Predicting the acoustic response of targets in an ocean environment based on modal analysis of finite element calculations

Abstract

Low frequency sound is a viable means for the detection of elastic targets in contact with the ocean floor. The incoming sound, with wavelengths on the order of the target dimensions, can excite resonant modes of the target leading to enhancements in the scattered field. A hybrid model has been developed to predict the acoustic scattering from cylinders, pipes and unexploded ordnance (UXO) in proud or buried configurations in the ocean sediment. The model exploits the symmetry by decomposing the 3-D problem into a sum of 2-D independent Fourier modal sub-problems. This hybrid modeling technique has been shown to agree well with experimental measurements conducted in a pond [A.L. España et al., J. Acoust. Soc. Am. 130, 2330 (2011)]. Presently, these hybrid model results are used to examine the target response on a mode-by-mode basis. A modal map is generated by keeping track of the number of dominant modes contributing to the bright features observed in the acoustic template. For features that are predominantly due to one or two modes, simple analytical models can be used to predict their evolution as a function of target/sensor geometry within the ocean waveguide. [Work supported by ONR and SERDP.]