Variability of Doppler shift in the ocean bottom reverberation data from the East Sea of Korea

Abstract

Sea trials measuring the ocean bottom reverberation using an active towed array sensor (ATAS) equipped with triplet receiver array were conducted in the East Sea of Korea during August, 2015. The continuous-wave waveforms with center frequency 3 kHz and pulse lengths of 0.1, 0.3, and 1 s are used as source signals, respectively. Ship velocity was between 4 and 5 knots and the water depth varies from 500 to 1250 m. Also, sound speed profiles were obtained by expendable bathythermographs (XBTs) during the ship maneuvering, where the depths of minimum sound speed appeared between 208 and 306 m. From the measurement results, we observed variability and spreading of reverberation Doppler shifts, which depends on the bottom bathymetry, platform motion, source depth, and receiver array depth. In particular, significant asymmetry of the Doppler spectrum occurred at the depths of the source and receiver below 120 m. The instantaneous frequency Doppler of reverberation signal in each beam direction provides information on the bathymetry and scattering characteristics of the bottom. This frequency shift is traced in time and analyzed.