Application of a Passive Acoustic Method for Detection and Estimation of Shallow-Water Bubble Gas Emissions

Resumo

A passive acoustic method for estimating the flow of underwater bubble gas emissions for shallow water areas has been proposed and tested. The method is based on the connection between the frequency of the acoustic signal produced by a gas bubble when it separates from the underwater outlet channel and the size of the bubble. In the conducted laboratory experiments acoustic signals with frequencies in the range from 2.7 to 0.4 kHz were recorded during generation of bubbles with a size from 2 to 15 mm within the fluid. The analysis of acoustic recordings made near existing shallow-water seeps in Laspi Bay showed a series of short audio signals produced by the released methane bubbles of 0.5–2s duration, grouped into packages containing approximately ten pulses. For the two investigated seeps, the frequency peaked at 1 and 1.4 kHz. According to a theoretical estimate, the bubbles generating such a signal are 7 and 5 mm in diameter, respectively. Taking the intensity of the bubble discharge into account, the calculated gas flows were 40 and 6 liters per day, respectively.