Modal analysis of broadband acoustic receptions at megameter ranges

Normal modes, the eigenfunctions of the ocean waveguide, are useful in underwater acoustics because the lowest modes provide an efficient description of the most energetic signals at long ranges. Understanding the structure of the mode arrivals at megameter ranges is crucial to tomographic and matched field processing applications, but there have been few opportunities for experimental observations of these signals. This paper presents a short-time Fourier framework for broadband mode processing and applies it to data from the Acoustic Thermometry of Ocean Climate (ATOC) experiment. In doing so, this work explicitly addresses the issues associated with broadband signals that have been largely ignored in previous work and provides the first detailed analysis of the modal content of receptions at 3515 km range.