Nearly two decades of ambient noise measurements at seven open-ocean sites in the North Pacific Ocean basin have revealed a complex pattern of long-term trends. The trends in the Northeastern Pacific Ocean show a significant decrease of almost 2 dB/decade. Along the Aleutian archipelago, the levels are either slightly increasing or remaining flat. Levels in two north central Pacific Ocean sites are essentially flat. The mechanisms driving these trends appear to be more subtle than simply the number of merchant ships or the local wind speed. Climatically-influenced basin-scale acoustic propagation conditions may have an important role.

Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in the Philippine Sea are compared to Monte Carlo predictions using a parabolic equation and path-integral predictions. Two simultaneous m-sequence signals are used, one centered at 200 Hz, the other at 300 Hz; both have a bandwidth of 50 Hz. The internal wave field is estimated at slightly less than unity Garrett-Munk strength. The observed spreads in all the early ray-like arrivals are very small, <1 ms (for pulse widths of 17 and 14 ms), which are on the order of the sampling period. Monte Carlo predictions show similar very small spreads. Pulse spread is one consequence of scattering, which is assumed to occur primarily at upper ocean depths where scattering processes are strongest and upward propagating rays refract downward. If scattering effects in early ray-like arrivals accumulate with increasing upper turning points, spread might show a similar dependence. Real and simulation results show no such dependence. Path-integral theory prediction of spread is accurate for the earliest ray-like arrivals, but appears to be increasingly biased high for later ray-like arrivals, which have more upper turning points.

Predictions of log-amplitude variance are compared against sample log-amplitude variances reported by White, Andrew, Mercer, Worcester, Dzieciuch, and Colosi [J. Acoust. Soc. Am. 134, 3347–3358 (2013)] for measurements acquired during the 2009 Philippine Sea experiment and associated Monte Carlo computations. The predictions here utilize the theory of Munk and Zachariasen [J. Acoust. Soc. Am. 59, 818–838 (1976)]. The scattering mechanism is the Garrett%u2013Munk internal wave spectrum scaled by metrics based on measured environmental profiles. The transmitter was at 1000 m depth and the receivers at nominal range 107 km and depths 600–1600 m. The signal was a broadband m-sequence centered at 284 Hz. Four classes of propagation paths are examined: the first class has a single upper turning point at about 60 m depth; the second and third classes each have two upper turning points at roughly 250 m; the fourth class has three upper turning points at about 450 m. Log-amplitude variance for all paths is predicted to be 0.04–0.09, well within the regime of validity of either Born or Rytov scattering. The predictions are roughly consistent with the measured and Monte Carlo log-amplitude variances, although biased slightly low. Paths turning in the extreme upper ocean (near the mixed layer) seem to incorporate additional scattering mechanisms not included in the original theory.

The smaller vertical scales of sound speed variability of several recent deep water Pacific
Ocean acoustic experiments are extracted from individual conductivity, temperature, depth
(CTD) casts taken along the acoustic paths of these experiments, close to the times of
the experiments. The sound speed variability is split into internal wave variability and
spice variability, as these two parts obey very different dynamics %u2013 the internal waves move
through the water and the spice field moves with the water. Larger scales are mostly
responsible for acoustic travel time fluctuations, but smaller scales are mostly responsible
for other important phenomena such as intensity and arrival angle fluctuations. A method
is presented to determine when the two components are separable. The internal wave
properties are consistent with a spectral model such as a generalized Garrett%u2013Munk model,
whereas the spice is very intermittent, and the measurements are not extensive enough
to confidently make a spice model for acoustic propagation purposes. Both the internal
wave results and the spice results are summarized as vertical wavenumber spectra over a
selected vertical depth interval, but with the spice, it must be understood that a spectral
model would be very different from the data, and that the three-dimensional horizontal%u2013
vertical spectrum would be pure conjecture. The spectral level of the (small-scale) spice,
averaged over all the profiles, is comparable to that of the internal waves, suggesting that it
is not significantly less important to acoustic propagation than are the (small-scale) internal
waves.

Ocean bottom seismometer observations at 5000 m depth during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 show robust, coherent, late arrivals that are not readily explained by ocean acoustic propagation models. These "deep seafloor" arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km. The travel times for six (of 16 observed) deep seafloor arrivals correspond to the sea surface reflection of an out-of-plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers. This out-of-plane bottom-diffracted surface-reflected energy is observed on the deep vertical line array about 35 dB below the peak amplitude arrivals and was previously misinterpreted as in-plane bottom-reflected surface-reflected energy. The structure of these arrivals from 500 to 3200 km range is remarkably robust. The bottom-diffracted surface-reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor.

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.

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° 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.

Second order mode statistics as a function of range and source depth are presented from the Long Range Ocean Acoustic Propagation EXperiment (LOAPEX). During LOAPEX, low frequency broadband signals were transmitted from a ship-suspended source to a mode-resolving vertical line array. Over a one-month period, the ship occupied seven stations from 50 km to 3200 km distance from the receiver. At each station broadband transmissions were performed at a near-axial depth of 800 m and an off-axial depth of 350 m. Center frequencies at these two depths were 75 Hz and 68 Hz, respectively. Estimates of observed mean mode energy, cross mode coherence, and temporal coherence are compared with predictions from modal transport theory, utilizing the Garrett–Munk internal wave spectrum. In estimating the acoustic observables, there were challenges including low signal to noise ratio, corrections for source motion, and small sample sizes. The experimental observations agree with theoretical predictions within experimental uncertainty.

Mode travel time estimation in the presence of internal waves (IWs) is a challenging problem. IWs perturb the sound speed, which results in travel time wander and mode scattering. A standard approach to travel time estimation is to pulse compress the broadband signal, pick the peak of the compressed time series, and average the peak time over multiple receptions to reduce variance. The peak-picking approach implicitly assumes there is a single strong arrival and does not perform well when there are multiple arrivals due to scattering. This article presents a statistical model for the scattered mode arrivals and uses the model to design improved travel time estimators. The model is based on an Empirical Orthogonal Function (EOF) analysis of the mode time series. Range-dependent simulations and data from the Long-range Ocean Acoustic Propagation Experiment (LOAPEX) indicate that the modes are represented by a small number of EOFs. The reduced-rank EOF model is used to construct a travel time estimator based on the Matched Subspace Detector (MSD). Analysis of simulation and experimental data show that the MSDs are more robust to IW scattering than peak picking. The simulation analysis also highlights how IWs affect the mode excitation by the source.

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.

In the spring of 2009, broadband transmissions from a ship-suspended source with a 284-Hz center frequency were received on a moored and navigated vertical array of hydrophones over a range of 107 km in the Philippine Sea. During a 60-h period over 19,000 transmissions were carried out. The observed wavefront arrival structure reveals four distinct purely refracted acoustic paths: One with a single upper turning point near 80 m depth, two with a pair of upper turning points at a depth of roughly 300 m, and one with three upper turning points at 420 m. Individual path intensity, defined as the absolute square of the center frequency Fourier component for that arrival, was estimated over the 60-h duration and used to compute scintillation index and log-intensity variance. Monte Carlo parabolic equation simulations using internal-wave induced sound speed perturbations obeying the Garrett–Munk internal-wave energy spectrum were in agreement with measured data for the three deeper-turning paths but differed by as much as a factor of four for the near surface-interacting path.

The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs.

Data collected during the 2004 Long-range Ocean Acoustic Propagation Experiment provide absolute intensities and travel times of acoustic pulses at ranges varying from 50 to 3200%u2009km. In this paper a subset of these data is analyzed, focusing on the effects of seafloor reflections at the shortest transmission range of approximately 50%u2009km. At this range bottom-reflected (BR) and surface-reflected, bottom-reflected energy interferes with refracted arrivals. For a finite vertical receiving array spanning the sound channel axis, a high mode number energy in the BR arrivals aliases into low mode numbers because of the vertical spacing between hydrophones. Therefore, knowledge of the BR paths is necessary to fully understand even low mode number processes. Acoustic modeling using the parabolic equation method shows that inclusion of range-dependent bathymetry is necessary to get an acceptable model-data fit. The bottom is modeled as a fluid layer without rigidity, without three dimensional effects, and without scattering from wavelength-scale features. Nonetheless, a good model-data fit is obtained for sub-bottom properties estimated from the data.

Ocean bottom seismometer observations during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 showed robust, coherent, late arrivals that were not observed on hydrophones suspended 750m and more above the seafloor and that were not readily explained by ocean acoustic propagation models. The DSFA arrival pattern on the OBSs near 5000m depth are a delayed replica, by about two seconds, of the arrival pattern on the deepest element of the DVLA at 4250m depth (DVLA-4250). Using a conversion factor from the seafloor vertical particle velocity to seafloor acoustic pressure, we have quantitatively compared signal and noise levels at the OBSs and DVLA-4250. Ambient noise and DSFA signal levels at the OBSs are so quiet that if the DSFA arrivals were propagating through the water column, perhaps on an out-of-plane bottom-diffracted-surface-reflected (BDSR) path, they would not appear on single, unprocessed DVLA channels. Nonetheless arrival time and horizontal phase velocity analysis rules out BDSR paths as a mechanism for DSFAs. Whatever the mechanism, the measured DSFAs demonstrate that acoustic signals and noise from distant sources can appear with significant strength on the seafloor at depths well below the conjugate depth.

The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50–90-Hz band that were recorded on a 1400-m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers ("modal group arrivals") is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.

A broadband low-frequency acoustics pilot study was conducted in the Philippine Sea in April/May of 2009. 19,071 M-sequences were transmitted over a period of 60 hours at a range of 107 km to a vertical array of hydrophones. Timeseries of acoustic intensity for arrivals corresponding to known paths are formed. Results from a simulation of acoustic propagation through a time-dependent internal wave perturbed sound speed field agree qualitatively with arrivals for two of the acoustic paths but not with the arrivals for the path which had a shallow upper turning point of ~60 m. Intensity fades of 5 to 10 dB which last for approximately 18 or 20 hours are observed in this shallow-turning path. Estimates from data of spectra and correlation times for acoustic intensity will be compared to Monte Carlo Parabolic Equation based predictions.

Investigators at the University of Washington's Applied Physics Laboratory collaborated with scientists from the Scripps Institution of Oceanography during the 2009 Philippine Sea Pilot Study/Engineering Test and the 2010 Philippine Sea Experiment. The focus of both efforts was to collect well controlled low-frequency acoustic propagation data and detailed environmental information. The data from these cruises are presently being analyzed in the interests of: horizontal statistics of ocean spice as measured on a towed conductivity-temperature-depth (pressure) chain, fluctuation measures of low-frequency broadband ocean acoustic signals, bottom properties, and associated theoretical developments. This presentation will outline the experimental plans for each year, discuss preliminary analysis results, and provide an introduction for more detailed presentations in the remainder of this session.

Short range propagation experiments in deep water provide volume-only weak scattering paths that can be used to test the limits of validity of fluctuation theories for low-frequency acoustical signals. Colosi et al. [J. Acoust. Soc. Am., 126, 1069-1083, 2009] suggested that Munk-Zachariasen theory (which uses first-order Rytov theory modified for the ocean environment) may be applicable in these cases; they used it to predict statistics for a 75-Hz signal propagated over a range of 87 km in the eastern North Pacific. Several scenarios used in the Philippine Sea experiments involving wideband signals may fall into this category: in 2009, ranges of 45 and 107 km were used (transmitter and receivers in the main sound channel) with carrier frequencies 82 and 284 Hz, and in 2010, continuous ranges from 25 to 43 km using a 61 Hz carrier (transmitter at 150 m and receivers near full ocean depth.) Predictions for all scenarios are presented, and comparisons are made against statistics observed for the 2009 107 km path.

Data collected during the 2004 long-range ocean acoustic propagation experiment form a unique set of measurements containing information about absolute intensities and absolute travel times of acoustic pulses at long ranges. This work primarily focuses on sound reflected from the seafloor at the shortest transmission range of the experiment, approximately 50 km. At this range, bottom-reflected energy interferes with arrivals refracted by the sound channel. Stable bottom-reflected arrivals are seen from broadband transmissions from acoustic source at 800 m depth with 75 Hz center frequency, and from a source at 350 m depth with 68.2 Hz center frequency. Standard acoustic propagation modeling tools such as parabolic equation solvers can be successfully used to predict acoustic travel times and intensities of the observed bottom-reflected arrivals. Inclusion of range-dependent bathymetry is necessary to get an acceptable model-data fit. Although there is no direct ground truth for actual sub-bottom properties in the region, a good fit can be obtained with credible sub-bottom properties. The potential for using data of this type for geoacoustic property inversion in the deep ocean will be discussed.

Measurements (1994-2007) from four cabled-to-shore hydrophone systems located off the North American west coast permit extensive comparisons between "contemporary" low frequency ship traffic noise (25-50 Hz) collected in the past decade to measurements made over 1963-1965 with the same in-water equipment at the same sites. An increase of roughly 10 dB over the band 25-40 Hz at one site has already been reported [Andrew et al., 2002]. Newly corrected data from the remaining three systems generally corroborate this increase. Simple linear trend lines of the contemporary traffic noise (duration 6 to 12 years) show that recent levels are slightly increasing, holding steady, or decreasing. These results confirm the prediction by Ross that the rate of increase in traffic noise would be far less at the end of the 20th century compared to that observed in the 1950s and 1960s.

An experimental transducer invented by Jack Butler of ImageAcoustics, Inc., and built by Massa Products, Inc., was loaned in 2005 to APL-UW by Jan Lindberg. APL-UW developed a ship-suspended system by integrating the transducer with a tuner built by Coiltron, Inc., a tracking pinger, an associated telemetry subsystem, and a pressure-balanced oil-filled battery. The transducer itself is a double-ported free-flooded device with sharp resonances at 210 and 320 Hz. The "band" spanning these resonances supports wideband signals, and for the PhilSea09 exercise a Q=2 m-sequence was used. The double-peaked response required considerable pre-equalization for controlled time resolution. For 2010, an experimental two-frequency "broadband dichromatic" signal was devised by superposing two high-Q m-sequences with carriers near the two resonance frequencies. To date, the system has logged about 100 h of high-resolution pulse interrogations of the deep Philippine Sea at ranges of 45, 107, and 500 km. Examples and results will be discussed.

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.

Ocean bottom seismometer observations during a long-range ocean acoustic propagation experiment in the North Pacific in 2004 showed robust, coherent, late arrivals that were not observed on hydrophones suspended 750 m and more above the seafloor and that were not readily explained by ocean acoustic propagation models. These deep seafloor arrivals were the largest amplitude arrivals on the vertical velocity channel for ranges from 500 to 3200 km. They appear to correspond to energy converted from ocean acoustic waves into seafloor interface (Scholte) waves at a small bathymetric high about 18 km from the receivers.

Some problems with this model are the following: (a) It is rare to observe discrete Scholte waves propagating 18 km or so on the seafloor. It is usually assumed that Scholte and shear waves decay rapidly in and along the seafloor because of strong scattering and in- trinsic attenuation. (b) There is apparently insignificant scattering back into the water column because this would have been observed on the DVLA. (c) Why is there no sign of this scattering process for ranges less than 500 km? (d) Although this model accurately predicts arrival times of the deep seafloor arrivals, the amplitudes are not consistent.

The Long Range Ocean Acoustic Propagation EXperiment (LOAPEX) carried out 75-Hz broadband transmissions at source depths of 800 m (on the sound channel axis) and 350 m to ranges of 50, 100, 250, 500, 1000, 1600, 2300, and 3200 km. These transmissions were received on a vertical array capable of resolving the first 10 acoustic normal modes. The observed mode energy distributions as a function of range are strongly related to the amount of internal wave scattering occurring across range.

A fundamental question is the predictability of the energy distributions from scattering theory and estimates of ocean internal wave spectra. Transport equations utilizing small angle forward scattering and the Markov approximation have been derived to describe the range evolution of cross-mode coherence. The theory agrees well with Monte-Carlo simulations suggesting that the scattering physics is well known. The LOAPEX observations are an excellent opportunity to test the predictive ability of the transport equations using real data. Issues in the model/data comparisons involve uncertainties in the internal wave spectrum and small sample sizes from LOAPEX.

The Office of Naval Research's (ONR) program in Ocean Acoustics maintains a thrust in deep-water acoustics and has recently funded two major efforts in the Philippine Sea known as PhilSea09 and PhilSea10. PhilSea09 was planned as a pilot study and an engineering test in preparation for the more extensive PhilSea10 experiment. However, PhilSea09 was also partially supported by ONR's program in undersea signal processing. This paper will summarize the Applied Physics Laboratory's involvement in PhilSea09, its support to the undersea signal processing program, and the Laboratory's efforts in PhilSea10.

The 2010 Philippine Sea Experiment cruise included two tows of the "towed CTD chain", an 800-m-long, ship-towed cable with 88 CTD sensor fins distributed along its length. The goal of this instrument's use in the experiment was to obtain high-resolution (in space and time) measurements of ocean conductivity and temperature in the upper 500–700 m of the deep-ocean water column, in order to characterize the horizontal spatial statistics of internal waves and "spice" (density-compensated sound-speed fluctuations). The effects of such ocean variability upon acoustic variability in long-range ocean acoustic propagation is an active area of current research.

Despite technical difficulties with the system, measurements at 3–5 s sampling periods were obtained on one to three dozen sensors distributed over 700 m depth. The first tow was 93 km for about 39 h and the second tow was 124 km for about 30 h. These datasets will be validated and/or augmented by measurements from a Microcat CTD mounted on the bottom of the cable on each tow and a second mounted at mid-cable during the second tow. A summary of the dataset and initial findings will be shown.

A team from the Applied Physics Laboratory of the University of Washington (APL-UW) conducted underwater sound propagation exercises from 5 to 29 May 2010 aboard the R/V Roger Revelle in the Philippine Sea. This research cruise was part of a larger multi-cruise, multi-institution effort, the PhilSea10 Experiment, sponsored by the Office of Naval Research, to investigate the deterministic and stochastic properties of long-range deep ocean sound propagation in a region of energetic oceanographic processes. The primary objective of the APL-UW cruise was to transmit acoustic signals from electro-acoustic transducers suspended from the R/V Roger Revelle to an autonomous distributed vertical line array (DVLA) deployed in March by a team from the Scripps Institution of Oceanography (SIO.) The DVLA will be recovered in March 2011.

Two transmission events took place from a location designated SS500, approximately 509 km to the southeast of the DVLA: a 54-hr event using the HX554 transducer at 1000 m depth, and a 55-hr event using the MP200/TR1446 "multiport" transducer at 1000 m depth. A third event took place towing the HX554 at a depth of 150 m at roughly 1–2 kt for 10 hr on a radial line 25–43 km away from the DVLA. All acoustic events broadcasted low-frequency (61–300 Hz) m-sequences continuously except for a short gap each hour to synchronize transmitter computer files. An auxiliary cruise objective was to obtain high temporal and spatial resolution measurements of the sound speed field between SS500 and the DVLA.

Two methods were used: tows of an experimental "CTD chain" (TCTD) and periodic casts of the ship's CTD. The TCTD consisted of 88 CTD sensors on an inductive seacable 800 m long, and was designed to sample the water column to 500 m depth from all sensors every few seconds. Two tows were conducted, both starting near SS500 and following the path from SS500 towards the DVLA, for distances of 93 km and 124 km. Only several dozen sensors responded during sampling. While the temperature data appear reasonable, only about one-half the conductivity measurements and none of the pressure measurements can be used. Ship CTD casts were made to 1500 m depth every 10 km, with every fifth cast to full ocean depth.

An acoustical transmitter was suspended at multiple depths to 800 m from the research vessel R/V Melville at several stations in the North Pacific in 2004. The 3-D position of the transmitter varied with time due to ship motion and subsurface currents. The transmitter 3-D position and velocity were subsequently estimated using a cable dynamics model forced by ship position, as measured by high-precision global positioning system (GPS), and subsurface currents, as measured by the onboard acoustical Doppler current profiler. These estimated positions and velocities varied in the horizontal up to 10 m from the station "center" position, and 0.5 m/s from zero, respectively. Auxiliary measurements indicate that these estimates were accurate along either horizontal coordinate to better than 2 m and 0.05 m/s, respectively. Transmitter motion dilates the apparent time base of the radiated signal, producing time-varying Doppler effects. Simulation and analysis are used to determine when the induced Doppler effect is significant, and a technique is presented that "de-dopplerizes" a received signal for arbitrary interplatform motion.

During an experiment in the northern Philippine Sea in 2009, low-frequency tones were transmitted from a shallow (15- and 60-m) source deployed from R/V Melville keeping station to a shallow (250-m) horizontal receiver array towed by R/V Kilo Moana approximately one convergence zone (CZ) away. Recordings were made during events in which the receiver ship maintained constant range in the convergence zone and during events in which the receiver ship transited radially through the CZ. The shallow CZ exhibits strong dependence on the bathymetry mid-way between the source and receiver array. In fact, the variability of the structure of the first CZ in this environment is significantly more strongly affected by the heterogeneous character of the bottom than water column fluctuations. Numerical modeling with a parabolic equation code is used to support the conclusions from the data analysis.

Measurements from four cabled-to-shore hydrophone systems located off the North American west coast permit extensive comparisons between "contemporary" low-frequency traffic noise (25–50 Hz) collected in the past decade to measurements made in the mid-1960s with the same in-water equipment at the same sites. An increase of roughly 10 dB over the band 25–40 Hz at one site has already been reported [Andrew et al., ARLO Acoust. Res. Let. Online 3 (2002)]. Newly corrected data from the remaining three systems corroborate this increase. Simple linear trend lines of the contemporary traffic noise (duration 6–12 years) show that the current levels either hold steady or decrease at three of the four sites. These results confirm the Ross prediction, at least at these sites, that the rate of increase in traffic noise would be far less at the end of the last century compared to that observed in the 1950s and 1960s.

Observations of scattering of low-frequency sound in the ocean have focused largely on effects at long ranges, involving multiple scattering events. Fluctuations due to one and two scattering events are analyzed here, using 75-Hz broadband signals transmitted in the eastern North Pacific Ocean. The experimental geometry gives two purely refracted arrivals. The temporal and vertical scales of phase and intensity fluctuations for these two ray paths are compared with predictions based on the weak fluctuation theory of Munk and Zachariasen, which assumes internal-wave-induced sound-speed perturbations [ J. Acoust. Soc. Am. 59, 818-838 (1976) ].

The comparisons show that weak fluctuation theory describes the frequency and vertical-wave-number spectra of phase and intensity for the two paths reasonably well. The comparisons also show that a resonance condition exists between the local acoustic ray and the internal-wave field, as predicted by Munk and Zachariasen, such that only internal waves whose crests are parallel to the local ray path contribute to acoustic scattering. This effect leads to filtering of the acoustic spectra relative to the internal-wave spectra, such that steep rays do not acquire scattering contributions due to low-frequency internal waves.

Receptions, from a ship-suspended source (in the band 50–100 Hz) to an ocean bottom seismometer (about 5000 m depth) and the deepest element on a vertical hydrophone array (about 750 m above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from 50 to 3200 km. In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), "deep seafloor arrivals," that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.

Over the decade 1996–2006, acoustic sources located off central California (1996–1999) and north of Kauai (1997–1999, 2002–2006) transmitted to receivers distributed throughout the northeast and north central Pacific. The acoustic travel times are inherently spatially integrating, which suppresses mesoscale variability and provides a precise measure of ray-averaged temperature. Daily average travel times at 4-day intervals provide excellent temporal resolution of the large-scale thermal field. The interannual, seasonal, and shorter-period variability is large, with substantial changes sometimes occurring in only a few weeks. Linear trends estimated over the decade are small compared to the interannual variability and inconsistent from path to path, with some acoustic paths warming slightly and others cooling slightly.

The measured travel times are compared with travel times derived from four independent estimates of the North Pacific: (1) climatology, as represented by the World Ocean Atlas 2005 (WOA05); (2) objective analysis of the upper-ocean temperature field derived from satellite altimetry and in situ profiles; (3) an analysis provided by the Estimating the Circulation and Climate of the Ocean project, as implemented at the Jet Propulsion Laboratory (JPL-ECCO); and (4) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) model. The acoustic data show that WOA05 is a better estimate of the time mean hydrography than either the JPL-ECCO or the POP estimates, both of which proved incapable of reproducing the observed acoustic arrival patterns. The comparisons of time series provide a stringent test of the large-scale temperature variability in the models. The differences are sometimes substantial, indicating that acoustic thermometry data can provide significant additional constraints for numerical ocean models.

Propagation of energy along the sound channel axis cannot be formally described in terms of geometrical acoustics due to repeated cusped caustics along the axis. In neighborhoods of these cusped caustics, a very complicated interference pattern is observed. Neighborhoods of interference grow with range and overlap at long ranges. This results in the formation of a complex interference wave — the axial wave — that propagates along the sound channel axis like a wave belonging to a crescendo of near-axial arrivals. The principal properties of this wave are calculated for the actual space-time configuration realized during a 2004 long-range propagation experiment conducted in the North Pacific. The experiment used M-sequences at 68.2 and 75 Hz, transmitter depths from 350 to 800 m, and ranges from 50 to 3200 km. Calculations show that the axial wave would be detectable for an optimal geometry — both transmitter and receiver at the sound channel axis — for a "smooth" range-dependent sound speed field. The addition of sound speed perturbations — induced here by simulated internal waves — randomizes the acoustic field to the extent that the axial wave becomes undetectable. These results should be typical for mid-latitude oceans with similar curvatures about the sound speed minimum.

This paper provides an overview of the experimental goals and methods of the Long-range Ocean Acoustic Propagation EXperiment (LOAPEX), which took place in the northeast Pacific Ocean between September 10, 2004 and October 10, 2004. This experiment was designed to address a number of unresolved issues in long-range, deep-water acoustic propagation including the effect of ocean fluctuations such as internal waves on acoustic signal coherence, and the scattering of low-frequency sound, in particular, scattering into the deep acoustic shadow zone. Broadband acoustic transmissions centered near 75 Hz were made from various depths to a pair of vertical hydrophone arrays covering 3500 m of the water column, and to several bottom-mounted horizontal line arrays distributed throughout the northeast Pacific Ocean Basin. Path lengths varied from 50 km to several megameters. Beamformed receptions on the horizontal arrays contained 10-20-ms tidal signals, in agreement with a tidal model. Fifteen consecutive receptions on one of the vertical line arrays with a source range of 3200 km showed the potential for incoherent averaging. Finally, shadow zone receptions were observed on an ocean bottom seismometer at a depth of 5000 m from a source at 3200-250-km range.

Large-scale, range- and depth-averaged temperatures in the North Pacific Ocean were measured by long-range acoustic transmissions over the decade 1996–2006. Acoustic sources off central California and north of Kauai transmitted to receivers throughout the North Pacific. Even though acoustic travel times are spatially integrating, suppressing mesoscale variability and providing a precise measure of large-scale temperature, the travel times sometimes vary significantly on time scales of only a few weeks. The interannual variability is large, with no consistent warming or cooling trends.

Comparison of the measured travel times with travel times derived from (i) the World Ocean Atlas 2005 (WOA05), (ii) an upper ocean temperature estimate derived from satellite altimetry and in situ profiles, (iii) an analysis provided by the Estimating the Circulation and Climate of the Ocean (ECCO) project, and (iv) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) show similarities, but also reveal substantial differences. The differences suggest that the data can provide significant additional constraints for numerical ocean simulations. The acoustic data show that WOA05 is a much better estimate of the time–mean hydrography than either the ECCO or POP estimates and provide significantly better time resolution for large-scale ocean variability than can be derived from satellite altimetry and in situ profiles.

The propagation of energy along the sound-channel axis cannot be described in terms of geometrical acoustics because of the presence of cusped caustics repeatedly along the axis. In neighborhoods of these cusped caustics a very complicated interference pattern is observed. Neighborhoods of interference grow with range and at long ranges they overlap. This results in the formation of a complex interference wave — the axial wave — that propagates along the sound-channel axis like a wave belonging to a crescendo of near-axial arrivals. In this paper, the axial wave is simulated for the LOAPEX CTD data measured at seven different ranges from the vertical line array. A signal with the center frequency of 75 Hz and 37.5-Hz bandwidth is used for computations. This signal well approximates one transmitted m-sequence in the LOAPEX experiment. The effect of environmental variability, induced by internal waves, on the axial wave is studied. The sound-speed fluctuations caused by ocean internal waves are obtained with the use of the buoyancy frequency profile measured in the LOAPEX. Calculations are based on the integral representation of the axial wave in a local coordinate system introduced in the vicinity of the range-variable sound-channel axis.

Large-scale temperatures in the North Pacific were measured by long-range acoustic transmissions from 1996–2006. Acoustic sources off California and Kauai transmitted to receivers distributed throughout the North Pacific from 1996–999. Kauai transmissions continued from 2002–2006. Acoustic travel time data are inherently integrating. This averaging suppresses mesoscale variability and provides an accurate measure of large-scale temperature, subject to the limitations of the ray path sampling. At basin scales, the ocean is highly variable, with significant changes occurring at time scales from weeks to years. The interannual variability is large compared to trends in the data. Willis, et al. used objective mapping techniques applied to satellite altimetry and hydrography to derive 0–750 m temperature fields for the global ocean. Travel times equivalent to the measured travel times can be calculated using these fields. The measured and calculated travel times are similar, but also show significant differences. Similar comparisions using travel times derived from the "Estimating the Circulation and Climate of the Ocean" (ECCO) model and a high-resolution Parallel Ocean Program (POP) model also show similarities and differences. The ECCO model was constrained by altimetric and profile data by data assimilation, suggesting that the acoustic travel times provide meaningful additional constraints on model behavior.

Internal waves introduce substantial fluctuations in the lowest mode signals recorded during long-range tomography experiments. Lacking a complete random model for the mode signals, tomographers typically resort to averaging the mode signals across receptions to mitigate internal wave effects. Chandrayadula et al. used the 2004 Long Range Ocean Acoustic Propagation EXperiment (LOAPEX) signals and parabolic equation (PE) simulations modeling the LOAPEX environment to derive range-dependent statistics such as temporal mean, temporal covariance, and intermodal correlation of the lower mode signals. This talk proposes several statistical signal processing techniques such as likelihood ratio detectors, minimum entropy deconvolution methods, and empirical orthogonal function detectors that are based on the derived statistics, to mitigate internal wave effects. The proposed methods are tested on both the LOAPEX signals and PE simulated mode signals and compared against simple averaging techniques.

Ambient sound spectra have been collected since 1994 from a decommissioned deep water military acoustic receiver located southwest of San Nicolas Island (southern California). A new spectrum is collected every 6 minutes. The dataset can be used to study level variability on scales from minutes to years. The calibration curves for this system, however, were considered suspect. Recently, a calibrated acoustic recorder was deployed near the receiver. Comparison of spectra for December 2003 provides a long-sought-after modern absolute calibration. With this new correction, the dataset can additionally be compared to levels measured in the 1960s by the same receiver. Although the corrected measurements corroborate (albeit by construction) the 1960–2000 ambient noise increase reported by McDonald et al., there is scant evidence of an increase over the last decade. This study again highlights the need for on-going long-term well-calibrated observation programs.

BASSEX exploited the signals transmitted from the SPICEX, LOAPEX, and NPAL Kauai sources to examine the forward scattering from the Kermit–Roosevelt Seamounts near the Mendocino Fault in the northeast Pacific. The receiver was the FORA (four octave research array) from Pennsylvania State University. This array was towed at 300 m and had approximate resolutions of 2 and 6 deg for the 250- and 75-Hz signal transmissions. These were beamformed and pulse compressed for analyzing the multipath receptions from the sources. Source–receiver ranges covered from 250 to 1600 km provide a very rich data set. Data for forward and backscattering from the slope near the NPAL source north of Kauai were also obtained. The analysis so far has demonstrated forward scattering shadow convergence zones as well as diffraction from the seamounts. The upslope/downslope path structure near the Kauai source has also been analyzed. Both ray path and full wave acoustic modeling have been used for path identification as well as a theoretical model for modal scattering from a cone within an acoustic wave guide.

During the North Pacific Acoustics Laboratory (NPAL-04) experiment broadband transmissions (50 Hz bandwidth, 75 Hz center frequency, 27.28 s period length) from a bottom-mounted source off the island of Kauai were received on a towed array (350 m depth, 200 m aperture) near the island and a large aperture vertical line array (VLA) at a range of 2469.5 km. Previous work [Heaney, J. Acoust. Soc. Am. 117(3), 1635-1642 (2005)] has demonstrated that bottom interaction near the source is a fundamental process in the evolution of the wave train. In this paper results from eight receptions at ranges from 2 to 300 km and the reception on the distant VLA is used to examine the wavefront evolution as it propagates into deep water. Results from a local geo-acoustic inversion will be presented as well as comparison of data with broadband parabolic equation simulations. The initial conclusion is that the first-order bottom bounce, off the seamount near the source, acts as an image source, contributing to a complex arrival pattern even at great distances.

Low-mode signals measured during long range tomography experiments, such as the Acoustic Thermometry of Ocean Climate (ATOC) and the North Pacific Acoustic Laboratory experiments, have a random arrival structure due to internal waves. At megameter ranges, the narrow-band mode amplitude is predicted to be Gaussian and uncorrelated with other modes. Wage et al. measured the centroid, frequency coherence, time spread, and time coherence for the broadband ATOC mode signals received at ranges exceeding 3000 km. The 2004 Long Range Acoustic Propagation EXperiment (LOAPEX) provided an opportunity to observe how the mode statistics evolves with range. This talk investigates the mean, temporal covariance, and intermodal correlation of the low modes at ranges between 50 and 3200 km using LOAPEX data. Broadband parabolic equation simulations were performed to model internal wave effects on the low-mode signals at the various LOAPEX stations. A kurtosis measure is used to study the amount of cross-modal scattering with respect to range. Statistics of the measured and simulated mode signals are compared.

We extract the low-frequency (70 Hz) mean acoustic pressure field, and the mean acoustic field up to time shifts, from the LOAPEX receptions on the vertical line arrays, for transmissions at various ranges. Means are taken over time intervals of order a day. Source and receiver motion is removed from the data, and possibly tides and other slow phenomena. The experimental results are compared to predictions from theoretical calculations assuming scattering by internal waves. The theoretical calculations make the Markov approximation that the sound-speed fluctuation correlations can be replaced by an operator at a single range. The calculations use modes rather than rays, because of the very low frequency, thus differing from the version of the Markov approximation that assumes delta-correlated sound-speed fluctuations.

Mode processing of transient signals in a deep ocean environment using a deficient vertical receiving array is considered. Here, "deficient" implies that the vertical array is too sparse to resolve modes and/or has gaps. It is well known that mode processing of cw signals using a deficient array leads to significant modal cross-talk, i.e., nonzero projections of mode n energy onto mode m, m≠n. It is shown, using simulated wave fields, that when phase-coherent processing is employed, transient signals are less sensitive to modal cross-talk than their cw counterparts. An explanation of this behavior is provided. Mode processing of a subset of the measurements made during the LOAPEX experiment at approximately 1 Mm range with f₀=75 Hz, ∆f=35 Hz is shown to give robust estimates of modal group time spreads for mode numbers 0 ≤ m ≤ 70.

The long-range ocean acoustic propagation experiment (LOAPEX), conducted in the NE Pacific Ocean, provided acoustic transmissions from a ship-suspended source at seven stations spanning a near zonal path of 3200 km. The transmissions were received on several bottom-mounted horizontal hydrophone arrays distributed about the NE Pacific Ocean Basin. The experiment and a simple inverse method are described and a preliminary thermal "snapshot" of the ocean basin is presented.

Four OBSs (ocean bottom seismometers), consisting of one hydrophone and one vertical component geophone channel each, were deployed below the deep vertical line array (DVLA) on the North Pacific Acoustic Laboratory/Long-range Ocean Acoustic Propagation Experiment (NPAL/LOAPEX). The OBSs were deployed at 2 km offset north, south, east, and west from the nominal DVLA location. Data, sampled at 500 sps, were recorded continuously from the deployment in September 2004 for over 100 days. The OBSs were on the seafloor in about 5000 m of water and would be in the shadow zone for long-range, ducted propagation in the sound channel. The goals of the deployment were (1) to quantify the amount of energy that leaks out of the sound channel into the shadow zone; (2) to measure the relative sensitivity (signal-to-noise) of seafloor hydrophones and vertical component geophones to long-range signals; and (3) to study the physics of earthquake generated T-phases. Leakage was observed out to 3200 km on the vertical component geophone and out to 1000 km on the hydrophone. The ratio of the pressure to vertical velocity varies between arrivals on the same trace.

The Long-range Ocean Acoustic Propagation EXperiment (LOAPEX), conducted in the NE Pacific Ocean, provided acoustic transmissions from a ship-suspended source at eight widely separated stations, and from a cabled acoustic source near the Island of Kauai, HI. The transmissions were received on several bottom-mounted horizontal hydrophone arrays distributed about the NE Pacific Ocean Basin and two, nearly colocated, vertical hydrophone line arrays spanning roughly 3500 m of the water column. Ranges varied from 50 km to several Mm. The goals of the experiment are (i) to study the evolution, with distance (range), of the acoustic arrival pattern and in particular the dependence of the spatial and temporal coherence; (ii) to investigate the nature of the deep caustics and the associated arrivals well below their turning depths; (iii) to analyze the effects of the ocean bottom near the bottom-mounted acoustic source cabled to Kauai; and (iv) to produce a thermal snapshot of the NE Pacific Ocean. The experiment goals, design, and methods are described as well as preliminary data results.

One of the main objectives of the NPAL 2004 experiment, LOAPEX (Long-range Ocean Acoustic Propagation EXperiment), which was conducted between 10 September and 10 October 2004, was to better understand the roles of scattering and diffraction in general. The LOAPEX measurement provided acoustic transmission data for ranges of 50, 250, 500, 1000, 1600, 2300, and 3200 km. By placing the source off-axis in order to avoid exciting low-order modes, we are able to study phenomena of the significant in-filling of acoustic energy into the finale region. Our focus will be on the transmissions for the off-axis source location (nominally 350 m depth), and the acoustic receptions as recorded on the 1400-m-long axial receiving array. The observation of the mean intensity of the wavefront arrival pattern at each range will be compared to deterministic ray and parabolic equation calculations. The following questions will be addressed here: (1) How does high angle acoustic energy from an off-axis source transfer energy to low angles in the axial region of the waveguide? (2) What are the relative contributions from diffraction and scattering? (3) How does this energy transfer scale with range?

The statistics of low-frequency, long-range acoustic transmissions in the North Pacific Ocean are presented. Broadband signals at center frequencies of 28, 75, and 84 Hz are analyzed at propagation ranges of 3252 to 5171 km, and transmissions were received on 700 and 1400 m long vertical receiver arrays with 35 m hydrophone spacing. In the analysis we focus on the energetic "finale" region of the broadband time front arrival pattern, where a multipath interference pattern exists. A Fourier analysis of 1 s regions in the finale provide narrowband data for examination as well. Two-dimensional (depth and time) phase unwrapping is employed to study separately the complex field phase and intensity. Because data sampling occured in 20 or 40 min intervals followed by long gaps, the acoustic fields are analyzed in terms of these 20 and 40 min and multiday observation times. An analysis of phase, intensity, and complex envelope variability as a function of depth and time is presented in terms of mean fields, variances, probability density functions (PDFs), covariance, spectra, and coherence. Observations are compared to a random multipath model of frequency and vertical wave number spectra for phase and log intensity, and the observations are compared to a broadband multipath model of scintillation index and coherence.

We examine statistical and directional properties of the ambient noise in the 10%u2013100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached %u223C150 dB re 1 %u03BC Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica.

Predictions of transverse horizontal spatial coherence from path integral theory are compared with measurements for two ranges between 2000 and 3000 km. The measurements derive from a low-frequency (75 Hz) bottom-mounted source at depth 810 m near Kauai that transmitted m-sequence signals over several years to two bottom-mounted horizontal line arrays in the North Pacific. In this paper we consider the early arriving portion of the deep acoustic field at these arrays. Horizontal coherence length estimates, on the order of 400 m, show good agreement with lengths calculated from theory. These lengths correspond to about 1° in horizontal arrival angle variability using a simple, extended, spatially incoherent source model. Estimates of scintillation index, log-amplitude variance, and decibel intensity variance indicate that the fields were partially saturated. There was no significant seasonal variability in these measures. The scintillation index predictions agree quite well with the dataset estimates; nevertheless, the scattering regime predictions (fully saturated) vary from the regime classification (partially saturated) inferred from observation. This contradictory result suggests that a fuller characterization of scattering regime metrics may be required.

In 1998–1999, a comprehensive low-frequency long-range sound propagation experiment was carried out by the North Pacific Acoustic Laboratory (NPAL). In this paper, the data recorded during the experiment by a billboard acoustic array were used to compute the horizontal refraction of the arriving acoustic signals using both ray- and mode-based approaches. The results obtained by these two approaches are consistent. The acoustic signals exhibited weak (if any) regular horizontal refraction throughut most of the experiment. However, it increased up to 0.4 deg (the sound rays were bent towards the south) at the beginning and the end of the experiment. These increases occurred during midspring to midsummer time and seemed to reflect seasonal trends in the horizontal gradients of the sound speed. The measured standard deviation of the horizontal refraction angles was about 0.37 deg, which is close to an estimate of this standard deviation calculated using 3D modal theory of low-frequency sound propagation through internal gravity waves.

This article analyzes the coherence of low-mode signals at ranges of 3515 and 5171 km using data from the Acoustic Thermometry of Ocean Climate (ATOC) and Alternate Source Test (AST) experiments. Vertical line arrays at Hawaii and Kiritimati received M-sequences transmitted from two sources: the 75-Hz bottom-mounted ATOC source on Pioneer Seamount and the near-axial dual-frequency (28/84 Hz) AST source deployed nearby. This study demonstrates that the characteristics of the mode signals at 5171-km range are quite similar to those at 3515-km range. At 75 Hz the mode time spreads are on the order of 1.5 s, implying a coherence bandwidth of 0.67 Hz. The time spread of the 28-Hz signals is somewhat lower, but these signals show significantly less frequency-selective fading than the 75-Hz signals, suggesting that at the lower frequency the multipaths are temporally resolvable. Coherence times for mode 1 at 75 Hz are on the order of 8 min for the 3515-km range and 6 min for 5171-km range. At 28 Hz mode 1 is much more stable, with a magnitude-squared coherence of greater than 0.6 for the 20-min transmission period.

Large-scale, depth-averaged temperatures have been measured by long-range acoustic transmissions in the North Pacific Ocean for the past nine years. Acoustic sources located off central California and north of Kauai transmitted to receivers distributed throughout the North Pacific from 1996 through 1999 during the Acoustic Thermometry of Ocean Climate (ATOC) project. The Kauai transmissions resumed in early 2002 and are now continuing as part of the North Pacific Acoustic Laboratory (NPAL) project; a six-year time series has been obtained so far. Even at long time and large spatial scales the ocean is highly variable. The paths from Kauai to California show a modest cooling trend (longer travel times) until the present time. A path to the northwest showed modest warming and a weak annual cycle from 1999 until early 2003, when a strong annual cycle returned. In retrospect, these changes stemmed from the warming of the central Pacific that occurred in this interval, possibly associated with the Pacific Decadal Oscillation (PDO).

Comparisons between measured travel times and those predicted using ocean models, constrained by satellite altimeter and other data, show significant similarities and differences. Comparison between upper-ocean Argo profiling float temperatures and the acoustically measured temperature along one path illustrates the strength of the integral measurements, with substantially lower uncertainty. The acoustic data ultimately need to be combined with sea-surface height Argo float data to determine the complementarity of the various data types. In particular, combining the acoustic and Argo data by inverse techniques will quantify the ability of the float data to resolve large-scale, upper-ocean heat content and the ability of the acoustic data to resolve abyssal temperature changes.

A method to obtain coherent acoustic wave fronts by measuring the space–time correlation function of ocean noise between two hydrophones is experimentally demonstrated. Though the sources of ocean noise are uncorrelated, the time-averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time-domain Green's function. A theoretical approach is derived for both volume and surface noise sources. Shipping noise is also investigated and simulated results are presented in deep or shallow water configurations. The data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the North Pacific Laboratory (NPAL) during time intervals when no source was transmitting.

Northeast Pacific blue whales seasonally migrate, ranging from the waters off Central America to the Gulf of Alaska. Using acoustic and satellite remote sensing, we have continuously monitored the acoustic activity and habitat of blue whales during 1994–2000. Calling blue whales primarily aggregate off the coast of southern and central California in the late summer, coinciding with the timing of the peak euphausiid biomass, their preferred prey. The northward bloom of primary production along the coast and subsequent northbound movements of the blue whales are apparent in the satellite and acoustic records, respectively, with the calling blue whales moving north along the Oregon and Washington coasts to a secondary foraging area with high primary productivity off Vancouver Island in the late fall. El Nino conditions, indicated by elevated sea-surface temperature and depressed regional chlorophyll-a concentrations, are apparent in the satellite records, particularly in the Southern California Bight during 1997/1998. These conditions disrupt biological production and alter the presence of calling blue whales in primary feeding locations. Remote sensing using acoustics is well suited to characterizing the seasonal movements and relative abundance of the northeast Pacific blue whales, and remote sensing using satellites allows for monitoring their habitat. These technologies are invaluable because of their ability to provide continuous large-scale spatial and temporal coverage of the blue whale migration.

Acoustic transmissions from a 75-Hz source near Kauai to a vertical line array near California were recorded as part of the North Pacific Acoustic Laboratory (NPAL) experiment. Extensive environmental measurements were also performed as part of the experiment and were intended to ensure correspondence between numerical simulations and the data. Despite the availability of this information, the process of identifying the recorded arrivals with predictions has not been a simple one. Since the source is near the seafloor at about 800 m depth, and the depth at the receiver is approximately 1800 m, acoustic interaction with the bathymetry has been explored as a possible complication. Ray simulations that allow for specular reflection from the bottom indicate that fully-refracted and bottom-interacting paths can reach the receiver range (about 3900 km) at similar travel times. The simultaneous presence of both kinds of acoustic energy could contribute to the identification difficulties. A series of parabolic equation simulations have been performed for different geoacoustic parameters in an attempt to correspond more closely to the data. The sensitivity of the predictions to the method used to extract and interpolate the sound speeds has also been investigated.

In our previous paper [J. Acoust. Soc. Am. 112, 2232], we obtained a time dependence of the horizontal refraction angle (HRA) of acoustic signals propagating over a range of about 4000 km in the ocean. This dependence was computed by processing of acoustic signals recorded during the North Pacific Acoustic Laboratory (NPAL) experiment using a ray-type approach. In the present paper, we consider the results obtained in signal processing of the same data using a modal approach. In this approach, the acoustic field is represented as a sum of local acoustic modes with amplitudes depending on a frequency and arrival angle. We obtained a time dependence of HRA for a time interval of about a year. Time evolution of HRA exhibits long-period variations which could be associated with seasonal trends in the sound speed profiles. The results are consistent with those obtained by the ray approach. Different horizontal angles within arrivals were impossible to resolve due to sound scattering by internal waves. A theoretical estimate of the angular width of the acoustic signals in a horizontal plane was obtained. It appears to be consistent with the observed variance of HRA data.

At long range, the low order acoustic modes constitute some of the most energetic arrivals. Prior to using these signals in tomographic or matched field inversions, it is important to understand their fluctuation statistics. Long vertical line arrays installed as a part of the Acoustic Thermometry of Ocean Climate (ATOC) experiment provided a unique opportunity to measure low-order acoustic mode signals at megameter ranges from a broadband source. The ATOC VLA's at Hawaii and Kiritimati received M-sequences transmitted from two sources: a bottom-mounted source on Pioneer Seamount and a near-axial source deployed nearby as a part of the Alternate Source Test (AST). The Pioneer source had a center frequency of 75 Hz, and the AST source had center frequencies of 28 Hz and 84 Hz. Ranges from the sources to the arrays at Hawaii and Kiritimati are on the order of 3.5 and 5.1 megameters, respectively. This paper compares the mode 1 arrivals at the two ranges and three center frequencies. Differences between the arrivals for the bottom-mounted and mid-watercolumn sources are investigated using broadband PE simulations. Temporal coherence of the mode 1 signals is discussed.

Acoustic measurements of large-scale, depth-averaged temperatures are continuing in the North Pacific Ocean in a follow-on to the Acoustic Thermometry of Ocean Climate (ATOC) project. Long-range acoustic transmissions resumed in January 2002 from a low-frequency acoustic source located north of Kauai to U.S Navy receivers distributed throughout the North Pacific Ocean. The source previously transmitted for about two years (1997-1999) as part of the ATOC project. Both the source and receivers are connected to shore by cable, providing near-real time data. It is anticipated that transmissions will continue for five years, as part of the North Pacific Acoustic Laboratory (NPAL) project. At these ranges acoustic methods give integral measurements of large-scale ocean temperature that provide the spatial low-pass filtering needed to observe small, gyre-scale signals in the presence of much larger, mesoscale noise. The paths to the east, particularly those paths to the California coast, show cooling relative to the earlier data. A path to the northwest showed modest warming until early 2003, when rapid cooling occurred. The acoustic rays sample depths below the mixed layer near Hawaii, but extend to the surface near the California coast and north of the Subarctic Front. The acoustic data will be compared to and ultimately combined with upper-ocean data from ARGO and sea-surface height data from satellite altimeters to detect changes in abyssal ocean temperature and to test the complementarity of the various data types. Acoustic travel-time data have been used previously in simple assimilation experiments and are now shown in comparison with assimilation products from state-of-the-art efforts from the ECCO (Estimating the Circulation and Climate of the Ocean) Consortium.

The Acoustic Thermometry of Ocean Climate (ATOC) provided an opportunity to observe signals propagating in the low-order modes of the ocean waveguide. Understanding the fluctuations of these mode signals is an important prerequisite to using them for tomography or other applications. In previous work, we characterized the cross-mode coherence and temporal variability of the low-order mode arrivals at 3515 km range [Wage et al., J. Acoust. Soc. Am. (in press)]. This study compares the mode arrivals for two different ranges: 3515 km and 5171 km, using data from the ATOC vertical line arrays at Hawaii and Kiritimati. We discuss the mode intensity and coherence statistics for each of the arrays and examine mean arrival time trends over the year-long deployment. Experimental results are compared to PE simulations of propagation through a realistic background environment perturbed by internal waves of varying strengths. The dependence of mode statistics on the path-dependent changes in the background sound speed and the parameters of the internal wave field is explored.

It has long been known that baleen (mainly blue and fin) whale vocalizations are a component of oceanic ambient sound. Urick reports that the famous "20-cycle pulses" were observed even from the first Navy hydrophone installations in the early 1950's. As part of the Acoustic Thermometry Ocean Climate (ATOC) and the North Pacific Acoustic Laboratory (NPAL) programs, more than 6 years of nearly continuous ambient sound data have been collected from Sound Surveillance System (SOSUS) sites in the northeast Pacific. These records now show that the average level of the ambient sound has risen by as much as 10 dB since the 1960's. Although much of this increase is probably attributable to manmade sources, the whale call component is still prominent. The data also show that the whale signal is clearly seasonal: in coherent averages of year-long records, the whale call signal is the only feature that stands out, making strong and repeatable patterns as the whale population migrates past the hydrophone systems. This prominent and sometimes dominant component of ambient sound has perhaps not been fully appreciated in current ambient noise models.

Acoustic measurements of large-scale, depth-averaged temperatures are continuing in the North Pacific as a follow on to the Acoustic Thermometry of Ocean Climate (ATOC) project. An acoustic source is located just north of Kauai. It transmits to six receivers to the east at 1-4-Mm ranges and one receiver to the northwest at about 4-Mm range. The transmission schedule is six times per day at four-day intervals. The time series were obtained from 1998 through 1999 and, after a two-year interruption because of permitting issues, began again in January 2002 to continue for at least another five years. The intense mesoscale thermal variability around Hawaii is evident in all time series; this variability is much greater than that observed near the California coast. The paths to the east, particularly those paths to the California coast, show cooling this year relative to the earlier data. The path to the northwest shows a modest warming. The acoustic rays sample depths below the mixed layer near Hawaii and to the surface as they near the California coast or extend north of the sub-arctic front. The temperatures measured acoustically are compared with those inferred from TOPEX altimetry, ARGO float data, and with ECCO (Estimating the Circulation and Climate of the Ocean) model output. This on-going data collection effort, to be augmented over the next years with a more complete observing array, can be used for, e.g., separating whole-basin climate change from low-mode spatial variability such as the Pacific Decadal Oscillation (PDO).

Spectra of omnidirectional ambient sound have been collected since 1994 at 13 locations around the North Pacific. Data were acquired for 3 minutes every 6 minutes and spectra calculated from 0–500 Hz in 1 Hz bands. With a million spectra per site, this database allows investigation into the statistical character of low-frequency ambient sound at multiple scales. At the shortest scales, the spectral levels in the shipping bands have a fluctuation spectrum similar to a 1/f process, with decorrelation times less than 20 minutes. At intermediate scales, the seasonal baleen whale component becomes the most dominant and repeatable feature. At the longest scales (averaging over the entire record) the ambient levels (at the Pt. Sur site) seem to have increased by up to 10 dB since the 1960s. The distribution of the levels (in decibels) generally indicates a short tail for quieter levels but a long tail for loud events. The Pt. Sur data set has also been used to validate the new dynamic ambient noise model (DANM), which shows good agreement in one-third octave bands to within a couple of decibels for January 1998. These and further results will be discussed.

Ocean ambient sound data from 1994 to 2001 have been collected using a receiver on the continental slope off Point Sur, California. A temporary, nearby receiving array was used for calibration purposes. The resulting data set is compared with long-term averages of earlier measurements made with the identical receiver over the period from 1963 to 1965. This comparison shows that the 1994 to 2001 levels exceed the 1963 to 1965 levels by about 10 dB between 20 and 80 Hz and between 200 and 300 Hz, and about 3 dB at 100 Hz. Increases in (distant) shipping sound levels may account for this.

As part of the North Pacific Acoustic Laboratory project, ambient sound data from 1994 to the present has been collected. Long-term averages of these data from a receiver on the continental slope west of Point Sur, CA, are compared to earlier measurements made at the same site over 1963–1965 by Wenz [Wenz, J. Underwater Acoust. 19 (1969)]. The levels Wenz reported fall below our 10% quantile from 5 Hz to 50 Hz, rise to the 50% quantile (i.e., the median) at 100 Hz, and again fall below the 10% quantile by 250 Hz. Wenz removed highly variable "transient" data before calculating his averages. We mimicked his processing with the NPAL data and obtained a result which is virtually indistinguishable from the median, which is approximately 1 dB below the (dB) mean of each one-third octave band. Hence, our median levels are directly comparable to Wenz's results, and this comparison shows that the 1994–2000 levels exceed the 1963–1965 levels by 9 dB or less below 100 Hz and again at 250 Hz, but are roughly similar at 100 Hz.

The low-order modes constitute some of the most energetic arrivals at long ranges. Understanding fluctuations of these mode arrivals is crucial to their use as observables in matched field processing and tomography. Both simulated and experimental data indicate that at megameter ranges, the low modes have complex arrival patterns due to internal-wave-induced coupling. Analysis of broadband receptions at 3515 km from the ATOC experiment has shown that mode coherence times are on the order of 6 minutes and that centroid statistics provide useful measures of arrival time trends over the course of several months [Wage et al., IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings, pp. 102-106, 2000]. The North Pacific Acoustic Laboratory (NPAL) experiment presents an opportunity for further research on broadband mode arrivals at megameter ranges. This study examines temporal coherence, intensity variations, and other mode statistics using data from the 40-element NPAL vertical line array. Experimental results are compared with PE simulations of propagation through internal waves of varying strengths, and the impact of the up-slope propagation near the receivers on the mode statistics is discussed.

Low-frequency (75-Hz) acoustic signals were repeatedly transmitted over a 1 year period and sampled vertically (with up to a 1400-m aperture) and horizontally (with a 3600-m cross-range aperture) by a distant billboard array (3900-km range) as described by the NPAL Group. The data are complicated by the fact that the sound interacts with the bottom near both the source and receiver. Vertical beamforming is used to filter the bottom interacting energy, and thus allow analysis of the fundamental acoustic properties. Subband and subarray processing is used to produce estimates of arrival times and angles or resolved ray arrivals. Time-series of acoustic travel times and arrival angles are then developed.

A comprehensive, long-range sound propagation experiment was carried out with the use of the billboard acoustic array of the North Pacific Acoustic Laboratory (NPAL) in 1999. The antenna consisting of five vertical line arrays was deployed near a California coast and received broadband acoustic signals transmitted from Hawaii over a distance of about 4000 km. Acoustic signals propagating over such a long distance might exhibit noticeable horizontal refraction. The paper will present results of processing the NPAL data to infer horizontal refraction angle (HRA) as a function of time. Different methods were used for determining HRA. The first approach employed cross correlation of the acoustic signals at different VLAs. Time delay corresponding to maximum of cross correlation is related to HRA assuming the angle is approximately the same for all rays (or modes). The second method used modal representation of the arriving broadband signals. The dependency of the amplitudes of acoustic modes on mode number, frequency, and arrival angle was determined independently within narrow frequency bins, and then the results were averaged over whole frequency range. This method allowed, in particular, to evaluate angular width of the arrived signal, which appeared to be of the order of a few milliradians.

While the NPAL array was primarily deployed to examine the spatial coherence of the Hawaii source, it is also a rich data set for ambient noise studies. Shipping noise, earthquakes and biologics all have been identified in the NPAL data. Moreover, ambient noise coherence is the primary issue in maximizing the SNR output of a sonar system. The first and second order statistics of data from the NPAL "noise only" segments have been analyzed with the following results: (i) There is a wide spread in the peak levels, most likely due to the proximity to shipping lanes. The maximum peak level in the recording band is 117 dB. (ii) Full broadband coherences tend to be low because of the presence of many ships. (iii) If one examines frequency bands of 1–2 Hz, then lines of individual ships can be identified and associated and they are very coherent across NPAL aperture. (iv) Vertical beamforming indicates relatively highly directional spectra at low grazing angles and "noise notch" for the spectra at higher frequencies. Horizontal beamforming has been difficult to implement due to element positioning errors and the large array transit time.

One of the main objectives of the NPAL experiment is to investigate the horizontal refraction and coherence of the acoustic wave fronts at long range. Given time series of acoustic arrival times and angles of resolved ray arrival arrivals, a detailed look at the acoustic wave fronts is possible. First and second order statistics (density functions and coherences) of the wave fronts are investigated. The wave fronts are shown to vary with time, frequency, depth and across the horizontal aperture.

Receptions of long-range acoustic transmissions by deep hydrophone arrays in the Pacific and Atlantic often have "ray-like" arrivals that occur in the shadow zone of the predicted time front. These "ray-like" arrivals can frequently be identified with the cusps of the predicted time front, but the receivers are up to 750 m below the depth of the cusps. Preliminary calculations show that the observed acoustic energy is not accounted for by errors in the sound speed, leakage of acoustic energy below the cusps as predicted by the full wave equation, or scattering due to internal waves. Data obtained during experiments in the Atlantic and Pacific will be reviewed. Experiments that have been conducted with receivers of vertical line arrays have not had receivers deep enough to observe this phenomena. The effect is seen when bottom-mounted or midwater acoustic sources are used. These data present a number of problems: If the ray paths are wandering all over the water column, why are predictions of ray travel times usually accurate? How does the energy loss associated with these data increase the attenuation of very long-range acoustic transmissions? Without knowing the forward problem, how can these data be used to determine oceanographic information?

The North Pacific Acoustic Laboratory program augmented the existing ATOC acoustic network with a sparse, two-dimensional receiving array installed west of Sur Ridge, California, from July 1998 through June 1999, to receive transmissions from the 75-Hz ATOC source north of Kauai. The NPAL array consisted of four 20-element vertical arrays, each with a 700-m aperture, and one 40-element vertical array with a 1400-m aperture. The arrays were deployed transverse to the 3900-km acoustic path from the Kauai source and had a total horizontal aperture of 3600 m. Data collected with the billboard array and the U.S. Navy SOSUS receivers are being used (i) to study the temporal, vertical, and horizontal coherence of long-range, low-frequency resolved rays and modes, (ii) to study 3-D propagation effects, (iii) to examine directional ambient noise properties, and (iv) to improve basin-scale ocean nowcasts via assimilation of acoustic data and other data types into models. In addition to acoustic data, environmental data along the path from the Kauai source to the billboard array were acquired by two oceanographic sub-surface moorings and by two XBT/CTD/ADCP transects along the path. The experiment will be described and some preliminary results presented.

Acoustic transmissions on basin scale ranges are being used to determine depth-dependent temperature variability. With travel time being the primary observable, stationary sources and nearly stationary receivers are experimental requirements. This has led to the use of bottom-mounted sources and receivers to reduce travel time variability. The NPAL (North Pacific Acoustics Laboratory) experiment has transmitted broadband acoustic pulses from two bottom-mounted sources near the sound channel axis. Recordings have been taken on the NPAL billboard array, a linear series of five vertical line arrays moored in 1800 m of water near Monterey, CA. Additional recordings have been taken from the SOSUS system throughout the Pacific basin. The effects of the near source and near receiver scattering are examined. In particular, near source scattering leads to excess high-angle energy entering deep water with a travel time delay of nearly 1 s due to the low group speeds of high-angle rays/modes in shallow water. We also compare the energetics of the arriving rays that have bounced once on the rising seafloor near the NPAL receivers. Comparisons of models and data for bottom interacting acoustics lead us to the perennial issue of geoacoustic parameters.

Current theories for wave propagation in random media can predict fluctuations for narrow-band (single-frequency) signals, but do not explicitly address wideband signals. Recent ocean acoustic tomography experiments, however, have employed wideband "m-sequence" signals. Processing here assumes a periodic signal, and hence, involves a unique set of Fourier series components. A technique is presented for decomposing such wideband signals into their Fourier components and then analyzing the space–time fluctuations of these "narrow-band" components. The technique is applied to AST96 data transmitted over 150-km and 1-Mm paths for single-point and two-point (temporal separation) statistics.

Time series of temperature have been measured acoustically in the northeast Pacific as part of the Acoustic Thermometry of Ocean Climate (ATOC) project. These time series are compared with other available data types. The acoustic time series of transmissions from the California and Kauai acoustic sources were obtained during 1996–1999. As a result of marine mammal protocols, the time series are intermittent; the California source was turned off in Fall 1998. Assuming that variations in sea-surface height observed by TOPEX/POSEIDON are caused by thermal expansion, the amplitude of the annual cycle of heat content derived from altimetry is larger than that found by the acoustic data, Levitus climatology, and monthly maps of ocean temperature from XBTs of opportunity. The heat content "anomalies" determined by the XBT maps are comparable in size to the differences between the XBT and acoustically derived heat content. A variety of problems with the XBT sampling may account for these differences. The 12-year time series of temperature derived from the Hawaiian Ocean Time series (HOT) data highlights the mesoscale noise in single-point sampling. However, thermal variability at 100-day time scales is observed in the acoustic data obtained between Hawaii and California using the Kauai source. Acoustic thermometry is complementary to altimetry and hydrography.

Spectra of ambient sound have been collected over the past 6 years at 13 locations around the North Pacific. Data were collected on single hydrophones for 3 min every 6 min and power spectra in 1-Hz bands from 0–500 Hz were calculated. Results from this 6-year data set are presented including probability statistics (e.g., the fraction of time a sound level is exceeded), whale, ship, and wind statistics, and long-term trends. These results build on those presented previously for 2 years [Curtis et al., J. Acoust. Soc. Am. 106, 3189-3200 (1999)].

The North Pacific Acoustic Laboratory program augmented the existing ATOC acoustic network with a sparse, two-dimensional receiving array installed west of Sur Ridge, CA, close to an existing U.S. Navy SOSUS array, during July 1998 to receive transmissions from the 75-Hz ATOC source north of Kauai. The NPAL array consisted of four 20-element vertical arrays, each with a 700-m aperture, and one 40-element vertical array with a 1400-m aperture. The arrays were deployed transverse to the 3900-km path from the Kauai source and had a total horizontal aperture of 3600 m. Data collected with the two-dimensional array and the U.S. Navy SOSUS receivers will be used to (i) study the temporal, vertical, and horizontal coherence of long-range, low-frequency resolved rays and modes, (ii) study 3D propagation effects, (iii) examine directional ambient noise properties, and (iv) to improve basin-scale ocean nowcasts via assimilation of acoustic data and other data types into models. Environmental data along the path from the Kauai source to the two-dimensional array were acquired by two oceanographic subsurface moorings and by two XBT/CTD/ADCP transects along the path, one at the beginning and one at the end of the experiment. We describe the experiment and offer some preliminary data.