Targets encountered during synthetic aperture sonar (SAS) surveys may exhibit elastic scattering behavior and re-radiate sound after initial interrogation. These re-radiated returns are often described as “late-time” energy, as they reach the sonar after the initial geometric returns. The range-general image reconstruction methods commonly used in SAS surveys do not properly account for late-time returns, causing late-time components to appear out-of-focus and away from the target’s true location. This presentation will discuss the development of a late-time focused reconstruction algorithm and its application to sonar surveys for unexploded ordnance (UXO). The proposed late-time focused method allows for selective application of range-general and range-specific reconstruction and removes far-field assumptions, both of which are improvements over previously developed techniques. Starting with modeled data, then data from an in-air sonar, and finally in-water data from a fielded sonar system, this work will demonstrate the qualitative differences between range-general imagery and late-time focused imagery. When using late-time focused reconstruction, late-time returns from resonant targets appear better-defined, in a spatially-tighter region near the target. Future work will substantiate the observed qualitative improvements with numeric image quality metrics and focus on applications to automatic target recognition.

This content is only available via PDF.