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Surveys to locate and monitor bubble gas seeps in Lake Baikal have been conducted mainly using the active echolocation methods since the 2000s. This paper compares the active acoustic method with a newly developed optical and passive acoustic methods using the example of the shallow-water gas seep Istok in the Selenga shoal. The optical method is based on processing of underwater video records of ascending bubbles while the passive acoustic uses spectral analysis of gas jet noise recorded using a hydrophone. Optical and passive acoustic methods enables estimation of the bubble size distribution function, which is necessary for the calculation of the bubble gas flux. The latter can be evaluated from active echolocation or optical data. By applying these methods, their possibilities and limitations are revealed as well as benefits of their combined use and ways to improve their accuracy in future.

Methane bubble fluxes in gas flares from bottom sediments in Lake Baikal were estimated for the first time using hydroacoustic methods. Earlier work has demonstrated the occurrence of gas seeps both inside and outside of areas where bottom simulating reflectors were identified in seismic profiles. Fluxes ranged from 14 to 216 tons per year, with the flux for the entire area of the central and southern basins ranging from 1,400 to 2,800 tons per year. Comparison with other water bodies showed that fluxes from the most intensive Baikal flares were similar to those in the Norwegian and Okhotsk seas. Gas hydrates decompose at the lower boundary of the gas hydrate stability zone due to sedimentation. Calculation of the amount of methane produced due to sedimentation gave a total of between 2,600 and 14,000 tons per year for the central and southern basins of the lake. Based on rough estimation, the total flux from shallow- and deep-water gas seeps is similar to the amount of methane produced due to sedimentation. This suggests that gas hydrates possibly occupy much more than 10 % of the pore volume near the base of the gas hydrate stability zone, or that there are other reasons for gas hydrate dissociation and bubble flux from these bottom sediments.

Bubbles in a subsurface ocean layer play a significant role in underwater acoustics and oceanography. Many works have been devoted to different methods of bubble density measurement. Among them, the nonlinear methods of bubble sizing based on combination acoustic scattering have a very high selectivity. These methods were utilized in several types of bubble counters developed in the Institute of Applied Physics for the last decade. In the devices the difference and the sum frequency waves of the incident ones generated by a bubble in the working volume are detected. Another approach to the problem based on the parametric generation of subharmonic of the incident wave frequency was also tested. The results of the measurements of bubble size distribution and density in the subsurface ocean layer and in the wakes of ships are given in the paper. Simulteneous measurements by different linear and nonlinear methods were done to make a comparison between them. The results of linear acoustic echosounding of a sub...

Phytoplankton plays a very important role in gas exchange in the ocean. But until now it has not been considered as acoustically significant as zooplankton. The existence of gas cavities in phytoplankton cells was suggested a long time ago but has not been detected so far because of early methods of observation. The review of recent works on acoustical investigations of gas bubbles associated with phytoplankton cells is given in this paper. Acoustical and related properties of phytoplankton suspensions were studied under laboratory conditions and in situ by several methods including acoustic 6.5‐MHz backscattering under ordinary conditions with additional compression up to 5 atm. It was observed that, under the sharp compression gas cavities dissolve and recover under the light exposition. The volume of gas cavity associated with a phytoplankton cell was found to be up to 0.3 of the volume of a cell for different alga species. Such a bubble may increase the acoustical cross section of a pure cell by 2–3 o...