Page 155 - Acquisition and Processing of Marine Seismic Data

146 2. MARINE SEISMIC DATA ACQUISITION

low- and high-frequency components from during the acquisition must be determined, to
approximately 3 Hz to the first ghost notch. The analyze whether the survey geometry is main-
spectrum can also be computed over different tained with respect to the nominal parameters.
time windows, such as the water column, shal- Navigation QC for in-sea equipment to docu-
low subsurface, target depth and deep window ment the nominal predetermined geometry
to compare the frequency content of the data typically comprises the following analyses:
from early to late arrivals.
• Vessel navigation/DGPS accuracy/gyro data
Spectral analysis is sometimes performed for
information
the first and last shot groups of the seismic line,
• Vessel speed and shot interval plots
and f-x and f-k spectra of the selected shots can
• Streamer depth/bird data/feathering
also be computed whenever a specific type of
information
noise is observed (Fig. 2.88BandC).The f-x
• Minimum offset check
spectrum can indicate the variations of spectral
• Acoustic network analysis
content of the data from near to far offsets along
• Streamer separation plots
the streamer(s), and it can also be used to eval-
• rGPS data
uate the local noise amplitudes, such as the
• Array and string separations
noise arising from the wing motions of the
• Source depth plots
depth levelers, or streamer bending noise, etc.
• Echosounder/TS dip/currentmeter data
The f-k spectrum can be used to analyze the
• Line deviations/coverage maps/fold
possible seismic interference or other types of
distribution
linear noise as well as the amplitudes of spa-
tially aliased data. Accuracy in the vessel positioning is impor-
tant since all the positions of in-sea equipment
are tied to the vessel reference position, which
2.6.3 Analysis of Navigation Data
is the only position determined directly from a
Offline analysis on the navigation data for all GPS system via satellite connection. Each seis-
dynamic offsets, rGPS data, and streamer acous- mic vessel is equipped with at least two DGPS
tics is completed using UKOAA navigation files receivers and two independent gyros; each is
and other specific navigation logs (if available) calibrated carefully before the survey. Modern
recorded during the acquisition. Most of the INSs provide error ellipses or scatter plots
navigation data analysis is done online and then (Fig. 2.89A) for DGPS quality checks and online
a detailed analyses and plotting of specific control of dilution of precision (DOP) during the
parameters are also performed during the off- acquisition. Today, horizontal error in determin-
line QC analyses. The only coordinate computed ing positions of the antenna locations of a DGPS
in real-time by a global positioning within the system is less than 1 m. For vessel positioning
whole spread is the vessel’s reference position, QC, readings for each gyro are plotted against
tied to the DGPS antenna offsets. Inline and shot point for each line (Fig. 2.89B).
crossline offsets of the other equipments/com- Shot interval is one of the most important
ponents are all nominal values determined parameters that must be maintained according
before the survey. Sources and streamers are to the predetermined survey parameters since
towed behind the vessel at these predetermined it affects the fold distribution. Variations in the
nominal layback distances. However, actual shot interval result in a nonuniform inline fold
locations of source centers, head and tails of distribution along the line. In order to document
the streamers and crossline distances such as that the shot intervals are within the survey
source/subarray and streamer separations acceptance limits, shot intervals for each sail line