journal of the Acoustical Society of America. 134:3230-3241.   10.1121/1.4818869   AbstractWebsite

During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79-535 Hz. The received energy arrives from two general paths-direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35 degrees spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain. (C) 2013 Acoustical Society of America.

Freeman, SE, D'Spain GL, Lynch SD, Stephen RA, Heaney KD, Murray JJ, Baggeroer AB, Worcester PF, Dzieciuch MA, Mercer JA.  2013.  Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea. Journal of the Acoustical Society of America. 134:3282-3298.   10.1121/1.4818843   AbstractWebsite

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 degrees 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. (C) 2013 Acoustical Society of America.

Worcester, PF, Dzieciuch MA, Mercer JA, Andrew RK, Dushaw BD, Baggeroer AB, Heaney KD, D'Spain GL, Colosi JA, Stephen RA, Kemp JN, Howe BM, Van Uffelen LJ, Wage KE.  2013.  The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea. Journal of the Acoustical Society of America. 134:3359-3375.   10.1121/1.4818887   AbstractWebsite

A series of experiments conducted in the Philippine Sea during 2009-2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010-2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010-2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers. (C) 2013 Acoustical Society of America.

Helble, TA, D'Spain GL, Hildebrand JA, Campbell GS, Campbell RL, Heaney KD.  2013.  Site specific probability of passive acoustic detection of humpback whale calls from single fixed hydrophones. Journal of the Acoustical Society of America. 134:2556-2570.   10.1121/1.4816581   AbstractWebsite

Passive acoustic monitoring of marine mammal calls is an increasingly important method for assessing population numbers, distribution, and behavior. A common mistake in the analysis of marine mammal acoustic data is formulating conclusions about these animals without first understanding how environmental properties such as bathymetry, sediment properties, water column sound speed, and ocean acoustic noise influence the detection and character of vocalizations in the acoustic data. The approach in this paper is to use Monte Carlo simulations with a full wave field acoustic propagation model to characterize the site specific probability of detection of six types of humpback whale calls at three passive acoustic monitoring locations off the California coast. Results show that the probability of detection can vary by factors greater than ten when comparing detections across locations, or comparing detections at the same location over time, due to environmental effects. Effects of uncertainties in the inputs to the propagation model are also quantified, and the model accuracy is assessed by comparing calling statistics amassed from 24 690 humpback units recorded in the month of October 2008. Under certain conditions, the probability of detection can be estimated with uncertainties sufficiently small to allow for accurate density estimates. (C) 2013 Acoustical Society of America.

2012
Lynch, SD, D'Spain GL.  2012.  Evidence of Doppler-shifted Bragg scattering in the vertical plane by ocean surface waves. Journal of the Acoustical Society of America. 131:2011-2022.   10.1121/1.3683247   AbstractWebsite

A set of narrowband tones (280, 370, 535, and 695 Hz) were transmitted by an acoustic source mounted on the ocean floor in 10 m deep water and received by a 64-element hydrophone line array lying on the ocean bottom 1.25 km away. Beamformer output in the vertical plane for the received acoustic tones shows evidence of Doppler-shifted Bragg scattering of the transmitted acoustic signals by the ocean surface waves. The received, scattered signals show dependence on the ocean surface wave frequencies and wavenumber vectors, as well as on acoustic frequencies and acoustic mode wavenumbers. Sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies. Deviations in vertical arrival angle from specular reflection agree with those predicted by the Bragg condition through first-order perturbation theory using measured directional surface wave spectra and acoustic modes measured by the horizontal hydrophone array. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3683247]

Helble, TA, Ierley GR, D'Spain GL, Roch MA, Hildebrand JA.  2012.  A generalized power-law detection algorithm for humpback whale vocalizations. The Journal of the Acoustical Society of America. 131:2682-2699.   10.1121/1.3685790   AbstractWebsite

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. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3685790]

2007
Lepper, PA, D'Spain GL.  2007.  Measurement and modeling of the acoustic field near an underwater vehicle and implications for acoustic source localization. Journal of the Acoustical Society of America. 122:892-905.   10.1121/1.2749410   AbstractWebsite

The performance of traditional techniques of passive localization in ocean acoustics such as time-of-arrival (phase differences) and amplitude ratios measured by multiple receivers may be degraded when the receivers are placed on an underwater vehicle due to effects of scattering. However, knowledge of the interference pattern caused by scattering provides a potential enhancement to traditional source localization techniques. Results based on a study using data from a multi-element receiving array mounted on the inner shroud of an autonomous underwater vehicle show that scattering causes the localization ambiguities (side lobes) to decrease in overall level and to move closer to the true source location, thereby improving localization performance, for signals in the frequency band 2-8 kHz. These measurements are compared with numerical modeling results from a two-dimensional time domain finite difference scheme for scattering from two fluid-loaded cylindrical shells. Measured and numerically modeled results are presented for multiple source aspect angles and frequencies. Matched field processing techniques quantify the source localization capabilities for both measurements and numerical modeling output. (c) 2007 Acoustical Society of America.

2006
D'Spain, GL, Batchelor HH.  2006.  Observations of biological choruses in the Southern California Bight: A chorus at midfrequencies. Journal of the Acoustical Society of America. 120:1942-1955.   10.1121/1.2338802   AbstractWebsite

This paper describes the characteristics of an underwater biological chorus recorded in the midfrequency band (1 - 10 kHz) in the Southern California Bight. The recordings were made in July, 2002 by a large-vertical-aperture, 131-element, 2D billboard array. The chorus, observed predominantly on two consecutive nights during the 8-day experiment, is composed of two bands of energy centered around 1.5 kHz and between 4 and 5 kHz. It causes a complete reversal in the vertical directional characteristics of the mid-frequency ambient sound field between day and nighttime periods; whereas the vertical structure during the day shows a notch in the horizontal direction with levels more than 10 dB below those at higher angles, the nighttime levels in the horizontal can exceed those at other vertical angles by more than 10 dB. These nighttime sounds also are strongly anisotropic in azimuth; they appear to come mainly from a popular Southern California fishing spot where the water depths exceed 75 in. Vertical beam-to-beam coherence squared estimates suggest the chorus source region exists on spatial scales greater than the multipath interference wavelengths of this environment. Individual sounds comprising the chorus, although difficult to separate from the background din, have a fluttering, rasping character. (c) 2006 Acoustical Society of America.

D'Spain, GL, Luby JC, Wilson GR, Gramann RA.  2006.  Vector sensors and vector sensor line arrays: Comments on optimal array gain and detection. Journal of the Acoustical Society of America. 120:171-185.   10.1121/1.2207573   AbstractWebsite

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 components in homogeneous, isotropic noise show that significant coherence exists at half-wavelength spacing between particle motion components. For angular intervals about broadside, an equal spacing of about one wavelength for all components provides maximum directivity index, whereas each of the component spacings should be different to optimize the directivity index for angular intervals about endfire. (c) 2006 Acoustical Society of America.

D'Spain, GL, Terrill E, Chadwell CD, Smith JA, Lynch SD.  2006.  Active control of passive acoustic fields: Passive synthetic aperture/Doppler beamforming with data from an autonomous vehicle. Journal of the Acoustical Society of America. 120:3635-3654.   10.1121/1.2346177   AbstractWebsite

The maneuverability of autonomous underwater vehicles (AUVs) equipped with hull-mounted arrays provides the opportunity to actively modify received acoustic fields to optimize extraction of information. This paper uses ocean acoustic data collected by an AUV-mounted two-dimensional hydrophone array, with overall dimension one-tenth wavelength at 200-500 Hz, to demonstrate aspects of this control through vehicle motion. Source localization is performed using Doppler shifts measured at a set of receiver velocities by both single elements and a physical array. Results show that a source in the presence of a 10-dB higher-level interferer having exactly the same frequency content (as measured by a stationary receiver) is properly localized and that white-noise-constrained adaptive beamforming applied to the physical aperture data in combination with Doppler bearnforming provides greater spatial resolution than physical-aperture-alone bearnforming and significantly lower sidelobes than single element Doppler beamforming. A new broadband beamformer that adjusts for variations in vehicle velocity on a sample by sample basis is demonstrated with data collected during a high-acceleration maneuver. The importance of including the cost of energy expenditure in determining optimal vehicle motion is demonstrated through simulation, further illustrating how the vehicle characteristics are an integral part of the signal/array processing structure. (c) 2006 Acoustical Society of America.

2005
Zimmerman, R, D'Spain GL, Chadwell CD.  2005.  Decreasing the radiated acoustic and vibration noise of a mid-size AUV. Ieee Journal of Oceanic Engineering. 30:179-187.   10.1109/joe.2004.836996   AbstractWebsite

An Odyssey IIb autonomous underwater vehicle (AUV) made by Bluefin Robotics, Inc., was acquired by the Marine Physical Laboratory Scripps Institution of Oceanography, to conduct research in underwater acoustics as well as provide a platform for other scientific studies. The original Odyssey IIb tail cone was replaced with a ducted fan, vectored thrust system installed on vehicles currently sold by Bluefin. In initial sea tests with the new thrust system, the acoustic self noise levels of the vehicle while underway were 20 to 50 dB higher than typical ocean background noise levels, preventing the vehicle's use as a receiver of low level sounds. Controlled tests were performed to characterize the radiated and vibration noise of the AUV propulsion and actuators. Once this baseline was established, changes were made, mostly to the tail cone propulsion, to decrease the vehicle's self noise. The resulting self noise levels of the AUV from 10 Hz up to 10 kHz measured while underway by a hydrophone mounted on the AUV's inner shroud now are at or below typical shallow water background noise levels except in three bands; below 250 Hz, around 500 Hz, and from 0.9 to 2.0 kHz. The goal of this paper is to describe these changes and their effects in lowering vehicle noise levels.

Sabra, KG, Roux P, Thode AM, D'Spain GL, Hodgkiss WS, Kuperman WA.  2005.  Using ocean ambient noise for array self-localization and self-synchronization. IEEE Journal of Oceanic Engineering. 30:338-347.   10.1109/joe.2005.850908   AbstractWebsite

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.

2003
Hedlin, MAH, Alcoverro B, D'Spain G.  2003.  Evaluation of rosette infrasonic noise-reducing spatial filters. Journal of the Acoustical Society of America. 114:1807-1820.   10.1121/1.1603763   AbstractWebsite

This paper presents results from recent tests of rosette infrasonic noise-reducing spatial filters at the Pinon Flat Observatory in southern California. Data from 18- and 70-m aperture rosette filters and a reference port are used to gauge the reduction in atmospheric wind-generated noise levels provided by the filters and to examine the effect of these spatial filters on spatially coherent acoustic signals in the 0.02- to 10-Hz band. At wind speeds up to 5.5 m/s, the 18-m rosette filter reduces wind noise levels above 0.2 Hz by 15 to 20 dB. Under the same conditions, the 70-m rosette filter provides noise reduction of up to 15 to 20 dB between 0.02 and 0.7 Hz. Standing wave resonance inside the 70-m filter degrades the reception of acoustic signals above 0.7 Hz. The fundamental mode of the resonance, 15 dB above background, is centered at 2.65-Hz and the first odd harmonic is observed at 7.95 Hz in data from the large filter. Analytical simulations accurately reproduce the noise reduction and resonance observed in the 70-m filter at all wind speeds above 1.25 m/s. Resonance theory indicates that internal reflections that give rise to the resonance observed in the passband are occurring at the summing manifolds, and not at the inlets. Rosette filters are designed for acoustic arrivals with infinite phase velocity. The plane-wave response of the 70-m rosette filter has a strong dependence on frequency above 3.5 Hz at grazing angles of less than 15degrees from the horizontal. At grazing angles, complete cancellation of the signal occurs at 5 Hz. Theoretical predictions of the phase and amplitude response of 18- and 70-m rosette filters, that take into account internal resonance and time delays between the inlets, compare favorably with observations derived from a cross-spectral analysis of signals from the explosion of a large bolide. (C) 2003 Acoustical Society of America.

2001
Kuperman, WA, D'Spain GL, Heaney KD.  2001.  Long range source localization from single hydrophone spectrograms. Journal of the Acoustical Society of America. 109:1935-1943.   10.1121/1.1339829   AbstractWebsite

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, Gee. Sec. Am., Memoir 27 (1948)]. (C) 2001 Acoustical Society of America.

Stevens, JL, Baker GE, Cook RW, D'Spain GL, Berger LP, Day SM.  2001.  Empirical and numerical modeling of T-phase propagation from ocean to land. Pure and Applied Geophysics. 158:531-565.   10.1007/pl00001194   AbstractWebsite

T-phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers. Several events located on Hawaii and the emerging seamount Loihi generated very large amplitude T phases that were recorded at both the preliminary IMS hydrophone station at Point Sur and land-based stations along the northern California coast. We use data from seismic stations operated by U. C. Berkeley along the coast of California, and from the PG&E coastal California seismic network, to estimate the T-phase transfer functions. The transfer function and predicted signal from the Loihi events are modeled with a composite technique, using normal mode-based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to land-based stations. The modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the California coast, which is used as input to the finite difference code TRES to model propagation onto land. We find both empirically and in the calculations that T phases observed near the conversion point consist primarily of surface waves, although the T phases propagate as P waves after the surface waves attenuate. Surface wave conversion occurs farther offshore and over a longer region than body wave conversion, which has the effect that surface waves may arrive at coastal stations before body waves. We also look at the nature of T phases after conversion from ocean to land by examining far inland T phases. We find that T phases propagate primarily as P waves once they are well inland from the coast, and can be observed in some cases hundreds of kilometers inland. T-phase conversion attenuates higher frequencies, however we find that high frequency energy from underwater explosion sources can still be observed at T-phase stations.

D'Spain, GL, Berger LP, Kuperman WA, Stevens JL, Baker GE.  2001.  Normal mode composition of earthquake T phases. Pure and Applied Geophysics. 158:475-512.   10.1007/pl00001192   AbstractWebsite

Understanding the nature of the coupling between the underwater acoustic field and the land seismic field is important for evaluating the performance of the T-phase stations in the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty. For upslope propagation in an ocean environment, the places where underwater acoustic field energy couples into the land seismic field are determined to first approximation by the local water depth and the normal mode composition of the acoustic energy. Therefore, the use of earthquake-generate T phases as natural probes of water-to-land coupling characteristics is aided by knowledge of their modal composition. Data collected by a 200 element, 3000-m-aperture vertical hydrophone array during a 1989 experiment in the deep northeast Pacific Ocean are used to determine the mode composition of T-phase arrivals from two mb 4.1 earthquakes near the west coast of the U.S., one occurring offshore and the other on land. Results from an eigenanalysis approach and conventional mode decomposition for the two events are consistent and show that at 5 Hz, the offshore event's arrivals have higher-order mode content compared to those from the event on land. Single hydrophone recordings at Pt. Sur of two m(b) 4.4 Hawaiian events in 1996 and 1997, one occurring offshore and the second on land, display time-frequency arrival structures that are explainable by the dispersion characteristics over the oceanic path. Although other effects due to complex source time functions and shear wave and dispersive propagation effects along the initial land path cannot be separated with these single element data, differences in these two events' arrival structures suggest differences in normal mode content consistent with those seen in the pair of 1989 events. Ocean-path dispersion also appears to play a significant role in determining the in-water arrival structure from a 1995 French nuclear test at Mururoa. Recordings of two Hawaiian events in 1997 by the T-phase station VIE and the seismic station at Berkeley illustrate that the water-land coupling confuses the relative timing between normal modes, resulting in apparent loss of information about the source.

2000
Thode, AM, D'Spain GL, Kuperman WA.  2000.  Matched-field processing, geoacoustic inversion, and source signature recovery of blue whale vocalizations. Journal of the Acoustical Society of America. 107:1286-1300.   10.1121/1.428417   AbstractWebsite

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 animal's 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 similar to 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. (C) 2000 Acoustical Society of America. [S0001-4966(00)02802-2].

Thode, AM, Kuperman WA, D'Spain GL, Hodgkiss WS.  2000.  Localization using Bartlett matched-field processor sidelobes. Journal of the Acoustical Society of America. 107:278-286.   10.1121/1.428304   AbstractWebsite

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. (C) 2000 Acoustical Society of America. [S0001-4956(oo)01001-8].

1999
D'Spain, GL, Kuperman WA.  1999.  Application of waveguide invariants to analysis of spectrograms from shallow water environments that vary in range and azimuth. Journal of the Acoustical Society of America. 106:2454-2468.   10.1121/1.428124   AbstractWebsite

The waveguide invariant summarizes in a single scalar parameter the dispersive propagation characteristics for a given environment. In this paper, an expression is derived for the invariant in shallow water environments where the bottom bathymetry varies with range and azimuth. It then is applied to the interpretation of broadband, single-element spectrograms collected during SWellEx-3, a shallow water acoustics experiment off the coast of San Diego. The unusual behavior of the interference patterns seen in the spectrograms from this experiment during the transmission of a 75-150-Hz pseudo-random noise sequence by a source towed over range-varying bottom bathymetry is well matched by the model predictions. (C) 1999 Acoustical Society of America. [S0001-4966(99)00911-X].

D'Spain, GL, Murray JJ, Hodgkiss WS, Booth NO, Schey PW.  1999.  Mirages in shallow water matched field processing. Journal of the Acoustical Society of America. 105:3245-3265.   10.1121/1.424653   AbstractWebsite

Broadband matched field processing (MFP)-derived estimates of 3D source location using data from hydrophone line arrays deployed in various geometries, i.e., vertical, horizontal, and tilted 45 degrees from vertical, are presented in this paper. These data were collected in two shallow water (100-200 m) experiments off the coast of San Diego. Results show that estimates of source range and depth remain surprisingly coherent in the presence of large mismatch in bathymetry, but are offset from the true position by as much as 100%. The offsets are independent of array geometry. In contrast, for estimates of source azimuth, bathymetry mismatch typically causes a degradation in MFP correlation rather than an appreciable offset. However, errors in the assumed tilt from vertical of an array can lead to large offsets in the estimated source azimuth, particularly as the nominal angle of the array from vertical becomes smaller. Predictions from a simple analytical model based on adiabatic normal modes in ideal waveguides provide good fits to the broadband MFP results. (C) 1999 Acoustical Society of America. [S0001-4966(99)03804-7].

1996
Hodgkiss, WS, Ensberg DE, Murray JJ, Dspain GL, Booth NO, Schey PW.  1996.  Direct measurement and matched-field inversion approaches to array shape estimation. IEEE Journal of Oceanic Engineering. 21:393-401.   10.1109/48.544050   AbstractWebsite

Accurate knowledge of array shape is essential for carrying out full wavefield (matched-field) processing, Direct approaches to array element localization (AEL) include both nonacoustic (tilt-heading sensors) and acoustic (high-frequency, transponder-based navigation) methods. The low-frequency signature emitted from a distant source also can be used in an inversion approach to determine array shape, The focus of this paper is on a comparison of the array shape results from these three different methods using data from a 120-m aperture vertical array deployed during SWellEx3 (Shallow Water evaluation cell Experiment #3). Located 2 m above the shallowest array element was a self-recording package equipped with depth, tilt, and direction-of-tilt sensors, thereby permitting AEL to be performed non-acoustically. Direct AEL also was performed acoustically by making use of transponder pings (in the vicinity of 12 kHz) received by high-frequency hydrophones spaced every 7.5 m along the vertical array, In addition to these direct approaches, AEL was carried out using an inversion technique where matched-field processing was performed on a multitone (50-200 Hz), acoustic source at various ranges and azimuths from the array. As shown, the time-evolving array shape estimates generated by all three AEL methods provide a consistent picture of array motion throughout the 6-h period analyzed.