Multicomponent marine geophysical data gathering system

Abstract

In one embodiment the invention comprises a particle velocity sensor that includes a housing with a geophone mounted in the housing. A fluid that substantially surrounds the geophone is included within the housing. The particle velocity sensor has an acoustic impedance within the range of about 750,000 Newton seconds per cubic meter (Ns/m 3 ) to about 3,000,000 Newton seconds per cubic meter (Ns/m 3 ). In another embodiment the invention comprises method of geophysical exploration in which a seismic signal is generated in a body of water and detected with a plurality of co-located particle velocity sensors and pressure gradient sensors positioned within a seismic cable. The output signal of either or both of the particle velocity sensors or the pressure gradient sensors is modified to substantially equalize the output signals from the particle velocity sensors and the pressure gradient sensors. The output signals from particle velocity sensors and pressure gradient sensors are then combined.

1 . A method of geophysical exploration comprising: 
 generating a seismic signal in a body of water;    detecting said seismic signal with a plurality of co-located particle motion sensor assemblies and pressure gradient sensors positioned within a seismic cable deployed in said body of water;    modifying the output signal of at least one of said particle motion sensor assemblies or said pressure gradient sensors to substantially equalize the output signals from said particle motion sensor assemblies and said pressure gradient sensors within at least a selected frequency range; and    combining the modified output signals from co-located particle motion sensor assemblies and pressure gradient sensors within at least said selected frequency range.    
   
   
       2 . The method of  claim 1  wherein the amplitude and phase of the output signals from said particle motion sensor assemblies and said pressure gradient sensors are substantially matched within said at least a selected frequency range  
   
   
       3 . The method of  claim 1  wherein modifying the output signals from either said particle motion sensor assemblies or said pressure gradient sensors is performed independently of the acoustic impedance of material through which said seismic signal travels.  
   
   
       4 . The method of  claim 1  wherein the output signals of said pressure gradient sensors and said particle motion sensor assemblies are substantially equalized during processing and combined.  
   
   
       5 . The method of  claim 1  wherein the amplitude and phase of the pressure gradient sensors and the particle motion sensor assemblies are equalized.  
   
   
       6 . The method of  claim 1  wherein output signals from said particle motion sensor assemblies and said pressure gradient sensor are combined to reduce spectral notches above frequencies of about 20 Hz.  
   
   
       7 . The method of  claim 1  wherein said particle motion sensor assemblies and pressure gradient sensors are positioned in the interior of a seismic cable having an inside diameter of about 55 millimeters.  
   
   
       8 . The method of  claim 1  wherein said particle motion sensor assemblies and pressure gradient sensors are positioned in the interior of a seismic cable having an inside diameter of about 66 millimeters.  
   
   
       9 . The method of  claim 1  wherein said seismic cable is deployed at a depth of less that six meters.  
   
   
       10 . The method of  claim 1  wherein said seismic cable is deployed at a depth of greater than nine meters.  
   
   
       11 . The method of  claim 1  wherein said particle motion sensor assemblies have an acoustic impedance with the range of about 750,000 Newton seconds per cubic meter to about 3,000,000 Newton seconds per cubic meter.  
   
   
       12 . The method of  claim 1  wherein said particle motion sensor assemblies have an acoustic impedance substantially equal to the acoustic impedance of the water in said body of water in which said cable is deployed.  
   
   
       13 . The method of  claim 1  wherein said seismic cable is a liquid-filled cable.  
   
   
       14 . The method of  claim 1  wherein said seismic cable is a gel-filled cable.  
   
   
       15 . The method of  claim 1  wherein said seismic cable is a solid cable.  
   
   
       16 . The method of  claim 1  wherein said cable is towed through said body of water.  
   
   
       17 . The method of  claim 1  wherein said cable is maintained at a substantially stationary position.  
   
   
       18 . The method of  claim 1  wherein at least a portion of the particle motion sensor assemblies are electrically interconnected in groups to generate a particle velocity output signals.  
   
   
       19 . The method of  claim 18  wherein at least a portion of the particle motion sensor assemblies are electrically interconnected in series in groups of at least three sensors.  
   
   
       20 . The method of  claim 18  wherein at least a portion of the particle motion sensor assemblies are electrically interconnected in parallel.  
   
   
       21 . The method of  claim 1  wherein particle motion sensor assemblies include sensors mounted in said cable in an orientation to detect signals in the vertical direction, the cross line direction and in-line direction.  
   
   
       22 . A method of geophysical exploration comprising: 
 deploying a seismic cable in a body of water, said seismic cable having a plurality of particle motion sensor assemblies included within said cable, said particle motion sensor assemblies having an acoustic impedance within the range of about 750,000 Newton seconds per cubic meter to about 3,000,000 Newton seconds per cubic meter; and    utilizing said seismic cable for detecting seismic data signals.    
   
   
       23 . The method of  claim 22  wherein said particle motion sensor assemblies have an acoustic impedance equal to about the impedance of the water in said body of water in which said seismic cable is deployed.  
   
   
       24 . A method of geophysical exploration comprising: 
 deploying a seismic cable in a body of water, said seismic cable having a plurality of motion sensor assemblies included within said cable, said particle motion sensor assemblies having a density less than 2 grams per cubic centimeter; and    utilizing said seismic cable for detecting seismic data signals.    
   
   
       25 . The method of  claim 24  wherein the density of said particle motion sensor assemblies is substantially equal to the density of the water in said body of water in which said seismic cable is deployed.  
   
   
       26 . A method of geophysical exploration comprising: 
 deploying a seismic cable in a body of water, said seismic cable having a plurality of particle motion sensor assemblies included within said cable;    wherein said particle motion sensor assemblies comprise a housing, a gimbal-mounted particle motion sensor mounted in said housing, a fluid within said housing substantially surrounding said particle motion sensor, said fluid having a viscosity selected to restrain noise-generating movement of said particle motion sensor and to allow said particle motion sensor to maintain a selected orientation as said housing is rotate; and    utilizing said seismic cable for detecting seismic data signals.    
   
   
       27 . The method of  claim 26  wherein said fluid has a viscosity of greater than about 500 centistokes and less than about 5000 centistokes.  
   
   
       28 . A method of processing marine seismic data to reduce spectral notches resulting from surface ghost reflections, comprising: 
 determining the amplitude and phase variation with frequency of the output of a particle motion sensor assembly of a co-located sensor pair comprising a particle motion sensor assembly and a pressure gradient sensor, independently of any variation in amplitude or phase with frequency of said particle motion sensor assembly resulting from impedance mismatch between said particle motion sensor assembly and a medium from which a seismic wave is coupled to said particle motion sensor assembly;    modifying the output signal of at least one of said motion sensor assemblies or pressure gradient sensors to compensate for said determined amplitude and phase variation, thereby generating modified output signals; and    summing said modified output signals from said pressure gradient sensor and said motion sensor assembly.