ASSESSMENT OF 3-D EFFECTS IN THE KEY WEST LWAD EXPERIMENT

The role of 3-D effects has long been debated in the ocean acoustics community. Typically, 3-D problems are handled in an approximation that uses a fan of bearing lines to build up a 3-D image of the acoustic field with many 2-D slices. This is the so-called N by 2-D approach and it is probably safe to say it is adequate for simple calculations of the SONAR range-of-the-day. On the other hand, acoustic models are now finding an important role in matched-field processing (MFP) where they are integrated with the signal processing to correct for the channel effects. These applications demand much more from the models but, somewhat surprisingly, recent broadband experiments have revealed that the models are up to the task. The main features of the echo patterns are well-predicted by such models and can be reliably exploited, at least in areas where the ocean bottom is fairly flat. The challenge now is to push MFP to more realistic scenarios with complicated topography and potential 3-D effects. One phase of the December 1997 Key West LWAD (Littoral Warfare Advance Development) experiment was designed to address exactly this issue. We use this Key West scenario as a backdrop to discuss modeling approaches combining beams and modes and the impact of the resulting 3-D variation on MFP for towed arrays.