Determining bubble size in aquatic sediments using wideband acoustic resonance and a bubble size distribution model: testing and application in Lake Kinneret, Israel

Free gas in natural aquatic sediments exists within discrete bubbles, which contribute to sediment destabilization and have implications for global warming. In this study, a micro-scale bubble model is used to characterize the shapes and sizes of methane bubbles in muddy aquatic sediments, governed by the mechanical properties of these muds. Building on this, a macro-scale bubble size distribution model was developed to determine the maximum equivalent spherical bubble size and cumulative gas content. An acoustic methodology, which examines the frequency-dependent reflection coefficient of sound from gassy sediments and reveals resonant behavior, was used to test the proposed model. Five acoustic measurements were conducted in Lake Kinneret, Israel, – one in 2016 and four in 2022 – at water depths ranging from 23 to 37 m. The transmitted chirp signal swelling from 300 Hz to a maximum of 15,000 Hz, was received by a nearby vertical line array consisting of up to seven hydrophones positioned near the source. Frequency analysis of the recorded signal components – including both bottom and surface reflections as well as the reverberant coda – revealed a spectral notch around 2584 Hz in 2016 and between 3027 and 4373 Hz in 2022, depending on the measurement location. These notches correspond to a maximum equivalent spherical bubble diameter of 7.95 mm in 2016 and 4.50–6.54 mm in 2022. These results are consistent with direct measurements of bubble size distributions obtained through X-ray computed tomography of frozen sediment cores collected in Lake Kinneret in 2016 by another research group, at locations matching the acoustic experiment sites.