Abstract: Observations of seabed bubbles of methane, an important greenhouse gas, at a natural marine hydrocarbon seep to evaluate the global methane budget per year is very important. The objective of this paper is to develop a new approach, in which the configuration and distribution of bubbles are adjusted before flow measurement, to measure cold seepage flow on seafloor by the wave form-amplitude parameters of acoustic transmission. After the spectrum analysis we lead to the height of the measurement window through which acoustic wave transmits to the opposite wave transducer is less than the distance which bubbles go up pass per second. And the bubble speed can be effectively measured by using the acoustic sensing system designed under the proposed height of the measurement window in the paper. The relationship between the bubble diameter and the measured acoustic wavelength is obtained under the Born approximation. Diameters of bubbles become smaller than 3 mm after they go through the honeycomb-core board and other structure parts, and the distribution of bubble populations is uniform. Spatial frequency spectrum analysis is applied to establish the relationship between geometry parameters of the piezoelectric transducer and the motion of bubbles, and the measurement accuracy of bubble velocity is expected to ± 2%. The nearly linear relationship between the amplitude of sound transmission and the gas bubble flow has been established through the experiment. It turns out to be an effective method of measuring the bubble flow.

Key words: hydrocarbon seeps on seafloor, bubble flow measurement, sound wave, flow pattern adjustment