Acoustic detection of gas bubbles in saturated sands at high spatial and temporal resolution
Limnol. Oceanogr. Methods 10:129-141 (2012) | DOI: 10.4319/lom.2012.10.129
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 transmitter-receiver 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.