Feasibility of classification of the materials through Laser-Induced Underwater Acoustic Signatures

Abstract

This study is aimed at the classification of materials such as metals (Al, Cu, Brass, and SS), ceramic (Glass, BK-7), polymers (Nylon, Teflon, PVC, Acrylic, and Foam) in the planar geometrical form, using laser-induced underwater acoustic signatures (LIUAS) both in temporal and spectral domains. In the time domain, classification of the materials is based on acoustic energy of LIUAS as a function of laser energy, and the thickness of the plate. The LIUAS is comprised of direct and reflected signals from the water-air and water-solid interfaces. In the frequency domain, the distinctive instantaneous frequency (IF) profiles of various materials are obtained from short-time Fourier transform (STFT). The STFT reveals that the LIUAS has a broad acoustic spectrum spanning from 10 - 800\u2005kHz, perpendicular to the laser propagation direction. From the STFT, we observed the direct signal has two IFs around 100 and 690\u2005kHz, and the reflected signal has only one IF at 100\u2005kHz. The reproducibility of LIUAS was confirmed by analyzing the direct acoustic signals from 20 laser pulses and their interactions with the SS target. The reflected signals from different target materials confirmed that the LIUAS plays a prominent role in the classification of underwater targets.