Acoustic detection of gas bubbles in saturated sands at high spatial and temporal resolution

Abstract

Small gas bubbles embedded near the surface of submerged sand affect physical, chemical, and biological processes, and so their detection and characterization is important. So far, a noninvasive method for the detection of such bubbles has not been available. We developed a portable acoustic detection technique that offers high spatial and temporal resolution. The measuring system consists of a hand‐held ultrasonic transmitterreceiver that we operated with transducers generating 1 MHz and 2.27 MHz pulses. Bubble detection was based on changes in amplitude and travel time of backscatter data collected from bubbles that were either manually injected into sediment or created by sedimentary microalgal photosynthesis. Laboratory experiments showed that individual bubbles of 6 mm diameter can be detected as deep as 60 mm beneath the sediment‐water interface. Vertical and horizontal resolutions of 0.4–1 mm and 8–13 mm were achieved, depending on the transducer used. Measurements in a coastal bay demonstrated that our technique discriminates between sands with and without bubbles and locates embedded gas. We found that gas accumulations develop in the upper 10 mm of shallow sandy sediment in shallow water during daytime, revealing that the combination of photosynthetic oxygen production and porewater transport in permeable sands can lead to bubble generation below the photosynthetically active layer.

Impact of Microphytobenthos Photosynthesis on the Characteristics of the Echo Signal from Baltic Sandy Sediments

“…The impact of the microphytobenthos photosynthesis on the acoustical backscattering properties of the Atlantic sandy sediments was previously demonstrated by Holliday et al (2004) and Wildman and Huettel (2012).…”

“…Bearing in mind the prior observations of diel oscillations of the backscattered energy due to the microphytobenthic photosynthesis (Holliday et al, 2004;Wildman, Huettel, 2012), the collected data were processed to reveal the diel fluctuation of the reverberation from the sandy bed. The example of acoustical echo (at 250 kHz) from the sediment-water interface is demonstrated in Fig.…”

“…Approximately 50,000 bubbles m −2 were common at study site in St. Joseph Bay, FL, in the research conducted by Wildman and Huettel (2012), and these bubbles significantly affect the sound propagation and scattering (Leighton, 1997;Medwin, 2005; Ainslie, Leighton, 2011). Therefore, the understanding of the benthic photosynthesis influence on the backscattering properties of the marine sediments under different local environmental conditions is a crucial task.…”

“…The physics of sound scattering from benthic animals and plants was studied mainly theoretically (numerical modelling) by Shenderov (1998), Stanton et al (2000), Stanton (2000), Stanton and Chu (2004), Gorska and Kowalska (2012). The impact of the benthic photosynthesis on the sandy sediment acoustical backscattering properties was studied both in the laboratory (Holliday et al, 2004;Wildman, Huettel, 2012) and in situ (Hermand, 2003;Wildman, Huettel, 2012) as well. The key finding was that the photosynthesis of the microphytobenthos (microscopic algae and cyanobacteria inhabiting the seafloor; Mac- Intyre et al, 1996) can significantly influence the echo signal characteristics (Holliday et al, 2004;Wildman, Huettel, 2012).…”

“…The impact of the microphytobenthos photosynthesis on the acoustical backscattering properties of the Atlantic sandy sediments was previously demonstrated by Holliday et al (2004) and Wildman and Huettel (2012).To account for the sensitivity of the hydroacoustical classification techniques to the backscattering properties of local marine sediments, it is important to understand the microphytobenthos photosynthesis impact for the Baltic Sea where the techniques are being actively developed now. This is the main motivation of the paper.…”