Fluctuating modal interference characteristics in shallow water with a seasonal thermocline

In temporal and spatial array signal processing, especially for a large-aperture horizontal line arrays (HLA), degradation of the horizontal longitudinal correlation of signals among different hydrophones due to multipath dispersion commonly limits the array gain. It is therefore important to reduce or cancel the dispersion effects between hydrophone signals. Modal filtering is one of the available approaches to utilize the interference structure of sound fields and to improve the signal processing performance. In this paper, modal filtering based on a warping transform from a nonlinear time-frequency signal to a linear one is used to separate modes from a broadband signal. Then, the contributions of modes to interference striation are determined in order to explain the difference of the waveguide invariant between modes. The variability of waveguide invariants in fluctuating shallow water with a strong seasonal thermocline and internal waves is examined experimentally. The experiment was performed with a sea-floor located HLA and a fixed sound source transmitting 160-250 Hz linear frequency modulated (LFM) signals, which were observed over a long period of time. The contribution of low-and high-order modes to interference patterns explains the variability of waveguide invariants in the measured data.