The broadband transport theory approach to model internal wave induced scattering across deep water acoustic time-fronts.

There are currently no models to fully predict the effects of internal wave induced scattering on acoustic pulses. Existing models, which predict time domain statistics, either use the ray-based path integral method or Monte Carlo type simulations. The path integral method fails to accurately predict all of the effects of scattering. The Monte Carlo methods base the statistics on ensemble averages and are not physics-based models. This paper overcomes these limitations by using the modes of the waveguide in a transport theory application. The transport theory equations have, thus far, been used only to explain diffusion of mode intensities and decorrelation due to internal waves at individual frequencies. This paper extends the current narrowband application predict mode correlations across different frequencies and, from that, the broadband time-front, time wander, travel time bias, and the amount of spread in intensity across time and depth. To validate these predictions, this paper uses separate parabolic equation simulations. The comparisons between the two are good, suggesting a success for the mode-based transport theory approach.

Long-term frequency changes of a potential baleen whale call from the central Indian Ocean during 2002-2019.

The multiple baleen whales of the central Indian Ocean use distinct calls, with their acoustic signatures marking their respective geographic distributions. This paper uses observations from Diego Garcia to track long-term changes (2002-2019) in calls produced by an unidentified whale. The calls around 20-45 Hz consist of closely spaced frequency tones that resemble a comb, followed by a downsweep. The observations show that while the average comb frequencies steadily increase, the downsweep portion decreases. Some frequencies disappear, while new ones appear. These frequency-observations are different from similar studies of other baleen whales in the region, which mostly show a decrease.

Observations of low-frequency, long-range acoustic propagation in the Philippine Sea and comparisons with mode transport theory.

The year-long Philippine Sea (2010-2011) experiment (PhilSea) was an extensive deep water acoustic propagation experiment in which there were six different sources transmitting to a water column spanning a vertical line array. The six sources were placed in an array with a radius of 330 km and transmitted at frequencies in the 200-300 Hz and 140-205 Hz bands. The PhilSea frequencies are higher than previous deep water experiments in the North Pacific for which modal analyses were performed. Further, the acoustic paths sample a two-dimensional area that is rich in internal tides, waves, and eddies. The PhilSea observations are, thus, a new opportunity to observe acoustic modal variability at higher frequencies than before and in an oceanographically dynamic region. This paper focuses on mode observations around the mid-water depths. The mode observations are used to compute narrowband statistics such as transmission loss and broadband statistics such as peak pulse intensity, travel time wander, time spreads, and scintillation indices. The observations are then compared with a new hybrid broadband transport theory. The model-data comparisons show excellent agreement for modes 1-10 and minor deviations for the rest. The discrepancies in the comparisons are related to the limitations of the hybrid model and oceanographic fluctuations other than internal waves.

Observations of phase and intensity fluctuations for low-frequency, long-range transmissions in the Philippine Sea and comparisons to path-integral theory.

In the Philippine Sea, from April 2010 to March 2011, a 330-km radius pentagonal acoustic transceiver array with a sixth transceiver in the center transmitted broadband signals with center frequencies between 172 and 275 Hz and 100 Hz bandwidth eight times a day every other day. The signals were recorded on a large-aperture vertical-line array located near the center of the pentagon at ranges of 129, 210, 224, 379, 396, and 450 km. The acoustic arrival structures are interpretable in terms of ray paths. Depth and time variability of the acoustic observations are analyzed for six ray paths (one from each transceiver) with similar vertical sampling properties in the main thermocline. Acoustic-field statistics treated include: (1) variances of phase and intensity, (2) vertical coherence and intensity covariance, (3) glinting and fadeout rates, and (4) intensity probability density functions. Several observed statistics are compared to predictions using Feynman path-integral theory assuming the Garrett-Munk internal-wave spectrum. In situ oceanographic observations support this assumption and are used to estimate spectral parameters. Data and theory differ at most by a factor of two and reveal the wave propagation regimes of unsaturated, partially saturated, and fully saturated. Improvements to the evaluation of path-integral quantities are discussed.

Coupled mode transport theory for sound transmission through an ocean with random sound speed perturbations: coherence in deep water environments.

Second moments of mode amplitudes at fixed frequency as a function of separations in mode number, time, and horizontal distance are investigated using mode-based transport equations and Monte Carlo simulation. These second moments are used to study full-field acoustic coherence, including depth separations. Calculations for low-order modes between 50 and 250 Hz are presented using a deep-water Philippine Sea environment. Comparisons between Monte Carlo simulations and transport theory for time and depth coherence at frequencies of 75 and 250 Hz and for ranges up to 500 km show good agreement. The theory is used to examine the accuracy of the adiabatic and quadratic lag approximations, and the range and frequency scaling of coherence. It is found that while temporal coherence has a dominant adiabatic component, horizontal and vertical coherence have more equal contributions from coupling and adiabatic effects. In addition, the quadratic lag approximation is shown to be most accurate at higher frequencies and longer ranges. Last the range and frequency scalings are found to be sensitive to the functional form of the exponential decay of coherence with lag, but temporal and horizontal coherence show scalings that fall quite close to the well-known inverse frequency and inverse square root range laws.

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.

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.