The propagation of broadband acoustic waves in shallow water channels is a complicated phenomenon that depends critically on the attributes of the water column and bottom structure, and it is characterized by the interference between propagation modes. In the far field, multiple interactions of the acoustic wave with the boundaries of the channel result in signal attenuation and multipath propagation which makes the interpretation of sonar signals difficult. For the passive case, it has been shown that the time-frequency structure resulting from this propagation can be explained with the principle of a waveguide invariant and that this description does not require precise knowledge of the propagation environment. The invariance principle has been successfully applied to the interpretation of passive sonar lofargrams where the signal travels a single path between the source and the receiver, but little work has been done for active sonar applications in which the acoustic wave follows a bistatic path. In this paper, experimental results from the Shallow Water Active Detection/Classification (SWAC) project are presented. Frequency analysis of the acoustic data reveals the presence of striations similar to those found in passive sonar lofargrams and this suggests the existence of a bistatic invariance principle. Physics-based simulations using the measured sound speed profile are also presented and they are in agreement with the observed results. The identification of a bistatic invariance principle for active sonar could lead to the implementation of robust processing methods and simultaneously decrease the complexity of tracking algorithms by adding constraints in the search domain.
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