| A method for positioning and characterizing marine gas seeps using an 18-kHz scientific split-beam echosounder (SBES) was developed and applied to SBES data collected in the northern Gulf of Mexico. A total of 161 plumes of presumed methane gas bubbles originating at approximately 1400 m depth were observed over 27 repeat surveys and grouped by proximity into 35 clusters. Profiles of mean target strength per vertical meter were calculated with compensation for SBES beam pattern and geometry of plume axis ensonification. These profiles were used as indicators of the fluxes and fates of gas bubbles acoustically observable at 18 kHz and showed significant variability between repeat observations at time intervals of 1 hour to 7.5 months. The minimum depths of acoustic plume observations averaged 875 m and frequently coincided with increased reverberation in layers of biological scatterers. Minimum depth estimates were limited by the SBES beam pattern in five instances. |
