Mapping the spatial distribution of seepage activity in the Coal Oil Point seep field (COP1609, EK80)

A broad acoustic survey of the Coal Oil Point seep field was conducted using a small dive support vessel (R/V Connell) from the University of California – Santa Barbara. The survey consisted of acoustically mapping specific seep sites within the Coal Oil Point seep field, commonly known as Platform Holly, Seep Tent, La Goleta, Patch and Trilogy. The survey was conducted with three downward-looking broadband echosounders: an MSI Constant Beamwidth Transducer (CBW) (30-100 kHz), a Simrad ES200-7C split-beam echosounder (150-250 kHz) and a Simrad ES333 split-beam echosounder (290-490 kHz). The echosounders were interfaced with an EK80 Wideband Transceiver and were deployed on a tow sled on the port side of the R/V Connell. A Garmin WAAS-enabled GPS was attached to the tow sled and was integrated into the Simrad EK80 software to record the position of the echosounders. A Seabird 19plus V2 CTD was used to collect profiles of salinity and temperature in the water column after each survey. A 300 kHz RDI ADCP was used for measuring water current profiles. During the last day of survey, 14 September 2016, a low-frequency single-beam transducer (1-10 kHz) was used to map a small subset of the seep field. All surveys were performed at vessel speeds of 4-6 knots depending on the weather conditions of the survey day.

On 31 August 2016, the instruments were mounted to the R/V Connell and were tested during an on-shore shake down.

On 1 September 2016, the tow sled and instruments were tested at-sea using various configurations in order to determine the optimal configuration to minimize noise interference with the echosounders.

On 2 September 2016, an EW 500 m by 500 m survey, with 10 m line spacing, was conducted at Patch. Gas seepage was visible throughout the survey, both in the acoustic data and near the surface of the water column. The sea surface was lightly oiled in some areas of the survey. The survey took four hours to complete and was followed with a CTD cast. Finally, a short (500 m) transect ADCP survey was conducted.

On 6 September 2016, a NS 500 m by 500 m survey, with 10 m line spacing, was conducted near Platform Holly. Gas seepage was visible predominantly near the platform, both in the acoustic data and significantly by eye near the platform. The sea surface was heavily oiled, especially near the platform. The survey took a total of four hours to complete and was followed with a CTD cast near the center of the survey area. An initial attempt at calibrating the echosounders was performed south of Platform Holly, however it was decided to move east of Patch for the calibration to avoid seeps during the calibration procedure. The calibration was performed using a 38.1 mm and 25.0 mm Tungsten-Carbide calibration spheres. A 12.67 mm Tungsten-Carbide calibration sphere was also used to attempt to calibrate the echosounders, however it was too small and light to use in the calibration site.

On 7 September 2016, an EW 500 m by 500 m survey, with 10 m line spacing, was conducted at Seep Tent. The sea surface was heavily oiled at the site. Gas bubbles were also observed at the sea surface in the site. The survey took four hours to complete and was followed by a second EW 500 m by 500 m survey, with 10 m line spacing, conducted at La Goleta. Oil and gas bubbles were observed at the sea surface throughout the survey area. The survey took approximate four hours and forty minutes to complete and was followed with a CTD cast near the La Goleta survey area.

On 8 September 2016, an EW 1000 m (EW) by 600 m (NS) survey, with 20 m line spacing, was conducted at Trilogy, followed by 6 evenly spaced NS lines across the survey area. The sea surface was slightly oiled in some locations in the survey area and three distinct locations within the seep area had significant quantity of gas bubbles at the sea surface. The survey took four and a half hours to complete and was followed with a CTD cast in the survey area. Following the CTD cast, ADCP operations were performed around Platform Holly. A NS line was conducted, once going north and then repeated going south, to the west of Platform Holly. A similar NS line was performed in both directions very close to the platform on the east side of the platform. It appeared that the lines were too close to the platform and the pilings caused issues with the data. Another NS line, heading north then south, was completed farther east from the platform. Another survey was conducted near Platform Holly. Survey lines were conducted as circles around the platform, starting as close as possible to the platform and extending the circle diameter until no seepage activity was observed in the acoustic data.

On 9 September 2016, a SE-NW 1300 m by 8500 m survey, with 412.5 m line spacing, was conducted. In addition, three groups of two lines, perpendicular to the broad survey lines, were conducted between seep sites. The broad survey encompassed the Platform Holly, Seep Tent, La Goleta and Patch seep sites. The survey took a total of five hours to complete and was followed with a CTD cast near the Patch survey site. A NS transect line was conducted with the ADCP near Patch after the CTD cast.

On 12 September 2016, a SE-NW 600 m by 850 m survey, with 20 m line spacing, was conducted to the northwest of Seep Tent. The survey took approximately one hour to complete. A CTD cast and a NS ADCP survey were performed after the survey near Seep Tent. A transit line was conducted between the ADCP survey site and Platform Holly. Once the vessel reached Platform Holly, another ADCP survey was conducted near the platform. An acoustic survey was conducted, with random lines, near Platform Holly. The survey was conducted with the intent of locating oil surfacing near the gas seep.

On 13 September 2016, a broad SE-NW survey with two lines centered between the previous broad survey lines was conducted. Two NS survey lines were conducted near Platform Holly. Two NS survey lines were conducted through the center and west part of La Goleta and then two additional NS lines were conducted near Patch. A CTD cast was performed after the survey, in the vicinity of the Patch seep site.

On 14 September 2016, two broad SW-NE survey lines were conducted using the low-frequency (1-10 kHz) single-beam echosounder. One NS survey line was conducted to the west of Platform Holly and then acoustic data was collected while the vessel drifted around the west side of the platform. Additional NS survey lines were conducted at Seep Tents, La Goleta and Patch. Acoustic data while the vessel drifted was also collected at these survey sites. A CTD casts was performed at the end of the acoustic survey.