Bubble vent localization for marine hydrocarbon seep surveys

Abstract

Commercial success of marine seep hunting exploration campaigns involves acquisition of high-quality bathymetry and backscatter along with targeted coring of shallow geochemical sampling of seep sediments. The sharp lateral chemical gradient encompassing seafloor seeps requires accurate identification of seep sites from high-resolution acoustic data. Active seafloor seeps featuring plumes of gas bubbles and oil droplets rising into the water column can be imaged with modern multibeam echosounders providing an effective approach to remotely characterizing seafloor seeps. Interpreting the seafloor position of gas plume emissions in multibeam data using existing mapping methodology is hindered by slow processing due to large files sizes, a manual “by eye” qualitative assessment of each sonar ping searching for plume anomalies, skill and fatigue of the geoscientist, and environmental or acquisition artifacts that can mask the precise location of gas emission on the seafloor. These limitations of midwater backscatter mapping create a qualitative data set with varying inherent positional errors that can lead to missed or incorrect observations about seep-related seafloor features and processes. By vertically integrating midwater multibeam amplitude samples, a 2D midwater backscatter raster can be generated and draped over seafloor morphology, providing a quantitative synoptic overview of the spatial distribution of gas plume emission sites for more refined seafloor interpretation. We reprocess multibeam midwater data set from NOAA Cruise EX1402L2 in the northwestern Gulf of Mexico using a vertical amplitude stacking technique. Constructed midwater backscatter surfaces are compared with digitized plume positions collected during the survey for a comparison into assessing uncertainty in mapping approaches. Our results show that the accuracy of manually digitizing gas emission sites varies considerably when compared with the midwater backscatter amplitude maps. This quantitative plume mapping technique offers multiple advantages over traditional geopicking from cost effectiveness, offshore efficiency, repeatability, and higher accuracy, ultimately improving the detectability and sampling of active seafloor seeps through precisely located cores.