Page 149 - Acquisition and Processing of Marine Seismic Data

140 2. MARINE SEISMIC DATA ACQUISITION

gun umbilicals, are checked and verified. 2.6.2 Analysis of Seismic Data
Manual triggering, termed solenoid click test,
is performed for each gun in the array to One of the fundamental processes per-
ensure that all the signal connections and formed during the offline QC applications is
solenoid valves are functional. the detailed analysis of the collected seismic
b. Array leakage test: Air leakage can occur from data, which consists of analyzing the data itself
by a fast-track processing, analyzing the spe-
loosely connected or damaged air hoses or
cific noise types embedded in the data, and
directly from the inside of the air guns if the
o-rings are corroded or not properly evaluating the spectral content. The basic pur-
emplaced. The whole system can be pose is to assess the impact of noise in the seis-
pressurized before the survey to observe if mic data being collected and to document the
any air leakage exists within the array. The probable acquisition issues in order to take
strings must maintain the applied pressure at the necessary precautions associated with
90% of the full 2000 psi at least for 15 min acquisition and recording to prevent any agents
that degrade the overall data quality and
with all valves closed. Defective components
decrease the S/N ratio of the data, typically
should be replaced or repaired before the
by evaluating the data in a line-by-line basis.
survey.
Definitions of shot/trace issues associated with
c. Gun signature tests: When the gun array is in
the seismic data are determined before the sur-
water before the regular shooting is initiated,
vey. For instance, a trace will be defined as a
each gun is fired individually to observe and
dead (or bad) or noisy trace if it falls into any
record its near-field signature. PBR values,
of the categories given in Table 2.15, and a shot
signal amplitudes and bubble periods of the
record is considered a bad record or misfire if it
guns with the same volume are compared to
understand if the signal from each gun is meets the specifications given in Table 2.16.
consistent. Array far-field signature modeling Analysis of the acquired data is done by an
and radiation patterns in along- and across- onboard processing application, termed fast-
side directions are calculated by array track processing. Modern seismic vessels have
modeling to analyze if the source signature of processing systems and hardware to fully pro-
the whole array is acceptable. cess 2D and 3D seismic data onboard to provide
a full onboard processing if claimed by the cli-
d. Drop-out tests: Consistency of seismic source
ent at an extra cost. Onboard processing is usu-
is one of the key factors affecting the overall
data quality and repeatability for 4D studies. ally performed for quality control purposes and
Before the survey, modeling of the seismic it typically comprises basic processing steps,
source (and field tests, if possible) is such as
performed to understand if the spare guns • SegD data loading
can compensate the gun drop-outs during the • Resampling from 2 to 4 ms (if necessary)
shooting. This provides gun drop-out • Seismic and navigation merge
specifications for each string, which shows • Removal of recording delay (if necessary)
how the generated peak-to-peak (P-P) signal • Gun and streamer static correction
amplitude and the ratio of P-P to bubble • Shot and channel edit
amplitude (PBR) deviate from its nominal • Low-cut filter for swell noise attenuation
source signature in case of gun failures. (typically 4–8 Hz, 18 dB/Oct)
Drop-out specifications document when a • Spherical divergence correction
spare gun is turned on, and when a specific • Near trace section/single fold cube/
gun needs maintenance. brute stack