Measuring temperature changes of the deep oceans, important for determining the oceanic heat content and its impact on the Earth's climate evolution, is typically done using free‐drifting profiling oceanographic floats with limited global coverage. Acoustic thermometry provides an alternative and complementary remote sensing methodology for monitoring fine temperature variations of the deep ocean over long distances between a few underwater sources and receivers. We demonstrate a simpler, totally passive (i.e., without deploying any active sources) modality for acoustic thermometry of the deep oceans (for depths of ~ 500–1500 m), using only ambient noise recorded by two existing hydroacoustic stations of the International Monitoring System. We suggest that passive acoustic thermometry could improve global monitoring of deep‐ocean temperature variations through implementation using a global network of hydrophone arrays.
The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping fluid–solid interfaces. The fluid may correspond to the ocean or a sediment layer that only supports compressional waves. The solid may correspond to ice cover or a sediment layer that supports compressional and shear waves. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying single-scattering approximations to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.
The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping solid–solid interfaces and variable topography. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying a single-scattering approximation to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.
A long range Underwater Navigation Algorithm (UNA) is described that provides a geolocation underwater while submerged without having to surface for a Global Navigation Satellite System (GNSS) position. The UNA only uses measured acoustic travel times from a constellation of underwater acoustic sources analogous to the constellation of satellites in GNSS. The UNA positions are calculated without any a priori track, position or sound speed information, and thus provide a “Cold Start” capability. The algorithm was tested using data from the 2010–2011 Philippine Sea Experiment in which six sources were deployed in a pentagon ∼400 km on a side. 502 positions of hydrophones in a bottom-moored vertical line array at depths of 485–3037 m drifting in a tidal watch circle up to 600 m in diameter were computed. The sources were 129–450 km from the hydrophone receivers. The mean UNA position error from ground truth was 58 m with a standard deviation of 32 m. The UNA Cold Start Algorithm position can be used as the point in the ocean for calculating acoustic model runs from the source positions with a four-dimensional sound speed field from a general circulation model to improve the accuracy.
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