Marine Siren Seismic Source

Abstract

A very low frequency marine seismic source has a reservoir of water ( 47 ) feeding water to an aperture communicating with the surrounding water ( 13 ). The rate of water flow through the aperture is controlled by a rotor disc ( 11 ) and stator disc ( 9 ) having holes which overlap to a greater or lesser extent as the rotor rotates. The modulation of the flow of water produces modulated pressure signal which is radiated into the surrounding water. The device is intended to produce acoustic signals over a band extending down to 0.5 Hz or lower.

1 .- 29 . (canceled) 
     
     
         30 . A marine seismic source suitable for producing seismic signals over a band of frequencies comprising:
 a reservoir for containing water;   a flow path between the reservoir and a body of water, wherein water in the reservoir flows into the body of water via the flow path;   a modulating mechanism to modulate an area of the flow path whereby the flow of water from the reservoir into the body of water to produce a modulated pressure signal in the body of water; and   a buffer configured to minimize fluctuations in static pressure of the water within at least one of the reservoir and flow path as the area of the flow path is modulated, the buffer comprising a plenum for containing gas in communication with at least one of the reservoir and flow path.   
     
     
         31 . The seismic source according to  claim 31 , wherein the buffer is arranged such that water from the reservoir or flow path can flow into or out of the plenum in contact with the gas as the area of the flow path is modulated. 
     
     
         32 . The seismic source according to  claim 31 , the source being adapted to be drawn through the body of water to cause water to flow from the reservoir and along the flow path, the reservoir comprising a body of water in which the source is immersed. 
     
     
         33 . The seismic source according to  claim 31 , further comprising a pressurizing mechanism to increase and reduce the pressure of the gas within the plenum. 
     
     
         34 . The seismic source according to  claim 34 , further comprising a tube which defines the reservoir and flow path. 
     
     
         35 . The seismic source according to  claim 35 , wherein the plenum extends around the tube. 
     
     
         36 . The seismic source according to  claim 31 , wherein water is caused to flow from the reservoir into the body of water by of pressurized gas. 
     
     
         37 . The seismic source according to  claim 37 , wherein the pressurized gas is held in a pressure vessel, the pressure vessel and reservoir being adapted to be held within the body of water. 
     
     
         38 . The seismic source according to  claim 38 , wherein the pressure vessel and reservoir are joined and separated by a wall provided with a valve to allow the passage of gas from the pressure vessel into the reservoir. 
     
     
         39 . The seismic source according to  claim 39 , further comprising a mechanism to discharge gas from the reservoir without the discharge gas entering the external body of water in order to refill the reservoir with water and wherein water is caused to flow from the reservoir into the body of water by pressurized gas held in a pressure vessel, the pressure vessel and reservoir being adapted to be held within the body of water wherein the pressure vessel and reservoir are joined and separated by a wall provided with a valve to allow the passage of gas from the pressure vessel into the reservoir. 
     
     
         40 . The seismic source according to  claim 31 , further including a mechanism to vary the flow of water from the reservoir in proportion to the period of the radiated signal to maintain constant the energy radiated per unit bandwidth of the output signal. 
     
     
         41 . The seismic source according to  claim 31 , wherein:
 the flow path comprises a duct arranged to extend downwards into the body of water;   the duct is longer than it is wide;   the modulating mechanism is located at or towards the lower end of the duct; and the seismic source comprising a plenum in communication with the duct at a point above the modulating mechanism, the plenum being capable of holding a sufficient volume of gas at a pressure substantially equal to that of the water at the point at which the duct area is modulated so as to substantially reduce or prevent fluctuations in the pressure of the water at the said point of communication.   
     
     
         42 . The seismic source according to  claim 31 , further comprising a fluid pump configured to drive water along the flow path. 
     
     
         43 . The seismic source according to  claim 31 , wherein the modulating mechanism comprises a valve having one or more vanes rotatable between an open and closed position about an axis normal to the direction of fluid flow through the flow path. 
     
     
         44 . A marine seismic source suitable for producing seismic signals over a band of frequencies comprising:
 a reservoir of water;   a flow path to provide water communication between the reservoir and a body of water;   a modulating mechanism to modulate the area of the flow path thereby modulating the flow of water into the body of water so as to produce a modulated pressure signal in the body of water; and   a buffer configured to minimize fluctuations in static pressure of the water within at least one of the reservoir and flow path as the area of the flow path is modulated, the buffer comprising a plenum for containing gas in communication with at least one of the reservoir and flow path and arranged such that water from the reservoir or flow path can flow into or out of the plenum in contact with the gas as the area of the flow path is modulated.   
     
     
         45 . The seismic source according to  claim 44 , wherein the modulating mechanism comprises a valve having one or more vanes rotatable between an open and closed position about an axis normal to the direction of fluid flow through the flow path. 
     
     
         46 . A method of producing seismic signals in a body of water over a band of frequencies, comprising:
 directing a flow of water from a reservoir into the body of water through a duct;   modulating the flow of water by modulating the area of the duct through which the water flows;   minimizing fluctuations in static pressure of the water within at least one of the reservoir and flow path as the area of the flow path is modulated, the minimizing by a buffer mechanism comprising a plenum for containing gas in communication with at least one of the reservoir and flow path and arranged such that water from the reservoir or flow path can flow into or out of the plenum in contact with the gas as the area of the flow path is modulated.   
     
     
         47 . The method of producing seismic signals according to  claim 46 , wherein the seismic source is moved through the body of water so as to cause water to flow from the reservoir through the duct and into the body of water, the reservoir comprising a body of water in which the source is immersed. 
     
     
         48 . The method according to  claim 47 , wherein the reservoir and duct are formed from a tube which is held substantially parallel to the surface of the body of water. 
     
     
         49 . The method according to  claim 46 , wherein the modulating of the flow of water comprises controlling a valve having one or more vanes rotatable between an open and closed position about an axis normal to the direction of fluid flow through the flow path.