Bottom backscattering strengths at low grazing angles using adaptive beamforming

Bottom backscattering strengths have been measured between 70 and 1000 Hz using broadband, omnidirectional SUS charges as sources and a horizontal line array as a receiver in the Critical Sea Test (CST) series of at-sea tests. When conventional beamforming is used to form beams in this geometry, bottom fathometer returns often contaminate the backscattering results for grazing angles below about 30 deg. The use of an adaptive beamformer (ABF) combined with examination of near-endfire beams allows scattering strengths to be reliably determined down to considerably lower grazing angles in many cases, over the entire range of frequencies measured. The ABF technique and its parameters and limitations will be discussed briefly. Examples of low-grazing-angle backscattering-strengths results will be shown for a variety of sites visited during various CST tests. [Work supported by ONR.]