Research paperMegaplume bubble process visualization by 3D multibeam sonar mapping

- Multibeam sonar used to derive high resolution 3D visualization of a marine megaplume.
- First detailed 3D image of marine megaplume downwelling and intrusion layer formation.
- Evidence supportive that currents help stabilize plumes against massive detrainment.
- Multibeam sonar has applications including better modeling seep methane fate.
- Intrusion layer forms above thermocline, elevated around the plume by upwelling flow.

MultiBeam echosounder data were collected during a surface-ship survey of the 22/4b well site in the North Sea in September 2011 using a Teledyne-Reson 7125. Modern multibeam echosounders are instrumental in providing detection and accurate localization of weak to strong bubble plumes. Two survey profiles effectively insonified the bubble plumes rising from the main crater at the well site, providing snapshot data of bubble plume processes. Additionally, three profiles insonified bubble plumes rising from, in, and to the south of a secondary crater, 1.2 km southeast of the main crater. Data processing included a simple algorithm that muted mislocated echoes from incomplete sidelobe suppression. The data processing produced a Cartesian volume of echo intensity from the water column and seabed.Plume geometry was analyzed to investigate a number of important large-scale plume processes, including plume bubble detrainment due to currents and stratification, downwelling flows, sea surface interaction, plume heterogeneity, and other fluid transport processes. The data showed strong upwelling flows, with bubble vertical motions generally much faster than currents. One important finding was that megaplumes create intrusions above the general thermocline, in part because their extensive upwelling flow lifts the thermocline locally. As a result, the intrusion layer deposits dissolved gases in the upper wave-mixed layer of the water column where it is not isolated from the atmosphere, unlike dissolved gases in the lower water column.The analysis shows that high fidelity multibeam echosounder data can provide a wealth of remote sensing information on bubble plume characteristics and processes, with important applications, including blowout monitoring and response, better understanding of megaplumes such as used in lake destratification, and improved characterization of natural seep emission processes.