Page 165 - Acquisition and Processing of Marine Seismic Data

156 2. MARINE SEISMIC DATA ACQUISITION

sinceitaffectsthefrequencycontentofthefar-field areacanbepreparedtoevaluatetheareasofsource
seismic signature depending on the ghost notch problems and overall performance of the source
interference (Section 2.2.3), which determines the arrays.
high-frequency content of the source signal. Con- QC of auxiliary channels is also important
sistency in the source depth is crucial to maintain since the near-field hydrophone signatures are
thefrequencycontentoftheseismicsignalshot-by- recorded into the auxiliary channels for each shot
shot for each sequence (Fig. 2.101A). In order to together with the time break, gun aim point, and
ensure that the source depth is in survey limits, 1pps GPS time synchronization pulses for every
at least one depth sensor is attached to each gun second. For each completed line, near-field sig-
string to measure the depth of the guns from natures are displayed side by side for all guns
gun float to the depth sensors. Pressure variations along the line to ensure that each gun generates
also affect the amplitude and frequency content of a consistent seismic signal for each shot as well
the seismic signal. Plotting of the array pressure as to identify air leaks and gun timing errors
with respecttothe shot number provides informa- (Fig. 2.102). Plotting and analysis of auxiliary
tion on the gun drop-outs and problematic shot channels are done online and offline during the
records due to the abnormal pressure variations acquisition to reveal the possible bubble period
(Fig.2.101B).Inadditiontotheshotnumberversus changes that may be the indication of air leaks.
pressureorarraydepthplots,aerialmapsshowing Plotting of the auxiliary channels in stacked form
the depth and pressure variations in whole survey shows the gun firing errors and auto-fires.

FIG. 2.101 (A) Depth of port and starboard arrays along the line and (B) air pressure applied to both arrays for each
shot point.