Hark the Herald Bubbles Ring: A Direct Assessment of Bubble-Mediated Methane Flux Using Wideband Acoustics

Elizabeth Weidner
Master of Science Defense

CCOM/JHC

Thursday, Apr. 19, 2018, 8:00am
Chase 130
Abstract

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 split-beam echosounder. The extended bandwidth (16 - 26 kHz) affords vertical ranges resolution of approximately 7.5 cm, allowing for the differentiation of individual bubbles in acoustic data. Split-aperture processing provides phase-angle data used to compensate for transducer beam-pattern effects and precise location of 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) is similar to 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.

Bio

Elizabeth Weidner graduated from the University of Washington in 2012 with a B.S. in Oceanography. Before coming to CCOM/JHC, she worked as a geophysicist for C&C Technologies. As an McS student, she is interested in acoustical oceanography and the ocean carbon cycle. Her thesis is focused on quantifying marine seep gas flux using calibrated broadband split-beam echosounder data.