Evidence of low-frequency multiple scattering of methane gas bubbles at Coal Oil Point, Santa Barbara, California

Abstract

Coal Oil Point, located in the Santa Barbara Channel, CA, is known for its prolific natural hydrocarbon seepage. These hydrocarbons are released at depths between 30 and 80 m, shallow enough that the hydrocarbons reach the surface. Transport via bubbles and droplets through the water column enables the exchange of hydrocarbons between the ocean and the atmosphere. Hydroacoustic observations from previous surveys have been used to map hydrocarbon seep distributions and estimate the flux of hydrocarbons from the seafloor in Coal Oil Point. On September 14, 2016, this area was revisited and surveyed using low-frequency sub-bottom profilers operating from 1 to 10 kHz. The goal is to estimate the gas flux of hydrocarbons from the hydroacoustic measurements and compare them to historic estimates for this area. Visual observations of dense bubble cloud structures of these seeps were seen and evidence of multiple scattering effects were observed in the hydroacoustic measurements. We examine how acoustic scattering of natural hydrocarbon gas bubbles is effected by multiple scattering and how this process affects the quantification of gas flux.