North Pacific Acoustic Laboratory Research Articles

State Estimates and Forecasts of the Northern Philippine Sea Circulation including Ocean Acoustic Travel Times

AbstractThe 2010–11 North Pacific Acoustic Laboratory (NPAL) Philippine Sea experiment measured travel times between six acoustic transceiver moorings in a 660-km diameter ocean acoustic tomography array in the northern Philippine Sea (NPS). The travel-time series compare favorably with travel times computed for a yearlong series of state estimates produced for this region using the Massachusetts Institute of Technology General Circulation Model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system constrained by satellite sea surface height and sea surface temperature observations and by Argo temperature and salinity profiles. Fluctuations in the computed travel times largely match the fluctuations in the measurements caused by the intense mesoscale eddy field in the NPS, providing a powerful test of the observations and state estimates. The computed travel times tend to be shorter than the measured travel times, however, reflecting a warm bias in the state estimates. After processing the travel times to remove tidal signals and extract the low-frequency variability, the differences between the measured and computed travel times were used in addition to SSH, SST, and Argo temperature and salinity observations to further constrain the model and generate improved state estimates. The assimilation of the travel times reduced the misfit between the measured and computed travel times, while not increasing the misfits with the other assimilated observations. The state estimates that used the travel times are more consistent with temperature measurements from an independent oceanographic mooring than the state estimates that did not incorporate the travel times.

Automated matching of measured long-range acoustic arrivals from autonomous gliders with acoustic predictions

The acoustic arrival matching localization method estimates the range of a mobile receiver from a fixed broadband acoustic source by matching the arrival with an acoustic prediction of the received arrival at a known range, calculating an offset from the known range, and combining the estimates of range from multiple sources using least-squares to estimate subsurface position. This is a labor-intensive task performed in post-processing, after recovery of the mobile platform. Automation of this matching is not straightforward due to the presence of spurious arrivals and other variations in the measured arrivals compared with predictions. Automated methods for estimating the range errors and performing ray identification for specific peak arrivals were developed using both programmatic and machine learning approaches. These methods take advantage of the temporal structure of the measured arrivals and simultaneously estimate range error and resolve measured peaks by comparing against a reference acoustic propagation model that describes the progression of the acoustic arrival structure in depth, range, and reduced travel time. The programmatic and machine learning approaches will be tested on range dependent ray simulations and evaluated in the context of a dataset collected using Seagliders during the North Pacific Acoustic Laboratory (NPAL) Philippine Sea Experiment.

Three-dimensional bottom diffraction in the North Pacific.

A significant aspect of bottom-interaction in deep water acoustic propagation, from point sources to point receivers, is the diffraction (or scattering) of energy from discrete seafloor locations along repeatable, deterministic paths in three-dimensions. These bottom-diffracted surface-reflected (BDSR) paths were first identified on the North Pacific acoustic laboratory experiment in 2004 (NPAL04) for a diffractor located on the side of a small seamount. On the adjacent deep seafloor, ambient noise and propagation in the ocean sound channel were sufficiently quiet that the BDSRs were the dominant arrival. The ocean bottom seismometer augmentation in the North Pacific (OBSANP) experiment in June-July 2013 studied BDSRs at the NPAL04 site in more detail. BDSRs are most readily identified by the arrival time of pulses as a function of range to the receiver for a line of transmissions. The diffraction points for BDSRs occur on the relatively featureless deep seafloor as well as on the sides of small seamounts. Although the NPAL04 and OBSANP experiments had very different geometries the same diffractor location is consistent with observed arrivals in both experiments within the resolution of the analysis. On OBSANP the same location excites BDSRs for 77.5, 155, and 310 Hz transmissions.

Acoustic mode travel time variability in the Philippine Sea

During the 2009 NPAL (North Pacific Acoustic Laboratory) PhilSea deep water acoustic propagation experiment, a low frequency source transmitted broadband chirps to a mid-water spanning hydrophone array at a distance of 185 km every three hours for approximately one month. In addition, transmissions took place every five minutes during some time periods. The source and the receiver array contained conductivity and temperature sensors. The motions of the source and receiver moorings were measured using long-baseline acoustic navigation systems. The experiment site was oceanographically dynamic and contained significant internal tide activity. This work builds a model to compare the acoustic and oceanographic variabilities. The acoustic observations are used to estimate the mode travel times, and then related to the internal tide displacements at the source and the receiver arrays. This paper presents the work in three parts. The first part uses matched subspace detectors to estimate the mode travel times. The second uses the environmental observations to build an internal tide model. The third constructs a linear perturbation model to relate the internal tides to the adiabatic mode travel times. The discrepancies between the model and the observations are attributed to factors such as limitations of the perturbation model for the travel times, range variability not accounted for in the adiabatic mode propagation, and signal processing issues.

The North Pacific Acoustic Laboratory deep-water ocean acoustics experiments in the Philippine Sea: A review

A series of experiments with participants from many institutions investigated deep-water ocean acoustics in the oceanographically and geologically complex Philippine Sea during 2009-2011. These experiments employed various configurations of a newly developed distributed vertical line array (DVLA) receiver. The time fronts recorded on the DVLAs for transmissions from a variety of acoustic sources, both moored and lowered form shipboard, provide a rich data set on acoustic scattering and variability due to internal waves, internal tides, density-compensated temperature and salinity variations (spice), oceanic fronts, eddies, and bottom interaction. During 2010-2011, a water-column-spanning DVLA was embedded within an ocean acoustic tomography array of six transceivers, enabling the observation of acoustic time fronts and measurement of ambient noise over a 5000-m aperture. The observed travel-time time series were similar to time series computed from ocean state estimates made using an eddy-permitting regional ocean model constrained by non-acoustic data, and the travel-time differences have in turn been successfully used to further constrain the model. Acoustic Seagliders also recorded the tomographic transmissions, and the travel times have been used to test the feasibility and accuracy of long-range acoustic navigation (Underwater GPS). Analyses of the data sets collected during the NPAL Philippine Sea experiments are continuing.

Ray travel times in the northern Philippine Sea circulation from state estimates

The North Pacific Acoustic Laboratory (NPAL) Philippine Sea experiment deployed a variety of instruments in the northern Philippine Sea during April 2010 through April 2011, including six acoustic transceivers for reciprocal measurements of travel times between instruments. Five transceivers were moored in a pentagon approximately 660 km in diameter with the sixth transceiver moored in the center. Observed travel-time time series were compared with travel times computed from ocean state estimates made using an eddy-active regional implementation of the MITgcm that were constrained by satellite sea surface height and sea surface temperature observations and by temperature and salinity profiles from Argo, CTDs, and XBTs but not by the acoustic data. The similarities cross-validate the state estimates, while the differences provide a simple estimate of the novel information present in the travel times. Smoothness in the modeled sound speed field had a large effect on the ability to find eigenrays. The ocean state estimates were then re-computed to fit the acoustic travel times. The state estimate was able to match the travel times within their error bars and did not significantly increase the misfits with the other observations.

Intensity statistics for dual-band, 500 km, ocean acoustic transmissions in the Philippine Sea 2010 experiment

The May 2010 component of the North Pacific Acoustic Laboratory's Philippine Sea experiment included low-frequency transmissions over a 500 km path from the APL-UW ship-suspended multi-port source to a distributed vertical line array (DVLA) with 149 working hydrophones spaced over most of the water column. The multi-port source has two acoustic resonances at approximately 200 and 300 Hz. To accommodate those resonances in this experiment, two M-sequence signals at those center frequencies were transmitted simultaneously. Data recorded over a 60 h data window in the experiment show deep fades in both frequency bands, i.e., frequency-dependent variations of 10–20 dB in acoustic arrival intensities on the DVLA. The measurements are processed with Wiener filtering based on source-monitoring receptions of the transmission on a hydrophone 20 m from the source, to compensate for effects of the power amplifier and the transducer. Statistics of the received acoustic intensities are reported for both frequency bands. Correlations in the intensity between these broadly separated frequency bands allow one to explore questions of the role in arrival-splitting (micro-multipathing) in the deep fades observed in this experiment. [Work supported by ONR.]

Deep water towed array measurements at close range

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° 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.

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

The North Pacific Acoustic Laboratory (NPAL) Group has performed a series of experiments to study deep-water acoustic propagation and ambient noise in the northern Philippine Sea: (i) 2009 NPAL Pilot Study/Engineering Test (PhilSea09), (ii) 2010–2011 NPAL Philippine Sea Experiment (PhilSea10), and (iii) Ocean Bottom Seismometer Augmentation of the 2010–2011 NPAL Philippine Sea Experiment (OBSAPS). The goals are to (i) understand the impacts of fronts, eddies, and internal tides on acoustic propagation in this oceanographically complex and dynamic region, (ii) determine 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 and for understanding the local ocean dynamics, (iii) improve our understanding of the physics of scattering by small-scale oceanographic variability, and (iv) characterize the depth dependence and temporal variability of the ambient noise field. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed Distributed Vertical Line Array (DVLA) receiver capable of spanning the water column in deep water. The acoustic transmissions and ambient noise were also recorded by the towed Five Octave Research Array (FORA), by acoustic Seagliders, and by ocean bottom seismometers during OBSAPS. [Work supported by ONR.]

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.

Acoustic seagliders in the Philippine Sea: first steps towards moving-receiver tomography

Four Seagliders equipped with Acoustic Recorder Systems (ARS) received transmissions from moored, swept frequency (~200-300 Hz) acoustic sources as part of the ONR-sponsored North Pacific Acoustic Laboratory PhilSea10 Experiment. The gliders transited between the mooring sites from November 2010 until March 2011, diving between the surface and 1000-m depth and providing acoustic receptions at many ranges and depths with respect to the moored sources. The Seagliders utilized GPS positioning at the surface, but were underwater for up to 8 hours at a time, sometimes traveling several kilometers during a single dive. The precision to which the gliders can be located while underwater will be explored, based on acoustic arrivals from five moored sources, to resolve the fundamental ambiguity between position and sound speed. The ultimate goal is to use the Seagliders as additional acoustic tomographic receivers, thereby multiplying the number of acoustic paths in the tomography network.

Acoustic thermometry of ocean climate and the global ocean observing system.

The uniqueness and sustainability of acoustic measurements of basin‐scale temperature were demonstrated by the the Acoustic Thermometry of Ocean Climate and North Pacific Acoustic Laboratory programs in the North Pacific over the decade 1996–2006. Tomography has a role to play in the global observing system as a measurement type that complements altimetry and profiling floats. Tomography is a subsurface measurement of temperature; salinity contributes negligibly. Acoustic travel times are inherently precise integral measurements of temperature; mesoscale variability is supressed. Tomographic measurements offer one of the few ways to sample the abyssal ocean. The North Atlantic and Arctic have been highlighted by international conferences as regions that would be suitable for implementing tomographic arrays. Ocean basins can be measured by tomography using a few acoustic sources and receivers, employing platforms of opportunity such as existing or planned components of the ocean observing system. Extensive acoustic sampling can be achieved by sharing resources, while minimizing long‐term operation and maintenance costs. Because of the integral nature of the data, tomography is best employed in conjunction with numerical ocean models and data assimilation. [Dushaw et al., “a global ocean acoustic observing network,” OceanObs’09: Sustained Ocean Observations and Information for Society, ESA Publication No. WPP‐306].

Acoustic propagation and ambient noise in the Philippine Sea: The 2009 and 2010-2011 Philippine Sea experiments.

The 2010–2011 North Pacific Acoustic Laboratory Philippine Sea experiment (PhilSea10) combines measurements of acoustic propagation and ambient noise with the use of acoustic remote sensing to characterize the oceanographically complex Philippine Sea. The goals are to (i) understand the impacts of fronts, eddies, and internal tides on acoustic propagation; (ii) determine 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 and for understanding the local ocean dynamics; (iii) improve our understanding of the physics of scattering by small-scale oceanographic variability; and (iv) characterize the depth dependence and temporal variability of the ambient noise field. The PhilSea10 experiment was preceded by a pilot study/engineering test in April–May 2009 (PhilSea09) to obtain an initial look at propagation and ambient noise in the Philippine Sea, study short-term variability using long-duration transmissions, and test equipment to be used in 2010–2011. In both experiments, a combination of moored and ship-suspended low-frequency acoustic sources transmit to a newly developed distributed vertical line array receiver capable of spanning the water column in deep water. In 2009, the towed five octave research array also recorded the transmissions. [Work supported by ONR.]

Deep water ambient noise on the seafloor.

Long, continuous time series of ocean bottom seismometer (OBS) and co‐located hydrophone data from the seafloor offer an unprecedented opportunity to study the physical mechanisms of seafloor ambient noise in the frequency band from 1 to 100 Hz. The Ocean Seismic Network Pilot Experiment (OSNPE) in 1998 compared noise and signal levels for a hydrophone at the seafloor with three‐component observations from OBSs on the seafloor, buried 1 m into the soft sediment, and clamped in a borehole 10 m into basaltic basement. The Hawaii‐2 Observatory (H2O) acquired over 3 years of nearly continuous, real‐time data from a shallow‐buried seismic and seafloor hydrophone system. On the North Pacific Acoustic Laboratory experiment in 2004 (NPAL04) three vertical‐component geophone/hydrophones were deployed beneath vertical line arrays in the water column. In addition to ambient noise data, controlled source transmissions were received from ranges out to 3200 km. In the absence of shipping noise, whale calls, and local sea state, what controls the noise floor on the deep seafloor? Interesting observations include banded coherence structure between the vertical geophones and hydrophones, peaks in ambient noise levels due to resonances in the sediment layer, and Stoneley/Scholte wave particle motions. [Work supported by the NSF, ONR and WHOI.]

Comparison of fluctuations of a broad-band pulsed acoustic signal in the Philippine Sea to simulation.

The North Pacific Acoustic Laboratory (NPAL) has conducted several long‐range (>500‐km) experiments in the North Eastern and North Central Pacific Ocean. In April 2009, NPAL conducted an engineering test/pilot study in the more oceanographically energetic Philippine Sea in preparation for a longer‐duration experiment in 2010. Transmissions from a ship‐suspended source were to be recorded on a vertical line array at a comparatively shorter range of ≈100 km. Statistics of fluctuations of the acoustic field are found by simulated broad‐band signal propagation through a model of the Philippine Sea environment. Model parameters are chosen following analysis of environmental data taken during the engineering test. Simulation‐based predictions are compared to experimental data and to predictions of acoustic fluctuations in the less‐energetic North Eastern and North Central Pacific Ocean. [Support for this work was provided by the Office of Naval Research.]

Two-point coherence of acoustic noise recorded by the North Pacific Acoustic Laboratory billboard array.

Acoustic noise in the ocean contains extensive information about the noise sources and the propagation environment. In particular, two‐point correlation function of noise (NCCF) is known to have peaks which correspond to acoustic travel times between the two points provided the noise field is sufficiently diffuse. In this paper, measurements of acoustic noise performed during the 1998–1999 Billboard Array Experiment [Baggeroer et al., J. Acoust. Soc. Am. 117, 1643–1665 (2005)] are re‐examined with the goal of extracting environmental information. NCCF is evaluated by averaging time series of noise recorded on various vertical line arrays that comprise the Billboard Array. For any two hydrophones, NCCF is found to have a number of robust peaks. Possible generation and propagation mechanism responsible for various features of the NCCF are discussed. Statistical distributions of noise are utilized to differentiate between the NCCF peaks of different origins and to identify those peaks, which can be used to retrieve information about the sound speed without detailed knowledge of the noise sources. [Work supported by ONR.]

Complex-density, equivalent-fluid modeling of acoustic interaction with the seafloor

Acoustic interaction with the seafloor can generate both compressional and elastic shear waves in the solid. Accurate models of shear propagation are often computationally expensive and difficult to apply to long-range propagation. When the sound field in the water is of primary interest, an equivalent-fluid model of the seafloor, with parameters chosen to match the reflection coefficient of the actual elastic solid, can sufficiently characterize the effect of the bottom on energy in the water. The effective density of the seafloor material in this approach can be a complex number. Prior methods for generating equivalent fluids were intended for low shear speeds and low grazing angles. Recent developments in the technique were intended to extend its validity to higher shear speeds and a wider range of angles. These efforts were initially motivated by the need to simulate bottom-interacting arrivals for the broadband Kauai source in the North Pacific Acoustic Laboratory experiment at megameter ranges. The work to be presented involves a more detailed examination of the performance of the method, including comparisons to benchmark models and to shorter range data from this Kauai source collected as part of the Basin Acoustic Seamount Scattering Experiment.

Comparison of ocean-acoustic horizontal coherence predicted by path-integral approximations and parabolic-equation simulation results

A line-integral approximation to the acoustic path integral has been used to generate predictions for the characteristic length scale of horizontal, cross-range coherence in long-range ocean-acoustic propagation. These estimates utilize a single range-independent sound-speed profile and the mean variance, as a function of depth, of fractional sound-speed perturbations due to internal waves. The length scales predicted by the integral approximation have been compared to the values generated by parabolic-equation simulations through multiple realizations of Garrett-Munk internal waves. One of the simulation environments approximates the Slice89 experiment; transmissions from a 250-Hz source were simulated in a deep-water transect to a maximum range of 1000km. The second environment corresponds to one of the propagation paths in the North Pacific Acoustic Laboratory (NPAL) experiment. The source in this experiment was bottom-mounted near Kauai, Hawaii and the relevant receiver consisted of five vertical line arrays oriented transverse to the propagation path with cross-range separations ranging from approximately 500 to 3500m. The receiver was at a range of 3889.8km from the source. The predicted length scales are consistently shorter than the parabolic-equation results by 30%–80%, depending on the range and environment examined.

The SPICEX (Spice Experiment) component of the 2004 North Pacific Acoustic Laboratory (NPAL) experiment: An overview

The objectives of the SPICEX component of the 2004 North Pacific Acoustic Laboratory (NPAL) experiment were (i) to elucidate the relative roles of internal waves, ocean spice, and internal tides in causing acoustic fluctuations; (ii) to understand the acoustic scattering far into the geometric shadow zone beneath caustics previously seen on bottom-mounted SOSUS receivers (shadow-zone arrivals); and (iii) to explore the range dependence of the fluctuation statistics. Two vertical line array receivers deployed in close proximity in the central North Pacific Ocean, together spanning much of the water column, recorded transmissions from broadband 250-Hz sources moored at the sound channel axis (750 m) and at surface conjugate depth (3000 m) at ranges of 500 and 1000 km from June through November 2004. Extensive environmental measurements were made along the acoustic path. The acoustic data from SPICEX reveal the vertical structure of the deep shadow zone arrivals. Internal waves are able to account for much of the observed vertical scattering in the reception finale, but other mechanisms seem necessary to account for the early shadow zone arrivals. Receptions from the abyssal sources show scattering occurring predominantly along the acoustic time fronts, as previously suggested by simulations, rather than across them.

Ocean acoustic thermometry and the seasonal cycle of temperature in the North Pacific Ocean

With several years of long-range (several Mm) acoustic propagation data obtained during the Acoustic Thermometry of Ocean Climate (ATOC) and North Pacific Acoustic Laboratory (NPAL) projects, the seasonal cycle of ocean temperature in the North Pacific can be examined. Acoustic transmissions have been made from a source located off the northern Californian coast and from a source located north of Kauai, HI to several receivers of opportunity located in the North Pacific Basin. The acoustic data are a high signal-to-noise measure of large-scale temperature with excellent temporal resolution. Although only a few realizations of the seasonal cycle are available, it is clear that inter- and intraannual variabilities are large contributions to the time series, with signal amplitudes comparable to the seasonal cycle. Such variabilities are likely advective in origin. The time scales for some of the changes in temperature are short, sometimes of order weeks. Not all available acoustic paths are suitable for assessing the seasonal cycle, however. Near Hawaii, the acoustic sampling does not extend to the near-surface waters, so seasonal variations there are not measured. The acoustic results will be compared to measures of the seasonal cycle by satellite altimetry, profiling floats, and the ECCO numerical ocean model.