Low grazing-angle sea surface reverberation patchiness and statistics observed with a high-frequency multibeam sonar

Sea-surface reverberation, and in particular backscatter from near-surface bubbles, can significantly limit high-frequency (>10 kHz) active sonar performance. While accepted models for near-surface bubble backscatter exist, these have generally assumed horizontally uniform bubble layers. This study presents measurements using a horizontally-oriented 90 kHz multibeam sonar from a moving ship that show significant spatial variability in low grazing angle sea-surface reverberation. The surface scattering strength and scattering amplitude statistics under sea states 3 to 4 conditions are investigated. The time- and spatially-averaged background reverberation levels were in moderate agreement with well-known bubble layer models. However, the instantaneous backscatter amplitudes exhibited localized patchiness and non-Rayleigh statistical distributions, in agreement with either a K-distribution or a Rayleigh-K mixture model. This variability was attributed to scattering patchiness, bubble-induced extinction and refraction, and near-surface processes such as Langmuir circulation. K-distribution shape parameters from 4 to 15 were estimated from the sonar data, increasing with range as predicted by an effective scatter density model. These low shape factors lead to a larger probability of false alarm than might be predicted using Rayleigh models.