Modal structure of low-frequency, broadband receptions at megameter ranges

Abstract

Data from the Acoustic Thermometry of Ocean Climate (ATOC) experiment show that at 3515-km range, each low (axial) mode signal contains a series of arrivals, rather than a single dispersive arrival, which would characterize adiabatic propagation. Short-time Fourier transform (STFT) analysis demonstrates that this multipath structure exhibits frequency-selective fading and significant temporal variability. Despite the complexity of individual receptions, averaging the mode estimates for transmissions at successive four-hour intervals reveals some of the expected dispersion characteristics of a deep water channel. Although this evidence suggests that the modes retain travel-time information at megameter ranges, the complicated nature of the arrivals indicates that a better understanding of mode coupling due to internal waves is required for tomography and matched-field processing applications. This study investigates the temporal and frequency coherence of broadband modes by comparing PE simulations with the ATOC results. Since ambient noise and environmental uncertainty affect the accuracy of mode signal measurements for experimental data, this research also addresses certain practical issues and limitations involved in estimating mode coherence. [Work supported by ONR.]