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
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