Investigating interference patterns in bistatic bottom reverberation through two-dimensional cross-term analysis

As a major source of interference in bistatic sonar systems, bistatic ocean bottom reverberation has gained considerable attention in recent years. The beam-time response of bistatic reverberation reveals interference stripes that follow distinct patterns, potentially misleading target detection and necessitating a thorough analysis of their underlying mechanism. These interference stripes are generated by mode coupling between propagating modes along both the incident and scattering paths. While mode coupling has been extensively studied for propagation, reverberation involves a complex two-way propagation process that cannot be effectively analyzed using a one-way propagation model. Therefore, this paper introduces a two-dimensional (2-D) cross term analysis. The "2-D" refers to the coupling not only within the incident and scattering paths but also between the incident and scattering modes. An analytical formula is derived to predict the positions of bright stripes, enabling the investigation of how the sound speed profile and bathymetry affect the interference structure. The proposed 2-D cross term analysis is validated using bistatic reverberation data collected over the continental shelf, showing that variations in interference stripes align well with predictions from the 2-D cross term theory.