The physical understanding of the vertical upwelling velocity and bottom boundary layer dispersal of a hydrocarbon seep in the Gulf of Mexico is extremely limited due to paucity of direct long-term measurements and to the time variability of the bubble plumes and boundary layer dynamics. Here we address GoMRI RFP V Theme 1: "Physical distribution, dispersion, and dilution of petroleum (oil and gas), its constituents, and associated contaminants (e.g., dispersants) under the action of physical oceanographic processes, air-sea interactions, and tropical storms" by proposing to measure the vertical upwelling velocities of hydrocarbons from sea floor gas hydrates using novel acoustic forward scatter instrumentation and to improve our understanding of dispersal processes in the bottom boundary layer by making time-series measurements of 3-D velocity and hydrographic properties near a natural seep in the northern Gulf of Mexico. More specifically, we aim to 1) measure the vertical upwelling velocity of a natural hydrocarbon seep at GC600 or GC185 and its role in vertical transport of methane and oil to the surface and 2) investigate the turbulent bottom boundary layer dynamics that causes horizontal and vertical dispersal, including resuspension of hydrocarbon-containing deposits.
Intellectual Merit: Due to a paucity of direct measurements of vertical upwelling velocities of methane bubble plumes, our understanding of the relative role of the physical mechanisms at play is limited. We propose to measure vertical velocity and plume turbulence with an acoustic scintillation instrument to obtain the first in-situ time-series of vertical transport in a bubble plume rising from a natural seep. Acoustic scintillation is a non-intrusive method that can be used to measure vertical velocities of plumes and provides a spatial average, since the transmitter and receiver are moored outside the plume creating a cone of sound that insonifies the plume at a particular depth regardless of its position along the path. It offers a unique approach to the long-term monitoring of deep hydrocarbon plumes emanating from the seafloor and has been used successfully for monitoring hydrothermal plumes. In addition to enabling quantification of vertical fluxes, the resulting time series is expected to yield novel insight into the dynamics of rising bubble plumes. The bubble plumes can induce a turbulent flow and cause vertical transport of deep waters and the acoustic scintillation system is sensitive to turbulent flow advected by the upwelling velocities. Acoustic scattering theory through a bubble plume opens up a new way to use acoustic scintillation analysis to quantify the bubble plumes at hydrocarbon seeps.
For hydrocarbons that are not transported vertically out of the boundary layer by bubble plumes, nearsource dispersal is governed primarily by energetic dynamical processes in the bottom boundary layer, including sub-inertial flows, wave motions, as well as turbulence. In order to study those processes we propose to obtain measurements of the 3-dimensional velocity field as well as acoustic backscatter in the BBL with a bottom-mounted ADCP, augmented by temperature and salinity measurements at several levels in the BBL to quantify vertical gradients. In addition to allowing quantification of advective and eddy-diffusive heat, salt and density fluxes, the BBL data are expected to show evidence for sediment resuspension and provide novel insights into the dynamics in BBLs.
Broader Impacts: The new measurements collected in the context of the proposed project provide a useful complement to the measurements taken during ECOGIG-2. The proposed project includes training for a post-doctoral researcher. To date there is nobody that makes use of the acoustic scintillation method to monitor vertical velocities of plumes in the deep sea. The development of this custom instrumentation was achieved with collaboration with industry. Over $400,000 has been invested in the development and application of this self-contained (internally logging) and battery operated system. Expanding the instruments use to monitor hydrocarbon plumes over a long period of time would open up a whole new application.
Project Research Overview (2016):
An overview of the proposed research activities from the GoMRI 2016 Meeting in Tampa.
Direct link to the Research Overview presentation.