Seeps found offshore in the East Siberian Arctic Shelf may mark zones of degrading subsea permafrost and related destabilization of gas hydrates. Sonar surveys provide an effective tool for mapping seabed methane fluxes and monitoring subsea Arctic permafrost seepage. The paper presents an overview of existing approaches to sonar estimation of methane bubble flux from the sea floor to the water column and a new method for quantifying CH₄ ebullition. In the suggested method, the flux of methane bubbles is estimated from its response to insonification using the backscattering cross section. The method has demonstrated its efficiency in the case study of single- and multi-beam acoustic surveys of a large seep field on the Laptev Sea shelf.

The results of acoustic investigations of over 250 gas “flares” (GFs) located in the depth range of 160–1400 m in the vicinity of the northeastern slope of Sakhalin Island are presented. Pronounced features in the GFs distribution by the seabed’s depth have been identified, which allowed dividing them into four groups corresponding to four depth intervals. The number of GFs per seabed unit area decreases, and the power of the individual GFs increases with depth. The average methane flux per seabed unit of area has been estimated for each depth interval. It has been demonstrated that the total quantity of methane emitted from the seabed in the form of bubbles in the studied area is ∼0.1 million tons per year.

A remote acoustic method is presented for estimating the methane flux into water from rising bubbles. With the proposed method and data of hydroacoustic measurements in the Sea of Okhotsk, the dissolved methane concentration profile that forms in the water column in regions of methane bubble release is estimated. Comparison of the aforementioned methane concentration profile with results of direct measurements shows good quantitative and qualitative agreement. This testifies to the fair accuracy of the proposed acoustic method and is indicative of the dominant role of bubble transport in the formation of the concentration of dissolved methane in the water column in such regions.

The East Siberian Arctic Shelf (ESAS) is a key area of CH₄ venting in the Arctic Ocean. The ESAS region stores more than 80% of the world’s predicted subsea permafrost and associated permafrost-related gas hydrates. Gas emissions from subsea permafrost are controlled by its current thermal state, which, in turn, depends on environmental factors. The aim of the manuscript is to show that the thermal state of subsea permafrost and phase transitions of its pore moisture can be estimated remotely by echo soundings, which can resolve the structure of shallow bottom sediments. It has been found that the duration of the seabed acoustic response (echo duration, Δ) at frequencies of 50 and 200 kHz correlates with sediment temperatures and generally increases with cooling below 0.5 °C. This correlation, explained by assuming a layered structure of the bottom sediments, establishes the basis for high-frequency acoustic thermometry. The technique is an advantageous tool for many applications: fast contouring of low-temperature zones, remote measurements of seabed surface temperature, and estimation of the thickness of frozen sediments near the bottom. The latter estimates have implications for the distribution of subsea permafrost and the stability of gas hydrates on the Arctic shelf.

Relevance. Gas flares or seeps consisting of bubbles continuously rising from the seabed have been recorded throughout the World Ocean at depths ranging from several meters to three kilometers or more. Measurements of the gas composition of the rising bubbles shown that they are dominated by methane (CH4). The East Siberian Arctic Shelf contains more than 30% of the world CH4 and carbon dioxide reserves, preserved in bottom sediments by underwater permafrost. In the shallow seas of the East Siberian Arctic Shelf, the main mechanism for transporting CH4 from bottom sediments into water is bubble transport. Therefore, it is extremely important to estimate the amount of CH4 transported by rising bubbles from bottom sediments into the water column and the atmosphere. Aim. Estimation of CH4 quantity transported by chains of rising bubbles from the seabed to the atmospheric surface layers. Methods. The manuscript presents a study of the mechanism of gas exchange between rising bubbles and a liquid column, carried out using a specially designed stand that allows simulating local upwelling. Results and conclusions. The paper shows that chains of bubbles coming from the seabed with intensities of ~40 ml∙min–1 and ~110 ml∙min–1, taking into account the hydrostatic pressure, deliver 206 mg∙min–1 and 616 mg∙min–1 of CH4 to the bottom layer of the water column, respectively. The results obtained during laboratory studies allowed us to specify the flux of CH4 from bottom sediments to the atmospheric surface layers. Taking into account the quantity of gas exchange and local upwelling, the amount of methane delivered to the atmospheric surface layers was 69 mg∙min–1 and 286 mg∙min–1. The paper presents an acoustic estimate of the amount of CH4 transported by the seep, which includes the considered chains of rising bubbles. According to calculations, the flux of CH4 into water from this area in 2012 was ~40 g∙min–1. At the same time, the amount of CH4 transported by this seep into the atmospheric surface layers, taking into account local upwelling and gas exchange occurring between the rising bubbles and the liquid column, is ~24.5 kg per day or ~9 tons per year.

This paper presents the results of an acoustic survey of barite mineralization in the Deryugin Basin (Sea of Okhotsk) performed in 2002 and 2003 using echo sounders at operating frequencies of 12 and