Publications
Modal analysis of broadband acoustic receptions at 3515 km range in the North Pacific using short-time Fourier techniques
Abstract
In 1995-1996 the Acoustic Thermometry of Ocean Climate (ATOC) experiment provided an opportunity to study long-range broadband transmissions over a series of months using mode-resolving vertical arrays. A 75 Hz source off the California coast transmitted broadband pulses to receiving arrays in the North Pacific, located at ranges of 3515 km and 5171 km. This paper develops a short-time Fourier transform (STFT) processor for estimating the signals propagating in the lowest modes of the ocean waveguide and applies it to analyze data from the ATOC experiment. The STFT provides a convenient framework for examining processing issues associated with broadband signals. In particular, this paper discusses the required frequency resolution for mode estimation, analyzes the broadband performance of two standard modal beamforming algorithms, and explores the time/frequency tradeoffs inherent in broadband mode processing. Short-time Fourier analysis of the ATOC receptions at 3515 km reveals a complicated arrival structure in modes 1-10, This structure is characterized by frequency-selective fading and a high degree of temporal variability. At this range the first 10 modes have equal average powers, and the magnitude-squared coherence between modes different modes is effectively zero. The coherence times of the peaks in the STFT mode estimates are on the order of 5.5 minutes. An analysis of mean arrival times yields modal dispersion curves and indicates that there are statistically-significant shifts in travel time over 5 months of ATOC transmissions.
© 2003 Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires the permission of the authors and the Acoustical Society of America. The article (PDF) appeared in the Journal of the Acoustical Society of America, vol. 113(2), pp. 801-817, February 2003 and may also be found on the JASA website.