A comparative study of mode arrivals at megameter ranges for 28 Hz, 75 Hz, and 84 Hz sources

At long range, the low order acoustic modes constitute some of the most energetic arrivals. Prior to using these signals in tomographic or matched field inversions, it is important to understand their fluctuation statistics. Long vertical line arrays installed as a part of the Acoustic Thermometry of Ocean Climate (ATOC) experiment provided a unique opportunity to measure low-order acoustic mode signals at megameter ranges from a broadband source. The ATOC VLA's at Hawaii and Kiritimati received M-sequences transmitted from two sources: a bottom-mounted source on Pioneer Seamount and a near-axial source deployed nearby as a part of the Alternate Source Test (AST). The Pioneer source had a center frequency of 75 Hz, and the AST source had center frequencies of 28 Hz and 84 Hz. Ranges from the sources to the arrays at Hawaii and Kiritimati are on the order of 3.5 and 5.1 megameters, respectively. This paper compares the mode 1 arrivals at the two ranges and three center frequencies. Differences between the arrivals for the bottom-mounted and mid-watercolumn sources are investigated using broadband PE simulations. Temporal coherence of the mode 1 signals is discussed.