Long-range Ocean Acoustic Propagation Experiment Research Articles

Bottom-diffracted surface-reflected arrivals in the North Pacific

Bottom-diffracted surface-reflected (BDSR) arrivals were first identified in the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX) in the North Pacific (Stephen et al., 2013, JASA, 134, 3307-3317). In deep water, ambient noise and PE predicted arrivals are sufficiently quiet that BDSR arrivals, scattered from small seamounts, can be the largest amplitude arrivals observed. The Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June-July 2013 studied BDSRs at the same site in detail. They are most readily identified by their move-out on lines of transmissions and are clearest on the vertical component channel. There appear to be two classes of diffractors. The diffraction point for Type 1 BDSRs occurs on the side of small seamounts and is substantially out of the source-receiver sagittal plane (for example, the BDSR observed on NPAL04). The diffraction point for Type 2 BDSRs is essentially in the source-receiver sagittal plane and occurs on the relatively featureless abyssal plain. In at least one case the same BDSR is observed for 77.5, 155 and 310 Hz M-sequence transmissions. Other characteristics of BDSRs will be discussed. [Work supported by ONR.]

Experimental study of sound propagation using a deep-water vector hydrophone

Vector hydrophone can synchronously receive the sound scalar field and the vector field information at a single point, which increases the variety and the quantity of the received sound information. Vector field information is beneficial to improve the performance of underwater acoustic devices or systems. In summer of 2014, a Long-Range Ocean Acoustic Propagation Experiment was carried out in deep sea. A deep-water vector hydrophone designed by Harbin Engineering University was located at the depth of 3146 m, which measured all the three components of particle velocities as well as the sound pressure in the acoustic field. The scalar and vector receptions of convergence zone arrivals and deep shadow zone arrivals were acquired for hundreds of kilometers in the experiment. The analyzed transmission loss results of the experimental data are described and compared with the predictions by a 2-D parabolic equation (PE) model, which is also called RAM. The comparisons show that the RAM-PE model is efficient to simulate both scalar field and vector field in deep water. Moreover, experiment results demonstrate that the deep-water vector hydrophone performs well to measure sound pressure signal and sound vector signal in deep water.

Bottom-diffracted surface-reflected arrivals in the Philippine Sea

Bottom-diffracted surface-reflected (BDSR) arrivals were first identified in the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX) in the North Pacific (Stephen et al., 2013, JASA, 134, 3307–3317). For sources at long ranges in deep water, they can be observed throughout the water column, but at depths above the conjugate depth they are obscured by ambient noise and energy propagating in the ocean sound channel (PE predicted arrivals). In deep water (deeper than the conjugate depth), ambient noise and PE predicted arrivals are sufficiently quiet that BDSR paths, scattered from small seamounts, can be the largest amplitude arrivals observed. In the Ocean Bottom Seismometer Augmentation in the Philippine Sea (OBSAPS) Experiment in April–May 2011, we observed BDSR arrivals on ocean bottom seismometers at relatively short ranges (less than 50 km). The experiment was designed to further define the characteristics of the BDSRs and to understand the conditions under which BDSRs are excited and propagate. BDSR arrivals are distinct, with little indication of coda or reverberation, and the scattering point appears to be discrete. The BDSR mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor. [Work supported by ONR.]

Weakly dispersive modal pulse propagation in the North Pacific Ocean

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. [Work supported by ONR.]

Bottom interacting acoustics in the north Pacific

During the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX), a new class of acoustic arrivals was observed on ocean bottom seismometers (OBSs) for ranges from 500 to 3200 km. The arrivals were called deep seafloor arrivals (DSFAs), because they were the dominant arrivals on the ocean bottom seismometers (OBSs), but were very weak on the deep vertical line array (Deep VLA), located above 750 m from the seafloor. Stephen et al. [JASA 134, 3307–3317 (2013)] attributed some of these arrivals to bottom-diffracted, surface-reflected (BDSR) energy that scattered from a seamount near the Deep VLA and subsequently reflected from the sea surface before arriving at the OBSs. In the Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June to July 2013, we returned to the Deep VLA site with a near-seafloor Distributed Vertical Line Array (DVLA) that extended upward 1000 m from the seafloor and 12 OBSs. We transmitted to the instruments with a ship-suspended J15-3 acoustic source. The receiver locations and transmission program were designed to test the hypothesis that DSFAs correspond to BDSR energy, to further define the characteristics of the DSFAs, and to understand the conditions under which DSFAs are excited and propagate.

Observations and transport theory analysis of low frequency, acoustic mode propagation in the Eastern North Pacific Ocean

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.

Weakly dispersive modal pulse propagation in the North Pacific Ocean

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.

Reduced rank models for travel time estimation of low order mode pulses

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.

Bottom interacting sound at 50 km range in a deep ocean environment

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.

Deep seafloor arrivals in long range ocean acoustic propagation

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.

Deep seafloor arrivals in long range ocean acoustic propagation

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.

Piecewise coherent mode processing of acoustic data recorded on two horizontally separated vertical line arrays

Motivated by measurements made in the 2004 Long-Range Ocean Acoustic Propagation Experiment (LOAPEX), the problem of mode processing transient acoustic signals collected on two nearby vertical line arrays is considered. The first three moments (centroid, variance, and skewness) of broadband distributions of acoustic energy with fixed mode number (referred to as modal group arrivals) are estimated. It is shown that despite the absence of signal coherence between the two arrays and poor high mode number energy resolution, the centroid and variance of these distributions can be estimated with tolerable errors using piecewise coherent mode processing as described in this paper.

Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean: Low mode numbers

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.

Bottom reflections from rough topography in the long-range ocean acoustic propagation experiment (LOAPEX)

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. [Work supported by ONR.]

Observations of mode scattering from axial and off-axial sources in the North Eastern Pacific, Long Range Ocean Acoustic Propagation Experiment.

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.]

Statistics of acoustic normal mode energy for low frequency long range propagation in the ocean: Comparisons between observations and theory.

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 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. [Research performed while the author held a National Research Council Research Associateship Award at Naval Postgraduate School.]

Deep seafloor arrivals: An unexplained set of arrivals in long-range ocean acoustic propagation

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.

Comparing the effects of internal waves, mode coupling, and change in bandwidth of a radiated signal on low mode energy propagation.

Wideband signal propagation modeling is carried out for the actual space‐time configuration realized during the long‐range ocean acoustic propagation experiment (LOAPEX) conducted in the North Pacific in 2004. In particular, the experiment used the Kauai transmit station that was located at a range of 2432 km from a vertical line array (VLA), M‐sequence at 75 Hz, and a source depth of 800 m close to the depth of a sound‐channel axis. Two sound speed profiles were utilized to get the smooth two‐dimensional sound speed field. The first one was the profile obtained from the conductivity‐temperature‐depth (CTD) measurements at the Kauai station. The second sound speed profile from the VLA location was based on a CTD cast taken at station T50 (50 km from the VLA). To take into account the acoustic fluctuations due to internal waves, the buoyancy frequency profile based on LOAPEX CTD measurements made at station T50 was used. For simulations, 512 values of the horizontal wave number were utilized with the maximal mode number equal to 50. The low modes intensity received by the hydrophones close to the sound channel axis at the VLA is compared with the results of simulations. [Work supported by ONR and ONR Global.]

The interference component of the acoustic field corresponding to the Long-Range Ocean Acoustic Propagation Experiment

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.

Robust observables for mode tomography.

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.]