A wideband acoustic method for direct assessment of bubble-mediated methane flux

The bubble-mediated transport and eventual fate of methane escaping from the seafloor is of great interest to researchers in many fields. Acoustic systems are frequently used to study gas seep sites, as they provide broad synoptic observations of processes in the water column. However, the visualization and characterization of individual gas bubbles needed for quantitative studies has routinely required the use of optical sensors which offer a limited field of view and require extended amounts of time for deployment and data collection. In this paper, we present an innovative method for studying individual bubbles and estimating gas flux using a calibrated wideband from the Bolin Centre for Climate Research database: http://bolin.su.se/data/.and split-beam echosounder. The extended bandwidth (16 - 26 kHz) affords vertical range resolution of approximately 7.5 cm, allowing for the differentiation of individual bubbles in acoustic data. Split-aperture processing provides phaseangle data used to compensate for transducer beam-pattem effects and to precisely locate bubbles in the transducer field of view. The target strength of individual bubbles is measured and compared to an analytical scattering model to estimate bubble radius, and bubbles are tracked through the water column to estimate rise velocity. The resulting range of bubble radii (0.68-8.40 mm in radius) agrees with those found in other investigations with optical measurements, and the rise velocities trends are consistent with published models. Together, the observations of bubble radius and rise velocity offer a measure of gas flux, requiring nothing more than vessel transit over a seep site, bypassing the need to deploy time-consuming and expensive optical systems.