Echo Returns as a Function of the Mass and Elasticity of a Target

Abstract

Three classical targets were ensonified by underwater explosives; (1) a buoyant 36-in-diam hollow sphere with a thin wall; (2) a 17-in.-diam solid iron sphere; and (3) an almost neutrally buoyant 17-in.-diam hollow sphere with a thick wall. The echos from the air-filled thin-walled hollow sphere and the solid sphere were as expected. The thin-walled sphere returned a short time pulse whose polarity was negative (i.e., inverted) in the frequency range of interest. The solid iron sphere returned a positive pulse. The study was undertaken to determine the return from the thick-walled hollow sphere. Since a compliant thin-walled sphere returns a negative pulse and a solid stiff sphere returns a positive pulse, the return from a target with a construction in between was expected to be complex with no definable polarity in a broad-frequency spectrum whose upper cutoff is limited. The results show that the return is made up of several frequencies corresponding to at least 2 modes of vibration. The initial polarity is obscured in the noise.