Long-range Ocean Acoustic Propagation Experiment Research Articles

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.

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.]

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.]