1 1. APPENDIX A: SEISMIC EQUIPMENT EQUIPMENT: To achieve a good trade-off between penetration and resolution, a seismic source consisting of a 420 in 3 (7 litres) linear array of two GI-gun was adopted. The data were logged by a complete acquisition system composed by a 300 m long digital streamer interfaced to a Geometrics CNT-2 Recorder. The complete block diagram is illustrated in Figure A-1. Figure A-1. Equipment block diagram. SEISMIC SOURCE The GI GUN is made up of two independent chambers within the same casing, used to control and reduce bubble oscillations. The first chamber is called the Generator, as it generates the primary pulse and creates the bubbles. The second one is called the Injector, as it injects air inside the bubble. Each gun has its own reservoir, its own shuttle, its own set of exhaust ports, and its own solenoid valve. A common hydrophone provides both the time break and the shape of the near field signal. This gun phone is located inside the bubble and responds to the actual air blast. Basic principle Phase 1: The Generator (G) is fired. The blast of compressed air produces the primary pulse and the bubble starts to expand. Phase 2: When the bubble approaches its maximum size, it encompasses the Injector ports, and its internal pressure is far below the outside hydrostatic pressure. At this time, the Injector (I) is fired, injecting air directly inside the bubble. Due to the quasi-static state of the bubble, the timing of the Injector is not critical.
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1. APPENDIX A: SEISMIC EQUIPMENT
EQUIPMENT:
To achieve a good trade-off between penetration and resolution, a seismic source consisting of a 420 in3 (7 litres) linear array of two GI-gun was adopted. The data were logged by a complete acquisition system composed by a 300 m long digital streamer interfaced to a Geometrics CNT-2 Recorder. The complete block diagram is illustrated in Figure A-1.
Figure A-1. Equipment block diagram.
SEISMIC SOURCE
The GI GUN is made up of two independent chambers within the same casing, used to control and reduce bubble oscillations. The first chamber is called the Generator, as it generates the primary pulse and creates the bubbles. The second one is called the Injector, as it injects air inside the bubble. Each gun has its own reservoir, its own shuttle, its own set of exhaust ports, and its own solenoid valve. A common hydrophone provides both the time break and the shape of the near field signal. This gun phone is located inside the bubble and responds to the actual air blast.
Basic principle Phase 1: The Generator (G) is fired. The blast of compressed air produces the primary pulse and the bubble starts to expand.
Phase 2: When the bubble approaches its maximum size, it encompasses the Injector ports, and its internal pressure is far below the outside hydrostatic pressure. At this time, the Injector (I) is fired, injecting air directly inside the bubble. Due to the quasi-static state of the bubble, the timing of the Injector is not critical.
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Phase 3: The volume of air released by the Injector increases the internal pressure of the bubble, and prevents its violent collapse. The oscillations of the bubble and the resulting secondary pressure pulses are reduced and re-shaped.
The bubble represents a secondary blast of energy that eventually appears in the seismic trace as a fake signal following the primary reflection with a delay equal to the bubble period. In Figure A-2, the sensor response is illustrated (green line) when only the Generator is activated (top) and when both chambers (Generator Injector) are activated (bottom). The reduction of the bubble effect (between 130 ms and 160 ms in the top) is evident.
Figure A-2. Gun controller display, basic principle and GI gun underwater bubble.
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Table A-1. Source characteristics.
SOURCE ARRAY
Manufacturer SERCEL
Model 2 GI Gun (CSIRO)
Volume 210 cu.in. (3.44l)
Mode Harmonic 105 G / 105 I
Weight 1. 74 Kg
Firing cycle 1. 7 sec
Air pressure 2000 psi (140 bar)
ARRAY GEOMETRY
Figure A-3. Gun array deployment.
Air Compressor and high pressure manifold
Manufacturer Hamworthy (Wärtsilä)
Model 4TH565EW100
Number of compressors 2
Air delivery 678 m3/h
Speed 1200 RPM
Power 205 kV
Pressure max 207 bar
Number of stage 4
Total weight 2680 kg
Receiver tanks 8 x 50 litres
Regulation Fisher valve
4
Gun
synchronization
GUN CONTROLLER FEATURES AND TECHNICAL SPECIFICATIONS
Manufacturer Real Time systems
Model Big Shot
Installation Fixed
Number of modules 2
Number of guns 16
Gun types Bolt, Sleeve, G and G.I.
Timing resolution 0.1 msec
Record out 100 msec closure (programmable time)
Predicted fire out 5 ms TTL high going
Field Time Break Out Summed sensor or hydrophone signal
Sensors and hydrophone signature 16 bit D / A
Aim Point 25 – 75 msec after trigger
Fire pulse width 1 -80 msec
Delta error Q / C limits 0.1 – 5.0 msec
Auto-fire detect level 0 – 10 Volts
Sensor signal gain x 0.1 – x 5
Sensor detection look window 2 – 40 msec
Sensor peak threshold 0 – 10 Volt
Sensor peak type Threshold, Peak or Zero Cross
Figure A-4. Starboard side air compressor High-pressure manifold
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Figure A-5. The Big Shot gun controller main window. It is possible to see the first four bursts of energy perfectly centred on the 50 ms aim point (red vertical solid line), that is the delay between the navigation trigger arrival and the actual shot time.
Power Requirements 115/230 VAC, 3.0/1.5 A max, 50/60 Hz
Voltage to Streamer 36-72 VDC
I/O Communications 100Base TX Fast Ethernet, IEEE 802.3 compliant
Ethernet Connection RJ-45
Trigger Connection BNC
Trigger Requirements Contact closure, positive or negative TTL
Auxiliary Inputs 4 analog channels with 24-bit resolution
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Table A-4. Streamer characteristics.
FEATURES AND TECHNICAL SPECIFICATIONS 2
Deck Cable
Electrical Conductors 7 twisted 24GA pairs and 10 16GA conductors
Strain Member Vectran
Length 50 m
Diameter 18.6 mm
Weight 21 kg for 50 meters
Termination Water tight connector
Bend Diameter 12 inches Tow Cable
Electrical Conductors 7 twisted 24GA pairs and 10 16GA conductors
Strain Member Vectran
Break Strength Over 2200 kg (5000 lbs)
Length 70 m
Diameter 18.6 mm
Weight 21 kg for 50 meters
Active Section
Number of channels 8 per section
Group Interval 3.125 m
Section Length 25 m
Hydrophones per Group 4
Hydrophones per Section 32
Hydrophone Type Benthos RDA Geopoint
Group Sensitivity 9 V /bar
Jacket Material Clear polyurethane, 70 duro, 3.18 mm (1/8 in) wall thickness
Diameter 41 mm (1.6 in)
Ballast fluid Inert, high flashpoint, non-polluting silicone oil, 100 cSt to 3 cSt
Weight 1.35 kg (3 lbs) /m
Strain Member Vectran
Break Strength 2200 kg (5000 lbs)
Typical Towing Noise <7 μbars at 4.5 knots, 8 Hz low-cut filter, Beaufort 5-6 seas
Minimum Bend Radius 750 mm (30 in)
Compass / Bird Coil I/O Model 587
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Table 2 Streamer winch characteristics.
FEATURES AND TECHNICAL SPECIFICATIONS
Engine Lombardini DIESEL 15LD350
Drum diameter 1.5 m
Winch Weight 650 kg
Overall Weight (winch + 300 m long streamer + tow cable) 960 kg
Dimensions 200 cm X 125 cm X 200 (H) cm
Figure A-7. The GeoEel streamer wrapped around the winch drum
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Table A-6. DigiSCAN system characteristics.
DigiSCAN STREAMER DEPTH CONTROL Manufacturer
DigiCourse
Model DigiBIRD5010
Operating Range 0 to 122 m
Resolution 0.15 m
Number of birds 4
Power Supply 4 D cell Lithium batteries
Figure A-8. Shipboard control: Real time birds depth monitoring In water device: DigiBIRD
Figure A-9. Seismic acquisition workstation on board the RV Investigator: DigiSCAN shipboard control (A), navigation PC with PDS200 software (B), seismic acquisition PC with CNT 2 Marine controller software (C), CNT 1 topside (D) and QINSy / deck camera screen monitor (E).