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Measurements of natural hydrocarbon gas flux in the Coal Oil Point seep field (COP1710, EK80)
On 26 October 2017, acoustic data was collected in tandem with direct, in situ gas flux measurements. Acoustic data was collected using a Simrad ES200-7C split-beam echosounder that was interfaced with an EK80 Wideband Transceiver. The echosounder was operated over a frequency bandwidth of 160-260 kHz. Direct measurements of gas flux were obtained using a bubble catch device (BCD) that was designed to capture gas in an inverted graduated cylinder through a funnel. An underwater camera that was attached to the BCD was used to capture images of the accumulation of gas within the cylinder. The echosounder was pole-mounted to the port side of the R/V Connell, while the BCD was hand deployed from the starboard side of the vessel. A Garmin WAAS-enabled GPS system was interfaced with the EK80 software to provide position of the vessel throughout the survey. Collection of both acoustic data and direct gas flux measurements were obtained at three seep sites located within the Coal Oil Point seep field: Platform Holly, Seep Tent and La Goleta. Data collection was obtained by positioning the vessel in locations of high seepage activity (verified with the acoustic data) and then allowing the vessel to drift over the area until no seepage activity was observed. Both the acoustic data and the direct gas flux measurements were collected simultaneously throughout the survey. The BCD was deployed at a depth of 23 m at La Goleta and Seep Tent and at 15 m at Platform Holly. A total of 20 .MP4 files were obtained with the BCD. A preliminary test run of the BCD was conducted at the La Goleta seep site at 1532 UTC. Following the BCD test, both acoustic data and video images from the BCD were acquired at the La Goleta seep site. At approximately 1930 UTC, data collection at La Goleta was completed and the R/V Connell transited to the Seep Tent seep site. Data collection at Seep Tent commenced at approximately 1956 UTC and was completed at 2116 UTC. A CTD cast was performed after the survey near Seep Tent. A field calibration was done for the ES200-7C split-beam echosounder following the CTD cast with a 28.5 mm Tungsten-Carbide calibration spheres. At 2150 UTC, the R/V Connell arrived at Platform Holly in order to collect both acoustic and BCD measurements. Data collection with the BCD finished at 2244 UTC and a final acoustic survey in the vicinity of Platform Holly was conducted between 2256 UTC and 2357 UTC.