Ocean Acoustic Propagation Experiment Research Articles

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.

Deep seafloor arrivals: Scattering or multi-path from ocean thermal structure?

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?

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.]

LOAPEX: The Long-Range Ocean Acoustic Propagation EXperiment

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.

Deep shadow zone arrivals on the ocean bottom.

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.]

Sensitivity issues in acoustic mode tomography.

Ocean acoustic mode tomography relies on a high signal to noise ratio and an accurate measure of the time of arrival for the mode signals. The low‐order modes measured during tomography experiments such as the 2004 long range ocean acoustic propagation experiment (LOAPEX) are sensitive to environmental and experimental conditions. For example, internal waves can cause mode coupling and time of arrival perturbations. The source and the VLA move due to currents and tides, which results in Doppler spreading and frequency selective fading. Uncertainties in the signal affects the accuracy of tomographic methods. Previous work [Chandrayadula et al., J. Acoust. Soc. Am. 121, 3053 (2007)] mainly focused on building a channel model for the internal wave effects. This paper extends the previous work by building a more complete channel model that also includes the effects due to residual source/receiver motion. The statistics from the channel model are used to study the effect of uncertainties on the mode signals me...

Signal processing techniques for low-order acoustic modes

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 compared against simple averaging techniques. [Work supported by ONR.]

Effect of ocean internal waves on the interference component of the acoustic field in the Long-range Ocean Acoustic Propagation Experiment

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. [Work supported by ONR.]

The range evolution of the mean wavefront intensity for the long‐range ocean acoustic propagation experiment (LOAPEX) off‐axis source transmissions

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? [Work supported by ONR.]

Long‐range ocean acoustic propagation experiment (LOAPEX) acoustic modeling analysis using measured environmental data

One objective of LOAPEX concerns the range dependence of multiple forward scattering for broadband, long‐range acoustic transmissions. Using environmental data extracted during the experiment, a sound speed field is constructed, which includes sound speed fluctuations due to neutrally buoyant intrusions as well as internal waves. This sound speed field is used as input to a full wave simulation model. Range‐dependent results from simulations regarding the coherent field and depth scattering below lower turning point caustics are presented.

The Long-range Ocean Acoustic Propagation EXperiment (LOAPEX): An overview

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. [Work supported by ONR.]

Moving ship thermometry

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. [Work supported by ONR.]

Vertical structure and arrival times of the axial wave based upon conductivity, temperature, and depth data acquired during the Long‐range Ocean Acoustic Propagation Experiment

In the LOAPEX experiment at some distances from VLA the sound source was deployed to depths quite close to the sound‐channel axis. This took place at stations T1000, T500, T250, and T50 where, in particular, the source was deployed to 800 m and the depth of the sound‐channel axis at these ranges is about 680 m. The interference of the wave fields corresponding to near‐axial rays leads to formation of a complex interference wave (‘‘the axial wave’’) that propagates along the axis being a component of the axial crescendo. The talk will present the results on simulation of the axial wave as a function of depth of the receiver and the calculation of its arrival time at frequencies 56.25–93.75 Hz. In the time domain the pulse propagation modeling is carried out for the broadband acoustic signals with the pulse center frequency of 75‐ and 37.5‐Hz bandwidth. The obtained results are compared with those for the lowest mode in the adiabatic approximation. Calculations are made for the range‐dependent ocean model based upon LOAPEX CTD data and use the integral representation of the axial wave in a local coordinate system introduced in the vicinity of the range‐variable sound‐channel axis. [Work supported by ONR.]

Seafloor hydrophone and vertical geophone observations during the North Pacific Acoustic Laboratory/Long-range Ocean Acoustic Propagation Experiment (NPAL/LOAPEX)

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 OBS deployments were funded by contributions from ONR, NSF, and WHOI. Data analysis funded by ONR.]

The long-range ocean acoustic propagation experiment (LOAPEX) observable: Modal content as a function of arrival time. Part B. Simulation results

Following the approach described in Part A, we present the acoustic mode content for fixed frequency, range, travel time, and time windowsize by performing broadband full-wave simulations using a wide-angle, parabolic equation model. We use a realistic stratification and internal wave spectrum to predict the acoustic arrival structure. The modal content is extracted using the same technique as we intend to use on the LOAPEX vertical line arrays (especially the deep array). Comparing simulations with and without internal waves reveals the additional modal spreading due to scattering.

Prescription frequency-modulated signal design for the long-range ocean acoustic propagation experiment (LOAPEX)

A ‘‘prescription frequency-modulated’’ signal is a swept FM signal whose sweep rate and possibly envelope are adjusted according to some a priori prescription. An idea advanced by Birdsall and Metzger in 1994 for the ATOC long-range tomography experiment, the concept is being revisited for LOAPEX as an alternative to ‘‘m-sequences.’’ The prescription can be used to tailor the output spectrum to a preferred shape: the sweep rate is slower at frequencies where more spectral energy is needed, and faster where less is needed. The prescription can also compensate for the transducer response. In addition, the prescription can drive the envelope to predefined maximum levels at every frequency, producing a signal which is ‘‘optimal’’ with respect to drive level. This signal is in general far less stressful on transducers than standard m-sequences. Examples of prescription FMs used during the LOAPEX experiment will be presented. [Work supported by ONR.]

LOAPEX: The long-range ocean acoustic propagation experiment

This paper describes the recent long-range ocean acoustic propagation experiment (LOAPEX) and presents initial results. LOAPEX was conducted between 10 September and 10 October 2004, and was closely coordinated with two other experiments, BASSEX and SPICEX. LOAPEX provided low-frequency broadband acoustic transmissions to vertical hydrophone arrays covering most of the deep ocean water column, and to horizontal hydrophone arrays surrounding the Eastern North Pacific Ocean. Two of the primary objectives are to explain the strong acoustic arrivals that are received in the deep ocean shadow zone, and to better understand the roles of scattering and diffraction in general. Transmission paths to the vertical hydrophone arrays varied from 50 to 3200 km. [Work supported by the Office of Naval Research, Code 321OA.]

The long-range ocean acoustic propagation experiment (LOAPEX) observable: Modal content as a function of arrival time. Part A: Relevance

It is well known that for gradual enough changes in the sound speed structure, the mode number is nearly constant. Thus, changes in mode number are the most direct measure of scattering by smaller structures, such as internal waves. The mode number changes can be determined from data by separating the signal by arrival time. It is shown how the separation happens in an ideal case without scattering, allowing an optimum choice of the time window. The modal content in a small time window can also be estimated from the depth dependence of the intensity in the region of the turning depths of the modes. Each mode has a rapid falloff near its turning depth, and the extension beyond that depth measures the higher mode content. The deep vertical line array at LOAPEX measured this depth dependence. In addition, for certain times, all of the arrival is in the deep array, allowing unambiguous projections onto modes, especially if the source is above the sound axis. [Work supported by the Office of Naval Research.]