Split-beam echosounder observations of natural methane seep variability in the northern Gulf of Mexico
A method for georeferencing and characterizing midwater plumes of free gas bubbles observed with an 18-kHz scientific split-beam echosounder (SBES) was developed using water column backscatter data collected during two research cruises aboard NOAA Ship Okeanos Explorer in the northern Gulf of Mexico, a region of natural and widespread gas seepage. The method was applied to SBES observations of plumes of presumed methane gas bubbles originating from gas vent sites at 1400 m depth. Split-beam processing techniques were used to provide estimates of the seep source locations on the seabed. A total of 141 SBES plume observations from 27 repeat survey lines were grouped into 15 clusters according to their proximity to each other. For all observations at each cluster, profiles of mean target strength per vertical meter were calculated as indicators of the flux and fate of gas bubbles acoustically observable at 18 kHz. The scattering strength profiles accounted for the geometry of intersection between the transmit pulse and the plume axis and showed significant variability between repeat observations at time intervals ranging from 1 hour to 7.5 months. Estimates of the shallowest observable plume depth averaged 943 m and frequently coincided with increased ambient noise at the deeper boundaries of the deep scattering layer, a depth range characterized by increased populations of biological scatterers. The shallowest observed plume depths were limited by the echosounder field of view in four instances. This SBES processing method complements the techniques presently used in marine gas seep research and is applicable to existing SBES data not previously scrutinized for seep observations.
Kevin Jerram has been pursuing an M.S. in Ocean Engineering through UNH CCOM/JHC since 2011. He has participated in six oceanographic research cruises during this time, including four (one via telepresence) for seep mapping in the Gulf of Mexico. Prior to his graduate studies, Kevin received a B.S. in Mechanical Engineering from UNH and worked in engineering positions for Shoals Marine Laboratory on Appledore Island, Maine, and Ocean Classroom Foundation aboard SSV Westward. His other interests include seafloor (and seep) mapping in the Arctic, riding old motorcycles, and hiking slightly older mountains.