Vertical upwelling and bottom-boundary layer dispersal at a natural seep site

In January 2016, Dr. Daniela Di Iorio at the University of Georgia was awarded an RFP-V grant of $1,204,902 to lead the GoMRI project entitled, Vertical Upwelling and Bottom-Boundary Layer Dispersal at a Natural Seep Site which consisted of 1 collaborative institution and approximately 5 research team members. 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, near-source 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. Research Highlights As of December 31, 2019, there are 1 peer-reviewed publication, 2 pending peer-reviewed publications, 6 scientific presentations, and 4 datasets being submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are to be made available to the public. Significant outcomes of this project's research according to GoMRI Research Theme are highlighted below. Theme One: Publications and highlights: Razaz, M., Di Iorio, D., Wang, B., Daneshgar Asl, S., & Thurnherr, A. (2019). Variability of a Natural Hydrocarbon Seep and its Connection to the Ocean Surface. Nature Scientific Reports (in press). The extensive field survey carried out in 2017 in GC600, GoM, consisted of video observations of seep source physical conditions, acoustic Doppler measurements of near-bottom horizontal flows, acoustic forward scatter measurements of the vertical flow, and acoustic backscatter measurements of the water column and seafloor. Full water column horizontal currents were obtained from the nearest NDBC station 42369. These data sets were used to model the bubble rise through the water column for comparison to sea surface oil slick observations obtained from Sentinel-1A SAR satellite. Video time-lapse camera (VTLC). Two VTLCs were positioned at approximately 60 and 90 cm from the seep by the ROV Maxx on Sept 3, 2017. VTLC-A recorded 10 s of data every 3 h during Sept 3-28, 2017 and VTLC-B logged 15 s of data every 6 h over the period of Sept 3, 2017-Feb 2, 2018. A short video made from all the last 10 frames (1/3s) of each of 568 video bursts recorded by the VTLC-B exhibits day-to-day fine-scale spatial and temporal variability of the source over the campaign period. The records indicate that the seafloor and the seepage were very dynamic on daily to monthly time scales in terms of bubble rise velocities, mean bubble diameter and source emission rate. Acoustic Scintillation Flow Meter (ASFM). The ASFM, deployed in GC600 such that the acoustic propagation paths intersected the bubble plume at ~20 mab. The 100 μs transmit pulses with central frequency of 307 kHz were emitted sequentially with 25 ms separation at 10 Hz sampling frequency. The instrument logged data in burst sampling mode, 15 min every hour. Application of toroidal transducers with a 10° beam width ensured that the sound is propagated through the plume, regardless of where the plume bends. Comparing the ASFM, vertical velocity, with the VTLC results averaged over the same period suggests the increase in mass flux led to approximately 40% lower upward velocities at 20 mab compared to the source. Given the ASFM configuration at GC600, the ASFM was sensitive to turbulence length scales of approximately 0.6 m that lies within the inertial subrange of fine-scale turbulence. One implication of this scale is that the ASFM treats the water (continuous-phase) and bubbles (dispersed-phase) as a mixed turbulent medium advected by a vertical velocity that is weighted towards the center of the acoustic path. Therefore, we hypothesize that the measured values are reflecting the vertical velocity of the plume continuous-phase (upwelling velocity) and does not include the dispersed-phase. It is also noted that there is no evident correlation between the upwelling flow variability and the ambient currents induced by tide or those occurring at lower frequencies. Acoustic Doppler current profiler (ADCP). The primary sources of real-time current profile data for the NOAA-NDBC program in the GoM are the ADCPs suspended from or attached to oil production platforms and drilling rigs. The raw data and the decoded data with quality control flags are available on the NDBC website. We used the archived data from the NDBC station 42369 located at 27° 12.4000N 90° 16.9667W with 1371.9 m depth. At this station, three ADCPs positioned at approximate depths of 12 m (down- looking), 450 m (up-looking), and 450 m (down-looking) covered the range 16-132, 148- 420, 470-1050 m respectively sampling every 20 min. The 3D flow field was used for understanding the transport of oil/gas through the water column. Texas A&M Oilspill Calculator (TAMOC): We used the single bubble model (SBM) of the TAMOC model to track the trajectory and the evolution of bubble size, mass, and composition as a function of depth, which provides information on the final surfacing characteristics for these bubbles. The module takes into account background stratification, advection by ambient currents, dissolution, and heat transfer. The bubbles were assumed to be gaseous coated with oil in our simulations with sizes ranging between 1 and 15 mm (equivalent spherical diameter with a stride of 1 mm) at the source. The simulation spanned the period Oct 1-6, 2017, when a synthetic aperture radar (SAR) image was available for comparison. The simulation implies that depending on the bubble size and currents, bubbles take substantially different migration paths (as a result of diurnal tides and inertial oscillations at approximately the same frequency), which subsequently affect the transit time of the bubble in the water column. The average transit time of bubbles with d > 8 mm at the source is approximately 40% of that computed for bubbles with d = 4-8 mm. An effect of the decrease in transit-time is a reduction in the size of the surfacing footprint, since there is less time for dispersal by turbulence and currents. Satellite remote sensing: Synthetic Aperature Radar (SAR) satellites can readily detect oil slicks under favorable wind conditions. Nineteen SAR images were collected over GC600 and its surrounding region between Feb 2017 and Dec 2017. A semi- automated image processing routine, the Texture Classifying Neural Network Algorithm (TCNNA) was employed to delineate all the oil slicks. The average surface residence-time is computed to be 8.6 h, and the average oil slick surface area was 4.45×106 m2 over the GC600 domain, taking into account the wind and surface current conditions. Choosing a conservative and uniform oil film thickness of 0.1µm following the published standards, the oil seepage on the seafloor for the GC600 domain is calculated to be 14.4 cm3/s. While processing the SAR images, the deflection distance can be as high as 2.3 km. Considering the geographically dense seeps in this region, this coarse estimation degrades our ability to identify the venting source of oil slicks accurately. We are able to denote a specific slick on the surface as coming from Mega Plume because of the information from the seabed and modelling it through the water column. This outlines the value of independent measurements of the transport of oil/gas bubbles through the water column for identifying the surface footprint. If we consider the the Mega Plume SAR image, the discharge is estimated 2.4 cm3/s, which compares favorably to our seep measurement of 3.0 cm3/s for Oct 2, 2017. B. Razaz, M., Di Iorio, D., Wang, B., & MacDonald, I. (2019). Temporal variations of a natural hydrocarbon seep using a deep-sea camera system. Journal of Atmospheric and Oceanic Technology (resubmission in review). To explore the time-space variability of a natural seep at the seafloor, two VTLCs were deployed using the ROV near the Mega Plume vent. The cameras were positioned approximately 60 cm and 90 cm from the vent cluster. To prevent strong crossflow from swaying the plume outside the camera frame and also due to technical constraints including limited illumination at heights above the bed, we directed the face of the cameras 30° down from the horizon to capture both the vents on the seafloor and the bubbles. Camera B logged 15 s of data every 6 h starting Sept 3, 2017 and continued through Feb 2, 2018 (568 records in 153 d). We developed a sequence of novel semi- supervised algorithms based on standard Mathematica® and ImageJ® routines to resolve the rise velocity (20-22 cm/s), its corresponding bubble size (5-11 mm), and the volumetric hydrocarbon flow rate (1-12 cm3/s). Qualitatively, the variation in the volumetric hydrocarbon flow rate compares well with low frequency variations in the total number of bubbles identified and in the bubble sizes. C. Razaz, M., D. Di Iorio, A.N. Thurnhurr (2019), Deep sea bottom boundary layer dynamics and turbulence characteristics. Journal of Geophysical Research: Oceans (In preparation). Acoustic Doppler current profiler (ADCP). A 600 kHz ADCP was positioned approximately 68 m from the Mega Plume source at ~1180 m depth and was programmed to ping for 15 min at 0.76 s interval every hour with 2 m range cells. All the data, with no averaging over pings, were stored in beam coordinates. Data were transformed into Earth coordinates and averaged over the 15 min burst to give hourly mean flow measurements which exhibits a strong dependency of currents on the spring-neap tide and highly energized flows reaching 20-25 cm/s. Daily averaged currents were primarily directed toward the south which is aligned approximately with the escarpment stretching N-S. In principle, the turbulent kinetic energy (TKE) dissipation rate, e, for the background flow, can be estimated from diverging ADCP beams using structure function methods following a r2/3 law, where r is the spacing between beam velocities, provided the assumption of local isotropy holds. Momentum fluxes (u'w'and v'w') were directly computed from velocities recorded in beam coordinates, using the variance method under the assumption that turbulence is homogenous and isotropic across the beams. Together with the mean shear (dU/dz, dV/dz), production of TKE was calculated. The TKE dissipation rates compare well to measurements of TKE production, P. Turbulence levels range from 10-9 to as high as 10-6 W/kg and correlate well with the highly energized current speeds.