Frequency warping for waveguide characterization with a single hydrophone

This paper presents a new signal processing tool: frequency warping, and its application in waveguide characterization. It can be applied in the context of signals recorded in shallow water (0-400 m) for an impulsive low-frequency source (0-200 Hz) and a single static receiver. In this configuration, propagation is described by modal theory: the recorded pressure field can be decomposed into several modes. Modes are non linear-frequency modulations which share a common frequency band. When the radial distance between source and receiver is smaller than 15 km, modes are also overlapped in time on the receiver. In this case, the recorded signal cannot be represented using classical time frequency representations and adaptive signal processing is required. Frequency warping processing transforms a give mode into a Dirac in time, using a priori information of the environment. As it is sensitive to environment mismatch, it can also be used to perform waveguide characterization. First, modal propagation is quickly reviewed. Secondly, it is shown that environment information is embedded in the time-frequency structure of the modes, but that adaptive signal processing is required to access it. Then, frequency warping processing is presented, both theoretically and experimentally. Finally, it is shown that frequency warping can be used to perform environment characterization.