Page 50 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 41

ensure that the standard deviation of the GPS distance before the actual line start position,
error is within the acceptable limits of the survey. which is termed the run-in distance. At every
The rGPS (relative GPS) system must be checked shot point location determined by the integrated
and calibrated. All other static offsets around the navigation system as the vessel proceeds along
vessel, such as the locations of the aforemen- the line, the shooting is repeated and data is
tioned sensors must be verified and accurately recorded. The shooting and recording stops at
set into the integrated navigation system. All a certain distance after the actual line end posi-
these measurements, calibrations and verifica- tion, which is termed the run-out distance. The
tions are fulfilled when the vessel is at the dock. seismic vessels need a large turning area to
Several experts must work together to keep run into the next line, which is planned by the
the all the seismic system components fully ope- navigators and generally controlled and care-
rational during the recording. First, data acqui- fully monitored by the navigation system.
sition parameters such as sampling rate, record During the line change, vessel speed is generally
length, shot interval, etc. are correctly set in the kept constant at acquisition speed.
navigation and recording units by navigators Typically, the following equipment is mobi-
and observers. The vessel is kept at a constant lized for a 2D seismic survey (Fig. 2.3A):
speed of approximately 4–4.5 knots and then tail
• Seismic vessel
buoys and streamers are deployed first, along
• Seismic recorder
with the necessary navigational units attached,
• Onboard processing/QC system
such as depth levelers (birds), acoustic rangers
• Streamer
and streamer positioning or steering equipment.
• Seismic sources
After deploying the dilt floats (streamer front
• Gun controller
buoys) and paravanes, observers watch the posi-
tions of all the in-sea equipment to settle using • Streamer depth controller
rGPS and acoustic ranger inputs. Finally, the • Air compressors
air compressors are started and air gun arrays • Integrated navigation system
• DGPS/rGPS systems
are deployed. After positioning of all the equip- • Tail buoy
ment, several tests for streamers and guns are • Lead-in and deck cables
carried out to ensure that all systems are func-
tioning properly before the data recording is The following additional equipment is also
initiated. Gun mechanics assure that all air guns required for 3D surveys (Fig. 2.3B):
are working properly, with no air leakage within
• Paravanes
thepressurizedcomponents,andthatguntiming
• Dilt floats
for all guns of all strings is in required tolerance
• Lateral streamer positioning equipment
limits. Navigators check that all the streamer
• Acoustic rangers
cables are at their predetermined positions, all
• Other equipment (velocimeters, separation
the acoustic ranging systems are properly func-
ropes, etc.)
tioning, and depth and lateral positions of all
streamers are correct. Observers get some noise
records without firing the guns to analyze the 2.1.1 Seismic Vessels
static noiselevelonthe streamers,and then check
the test shots and visually inspect the data and Seismic vessels are specific ships dedicated to
noise levels on all streamers before the recording. use solely for 2D or 3D seismic data acquisition
If all systems are working properly, then the in the ocean. Since in-sea equipment deployed
vessel is ready to run into the first sail line. during the acquisition consists of large spreads,
The shooting and recording starts at a certain commonly up to 16 streamers each 8–10 km