Date of Award
2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Ocean Engineering
Specialization
Acoustics
Department
Ocean Engineering
First Advisor
Lora J. Van Uffelen
Abstract
Acoustic propagation in the Arctic is evolving as surface temperatures increase and ice melts, resulting in a changing sound propagation environment. This study investigates the acoustic arrival structure in the Canada Basin and its connection to the underlying sound speed profiles. Two Seagliders were deployed in the summer of 2016 and 2017, diving in a sawtooth pattern collecting conductivity, temperature, and depth (CTD) data as part of the Canada Basin Acoustic Glider Experiment (CABAGE). These data are used to calculate sound speed profiles used to predict and understand acoustic arrival structures.
The warming of the Pacific Summer Water and surface layers in the Arctic has caused a double-ducted sound speed profile to develop in the summer months. The lower duct, centered on cold Pacific Winter Water and known as the Beaufort duct, has been a useful tool in Arctic acoustic experiments as it allows for long-range acoustic transmissions with little interaction from the sea surface or seafloor. The upper subsurface duct exists due to the warmer surface temperatures in the summer months. The double-ducted feature, bounded on the bottom by a warm, salty Atlantic Water layer, results in a 'shallow water' acoustic propagation environment in which the lower-order modes, or lower-angle rays, arrive first. These final arrivals make up the Reverse Geometric Dispersion (RGD) feature and arrive concurrently with acoustic arrivals that follow a 'deep water' arrival pattern in which the high-order modes, or high-angle rays, arrive first. This feature is observed in the predicted acoustic time fronts and recorded acoustic data from CABAGE.
The persistence and strength of the double-ducted sound speed profile is analyzed using duct parameters. These measure the depth span, sound speed span, and integrated area within this span of the duct. The first chapter applies these parameters to the double-ducted portion of the sound speed profile. To define the bottom bound, the isohaline at 34.2 PSU is used to mark the depth of the Atlantic Water layer. This is shown to provide a good estimate of the depth span of the resulting RGD feature. The integrated duct parameter is able to capture the unique structure of the double-duct better than a single difference measurement and correlates well with the resulting range-independent RGD feature.
In the second chapter, acoustic propagation performed using multiple ocean models and datasets, including the sound speed profiles derived from CABAGE data, are used to investigate how well this RGD feature can be predicted, if at all, using existing resources including the World Ocean Atlas (WOA), the Navy Global Hybrid Coordinate Oceanographic Model (HYCOM), and the Arctic Subpolar gyre sTate Estimate (ASTE). Measurements are sparse in the Arctic compared to areas of easier access and this analysis shows the importance of including in situ ocean measurements into the ocean models in order to predict the acoustic arrival structures reflected in the CABAGE data.
The acoustic arrival structure in the Canada Basin is more complicated than just a reversal in arrival dispersion. Typically, normal modes are used to analyze acoustic propagation in the Arctic. In chapter three, three unique regimes are described and analyzed using ray trace and normal mode propagation models. Using the ray traces, the driving factors of the arrival patterns are able to be defined. Action is also calculated for the range-independent, background sound speed profile and is shown to be a useful tool in predicting the acoustic arrival structure without the need to run a full acoustic propagation model.
Recommended Citation
Desrochers, Jessica, "REVERSE GEOMETRIC DISPERSION IN THE BEAUFORT SEA: DISSECTING SOUND PROPAGATION AT LONG RANGES IN DUCTED PROFILES" (2025). Open Access Dissertations. Paper 4498.
https://digitalcommons.uri.edu/oa_diss/4498