The in‐phase (I) and quadrature (Q) components of the seafloor backscattered acoustic returns received by a Sea Beam multibeam echo‐sounder over a manganese nodules field in the tropical northeastern Pacific have been digitized and recorded on magnetic tape. Because both theIandQcomponents of the signals were present, the sidelobe interference inherent in multibeam systems [C. de Moustier, J. Acoust. Soc. Am.79, 316–331 (1986)] was able to be removed, and a complete angular dependence function of acoustic backscattering between ± 20° of vertical incidence was obtained. These results and the sidelobe interference removal method are discussed. [Research funded by ONR.]