Abstract:
A seismic vibrator includes a baseplate having a surface configured to couple to a ground surface. A driver is coupled to the baseplate and is configured to move the baseplate in a vibratory manner. A decoupling system is coupled to a part of the baseplate other than the ground-contacting surface. The decoupling system includes a first layer having a Young&#39;s modulus greater than that of a second layer coupled to the first layer. The second layer is coupled to the baseplate. The Young&#39;s moduli, thicknesses and masses of the first and second layer are selected to provide the decoupling system with a resonant frequency of at most, a spatial aliasing frequency of seismic sensors deployed on the ground surface or a lowest seismic frequency of interest.

Description:
BACKGROUND 
       [0001]    The present disclosure is related generally to the field of vibrators used as seismic energy sources. More specifically, the present disclosure is related to vibrators having capability to suppress airwaves generated by the operation of such vibrators. 
         [0002]    Seismic surveys for oil and gas exploration commonly use seismic vibrators to generate seismic energy that is transmitted into the Earth&#39;s subsurface. Airwaves (also often referred to as “airblast” or “air-coupled” waves, hereinafter “airwaves”) are coherent noise trains produced by a surface seismic source, propagating at the speed of sound in air. Airwaves may be entirely coupled with the air, or they may be partially coupled with the near surface if the phase velocity of Rayleigh waves and the speed of sound in air are the same. Seismic vibrators usually operate above the ground surface, with vibrational energy transmitted into the subsurface through a baseplate resting on the ground surface. In such seismic surveys, it is common to make use of a vibrator mounted on a truck. Because the majority of the vibrator is exposed to the air, including the upper surface of the baseplate, some of the vibrational energy during operation is transmitted through the air as sound waves. 
         [0003]    Such air-coupled sound waves are often of sufficient intensity to detrimentally affect the measurement of seismic signals of interest, specifically those seismic signals reflected from acoustic impedance boundaries in the subsurface. The reflected seismic signals are small in magnitude and waves propagating through the air may cause slight vibrations of seismic sensors (typically geophones or accelerometers) deployed proximate the ground surface, or vibrations of the ground itself. Such vibrations are typically of relatively high amplitude, and may result in such air-coupled waves being detected by the seismic sensors and recorded. Because air waves can cause the ground itself to vibrate, burial or shielding of the seismic sensors often times fails to adequately address the problem. 
         [0004]    Airwave noise is strongest at higher frequencies, typically 30 Hz and above. The actual value of the frequency range at which air-wave coupled Rayleigh waves are more energetic depends on the elastic properties of formations proximate the Earth&#39;s surface. 
         [0005]    Signal processing and hardware techniques have been used to attenuate the effects of airwaves on detected seismic signals. Signal processing techniques for removing Rayleigh waves (typically having a frequency less than 15 Hz.) have proven ineffective at the frequencies associated with airwaves because typical seismic sensor spacing results in spatial aliasing. Spatial aliasing of surface waves can be mitigated using point receiver acquisition, as contrasted with the typical practice of summing signals from subsets of the seismic sensors to attenuate the effects of near surface propagating seismic waves, but because of low propagation velocity of typical airwaves, the higher and more energetic frequencies thereof tend to remain spatially aliased. 
         [0006]    It is desirable to have an improved method and apparatus for reducing the effects of airwaves on seismic signals detected from a vibrator-type seismic energy source. 
       SUMMARY 
       [0007]    One aspect of the present disclosure is a seismic vibrator including a baseplate having a surface configured to couple to a ground surface. A driver is coupled to the baseplate and is configured to move the baseplate in a vibratory manner. A decoupling system is coupled to a part of the baseplate other than the ground-contacting surface. The decoupling system includes a first layer having a Young&#39;s modulus greater than that of a second layer coupled to the first layer. The second layer is coupled to the baseplate. The Young&#39;s moduli, thicknesses and masses of the first and second layer are selected to provide the decoupling system with a resonant frequency of at most, a spatial aliasing frequency of seismic sensors deployed on the ground surface or a lowest seismic frequency of interest. 
         [0008]    A method for imparting seismic energy into the ground according to another aspect of the present disclosure includes driving a baseplate in contact with the ground in a vibratory manner. Motion of a part of the baseplate not in contact with the ground is coupled to a decoupling system comprising a first layer in contact with a second layer. The second layer is in contact with the part of the baseplate not in contact with the ground. A Young&#39;s modulus, thickness and mass of the first and second layers are selected so that the decoupling system has a resonant frequency of at most, a spatial aliasing frequency of seismic sensors deployed on the ground surface or a lowest seismic frequency of interest. 
         [0009]    Other aspects and advantages will be apparent from the description and claims which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows an example seismic vibrator having a baseplate and decoupling system assembly. 
           [0011]      FIG. 2  shows an example seismic vibrator baseplate having a decoupling system associated therewith. 
           [0012]      FIG. 3  shows an equivalent physical system to the vibrator baseplate and decoupling system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A non-limiting example seismic vibrator is illustrated in  FIG. 1  at  10 . The vibrator structure shown in  FIG. 1  is only one example of seismic vibrators that may be used with a baseplate and decoupling system assembly, the latter explained in more detail below. Therefore, the example vibrator shown in and explained with reference to  FIG. 1  is not to be construed as limiting the scope of any present or subsequent claims related to the present application. The seismic vibrator  10  transmits force to the ground  30  using a base plate and decoupling system assembly  20  and a reaction mass  50 . The vibrator  10  may be mounted on a carrier vehicle (not shown) that uses a mechanism and bars shown at  12  and  14  to lower the vibrator  10  to the ground. With the vibrator  10  lowered, the weight of the vehicle (not shown) can hold the baseplate and decoupling system assembly  20  engaged with the ground  30  so seismic source signals can be transmitted into the subsurface. 
         [0014]    The baseplate and decoupling system assembly may be moved in a vibratory manner by a driver. The driver may include components according to the following non limiting example. The reaction mass  50  may be positioned directly above base plate and decoupling system assembly  20 . Stilts  52  may extend from the base plate and decoupling system assembly  20  and through the reaction mass  50  to stabilize the stilts  52 . Internally, the reaction mass  50  may have a cylinder  56  formed therein. A vertically extending piston  60  may extend through the cylinder  56 , and a head  62  on the piston  60  divides the cylinder  56  into upper and lower chambers. The piston  60  may be connected at its lower end to a hub in a lower cross piece  54 L and may extend upward through cylinder  56 . The piston  60  upper end connects to a hub on an upper cross piece  54 U, and the cross pieces  54 U  54 L may be connected to the stilts  52 . 
         [0015]    To isolate the baseplate and decoupling system assembly  20  from the bars  14 , the bars  14  may have feet  16  with isolators  40  disposed between the feet  16  and the base plate and decoupling system assembly  20 . As shown, two isolators  40  are disposed under each foot  16 . In addition, the feet  16  may have tension members  42  interconnected between the edges of the feet  16  and the base plate  20 . The tension members  42  are used to hold the base plate  20  when the vibrator  10  is raised and lowered to the ground. Finally, shock absorbers (not shown) may also be mounted between the bottom of the feet  16  and the base plate  20  to isolate vibrations therebetween. 
         [0016]    During operation, a controller  80  may receive signals from a first sensor  85  coupled to the upper cross piece  54 U and may receive signals from a second sensor  87  coupled to the reaction mass  50 . Based on feedback from these sensors  85 ,  87  and a desired driver signal (usually a “sweep” or “chirp”) for operating the vibrator  10 , the controller  80  generates a drive signal to control a servo valve assembly  82 . Driven by the drive signal, the servo valve assembly  82  alternatingly routes high pressure hydraulic fluid between a hydraulic fluid supply  84  and upper and lower cylinder piston chambers via ports in the reaction mass  50 . As hydraulic fluid alternatingly accumulates in the piston  60  chambers located immediately above and below the piston head  62 , the reaction mass  50  reciprocally vibrates in a vertical direction on the piston  60 . In turn, the force generated by the vibrating reaction mass  50  may transfer to the base plate and decoupling system assembly  20  via the stilts  52  and the piston  60  so that the base plate and decoupling system assembly  20  vibrates at a desired amplitude and frequency or sweep to impart a seismic source signal into the ground  30 . It will be appreciated by those skilled in the art that the motion of the driver may be coupled directly to the baseplate ( 23  in  FIGS. 2 and 3 ), while motion of the baseplate may be coupled to one of two layers which form the decoupling system ( 25  in  FIG. 2 ) for at least part of the baseplate not in contact with the ground surface  30 . 
         [0017]    As the moving reaction mass  50  acts upon the baseplate and decoupling system assembly  20  to impart a seismic source signal into the subsurface, the signal travels through the earth, reflects at discontinuities and formations, and then travels toward the Earth&#39;s surface. At the surface, an array of seismic sensors  13  coupled to the ground  30  detects the reflected signals, and a recording device (not shown) records the signals from the seismic sensors  13 . The recording device (not shown) may use a correlation processor or other processor to correlate the computed ground force supplied by the seismic source to the seismic signals received by the seismic sensors. The vibrator  10  may have a hydraulic pump subsystem with hydraulic lines that carry hydraulic fluid to the servo valve assembly  80 , and a cooler may be present to cool the hydraulic subsystem. 
         [0018]    A local sensor (e.g., accelerometer or geophone)  85  may be positioned on the upper cross piece  54 U of the vibrator  10 , which may be positioned above the reaction mass  50 . Affixed at a location  55  on the upper cross piece  54 U, the local sensor  85  couples to the baseplate and decoupling system assembly  20  through the stilts  52 . 
         [0019]    In operation, the controller  80  may measure the signal imparted into the ground  30  using signals from the local sensor  85 . The measured signals may be transmitted to a correlation processor or other processor (not shown), which may also receive the signals from the seismic sensors  13 . The seismic sensors  13  may be separated by a distance shown at d in  FIG. 1 . The distance d is related to the spatial aliasing frequency of the airwave. Depending on the distance d, parameters for components of the baseplate and decoupling system assembly  20  may be selected as explained below. 
         [0020]    The baseplate and decoupling system assembly may include a two-layer decoupling system affixed to the upper surface of the baseplate. Referring to  FIG. 2 , the decoupling system  25  may be made from a first layer  21  of “stiff” material, for example, steel, or material having a similar value of Young&#39;s modulus as steel and a second layer  22  of “soft” material, for example, rubber or other elastomer coupled to an upper surface of the baseplate  23 . The material used to make the second layer  22  may have a Young&#39;s modulus similar to that of rubber, and the Young&#39;s modulus thereof is generally low enough with respect to the Young&#39;s modulus of the first layer so that the stiffness of the first layer can be considered infinite and its viscosity zero. The materials used for the first layer  21  and the second layer  22  and their respective masses and thicknesses may be selected such that the decoupling system  25  has a resonance frequency lower than the dominant frequency at which airwaves generated by the baseplate  23  affect seismic signals detected by the seismic sensors ( 13  in  FIG. 1 ). The dominant frequency may be selected as follows. 
         [0021]    The materials and their thicknesses of the first  21  and second  22  layers may be chosen such that at least one of the two following conditions is met: (1) the decoupling system  25  has a resonance frequency close to as low as the minimum frequency used in seismic exploration (e.g., 4 Hz) to attenuate the airwave in the entire seismic frequency band of interest; or (2) the decoupling system  25  has a resonance frequency lower than the frequency at which the airwave is spatially aliased. In the latter case, the critical frequency range in which the airwave is spatially aliased will be suppressed. The lower frequency components of the airwave, which are generally properly spatially sampled, can be attenuated using techniques such as those used for the attenuation of ground surface propagating waves. 
         [0022]    An advantage of designing the decoupling system  25  to meet condition 1 is that the airwave may be suppressed within the entire seismic frequency band. An advantage of designing the decoupling system  25  to meet condition 2 is that a thick first layer of stiff material, which may have substantial mass, is not required. The determination of the properties and thicknesses of the materials depicted in  FIG. 2  can be obtained using the equivalent mechanical model depicted in  FIG. 3 . Because the decoupling system  25  is significantly lighter than the baseplate  23 , the interaction between the baseplate  25  and the ground  30  can be ignored. If a stiff material such as steel is used for the first layer  21  and soft rubber is used for the second layer  22 , the stiffness of the steel can be considered infinite and its viscosity zero. The remaining parameters of the mechanical model shown in  FIG. 3  can be determined for the desired resonance frequency and damping factor. 
         [0023]    Assuming M 3 +M 2 =M 3 , K 3 =∞, D 3 =0, the equation of motion for the system shown in  FIG. 3  can be written as: 
         [0000]        M   3   {umlaut over (x)}   d1   =−D   2 ( {umlaut over (x)}   d1   −{dot over (x)}   b )= M   3   g−k   2 ( x   d1   −x   b   l   0 )+ M   3   g   (1)
 
         [0000]    where x d1  is the vertical location of the top of the second layer in an inertial system. Because the stiffness of the first layer is assumed to be infinite, then
 
x d1 =x d2 +Δ s , wherein Δ s  is the thickness of the first layer. l 0  is the thickness of the second layer when the first layer is not applied to it. It is related to the stiffness of the second layer, x 0 , when the first layer is added via the equilibrium equation:
 
         [0000]        M   3   g=k   2 ( l   0   −x   0 )  (2)
 
         [0024]    Eq. (1) can be used to eliminate l 0  in eq. (1) to obtain: 
         [0000]        M   3   {umlaut over (x)}   d1   =−D   2 ( {dot over (x)}   d1   −{dot over (x)}   b )− k   2 ( x   d1   −x   b   +z   0 )  (3)
 
         [0025]    Rewriting eq. (3) relative to the equilibrium position x 0 , one obtains the following expression: 
         [0000]        {umlaut over (x)}   d1 +2ε {dot over (x)}   d1 +ω 0   2   x   d1 =ω 0   2   x   b +2ε {dot over (x)}   b   (4)
 
         [0000]    that express in the time domain the relationship between the baseplate and the top of the second layer displacement. The resonance pulsation is ω 0 =√{square root over (k 2 /M 3 )}, the damping term is 2ε=D 2 /M 3 . Eq. (4) can be rewritten in the frequency domain as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       X 
                       
                         d 
                          
                         
                             
                         
                          
                         1 
                       
                     
                     
                       X 
                       b 
                     
                   
                   = 
                   
                     
                       1 
                       + 
                       
                         2 
                          
                         
                           ω 
                           
                             ω 
                             0 
                             2 
                           
                         
                          
                         ɛ 
                       
                     
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             ω 
                             
                               ω 
                               0 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         2ɛ 
                          
                         
                           ω 
                           
                             ω 
                             0 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0026]    Examples of material properties and thicknesses to obtain resonant frequencies meeting at least one of the two conditions described above are listed in TABLE 1 and TABLE 2, respectively. In both cases shown here, the baseplate area is 2.7 m 2 . 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Resonant Frequency at Bottom of Seismic Frequency Band 
               
             
          
           
               
                 First layer (Steel Plate) 
                 Second layer (Decoupler) 
               
               
                   
               
             
          
           
               
                 Plate 
                 0.008 
                 m 
                 Stiffness per unit area 
                 50000 
                 N/m 3   
               
               
                 Thickness 
               
               
                 Steel Density 
                 7800 
                 kg/m 3   
                 Stiffness 
                 135000 
                 N/m 
               
               
                 Plate Mass 
                 168.48 
                 kg 
                 Thickness 
                 50 
                 mm 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Resonant Frequency at Spatial Aliasing Limit 
               
             
          
           
               
                 First layer (Steel Plate) 
                 Second layer (Decoupler) 
               
               
                   
               
             
          
           
               
                 Plate Thickness 
                 0.004 
                 m 
                 Stiffness per unit 
                 500000 
                 N/m 3   
               
               
                   
                   
                   
                 area 
               
               
                 Steel Density 
                 7800 
                 kg/m 3   
                 Stiffness 
                 1350000 
                 N/m 
               
               
                 Plate Mass 
                 84.24 
                 kg 
                 Thickness 
                 20 
                 mm 
               
               
                   
               
             
          
         
       
     
         [0027]    The resonance frequencies of the decoupling system example in TABLE 1 is 4.51 Hz and the resonance frequency of the decoupling system example in TABLE 2 is 20.15 Hz. 
         [0028]    A seismic vibrator having a baseplate and decoupling system assembly may provide seismic data less affected by airwaves or that may be more easily processed to reduce the effect of airwaves therein. 
         [0029]    While the present application has described aspects with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the present claims or any subsequent related claims in connection with this disclosure.