Acoustic and optical determination of bubble size distributions – Quantification of seabed gas emissions. (June 2021)

• Record Type: Journal Article
• Authors: Li, Jianghui; White, Paul R.; Roche, Ben; Bull, Jonathan M.; Leighton, Timothy G.; Davis, John W.; Fone, Joseph W.
• Is Part Of: International journal of greenhouse gas control. Volume 108(2021)
• Publication Date: 2021-06
• Subjects: Bubbles -- Identification -- Underwater acoustics -- Greenhouse gas -- CO2 -- Marine Carbon Capture and Storage -- CCS; Greenhouse gases -- Environmental aspects -- Periodicals; Air -- Purification -- Technological innovations -- Periodicals; Gaz à effet de serre -- Périodiques; Gaz à effet de serre -- Réduction -- Périodiques; Air -- Purification -- Technological innovations; Greenhouse gases -- Environmental aspects; Periodicals; 363.73874605
• DOI: 10.1016/j.ijggc.2021.103313
• Languages: English
• ISSNs: 1750-5836

Abstract

Highlights: Passive acoustic techniques can be used to identify and quantify underwater gas release. We propose an adaptive single bubble identification technique, incorporating bubble acoustic characteristics of time, frequency and strength. In a controlled gas release experiment, the bubble radius is acoustically determined as 0.15–0.3 cm, and optically determined as 0.2–0.5 cm. The gas flux is acoustically estimated as 32–88 kg/day in response to a known gas injection flow rate 143 kg/day. Abstract: Passive acoustic techniques can be used to identify and quantify underwater gas release at natural sites, or at locations related to anthropogenic activities. There are still significant issues in extracting bubble signals from background noise, particularly for bubble counting and sizing techniques relying on inversion of the time-averaged acoustic spectrum. In this work we propose an adaptive single bubble identification technique, which incorporates bubble acoustic characteristics including pulsation time interval, frequency bandwidth and radiation strength. The method applies a cross-spectrogram, enabling an increase in signal-to-noise ratio resulting in a reduction of the false alarm rate on bubble identification. We demonstrate this technique using an array of hydrophones to determine the bubble size distribution and gas flux at a controlled CO2 release site, 4 m beneath the seabed, at 120 m water depth in the central North Sea. The results show that the bubble radius, as estimated using acoustics has a distribution with a peak in the 0.15–0.3 cm range, while an estimate based on optical method suggests a range of 0.2–0.5 cm. The gas flux is acoustically estimated as 32–88 kg/day in response to a known gas injection flow rate 143 kg/day, indicating 22–62% of the injected CO2 is emitted from the seabed in gaseous form, with the remainder being trapped, or dissolved.