The device of seismic prospecting, in particular a seismic source used in seismic off-shore prospecting

摘要

1,107,864. Geophysical prospecting. MOBIL OIL CORPORATION. 23 March, 1965 [23 March, 1964], No. 12235/65. Heading H4D. [Also in Division F1] An underwater seismic source comprises a chamber from which a pressurized gas, which may be compressed air or diesel-fuel combustion products, is released through a valve to generate an acoustic pulse. In the embodiment of Fig. 3, a-chamber 10 is provided with a valve 13 which slides on a tube 50 and which comprises a lower head 14 for sealing chamber 10 from the water, and an upper head 16 for defining with a fixed member 35 a secondary chamber 23. Chamber 23 is permanently vented to the atmosphere through narrow conduits 40, 43. In operation, compressed air passed through a conduit 56 to tube 50 enters the hollow stem, 15, of valve 13 through an aperture 52 in tube 50 and raises valve 13 to its closed position. Chamber 10 is then pressurized through a conduit 72, and the chamber pressure acting on valve 13, whose upper head 16 is of greater diameter than its lower head 14, is sufficient in itself to maintain valve 13 in the closed position. To initiate an acoustic pulse, a solenoid valve 24 is opened to apply the chamber 10 pressure to chamber 23, thereby driving a valve 13 downwards and releasing a bubble of compressed air from chamber 10 into the water. Valve 13 is returned to its closed position while the bubble encompasses the lower end of chamber 10, thereby preventing entry of water into the chamber. To reduce secondary pressure pulses arising from oscillations of the bubble, air at atmospheric pressure is drawn into the rising bubble during its first contraction through tubes (80) (Fig. 1) whose upper ends communicate with the atmosphere and whose lower ends are provided with sprung flap-valves (83) in communication with the bubble. In the embodiment of Fig. 5, compressed air is fed to the main chamber, 100, through conduit 104, and diesel fuel is injected through nozzles 105. The mixture is ignited by heating elements 106, and when the chamber pressure reaches a maximum value an acoustic pulse is initiated (as in the Fig. 3 embodiment) by applying the main chamber 100 pressure through a solenoid valve, 124, to a secondary chamber, 119. The doubleheaded valve, 101, is returned to its closed position by a spring 116. A further valve (not shown) may be provided in chamber 100 to enable fresh air from conduit 104 to flush out exhaust gases. In modifications of the Fig. 5 embodiment, the air and diesel-fuel may be mixed before being fed to chamber 100, or a charge of propane gas from a supply (161) (Fig. 7) may be ignited by spark-plug(s) (163) to promote the combustion of the air-dieselfuel mixture. In either embodiment the motion of the double-headed valve (13 or 101) at either end of its travel may be braked by hydraulic or hydraulic-and-spring dampers, and solenoid valves controlled by motor-driven cam-operated switches are used to control the feed of compressed air or combustible fuel to the chambers. Numerical examples of pressures and dimensions are given.