Estimation of parameters of gassy layer in sediment in shallow water using measurement of angular dependence and spectrum of reflection coefficient of wide-band signals

Abstract

Method of estimation of gassy layer parameters in sediment (sound speed and thickness) on the base of measurement of angular and frequency dependencies reflection coefficient of wideband signals is presented. Experiments were carried out in Lake Kinneret (Israel), which is characterized by remarkable organic content in sediment producing methane bubbles. Direct chemical, biological analysis and usage of frozen cores show comparatively narrow layer (a few tens of cm) changing in dependence on season. Principal point for acoustical method is low sound speed in bubble layer (up to 300–400 m/s) and big reflection coefficient from the corresponding half-space (0.6–0.7 for vertical incident). Due to narrow gassy layer, there should be half wavelength resonances at the corresponding frequencies. In experiments, wideband signals were used (500–2500 Hz) and receiving system: single hydrophone (1 m from source) and vertical line array. Estimation of layer’s parameters was carried out using measured reflection coefficient as a function of frequency and angle of reflection. Resonance frequencies were 800 Hz and 1600 Hz. The corresponding analysis of data give values of thickness 25–30 cm, with the sound speed in layer 400–450 m/c. Results are compared with experimental data obtained by direct measurements. [Work was supported by ONRG and ISF.]