Split-beam echo sounder observations of natural methane seep variability in the northern Gulf of Mexico

Abstract

A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 kHz scientific split-beam echo sounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 kHz and showed significant variability between repeat observations at time intervals of 1 h-7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18 kHz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances.

Modern Assessment of Natural Hydrocarbon Gas Flux at the Coal Oil Point Seep Field, Santa Barbara, California

“…There is a high degree of variability in volumetric gas flow rates, both between investigators and the different methods used in each individual study, and for sites visited multiple times (e.g., Clark et al, , visited La Goleta in 2002, 2003, and 2005). This variability is consistent with other studies for which seep activity has been observed to vary over timescales of seconds to minutes and hours to years (Boles et al, ; Clark et al, ; Clark et al, ; Hornafius et al, ; Jerram et al, ; Kinnaman et al, ; Leifer & Boles, ; Quigley et al, ). Quigley et al () reported a decrease of seep activity of 80% in the vicinity of Platform Holly in a 22‐year period, by comparing the area of detected seeps (but not volumetric gas flow rates) to those of Fischer and Stevenson (), and that reduction has been hypothesized to be linked to petroleum extraction from Platform Holly.…”

A New Look at Seafloor Venting: Natural Gas Hydrate Derivatives

“…In the gas hydrate stability pressure and temperature field maintaining a hydrate shell requires a minimum dissolved concentration of gas in the water (Chen et al 2013), suggesting that hydrate-shelled bubbles may ascend as part of a multiphase plume. The shell encapsulates, and effectively armors, the bubble from dissolution until it reaches the top gas hydrate stability (TGHS) where the NGH quickly dissociates and dissolves (Rehder et al 2002, Levine 2015, Warzinski et al 2014, Jerram, Weber, and Beaudoin 2015. The shell encapsulates, and effectively armors, the bubble from dissolution until it reaches the top gas hydrate stability (TGHS) where the NGH quickly dissociates and dissolves (Rehder et al 2002, Levine 2015, Warzinski et al 2014, Jerram, Weber, and Beaudoin 2015.…”

“…Characterizing emissions is improving our understanding of the transport processes and allowing estimates and quantification of fluxes (Wilson, Leifer, and Maillard 2015, Leifer and Judd 2015, Leifer 2015, von Deimling et al 2015, Vielstädte et al 2015, Weber et al 2014, Wiggins et al 2015, Nauw, Linke, and Leifer 2015, Jerram, Weber, and Beaudoin 2015, Frouzova, Tušer, and Stanovsky 2015 that will provide more detail to the CH 4 budgets involved. Furthermore, following the Deep Water Horizon disaster in the Gulf of Mexico detailed mapping of the region near the wellhead revealed multiple sources of natural hydrocarbons (Smith et al 2010).…”

Acoustic mapping of mixed layer depth

“…Not long after the first military applications, acoustic water column mapping with echo sounders was applied to fisheries science, for which the detection and quantification of fish distributions were the primary focus (Kimura, 1929;MacLennan, 1990). The applications of acoustic water column mapping have broadened in recent years to include marine ecosystem acoustics (Benoit-Bird and Lawson, 2016;Godø et al, 2014), observations of gas bubbles and oil droplets associated with natural seeps (Jerram et al, 2015;Merewether et al, 1985), and fossil fuel production (Hickman et al, 2012;. Acoustic imaging of the water column has also been used within the field of physical oceanography; single-beam echo sounders can capture fine-scale oceanographic structures typically attributed to biological scattering or turbulent microstructures (Klymak and Moum, 2003;Pingree and Mardell, 1985;Trevorrow, 1998).…”