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During the 1991 Heard Island Feasibility Test, a vertical hydrophone array deployed off Monterey, CA, recorded transmissions from a low-frequency acoustic source nearly 18,000 km away. By determining the modal structure of the received transmissions, it is possible to characterize the physics of such long range propagation. This thesis focuses on the determination of the modal, or vertical, structure of the signal. It was necessary to first develop a conditioning scheme to address several data quality issues, including very low signal levels (-15 dB SNR on a single channel), large transient spikes, and a limited set of operational channels. Very narrowband filtering was used to obtain a 25 dB increase in SNR. Doppler shifts for each transmission event were predicted from available parameters and were found to be within ±2 mHz of the measured shifts. The modal analysis employed two methods: comparing variations in signal energy with depth to the vertical extent of the modes, and fitting the data using a least squares modal decomposition. The least squares performance given a subsampled basis set of modes was studied and improved upon through the use of diagonal loading. Lack of array orientation data hindered the analysis, and least squares fitting was used to estimate the most likely orientation. The least squares analysis indicated the presence of modes at least up to mode 7, possibly higher. This is significant in that predictions prior to the experiment were that all but the lowest modes would be attenuated by boundary interactions along the path. Results from independent analyses of the same data also support the conclusion that the signal structure is quite complex. Thesis Supervisor: Prof. Arthur B. Baggeroer Massachusetts Institute of Technology Acknowledgments I would like to first acknowledge my advisor and thesis supervisor, Prof. Arthur Baggeroer. His guidance with this work has been invaluable, and his patience with my learning curve deeply appreciated. Many thanks to my fellow colleagues Kathleen Wage and Peter Traykovski for their helpful discussions, support, and proof reading. A special thanks to those at WHOI Eddie Scheer for his help with the computers and Keith von der Heydt for his information on the technical details of the array. Also, what thesis experience would be complete without a bunch of officemates to thank Joe, Matt, Tarun, Dave, Diane, and the rest of the 5-435 crowd. Many thanks to my parents, brothers, and grandparents for their continued interest and support. I am particularly grateful for their enthusiasm regarding my work. Finally, and most importantly, I wish to thank my wife Kelly Poort, not only for putting up with my long hours and extended stays in Woods Hole, but also for providing words of encouragement, motivation, and understanding at just the right moments. I certainly could not have made it this far without you, Kel! The Vertical Line Array effort for the Heard Island Feasibility Test was supported by the Monterey Bay Aquarium Research Institute (MBARI), the Naval Postgraduate School (NPS), and Science Applications International Corporation (SAIC). Funding for this work was provided by the Department of Energy Grant #DE-FG0291ER61100 (Vertical Arrays for the Heard Island Acoustic Feasibility Experiment on Monitoring Global Warming), and the University of California Scripps ATOC Agreement, PO#10037359.

Modal analysis of vertical array receptions for the Heard Island Feasibility Test