Page 69 - Acquisition and Processing of Marine Seismic Data

60 2. MARINE SEISMIC DATA ACQUISITION

buoys relative to the vessel reference position. software receives the positioning information
The accuracy of rGPS systems is commonly less from the aforementioned peripheral systems to
than 1 m. compute the navigation for the vessel reference
INS software has also an interface to the position (VRP), as well as the exact positions of
acoustic ranging system to display and log the predefined offsets around the vessel with
positions of the streamers. Acoustic ranging respect to the VRP. INS also computes the source
instruments (Section 2.3.4) use acoustic signals and all available receiver positions in real-time.
to communicate with each other in order to keep INS software packages perform navigation
the streamer separation in the determined lat- data logging during the acquisition for further
eral range. Since the communication is done in navigation processing, offline QC and binning
through acoustical means, we definitely need purposes.The logdatafilesaregenerallyinaspe-
to know the sound velocity in the sea water dur- cific format termed UKOAA files. Raw and pro-
ing the survey, since it varies with location and cessed navigation data files are commonly
time (Section 1.1). Therefore, a couple of velo- distributed in UKOAA P2/94 and UKOAA P1/
cimeters are generally attached to the head 90 format, respectively. While raw P2 files con-
and tail of one or two streamers for 3D surveys tain raw survey observations of navigation, gen-
to measure the sound velocity in real-time as an erally in range and bearing format such as
input to INS to accurately compute the acoustic heading, rGPS or acoustic ranging data, pro-
ranges between the streamers. In some cases, cessedP1filescomprisesourceandreceiverposi-
specific sensors may be used to measure temper- tions as well as common midpoint (CMP)
ature (T) and salinity (S) as a function of water coordinates. The information logged by INS
depth, which is termed TS dips. These measure- may vary for each specific survey and for each
ments are used to compute the sound velocity in specific seismic system. However, it commonly
the water column, which is entered into the INS includes positions of the vessel reference posi-
for acoustic ranging computations in real-time. tion, shot and receiver locations, center of source
In some cases, vessels also have acoustic Dopp- (COS), heading, streamer depths from birds,
ler current profiler (ADCP) inputs to the INS to locations of the CDPs, vessel speed, system tim-
obtain the local surficial currents to predict the ing information, lateral separations of sourceand
streamer feathering. streamers, etc. Using the navigation logs, fold
A GPS receiver provides only the position of distribution is computed in regular intervals in
its antenna location. A seismic vessel, however, 3D surveys.
has several offset positions for various sensors,
such as gyrocompass or motion sensor, or spe- 2.1.8 Tail Buoys, Dilt Floats and
cific locations on and around the vessel super- Paravanes
structure, such as the location of vessel
reference position, location of vessel stern, trans- Each streamer ends up with a tail buoy
ducer of the single-beam echosounder or center (Fig. 2.18A and B), which is equipped with a
of source arrays, etc. (Fig. 2.17). Lateral and ver- flashing beacon for night time vision, a radar
tical separations between the equipment are reflector to be easily detected by the nearby ves-
termed nominal offsets, which define the prede- sels, and an rGPS transponder (Fig. 2.18C) to
termined towing and acquisition configuration monitor the location and direction of streamers
of the seismic spread with respect to the center during the acquisition. Some tail buoys are pow-
of navigation or vessel reference point. These ered from specific tail buoy power modules
offsets are set into the INS to calculate their exact mounted at the tail of the streamers, while others
positions during the survey for logging. INS have their own power generator from a small-