Investigating bubble transport and fate in the watercolumn with calibrated broadband split-beam echosounder data

Abstract

The transport and eventual fate of gas bubbles in the marine environment is a topic of interest to researchers in numerous fields. Acoustic systems are commonly used to study bubble ebullition sites as they provide synoptic measurements of the watercolumn. However, the visualization of individual bubbles has traditionally required the use of point-source equipment, such as vehicle-mounted cameras. Here, we present an acoustic methodology for studying individual bubbles using a calibrated broadband split-beam echosounder. The extended bandwidth (14–24 kHz) provides a vertical resolution on the order of 10 cm, which allows for the discrimination of individual bubbles in the echogram. Split-beam phase differentiation provides phase-angle data which can be used to compensate for beam-pattern effects and precisely locate bubbles within the watercolumn. Bubble target strength is measured and compared to analytical models to estimate bubble radius, and bubbles are tracked through the watercolumn to estimate rise velocity. The resulting range of bubble radii (1–6 mm in radius) is similar to those found in other investigations, and the rise velocities are consistent with published models. Together, the observations of bubble radius and rise velocity offer a measure of gas flux.