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A mechanism is presented by which the observed acoustic intensity is made to vary due to changes in the acoustic path that are caused by internal-tide vertical fluid displacements. The position in range and depth of large-scale caustic structure is determined by the background sound-speed profile. Internal tides cause a deformation of the background profile, changing the positions of the caustic structures-which can introduce intensity changes at a distant receiver. Gradual fades in the acoustic intensity occurring over timescales similar to those of the tides were measured during a low-frequency (284-Hz) acoustic scattering experiment in the Philippine Sea in 2009 [White et al., J. Acoust. Soc. Am. 134(4), 3347-3358 (2013)]. Parabolic equation and Hamiltonian ray-tracing calculations of acoustic propagation through a plane-wave internal tide environmental model employing sound-speed profiles taken during the experiment indicate that internal tides could cause significant gradual changes in the received intensity. Furthermore, the calculations demonstrate how large-scale perturbations to the index of refraction can result in variation in the received intensity.

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–Munk 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.

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.

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 article of record as published may be found at https://doi.org/10.1121/1.4820882 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. This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014-06-1-0245, N00014-08-1-0195, and N00014-11-1-0194.

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.

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 km. 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 km. 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.

Nearly two decades of low-frequency (20–500 Hz) 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. Comparisons with very sparse measurements made over the last 5 decades suggest that the mid-latitude noise levels may have peaked in the 1990s. These measurements also show, however, that the noise level is still rising elsewhere. 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 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.

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 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.

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.

Long-term statistics of ambient sound in an ocean basin have been derived from 2 years of data collected on 13 widely distributed receivers in the North Pacific. The data consist of single hydrophone spectra (1–500 Hz in 1-Hz bands) averaged over 170 s and recorded at 5-min intervals. Cumulative probability distributions of the ambient sound level show that for the open-ocean arrays at 75 Hz, sound levels are 3 dB higher than the median level 10% of the time and 6 dB higher 1% of the time. For the coastal arrays, sound levels are 7 dB higher than the median level 10% of the time and 15 dB higher 1% of the time. The clearest feature in many of the spectrograms is a strong annual cycle in the 15–22 Hz band with peak signal levels up to 25 dB above the sound floor; this cycle is attributed to the presence and migration of blue and fin whales. On average, whales are detected 43% of the time. Ships are heard 31%–85% of the time on the coastal receivers and 19%–87% of the time on the open-ocean receivers, depending on the receiver. On average, ships are detected 55% of the time. The correlation coefficient between the sound level in the 200–400 Hz band and wind speed, determined from satellite and global meteorological analysis, is on average 0.56 for the coastal receivers and 0.79 for the open-ocean receivers. For some receivers, the sound level in the 12–15 Hz band is correlated with the sound level in the 200–400 Hz band, with a correlation coefficient of 0.5.

During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km; receptions occurred over a period of six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and exponential distributions. The observed PDFs are closer to the log-normal distribution, and variances of log intensity are between (3.1 dB)2 (with a scintillation index of 0.74) for late-arriving timefronts and (2.0 dB)2 (with a scintillation index of 0.2) for the earliest timefronts. Fluctuations of the pulse termination time of the transmissions are observed to be 22 ms rms. The intensity PDF of nonidentified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov–Smirnov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB)2 and the scintillation index is 0.92. As a group, the observations suggest that the propagation is on the border of the unsaturated and partially saturated regimes. After improving the specification of the ray weighting function, predictions of travel-time variance using the Garrett–Munk (GM) internal-wave spectrum at one-half the reference energy are in good agreement with the observations, and the one-half GM energy level compares well with XBT data taken along the transmission path. Predictions of pulse spread and wave propagation regime are in strong disagreement with the observations. Pulse time spread estimates are nearly two orders of magnitude too large, and Λ–Φ methods for predicting the wave propagation regime predict full saturation.

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surroga...

Most marine sediments exhibit transverse isotropy (TI) that can have a significant effect on the signal properties of strongly bottom interacting sound. Locally, transverse isotropy has the greatest effect on the fundamental and near fundamental modal overtones. The local shallow water TI modes have reduced amplitude in the sediment relative to the corresponding shallow water modes for an isotropic bottom. Even a small departure from isotropy (2.4%) can have a significant (15%) effect on the phase velocity of bottom interacting modes. Calculations of mode–mode coupling coefficients for a range‐dependent medium indicate that mode coupling is more strongly confined to modal nearest neighbors for a TI medium characterized predominantly by shear wave anisotropy, when compared to the corresponding isotropic medium. As the frequency increases, the strongest coupling occurs between higher overtones and also becomes more strongly peaked around nearest neighbors. The coupled mode theory of Maupin [Geophys. J. 93, 173–185 (1988)] is employed to model the coupling. This theory can treat smooth gradients and sloping layer boundaries for all five of the bottom elastic moduli in a TI medium, the densities, and the range dependence of the water column itself. This coupled mode formulation also properly accounts for the continuity of stress and displacement boundary conditions in an exact way at irregular interfaces.

Broadband acoustic signals were transmitted from a moored 250‐Hz source to a 3‐km‐long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound‐speed field along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), conductivity‐temperature‐depth (CTD), and air‐launched expendable bathythermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that extends over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic travel times with 1‐ms precision. The time‐mean travel times of the early raylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time‐mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound channel axis, are consistent with ray predictions based on the direct measurements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizontal wavelengths shorter than 50 km, which is not resolved by the direct measurements, significantly (25 ms peak‐to‐peak) affects the time‐mean ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100–200 m off the sound channel axis. Nongeometric effects, such as diffraction at caustics, partially account for this observation.

A 1000‐km acoustical transmission experiment has been carried out in the North Pacific, with pulses broadcast between a moored broadband source (250‐Hz center frequency) and a moored sparse vertical line of receivers. Two data records are reported: a period of 9 days at a pulse rate of one per hour, and a 21‐h period on the seventh day at six per hour. Many wave‐front segments were observed at each hydrophone depth, and arrival times were tracked and studied as functions of time and depth. Arrivals within the final section of the pulse are not trackable in time or space at the chosen sampling rates, however. Broadband fluctuations, which are uncorrelated over 10‐min sampling and 60‐m vertical spacing, are observed with about 40 (ms)2 variance. The variance of all other fluctuations (denoted as low‐frequency) is comparable or smaller than the broadband value; this low‐frequency variance can be separated into two parts: a wave‐front segment displacement (with vertical correlation length greater than 1 km) that varies substantially between rays with different ray identifiers, and a distortion (with vertical correlation length between 60 m and 1 km) of about 2 (ms)2 variance. The low‐frequency variance may be explained as the effect of internal waves, including internal tides. The variance of the broadband fluctuations is reduced somewhat but not eliminated if only high‐intensity peaks are selected; this selection does not affect the statistics of the low‐frequency fluctuations.

The statistics of intensity fluctuations in dual-band, low-frequency, ocean acoustic transmissions across 500 km in the Philippine Sea 2010 experiment appear to be at odds with the standard micro-multipath theory predicting them. M-sequence encoded acoustic signals at 200 Hz and 300 Hz were transmitted for 54 h from a ship-suspended multiport source to a distributed vertical line array (DVLA) with 149 working hydrophones covering most of the 5500 m water column. Histograms of the received intensity fluctuations are approximately exponential. Intensity fluctuations at the two frequencies are strongly correlated. Scintillation indices and variances of log intensity are high (>1 and >5.57dB, respectively), suggesting the measurements are not yet in a saturated regime but still significantly scattered. However, for all this, the pulses received at the DVLA are virtually the same width as those recorded next to the source; no pulse spreading or arrival splitting is seen. So, the fading observed in the data doe...

A long-term observation program begun in approximately 1994 has amassed time-series of ambient sound short-time spectra from omni-directional hydrophones deployed on the ocean floor at seven locations in the northern Pacific Ocean. Each time-series consists of spectra estimated every 5 min over a useful band of 10–500 Hz, thereby encompassing the vocalizations of baleen whales, the anthropogenic contribution of ship traffic noise, and wind/wave noise due to sea surface processes. Simple linear trend lines show that traffic noise in the northern and northwestern reaches of the Pacific has increased by as much as 3 to 4 dB during this program. In the eastern North Pacific, however, the ambient sound shows a decrease of about 3 dB. The number of ships in the world merchant fleet increased by approximately 25% over this period. This change is insufficient to explain the increases in noise levels along northern and northwestern regions, and provides no explanation for the decreases observed in the north-east. The traffic noise field is evidently dependent on more complex temporal and geographical patterns of shipping traffic. [Work supported by ONR.]A long-term observation program begun in approximately 1994 has amassed time-series of ambient sound short-time spectra from omni-directional hydrophones deployed on the ocean floor at seven locations in the northern Pacific Ocean. Each time-series consists of spectra estimated every 5 min over a useful band of 10–500 Hz, thereby encompassing the vocalizations of baleen whales, the anthropogenic contribution of ship traffic noise, and wind/wave noise due to sea surface processes. Simple linear trend lines show that traffic noise in the northern and northwestern reaches of the Pacific has increased by as much as 3 to 4 dB during this program. In the eastern North Pacific, however, the ambient sound shows a decrease of about 3 dB. The number of ships in the world merchant fleet increased by approximately 25% over this period. This change is insufficient to explain the increases in noise levels along northern and northwestern regions, and provides no explanation for the decreases observed in the north-east. ...

The oceanography of the Philippine Sea is partially characterized by energetic mesoscale and strong locally generated internal tides. Despite the simplification of range-independence and the exclusion of internal tides from Monte Carlo parabolic equation (MCPE) simulations, predictions of scintillation index, variance of log-intensity, and the distribution of intensity for acoustic paths with upper-turning-points (UTP) below the extreme upper ocean generally agree with measurements made during an experiment in 2009. These measures of the fluctuations did not appear to be strongly influenced by the number of UTPs in the path, though a compensating effect due to differences in UTP position cannot be ruled out. Enhanced variability in the form of deep fades is observed for paths turning in the extreme upper ocean; this enhanced variability is not predicted by the MCPE model employed. Seaglider-based observations of mixed-layer depth (from 2010 to 2011) and moored measurements of internal-tide-related sound-speed perturbations are presented. A plane-wave internal-tide model and results from acoustic mode propagation through range-independent profiles measured in situ are compared with the observed character of the intensity fades.

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.]

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. [Funding provided by ONR]

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. [Work supported by ONR.]

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. [Work supported by ONR.]

T-phases are earthquake signals that have propagated, at least partially, in the ocean sound channel. T-phase hydrophone networks detect much smaller earthquakes over basin scales than land-based networks and they detect many more earthquakes than comparable regional scale seismic land networks. Furthermore, since T-phases travel at lower velocities than seismic phases, they result in much more precise locations of events given the same timing accuracy. T-phases are typically spread over 10's of seconds, and a common problem, however, is precisely identifying the arrival time of an event. T-phase stations usually consist of single hydrophones moored near the sound channel axis and the depth dependence of the T-phase envelope and frequency content is rarely studied. In the North Pacific Ocean, from 2004 to 2005, ambient noise and earthquakes were observed at an ocean acoustic observatory consisting of a vertical hydrophone array (from about 750 m above the seafloor to 375 m from the surface) and three co-located ocean bottom seismometers. This data set provides a unique opportunity to observe earthquake signals and their characteristics throughout the water column and to provide ground-truth to theoretical predictions on the excitation and propagation mechanisms of T-phases. In at least one case, a T-phases from a distant earthquake was readily observed even at the seafloor, well-below the conjugate depth.

During an experiment in the northern Philippine Sea in 2009, a ship towing Penn State's Five-Octave Research Array (FORA) at approximately 120 m depth drove counter-clockwise in an arc, maintaining constant range at one convergence zone (CZ) from a second ship holding station with an acoustic source deployed at 15 and 60 m. In addition, the FORA was towed at various depths in a star pattern about the station-keeping source ship, thereby sampling the first CZ in range, depth, and azimuth. Throughout the experiment, sound speed profiles were measured using expendable bathy-thermographs, expendable sound velocimeters, and conductivity/temperature versus depth sensors, and detailed bathymetric data were collected using the multibeam systems aboard these and other ships. By incorporating this extensive environmental information into numerical models, variability observed in these measurements of the range and structure and asymmetry of the distribution of received levels of the first CZ resulting from a shallo...

Over the course of an experiment in the northern Philippine Sea in 2009, weather and sea surface conditions varied from calm and smooth to stormy and very rough. A ship with an acoustic source deployed at 15 and 60 m held station while a second ship towed Penn State's Five Octave Research Array (FORA) at 120 m depth in an arc, maintaining constant range at the first convergence zone. With the source and receivers so near the ocean surface and with zero closing speed, these events offer an opportunity to extract information about the acoustic waves' interaction with the temporally varying ocean surface. Additionally, the FORA was towed at various depths in a star pattern about the stationary source ship, and a vertical acoustic array was deployed near the source ship. Using meteorological data and a model for ocean waves given weather conditions and fully developed seas, these events offer an opportunity to separate in the data-adaptive beamformer output the arrivals scattered and Doppler shifted by the ro...

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 propertie...

Experimental observations of the axial modes across range are crucial to verify and complement existing theories for internal‐wave induced mode scattering. The 2004 Long Range Ocean Acoustic Propagation Experiment (LOAPEX) was conducted with the main objective of analyzing internal‐wave induced mode fluctuations at ranges of 50–3200 km. During LOAPEX, a 75 Hz broadband source made transmissions at an off‐axial depth of 350 m and an axial‐depth of 800 m. The transmissions were received across a 40 element vertical array capable of resolving the first ten acoustic modes. The mode arrivals from the two source depths significantly differ from each other. For the axial source, the observed mode arrivals are strongly influenced by the low modes directly excited at the source or by nearest neighbor coupling. The shallow source in contrast excites the low modes at a much lower energy, and thus the axial arrivals from the shallow source transmissions are due to scattering from the high modes. This talk discusses the LOAPEX mode statistics for the two source depths. The observed mode statistics are compared with predictions from theory and simulations. [Work sponsored by a National Research Council Research Associateship Award at Naval Postgraduate School.]

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’ 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. [Work sponsored by ONR.]

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 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 th...

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 intrinsic attenuation. (b) There is apparently insignificant scattering back into the water column because this would have been observe...

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.]

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 [J. Acoust. Soc. Am. 126, (2009)]. 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 abilit...

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 bichromatic” 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. [Work sponsored by ONR code 322OA.]

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.

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.]

Deep water tomographic inversions should include the low modes to improve estimates of the axial sound speed. Unfortunately, the low‐mode signals are strongly affected by internal wave fluctuations, making it very challenging to estimate their arrival times. This talk describes the use of matched subspace detectors (MSDs) to measure mode travel times for ranges up to 400 km. The MSD approach was first proposed by Scharf and Friedlander [IEEE Trans. Signal Process. 42, 2146–2157 (1994)] as a way to detect signals that lie within a particular subspace. In this work the subspace for each mode is defined by analyzing simulations of mode propagation through independent realizations of the internal wave field. Simulations show that the MSD‐based travel time estimation approach yields higher accuracy and less variance than other methods such as peak‐picking. The end result is improved inversions for sound speed near the sound channel axis. In addition to the simulation study, mode signals from the 2004 Long Range Ocean Acoustic Propagation EXperiment (LOAPEX) are used to invert for the sound speed profile (SSP) across the LOAPEX path. The inverted SSP is compared with environmental measurements made during LOAPEX. [Work sponsored by ONR.]

An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the NPAL04 long‐range ocean acoustic propagation experiment in the North Pacific. The observed intensity pattern of the OBS arrivals is significantly more complex than the waterborne arrivals seen on the deep vertical line array (DVLA). These “deep seafloor” arrivals occur later than the first PE predicted arrival; their arrival time is not predicted by acoustic PE propagation models, they do not correspond to decay from shallower turning points (as is the case for deep shadow zone arrivals), and they are not readily observed on the DVLA hydrophone just 750 m above the seafloor. The arrival structure in the observed data, in time and amplitude, varies substantially between three OBSs that are separated by less than 4 km. Could these unexplained arrivals be scattering or horizontal multi‐path from persistent ocean thermal structure?

To gain insight into the physical mechanisms responsible for deep shadow zone arrivals in long range ocean acoustic propagation we have characterized some key features of the replica correlated signals observed on an ocean bottom seismometer (OBS) deployed on the 2004 long‐range ocean acoustic propagation experiment. We compare the vertical seismometer data from an OBS at 4973‐m depth beneath the deep vertical line array (DVLA) with the deepest hydrophone on the DVLA at 4270 m depth. The results of the preliminary analysis show (1) that the vertical geophone channel has more arrivals than the deep DVLA hydrophone channels, (2) that the first arrivals on the OBS geophone and DVLA deep hydrophones correspond to energy in the first deep arriving path predicted by the parabolic equation solution, (3) that the later arrivals which are much larger in amplitude are not explained by the parabolic equation method, and (4) that it is the later arrivals that contribute to the long range (up to 3200 km) shadow zone “receptions.” Possible mechanisms for the later arrivals and some consequences of the observation are discussed. [This work was supported by the Office of Naval Research.]

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 (McDonald et al., JASA, 120(2), August 2006). 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 1960’s 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. [Work...

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 [Chandrayadula et al., J. Acoust. Soc. Am. 120, 3062 (2006)]. 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...