Estimation of gassy sediment parameters in shallow water using correlation functions of shipping noise

Results of experimental research of low frequency (<500 Hz) shipping noise modal structure in shallow water waveguide with gassy sediments are presented. Experiments were carried out in the Sea of Galilee (Lake Kinneret, Israel) having maximal depth ~ 40.5 m and remarkable concentration of methane bubbles in the upper sedimentary layer. Moving R/V "Hermona" was used as a low frequency noise source with a wideband spectrum. As a receiving system, two 27-m vertical line arrays (VLAs) with 10 hydrophones at each were deployed. They spanned a lower part of the water column with interval ~3 m. The VLAs were fixed in the centre of the lake with the distance ~40 m between them. R/V "Hermona" was moving along a straight line connecting VLAs at the range of up to 1 km from the arrays. Mode filtering procedure using two synchronized arrays is proposed to extract complex eigenvalues of each mode, i.e., including modal attenuation. The extracted attenuation coefficients demonstrate a considerable increase in the neighbourhood of cut-off frequency for all normal modes, revealing that the sound speed in the bottom is much less than that in water, and all modes are leaky.