Page 148 - Acquisition and Processing of Marine Seismic Data
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2.6 QC IN DATA ACQUISITION 139
streamer tests are reported in the final acquisi- The main purpose of RMS noise and DC offset
tion report in daily, monthly and end-of-job test tests is to measure the noise generated by the
results, separately. In each specific report, the internal circuitry and A/D converter of the
channel numbers and corresponding test(s) that digitizers. A harmonic distortion test is done
they fail as well as the dead channels are listed by sending a sine wave with 3 dB amplitude
accordingly. Although several other tests are generally at 31.25 Hz frequency to the input
also possible depending on the specifications circuitry and A/D converter to measure the
of the seismic system employed, typically the harmonic distortion level generated by the
following controls on the recording system input circuitry. An instrument gain and phase
and components are executed before the initial- test is used to find out the deviation of the
ization of the survey: gain and phase of the digitizers’ A/D
converters from 0 to the high-frequency cut-
• Verification of recording parameters,
off value. A hydrophone discharge/leakage
recording format and headers
test reveals the timing and slope of the
• Control of the recording unit, streamer and
response to a pulse sent to the hydrophones.
telemetry
A crosstalk test is performed to check the
• Hydrophone and digitizers’ A/D tests
unwanted signals within the neighboring
• RMS noise (typically 5 μV, maximum 8 μV)
channels in the digitizing modules. The test is
• DC offset (typically 6 μV, maximum 10.0 μV)
achieved in two stages using a sine wave with
• Harmonic distortion (typically 100.0 dB)
3 dB amplitude generally at 31.25 Hz
• Gain accuracy (typically 0.02%, maximum
frequency: first, all even channels and then all
0.05%)
odd channels are tested separately. An
• Crosstalk ( 95.0 dB)
• Hydrophone discharge/leakage (typically impulse response test is used to obtain the
5MΩ, minimum 1 MΩ) response of each individual channel to a one
• Impulse response ( 0.08 dB) sample-long pulse.
2.6.1.3 Tests for Gun Array and
a. Recording parameters, recording format and Components
headers: Data recording parameters including A couple of start-up tests before the survey
recording length, sampling rate, seismic data are also performed on the air gun array(s) and
and sample formats, recording filter cut-off related equipment to ensure that all the individ-
frequencies and trace summing parameters if
ual guns as well as the whole gun array system is
required, are entered into the seismic
working properly. These simple tests include
recorder (Section 2.5.2). The information
written into the EBCDIC headers of the • Control of the gun line integrity and
seismic data is prepared. triggering
b. Control of the recording unit, streamer and • Array leakage test
telemetry: This step comprises testing of all • Gun signature tests
available digital components of the recorder • Drop-out modeling/tests
and streamer including telemetry, digitizer,
lead-in connections, etc. a. Gun line integrity and triggering tests: All the
c. A/D tests for hydrophones and digitizers: Modern connections between the guns, near-field
seismic recording systems automatically hydrophones, array depth sensors and the
perform several tests on the hydrophone and gun controller, as well as the high pressure
digitizing modules before the survey. hoses from air compressors to each air gun via