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Sagen, H, Worcester PF, Dzieciuch MA, Geyer F, Sandven S, Babiker M, Beszczynska-Moller A, Dushaw BD, Cornuelle B.  2017.  Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait. Journal of the Acoustical Society of America. 141:2055-2068.   10.1121/1.4978780   AbstractWebsite

An ocean acoustic tomography system consisting of three moorings with low frequency, broad-band transceivers and a moored receiver located approximately in the center of the triangle formed by the transceivers was installed in the central, deep-water part of Fram Strait during 2010-2012. Comparisons of the acoustic receptions with predictions based on hydrographic sections show that the oceanographic conditions in Fram Strait result in complex arrival patterns in which it is difficult to resolve and identify individual arrivals. In addition, the early arrivals are unstable, with the arrival structures changing significantly over time. The stability parameter a suggests that the instability is likely not due to small-scale variability, but rather points toward strong mesoscale variability in the presence of a relatively weak sound channel as being largely responsible. The estimator-correlator [Dzieciuch, J. Acoust. Soc. Am. 136, 2512-2522 (2014)] is shown to provide an objective formalism for generating travel-time series given the complex propagation conditions. Because travel times obtained from the estimator-correlator are not associated with resolved, identified ray arrivals, inverse methods are needed that do not use sampling kernels constructed from geometric ray paths. One possible approach would be to use travel-time sensitivity kernels constructed for the estimator-correlator outputs. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

Roux, P, Kuperman WA, Cornuelle BD, Aulanier F, Hodgkiss WS, Song HC.  2013.  Analyzing sound speed fluctuations in shallow water from group-velocity versus phase-velocity data representation. Journal of the Acoustical Society of America. 133:1945-1952.   10.1121/1.4792354   AbstractWebsite

Data collected over more than eight consecutive hours between two source-receiver arrays in a shallow water environment are analyzed through the physics of the waveguide invariant. In particular, the use of vertical arrays on both the source and receiver sides provides source and receiver angles in addition to travel-times associated with a set of eigenray paths in the waveguide. From the travel-times and the source-receiver angles, the eigenrays are projected into a group-velocity versus phase-velocity (Vg-Vp) plot for each acquisition. The time evolution of the Vg-Vp representation over the 8.5-h long experiment is discussed. Group speed fluctuations observed for a set of eigenrays with turning points at different depths in the water column are compared to the Brunt-Vaisala frequency. (C) 2013 Acoustical Society of America.

Dzieciuch, MA, Cornuelle BD, Skarsoulis EK.  2013.  Structure and stability of wave-theoretic kernels in the ocean. Journal of the Acoustical Society of America. 134:3318-3331.   10.1121/1.4818846   AbstractWebsite

Wave-theoretic modeling can be applied to obtain travel-time sensitivity kernels (TSKs) representing the amount ray travel times are affected by sound-speed variations anywhere in the medium. This work explores the spatial frequency content of the TSK compared to expected ocean variability. It also examines the stability of the TSK in environments that produce strong sensitivity of ray paths to initial conditions. The conclusion is that the linear TSK model is an effective predictor of travel-time changes and that the rays perform nearly as well as the full-wave kernel. The TSK is examined in physical space and in wavenumber space, and it is found that this is the key to understanding how the travel time reacts to ocean perturbations. There are minimum vertical and horizontal length scales of ocean perturbations that are required for the travel time to be affected. The result is that the correspondence between true travel times and those calculated from the kernel is high for large-scale perturbations and somewhat less for the small scales. This demonstrates the validity of ray-based inversion of travel time observations for the cases under study. (C) 2013 Acoustical Society of America.