Estimating bubble size in Lake Kinneret through reflection of wideband sound signals from gas-rich sediment environment

Gas-rich sediments cause permanent concern due to their contribution to sediment destabilization and global warming. In this paper, an effective media theory of gassy sediments previously suggested by the authors is tested experimentally in Lake Kinneret using wideband acoustic signals. Results of experiments and the corresponding acoustic data processing is presented. Ten 5-s long wideband (0.3-15 kHz) chirp pulses were radiated by an underwater transducer mounted directly at a 30-m long seven-channel vertical line array (VLA) deployed in the central part of the lake (the seafloor depth is 35 m). Received sound field timeseries consists of a sequence of pulse arrivals comprised of specular reflections from interfaces followed by reverberation codas caused by non-specular scattering from the interface roughness and volume inhomogeneity. Having studied the frequency dependence of the signal reflected from the bottom, a dip in the reflection coefficient was found at frequencies of 4-6 kHz. This suggests the existence of bubbles with an effective spherical diameter of about 3 mm, which is consistent with previous direct measurements of bubbles in the lake sediments.