Acoustic signatures of gassy sediments in two subtropical lakes - Lake Kinneret (Israel) and Lake Biwa (Japan)

Methane is common in organic-rich marine and freshwater sediments. Escape of this gas via ebullition is the most efficient mechanism of methane transport from the shallow sediments to the atmosphere. Therefore characterization and quantification of free gases in surface sediments is an important ecological issue. High spatial heterogeneity of methane distribution does not allow using conventional sampling methods for representative quantification of sedimentary gases over large areas, such that development of remote-sensing methodology is required to investigate gassy sediment distribution. In this paper we compare the results of acoustical sampling to study sound scattering in soft gassy sediments in two deep subtropical lakes-Lake Kinneret and Lake Biwa. Acoustic samplings were carried out using 70 kHz and 120 kHz single beam echosounders along standard transects. To determine the typical acoustic features of gassy sediments we conducted echo envelope parameterisation using different spectral, wavelet, fractal, statistical and energy parameters. A number of parameters appeared to be sensitive to the presence free gases in sediments. Comparison of the results obtained on two lakes allows detecting the effect of sediment type, its location, and water level change on acoustic properties of the top layer of gassy sediments.