Investigating acoustic signal propagation between the Kauaʻi Beacon source an

Acoustic tomography is a powerful technique for remote ocean sampling that measures ocean properties over integrated acoustic paths. The information gained can be used to refine ocean models and improve the understanding of oceanographic processes. Tomography has diverse applications, including the study of internal waves, temperature variability, gyre dynamics, tides, and other ocean phenomena. Acoustic tomography leverages the fact that sound speed is a function of temperature, salinity, and pressure. By observing changes in the travel time of a signal along a transect, one can invert perturbations in arrival time to detect sound speed variations, ultimately revealing temperature changes. The Kauaʻi-Beacon (KB) source was reactivated in March 2023 after a 15-year hiatus for long-range acoustic applications in the Pacific Basin, including tomography and navigation. KB is located on the seafloor off the coast of Kauaʻi at a depth of 810 m and transmits a 44-period m-sequence pseudo-random noise signal with a 75 Hz carrier frequency on a nominal 2% duty cycle schedule. Recent studies demonstrated successful reception of the signal at Wake Island, Monterey Bay, and the Ocean Observatories Initiative (OOI) sensor network, achieving 30 dB of processing gain. However, since KB is installed on the seafloor, local effects (e.g., from bathymetry and seabed properties) potentially affect the far-field signal; several studies from the early 2000s demonstrated a relatively complex acoustic near-field at KB. Fortuitously, the ALOHA Cabled Observatory (ACO) hydrophone, leveraged here as a “KB near-field receiver of opportunity”, receives the KB signal at 166 km from the KB source with precise timing. ACO is seafloor-mounted, 100 km north of Oʻahu at 4,728 m depth. This thesis aims to advance the understanding of underwater acoustic propagation in the acoustic near-field of KB by processing and investigating the KB signal as received at ACO. We analyze over 1000 KB transmissions received at the ACO over 24 months and examine the temporal variability in the receptions. This variability is compared to modeled results created using a range-dependent BELLHOP model. The model simulates acoustic propagation using ray theory and known ocean properties from the Hawaii Ocean Time-Series and World Ocean Atlas. Modeled acoustic arrivals are compared to ACO data, revealing similarities in the magnitude, period, and annual phase cycle of arrival time perturbations when tracking a single ray path over a yearly cycle. The thesis defense will detail the methods and preliminary findings of our KB-ACO investigations, including the effects of seasonal variability on the received signal. This work forms the foundation for future research into understanding the complete acoustic arrival pattern of the KB signal received at ACO, explicitly considering the bathymetric interactions along the 166 km transect [supported by the Office of Naval Research Ocean Acoustics program]. Time: Apr 16, 2025 09:00 PM Hawaii Join Zoom Meeting https://hawaii.zoom.us/j/82827911158 Meeting ID: 828 2791 1158 Passcode: Eliza2025

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Ocean and Resources Engineering, Mānoa Campus

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