Adaptive Mode Filtering for Arctic Environments

Ocean acoustic tomography uses measured mode and ray travel times to infer properties of the waveguide. Tomographic experiments often employ broadband sources transmitting pulse waveforms that can be processed by a vertical array to obtain the desired travel times. This paper focuses on the problem of mode filtering for tomography in arctic environments. Ideally a mode filter passes the desired mode with unity gain while also suppressing ambient noise and interference from other modes or rays. The challenge with implementing mode filters for tomography is that the structure of the interference is a function of arrival time due to dispersion in the waveguide. Thus, the mode filter has to adapt with time to reject noise and interference. In previous work [1] we developed a performance-weighted blended (PWB) adaptive mode filter for continous-wave signals using Buck and Singer's universal beamforming concept [2]. The PWB mode filter combines pseudo-inverse mode filters of different ranks based on their observed performance. This paper modifies the PWB approach to accomodate the pulse transmissions used in tomography. Simulations for an arctic waveguide show that the PWB filter achieves better performance than a single fixed rank filter and requires minimal data for training.