Estimation of gassy sediment parameters in shallow water using shipping noise signals in frequency domain near cut-off

Abstract

Modal structure of shipping noise in a shallow water waveguide is studied experimentally and theoretically. Experiments were carried out in the Sea of Galilee (Lake Kinneret, Israel) having maximal depth of 40.5 m and remarkable concentration of methane bubbles in the upper sedimentary layer. Receiving system consisted of 27-m vertical line arrays (VLAs) with ten hydrophones at each. They spanned a lower part of the waveguide 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. Modal filtration technique on two synchronized VLAs with a phase correction was proposed to extract complex eigenvalues of each mode, including modal attenuation, which strongly increase in the proximity of cut-off frequency for all normal modes. Based on frequency dependences of attenuation coefficients of lower modes, sound speed in gas-saturated bottom layer was estimated. For application of this technique, there is no need to know exact coordinate and speed of noise source, which seems to us to be an advantage of the method. [Work was supported by RFBR, Grant No. 20-05-00119.]