Overflow waters in the western Irminger Sea modify deep sound speed structure and convergence zone propagation.

Deep sound speed structure in the western Irminger Sea is found to be highly dynamic in comparison to the adiabatic (uniform) sound speed gradient underpinning data assimilation and modeling efforts around the globe. A beamed source parabolic equation model is used to illustrate how the resulting non-uniform sound speed structure at 1 to 1.5 km in depth and sound speed inversion near the seafloor produce observable effects on acoustic signals between a shallow source and shallow vertical line array at convergence zone ranges. Beamforming analysis shows that a uniform sound speed gradient leads to "ideal" interference patterns that do not capture or represent modeled convergence zone properties, such as location, strength, and sharpness. Overall findings suggest that in situ information about sound speed below 1 km is necessary for low frequency, long-range propagation studies, particularly in areas of complex thermohaline circulation.

Transmission loss for the Beaufort Lens and the critica frequency for mode propagation during ICEX-18.

Complexities of acoustic propagation in ducts have long been known, e.g., shallow water environments and deep waters off Gibraltar. The "Beaufort Lens" (Lens) is a duct north of Alaska with nominal depths between 60 and 200 m and is reachable by oceanographic instruments and underwater unmanned vehicles and submarines. Propagation within the ducts is governed by waveguide physics. The frequencies must be high enough to support the modes within them such that there is a "critical frequency" (CF) where modes start to "detach" from surface loss mechanisms. Therefore, transmission losses (TLs) can abruptly decrease once a mode "fits" within a duct. This paper describes an experimental part of Ice Exercise 2018 supported by the U.S. Navy's Arctic Submarine Laboratory. The signals were transmitted from Camp Sargo north of Prudhoe Bay to the submarines SSN Hartford, SSN Connecticut, and HMS Trenchant. The data indicate low TLs near 100 Hz and an abrupt 10 dB decrease in TLs 244-280 Hz, both suggesting CFs. Modeling suggests CFs for modes 1 near 100 Hz and a higher CF when modes 3-6 "cascade" into the Lens starting near 250 Hz. There are also abrupt increases in TLs at other frequencies, which are explained by nulls in the product of the mode functions.

The stochastic Cramér-Rao bound for source localization and medium tomography using vector sensors.

A direct version for the stochastic Cramér-Rao bound (CRB) for parameters of Gaussian signals with additive Gaussian noise is introduced. The formulation applies to passive and active radars/sonars/seismics/structures with vector observations from multiple sources. These sensors include pressure, vector velocity, and/or acceleration sensors for ocean and structural acoustics, seismometers, polarized receivers for electromagnetics, and vector current meters for oceanography. The observations may contain partially coherent signals such as multipath. The parameters represent (i) signal localization or (ii) tomographic ones. As such, their embedding is very general using a Green's function vector and is not limited to direction of arrival problems. This formulation leads to simplified expressions for the stochastic CRB using just three quadratic forms involving just the Green's function and its derivatives with the inverse of the noise matrix for the norm. The number of the parameters sets the dimensions of these quadratic forms, so performance studies over the parameter space can be done with much smaller matrices as the noise covariance is inverted just once. The formulas are applied to vector sensors in jamming with both analytical and numerical results. The results are also compared to often cited papers on the CRB.

Transarctic acoustic telemetry.

On April 9 and 13, 1999, two Arctic Climate Observation Using Underwater Sound tomography signals were transmitted from a 20.5-Hz acoustic source moored at the Franz Victoria Strait to an eight-element, 525-m vertical array at ice camp APLIS in the Chukchi Sea at a distance of ∼2720 km. The transmitted signal was a 20-min long, 255-digit m-sequence that can be treated as a binary-phase shift-keying communication signal with a data rate of 2 bits/s. The almost error-free performance using either spatial diversity (three elements) for a single transmission or temporal diversity (two transmissions) with a single element demonstrates the feasibility of ice-covered trans-Arctic acoustic communications.

Deep water towed array measurements at close range.

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.

Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea.

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.

The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea.

A series of experiments conducted in the Philippine Sea during 2009-2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010-2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010-2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining 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, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.

Statistics and vertical directionality of low-frequency ambient noise at the North Pacific Acoustic Laboratory site.

We examine statistical and directional properties of the ambient noise in the 10-100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached approximately 150 dB re 1 micro Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica.

On the kinematics of broadband multipath scintillation and the approach to saturation.

Utilizing a simple model in which the acoustic wave function is a sum of independent Gaussian wave packets, the relative intensity variance or scintillation index (SI) is analytically calculated. The model has an unspecified probability density function (PDF) for wave packet amplitudes and Gaussian PDFs for travel-time-induced and non-travel-time-induced phase shifts; amplitudes and both phase shifts are assumed to be mutually uncorrelated. It is shown that a proper treatment of the mean field is required to obtain the saturation value, SI = 1, in the limit of a large number of interfering wave packets. The analytic formulas for SI allow identification of important wave packet parameters in the approach to saturation. Criteria are identified for both broadband and narrow-band cases for which the approach to saturation is from above and below 1. It is demonstrated that the broadband approach to saturation is much slower than the narrow-band cases, since wave packets separated in time by more than an inverse bandwidth do not strongly contribute to interference. This effect is quantified by the time-bandwidth product. The model is also used to obtain an analytic expression for pulse time spread; it is shown that multipath conditions which favor a rapid approach to saturation do not favor large pulse spread.

Consistency and accuracy of perturbative inversion methods for group travel time data.

Perturbation theory for ocean acoustic modal group speed responses to small environmental changes is investigated with regard to its applicability to ocean acoustic tomography. Assuming adiabaticity, the inverse problem for each vertical eigenmode is an integral equation whose kernel involves the eigenfunction and its frequency derivative. A proof is given for the equivalence of two dissimilar forms of the integral equation. Numerical examples are given for the inversion kernel for four types of sound-speed profiles, and then the parameter range (amplitude and scale size) in which perturbation theory is accurate is examined. It is found that the range of validity is determined not only by the amplitude of the perturbations, but also by their vertical scale size.

Modal analysis of broadband acoustic receptions at 3515-km range in the North Pacific using short-time Fourier techniques.

In 1995-1996 the Acoustic Thermometry of Ocean Climate (ATOC) experiment provided an opportunity to study long-range broadband transmissions over a series of months using mode-resolving vertical arrays. A 75-Hz source off the California coast transmitted broadband pulses to receiving arrays in the North Pacific, located at ranges of 3515 and 5171 km. This paper develops a short-time Fourier transform (STFT) processor for estimating the signals propagating in the lowest modes of the ocean waveguide and applies it to analyze data from the ATOC experiment. The STFT provides a convenient framework for examining processing issues associated with broadband signals. In particular, this paper discusses the required frequency resolution for mode estimation, analyzes the broadband performance of two standard modal beamforming algorithms, and explores the time/frequency tradeoffs inherent in broadband mode processing. Short-time Fourier analysis of the ATOC receptions at 3515 km reveals a complicated arrival structure in modes 1-10. This structure is characterized by frequency-selective fading and a high degree of temporal variability. At this range the first ten modes have equal average powers, and the magnitude-squared coherence between the modes is effectively zero. The coherence times of the peaks in the STFT mode estimates are on the order of 5.5 min. An analysis of mean arrival times yields modal dispersion curves and indicates that there are statistically significant shifts in travel time over 5 months of ATOC transmissions.