IHA Application: Seismic Surveys in Cook Inlet by Marathon Oil ...

ecording, shooting, and possibly one or two smaller utility boats. The vessels are
generally smaller than those used in streamer operations, and the utility boats can
be very small, in the range of 10-15 m.
An OBC operation begins by laying cables off the back of the layout vessel.
Cable length typically is 4-6 km but can be up to 12 km. Groups of seismicsurvey
receivers (usually a combination of both hydrophones and vertical-motion
geophones) are attached to the cable in intervals of 25-70 m. Multiple cables are
laid on the seafloor parallel to each other using this layout method, with a cable
spacing of less than ½ mile, depending on the geophysical objective of the survey.
When the cable is in place, a vessel towing the source array passes over the cables
with the source being activated every 25 -50 m. The sound source levels (zero to
peak) associated with the OBC seismic survey are the same for most 2D and 3D
marine seismic surveys (233-240 dB re 1uPA at 1 m). The ship speed is typically
between 4-5 knots.
After one source line is acquired, the source vessel takes about 10-15 minutes to
turn around and pass over the next source path. When a cable is no longer needed
to record seismic survey data, it is recovered by the cable-pickup vessel and
moved to the next recording position. A particular cable can lay on the seafloor
anywhere from 2 hours to several days, depending upon operating conditions. An
OBC seismic survey covers a smaller area and spends several days in the area. In
contrast, 3D streamer seismic surveys cover a much larger area and only stay in a
particular area for a few hours.
2007 Fall Acquisition Program
The data acquisition proposal incorporates the use of a lightweight Sercel 408
recording system, several shallow draft vessels and a team of seasoned personnel
with extensive experience in Cook Inlet Sea transition zone operations. Veritas
(VTS) will use a light weight, helicopter-portable recorder which can be
positioned onshore or on the mother ship. The M/V Peregrine Falcon is self
contained and able to house their 24 hour crew compliment, although, smaller
cable support vessels will house their crew compliment on a mother ship offshore.
The recording staff (observers) will be capable of 24 hour recording and trouble
shooting, which will allow acquisition to proceed efficiently throughout the short
window, and minimize the exposure period to marine mammals.
Mobilization
The mobilization effort for the survey will involve simply moving the M/V
Peregrine Falcon and 3-4 bow pickers from the Union Oil Company of California
Granite Point seismic operation to the North Ninilchik project area. The
mobilization will occur immediately after seismic operations are completed at the
Granite Point project area, probably in late September.
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