Multi-frequency imagery based on the non-linearity of propagation. Applications to characterization of the seafloor.

Classical acoustical surveying systems gather data about the seafloor usually on a narrow frequency bandwidth: a single system does not provide frequency diversity in the collected acoustic backscattered intensities. Gathering data at different frequencies has been revealed to be an interesting asset for sea floor characterization. At the present time, to collect multi- frequency information implies the use of different systems. Consequently, the acquired data are not perfectly matched in time and space, and their fusion is problematic. The feasibility of an original system that takes advantage of nonlinear propagation to achieve a multi-frequency source is presented. The principle is to generate a harmonic pulse with sufficient energy so that the saturation phenomenon emerges (Shooter et al., 1974). As a result, a single source generates a whole set of beams at the harmonic frequencies, and all these beams are perfectly superposed both in time and in space. First, a preliminary study in which a simplistic theoretical model based on a generalized Burgers’ equation is introduced. This was useful in the nonlinear propagation study and array sizing. Three sources conceived in our laboratory have been tested in pool facilities to study the multi-frequency emission feasibility. A special attention was focused on experimental development; however a comparison between numerical and experimental results is made. The research was then extended further to develop a Multi-frequency Side Scan Sonar prototype. A broadband receiver was realized within the purpose of one-day sea trials. A thorough analysis of the system and the future prospective are presented.