Lora J. Van Uffelen

The vertical structure of shadow‐zone arrivals at long range in the ocean

Multimegameter-range acoustic data obtained by bottom-mounted receivers show significant acoustic energy penetrating several hundred meters into geometric shadow zones below cusps (caustics) of timefronts computed using climatological databases [B. D. Dushaw et al., IEEE J. Ocean. Eng. 24, 202-214 (1999)]. This penetration is much larger than predicted by diffraction theory. Because these receivers are horizontal arrays, they do not provide information on the vertical structure of the shadow-zone arrivals. Acoustic data from two vertical line array receivers deployed in close proximity in the North Pacific Ocean, together virtually spanning the water column, show the vertical structure of the shadow-zone arrivals for transmissions from broadband 250-Hz sources moored at the sound-channel axis (750 m) and slightly above the surface conjugate depth (3000 m) at ranges of 500 and 1000 km. Comparisons to parabolic equation simulations for sound-speed fields that do not include significant internal-wave variability show that early branches of the measured timefronts consistently penetrate as much as 500-800 m deeper into the water column than predicted. Subsequent parabolic equation simulations incorporating sound-speed fluctuations consistent with the Garrett-Munk internal-wave spectrum at full strength accurately predict the observed energy level to within 3-4-dB rms over the depth range of the shadow-zone arrivals.

The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea

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.

Effects of upper ocean sound-speed structure on deep acoustic shadow-zone arrivals at 500- and 1000-km range

Deep acoustic shadow-zone arrivals observed in the late 1990s in the North Pacific Ocean reveal significant acoustic energy penetrating the geometric shadow. Comparisons of acoustic data obtained from vertical line arrays deployed in conjunction with 250-Hz acoustic sources at ranges of 500 and 1000 km from June to November 2004 in the North Pacific, with simulations incorporating scattering consistent with the Garrett-Munk internal-wave spectrum, are able to describe both the energy contained in and vertical extent of deep shadow-zone arrivals. Incoherent monthly averages of acoustic timefronts indicate that lower cusps associated with acoustic rays with shallow upper turning points (UTPs), where sound-speed structure is most variable and seasonally dependent, deepen from June to October as the summer thermocline develops. Surface-reflected rays, or those with near-surface UTPs, exhibit less scattering due to internal waves than in later months when the UTP deepens. Data collected in November exhibit dramatically more vertical extension than previous months. The depth to which timefronts extend is a complex combination of deterministic changes in the depths of the lower cusps as the range-average profiles evolve with seasonal change and of the amount of scattering, which depends on the mean vertical gradients at the depths of the UTPs.

Observations of sound-speed fluctuations in the western Philippine Sea in the spring of 2009

As an aid to understanding long-range acoustic propagation in the Philippine Sea, statistical and phenomenological descriptions of sound-speed variations were developed. Two moorings of oceanographic sensors located in the western Philippine Sea in the spring of 2009 were used to track constant potential-density surfaces (isopycnals) and constant potential-temperature surfaces (isotherms) in the depth range 120-2000 m. The vertical displacements of these surfaces are used to estimate sound-speed fluctuations from internal waves, while temperature/salinity variability along isopycnals are used to estimate sound-speed fluctuations from intrusive structure often termed spice. Frequency spectra and vertical covariance functions are used to describe the space-time scales of the displacements and spiciness. Internal-wave contributions from diurnal and semi-diurnal internal tides and the diffuse internal-wave field [related to the Garrett-Munk (GM) spectrum] are found to dominate the sound-speed variability. Spice fluctuations are weak in comparison. The internal wave and spice frequency spectra have similar form in the upper ocean but are markedly different below 170-m depth. Diffuse internal-wave mode spectra show a form similar to the GM model, while internal-tide mode spectra scale as mode number to the minus two power. Spice decorrelates rapidly with depth, with a typical correlation scale of tens of meters.

Estimating uncertainty in subsurface glider position using transmissions from fixed acoustic tomography sources

Four acoustic Seagliders were deployed in the Philippine Sea November 2010 to April 2011 in the vicinity of an acoustic tomography array. The gliders recorded over 2000 broadband transmissions at ranges up to 700 km from moored acoustic sources as they transited between mooring sites. The precision of glider positioning at the time of acoustic reception is important to resolve the fundamental ambiguity between position and sound speed. The Seagliders utilized GPS at the surface and a kinematic model below for positioning. The gliders were typically underwater for about 6.4 h, diving to depths of 1000 m and traveling on average 3.6 km during a dive. Measured acoustic arrival peaks were unambiguously associated with predicted ray arrivals. Statistics of travel-time offsets between received arrivals and acoustic predictions were used to estimate range uncertainty. Range (travel time) uncertainty between the source and the glider position from the kinematic model is estimated to be 639 m (426 ms) rms. Least-squares solutions for glider position estimated from acoustically derived ranges from 5 sources differed by 914 m rms from modeled positions, with estimated uncertainty of 106 m rms in horizontal position. Error analysis included 70 ms rms of uncertainty due to oceanic sound-speed variability.

The Aloha Cabled Observatory.

Sustained observation of the ocean is difficult. Ocean science requires new and varied ways to observe the ocean, each with its own strengths and weaknesses, in order to advance our understanding and lay the foundations for predictive models and their applications.

Localization and Subsurface Position Error Estimation of Gliders Using Broadband Acoustic Signals at Long Range

Broadband acoustic source transmissions recorded on Seagliders at ranges up to 700 km are used to estimate subsurface glider position. Because the sources transmitted at 9-min intervals the glider moved appreciably between source receptions. Source-glider ranges estimated from acoustic arrivals were combined using least squares analysis to estimate glider position and velocity during each reception period. The analysis was applied to 387 sets of source transmissions using three different flight models of glider subsurface motion for initial position input values. The offsets between the position estimated from the flight models and the acoustically derived position resulting from the inversions were 600-900-m root mean square (rms) depending upon the model and input parameters. The offsets were tripled if the positions from the flight models were not corrected for a dive-averaged current (DAC). Estimates of a posteriori errors ranged from 78-105-m rms and from 9.1-11.6-cm/s rms for glider position and velocity, respectively. Data residuals were on the order of 50-m rms, a dramatic reduction from 178-m rms, which was documented for the case neglecting the motion of the glider between subsequent source transmissions (Van Uffelen , J. Acoust. Soc. Amer., vol. 134, pp. 3260-3271, 2013). Overall horizontal glider speed was estimated to be approximately 21-cm/s rms.

Acoustic Seagliders in PhilSea10: Preliminary results

In November 2010, four acoustic Seagliders were deployed in the Northern Philippine Sea in the vicinity of an acoustic tomography array as part of the PhilSea10 project with the goal of characterizing this oceanographically complex and highly dynamic region. The gliders were flown between the moored transceivers of the pentagonal tomography array with a radius of approximately 330 km until their recovery in April 2011. During this mission they collected oceanographic and acoustic data in the upper 1000 m of the water column. Temperature, salinity, and pressure data collected by the Seagliders provide a time-evolving characterization of the sound-speed environment in the variable upper ocean between the transceivers. The gliders were also equipped with an integrated Acoustic Recorder System (ARS). The ARS was scheduled to record transmissions from the moored acoustic tomography sources, measuring the arrival structure between the various moorings in order to near-continuously map the arrival pattern as a function of range and depth. Spectrograms show the arriving linear frequency modulated signals from the sources, as well as other ocean sounds. With travel times determined from this data, we will determine whether, given the joint nature of the combined positioning/tomography problem, it is possible to use Seagliders equipped with an acoustic receiver as mobile nodes in the tomography array, thereby enhancing the resolution of the tomography system.

NPAL04 OBS data analysis part 1 : kinematics of deep seafloor arrivals

Funding was provided by the Office of Naval Research through Contract No. N00014-06-1-0222.

Absolute intensities of acoustic shadow zone arrivals

Qualitative observations from bottom‐mounted US Navy SOSUS receiving stations in the North Pacific reveal anomalously deep acoustic arrivals at travel times directly corresponding with timefronts expected to have turned much higher in the water column. The vertical structure of these shadow zone arrivals was studied during SPICEX, a long‐range propagation experiment conducted from June to November 2004 in the North Pacific, utilizing moored sources 500 and 1000 km distant from two vertical line array receivers, which together virtually spanned the full ocean depth. Comparison of the measured absolute intensities of shadow zone arrivals with Monte Carlo parabolic equation simulations suggest that the amount of internal wave scattering associated with the standard Garrett‐Munk (GM) internal wave spectrum is not adequate to account for the extent of scattering into the acoustic shadow evident in the experimental data, suggesting either that the GM spectrum is not an appropriate representation of the internal...