Marine Seismic Vibrators and Methods of Use

Abstract

Embodiments relate to marine seismic vibrators and associated methods of use. An embodiment provides a marine seismic vibrator comprising: a shell comprising endbeams and shell side portions coupled to the endbeams, wherein each of the shell side portions has a midline between the endbeams, wherein each of the shell side portions has a thinner portion at the midline to force each of the shell side portions to bend at the midline; a driver disposed within the shell; and a pair of spring elements disposed within the shell on either side of the driver, wherein the pair of spring elements are coupled to the driver and to the shell such that movement of the driver is transferred to the shell by way of the spring elements, wherein the pair of spring elements have a second mode of oscillation that provides a second resonance frequency within the operational frequency range.

What is claimed is: 
     
         1 . A marine seismic vibrator comprising:
 a shell comprising endbeams and shell side portions coupled to the endbeams, wherein each of the shell side portions has a midline between the endbeams, wherein each of the shell side portions has a thinner portion at the midline to force each of the shell side portions to bend at the midline;   a driver disposed within the shell; and   a pair of spring elements disposed within the shell on either side of the driver, wherein the pair of spring elements are coupled to the driver and to the shell such that movement of the driver is transferred to the shell by way of the spring elements, wherein the pair of spring elements have a second mode of oscillation that provides a second resonance frequency within the operational frequency range.   
     
     
         2 . The marine seismic vibrator of  claim 1 , wherein the shell side portions are hingedly coupled to the endbeams. 
     
     
         3 . The marine seismic vibrator of  claim 1 , wherein the pair of spring elements are hingedly coupled to the endbeams. 
     
     
         4 . The marine seismic vibrator of  claim 1 , wherein the first resonance frequency of the marine seismic vibrator is within a two octave band of a low end of the operational frequency range, and wherein the second resonance frequency of the marine seismic vibrator is within a four octave band of the first resonance frequency. 
     
     
         5 . The marine seismic vibrator of  claim 1 , wherein the shell has a spring constant selected to provide a first resonance frequency within an operational frequency range of about 1 Hz and about 300 Hz. 
     
     
         6 . The marine seismic vibrator of  claim 1 , wherein the shell comprises a pair of the shell side portions on opposing sides of the driver from one another. 
     
     
         7 . The marine seismic vibrator of  claim 6 , further comprising a fixture coupled to the endbeams, wherein the driver is coupled to the fixture. 
     
     
         8 . The marine seismic vibrator of  claim 7 , wherein the driver comprises a pair of drive coils on opposing sides of the fixture, wherein the drive coils are each coupled to a different one of the pair of the spring elements. 
     
     
         9 . The marine seismic vibrator of  claim 1 , wherein the driver comprises a single, bi-directional linear actuator. 
     
     
         10 . The marine seismic vibrator of  claim 1 , wherein the driver comprises more than one uni-directional driver. 
     
     
         11 . The marine seismic vibrator of  claim 1 , wherein the shell side portions each comprise at least one of carbon fiber or glass fiber reinforced plastic. 
     
     
         12 . The marine seismic vibrator of  claim 1 , wherein the spring element comprises at least one of carbon fiber or glass fiber reinforced plastic. 
     
     
         13 . An array of marine seismic vibrators comprising:
 one or more low frequency marine seismic vibrators, wherein the one or more low frequency marine seismic vibrators each comprise:
 a first shell having a spring constant selected to provide a first resonance frequency within a first operational frequency range of about 5 Hz and about 25 Hz, wherein the first shell comprises endbeams and shell side portions coupled to the endbeams, wherein each of the shell side portions has a midline between the endbeams, wherein each of the shell side portions has a thinner portion at the midline to force each of the shell side portions to bend at the midline; 
 a first driver disposed within the shell; and 
 a first pair of spring elements disposed within the first shell on either side of the first driver, wherein the first pair of spring elements are coupled to the first driver and to the first shell such that movement of the first driver is transferred to the first shell by way of the first spring elements, wherein the first pair of spring elements have a second mode of oscillation that provides a second resonance frequency within the operational frequency range; and 
   one or more high frequency marine seismic vibrators, wherein the one or more high frequency marine seismic vibrator each comprise:
 a second shell having a spring constant selected to provide a first resonance frequency within an second operational frequency range of about 25 Hz to about 100 Hz, wherein the second shell comprises endbeams and shell side portions coupled to the endbeams, wherein each of the shell side portions has a midline between the endbeams, wherein each of the shell side portions has a thinner portion at the midline to force each of the shell side portions to bend at the midline; 
 a second driver disposed within the second shell; and 
 a second pair of spring elements disposed within the second shell on either side of the second driver, wherein the second pair of spring elements are coupled to the second driver and to the second shell such that movement of the second driver is transferred to the second shell by way of the second spring elements, wherein the second pair of spring elements have a second mode of oscillation that provides a second resonance frequency within the operational frequency range. 
   
     
     
         14 . The array of  claim 13 , wherein the first resonance frequency of the one or more low frequency marine seismic vibrators is within a two octave band of a low end of the operational frequency range, and wherein the second resonance frequency of the one or more low frequency marine seismic vibrators is within a four octave band of the first resonance frequency. 
     
     
         15 . The array of  claim 13  further comprising one or more additional marine seismic vibrators having an operational frequency from about 100 Hz to about 200 Hz. 
     
     
         16 . The array of  claim 13  wherein the array comprises four of the low frequency marine seismic vibrators and eight of the high frequency marine vibrators. 
     
     
         17 . The array of  claim 13  further comprising an air supply located on a survey vessel for supplying air for pressure compensation in the low frequency marine vibrators and the high frequency marine vibrators. 
     
     
         18 . A method for subsurface seismic exploration comprising:
 towing a marine seismic vibrator through a body of water, wherein the marine seismic vibrator comprises a shell, a driver disposed within the shell, and a pair of spring elements disposed within the shell on either side of the driver, wherein the shell comprises endbeams and shell side portions coupled to the endbeams, wherein each of the shell side portions has a midline between the endbeams, and wherein each of the shell side portions has a thinner portion at the midline;   operating the marine seismic vibrator in the body of water, wherein linear movement of the driver causes movement of the pair of spring elements that is then transferred to the shell, wherein the shell provides a first resonance frequency of the marine seismic vibrator, and wherein the spring element has a second mode of oscillation that provides a second resonance frequency of the marine seismic vibrator, wherein the thinner portion of the shell side portions causes the shell side portions to bend at the midline; and   detecting seismic signals originating from the marine seismic vibrator.   
     
     
         19 . The method of  claim 18 , wherein the first resonance frequency of the marine seismic vibrator is within a two octave band of a low end of the operational frequency range, and wherein the second resonance frequency of the marine seismic vibrator is within a four octave band of the first resonance frequency. 
     
     
         20 . The method of  claim 18 , wherein the marine seismic vibrator is operated at an operational frequency range from about 5 Hz to about 100 Hz.