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. |