Abstract
An ocean acoustic tomography system consisting of three moorings with low frequency, broadband 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 α 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.
Highlights
Arctic Modeling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES) project (Skarsoulis et al, 2010; Dushaw et al, 2016; Sagen et al, 2016).The sound-speed field in Fram Strait has a weak sound channel at depths of 550–700 m that gives relatively little geometric dispersion, making it difficult to resolve and identify individual arrivals in the early part of the arrival patterns
A full understanding of the acoustic propagation and its relation to the corresponding measured acoustic arrivals is important for development of tomographic inversion and assimilation techniques; this is the forward problem of ocean acoustic tomography
The structure of the acoustic arrival pattern, which determines whether individual arrivals can be resolved and identified, depends critically on the background sound-speed profile
Summary
Arctic Modeling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES) project (Skarsoulis et al, 2010; Dushaw et al, 2016; Sagen et al, 2016). The sound-speed field in Fram Strait has a weak sound channel at depths of 550–700 m that gives relatively little geometric dispersion, making it difficult to resolve and identify individual arrivals in the early part of the arrival patterns This was first seen in the transmissions from a source on the eastern side of the Strait to a receiver in the center during DAMOCLES (Skarsoulis et al, 2010; Dushaw et al, 2016). The conclusion was that the arrivals in Fram Strait were expected to be sufficiently stable to use acoustic remote sensing for monitoring mean ocean temperature (Naugolnykh et al, 1998) These results were sufficiently encouraging that the 2008–2009 pilot study in Fram Strait was conducted as part of the DAMOCLES project.
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