Abstract:
A vibrator truck includes a vehicle and a vibration assembly mobile relative to each other by a lift system having at least one footbase and configured to lift up/down the vibration assembly. The lift system includes at least one flexible strap and a coupling device configured to couple the at least one flexible strap to the vibratory part and the at least one footbase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority from and the benefit of U.S. Provisional Application No. 62/158,927, filed May 8, 2015, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The field of the disclosure is that of acquiring geophysical and seismic data on the ground, in particular for the purpose of exploiting fields of hydrocarbons. 
     In order to collect geophysical data, one or more seismic sources in contact with the ground are activated to propagate series of omnidirectional seismic waves. The series of waves reflected by the strata of the subsurface are then detected by sensors which generate a signal characterizing the reflection of the waves on the geological interfaces of the subsurface. 
     More specifically, the disclosure relates to a vibrator truck configured as a seismic source. 
     TECHNOLOGICAL BACKGROUND 
     Traditionally, and as shown in  FIG. 1 , a vibrator truck  1000  comprises a vehicle (also referred to as “mobile platform”)  1001  and a vibration assembly (also referred to as “shaker”)  1002  which are mobile relative to each other by a lift system assembly (also referred to as “ground-hugging assembly”). 
     The function of the vibration assembly is to apply, to the ground, a variable force (of the sinusoidal type for example) of predetermined nominal value. To this end, and referring to  FIG. 2 , the vibration assembly includes a vibrator part and a baseplate (also referred to as “support plate”)  210  for coupling to the ground. The baseplate  210  is used to transmit the vibrations generated by the vibrator part through the ground. In the shown example, the vibrator part includes a reaction mass  270 , a drive piston  260 , an upper plate  280  and stilt legs  225 . The stilt legs, located between the upper plate  280  and the baseplate  210 , are intended to more evenly distribute the pressure applied by the weight of the mobile platform onto the baseplate  210 . The drive piston  260  guides the reaction mass  270  in a relative movement with respect to the baseplate  210  when the servo valve is operated. The vibrator part is thus used to generate a seismic vibration wave and to transmit it into the ground. 
     The ground-hugging assembly is fixed to the vibration assembly, and mounted to slide on the chassis  100  of the vehicle  1001 :
         firstly to place the vibration assembly on the ground or raise it. In other words, the ground-hugging assembly behaves like a lift system configured to lift the vibration assembly up (for moving the vehicle between two vibration locations) or down (for placing it on the ground, at a vibration location); and   secondly to transfer the weight of the mobile platform onto the baseplate  210  and the top plate  280 , which thus applies a static pre-load in a direction along an axis  1 , to increase the power of the vibration transmitted to the ground.       

     In the example shown in  FIG. 2 , the lift system includes guide bushings  240  mounted in the chassis  100  of the vehicle in which lift (guide) columns  290  can slide, at least one footbase (also referred to as “beam”)  230  secured to the lower part of the guide columns  290  and more or less perpendicular to the lift (guide) columns  290 , and lift cylinders  220  mounted between the chassis  100  and the footbase  230  to control the transfer of the weight of the vehicle on the vibration assembly. 
     More details concerning the known technique (described briefly above) can be found in U.S. Pat. No. 7,499,378, which is incorporated herein by reference. In particular, a description of elements 3, 4, 200, 251-255 and 258 is not given herein but can be found in the aforesaid US patent. 
     In the phase of lifting up or down the vibration assembly, i.e. when the vehicle weight is not transferred on the baseplate, the connection between the ground-hugging assembly (and more precisely the at least one footbase) and the vibration assembly is ensured by chains mounted in hose-like pieces of rubber, thus giving the vibration assembly a sufficient degree of movement to adapt to a ground which is rarely flat. In the up position and in the phase of lifting up or down the vibration assembly, the chains support the load of the vibration assembly and are therefore tensioned. In the down position, when at least a part of the vehicle weight is applied to the vibration assembly, the chains no longer support load and are therefore released (slackened). 
     The rubber hoses, or sheaths (sleeves), around the chains prevent links of the chains to knock against each other (i.e. prevent the clicking of the links) during vibration in the down position, and thereby avoid producing a noise resulting in pollution for the seismic acquisition (slackened chains down). 
     This known solution of chains with rubber hoses is reliable and adapted. 
     However, for vibrator trucks having a great peak force output (e.g. 60,000 lbf (i.e. 264 000 N) or more), there are sometimes breaks (failures) of the chain attachments, requiring replacement operations, or preventive maintenance. In both cases the implementation or withdrawal of chains is a complicated operation. 
     Sometimes, the forces applied to the chains distort the fixing nuts and it becomes even more difficult to replace the chains which might need new brackets to be welded. 
     Moreover the rubber hoses are not always completely effective and a noise resulting from the clicking of links is sometimes observed. 
     SUMMARY 
     A particular aspect of the present disclosure relates to a vibrator truck comprising a vehicle and a vibration assembly mobile relative to each other by a lift system comprising at least one footbase and configured to lift up/down the vibration assembly, wherein the lift system comprises at least one flexible strap and a coupling device configured to couple the at least one flexible strap to the vibratory part and the at least one footbase. 
     The general principle is that of replacing the sheathed chains (i.e. the chains mounted in hose-like pieces of rubber) by flexible straps adapted to carry the load. 
     This provides several advantages, including:
         a more reliable solution (no breaking of anchorages and increased strength compared with the known solution);   the replacement operations and/or preventive maintenance are easier than with sheathed chains;   the aforesaid noise resulting from the clicking of links is suppressed.       

     According to a particular feature, the coupling device comprises the following components: a first fixing member, a second fixing member, and at least one winding axle, wherein each of said components is fastened to the at least one footbase or to the vibration assembly. Among said components, there is at least one pair comprising a fixing member and a winding axle which are fastened to two different elements among the at least one footbase and the vibration assembly. 
     According to a particular feature, the at least one flexible strap is coupled at each end to one of said first and second fixing members and winds along its length around said at least one winding axle. 
     According to a particular feature, the at least one flexible strap passes alternately from a component fastened to the at least one footbase to a component fastened to the vibration assembly. 
    
    
     
       LIST OF FIGURES 
       Other features and advantages of embodiments shall appear from the following description, given by way of indicative and non-exhaustive examples and from the appended drawings, of which: 
         FIG. 1 , already described, shows a side view of a known vibrator truck; 
         FIG. 2 , already described, shows a known implementation of a vibration assembly and a ground-hugging assembly; 
         FIGS. 3 and 4  are a side view and a perspective view respectively, of a linking assembly, between the baseplate of the vibration assembly and the footbase (or one of the footbases) of the ground-hugging assembly, according to an illustrative embodiment of the disclosure; 
         FIG. 5  is a perspective view detailing the linking assembly of  FIGS. 3 and 4 ; 
         FIG. 6  shows a preferred embodiment of a flexible strap comprised in the linking assembly of  FIGS. 3 and 4 ; and 
         FIG. 7 to 10  show alternative embodiments of the linking assembly. 
     
    
    
     DETAILED DESCRIPTION 
     In all of the figures of the present document, similar elements and steps are designated by the same numerical reference sign. 
     In the following description, considering again the context of  FIG. 2 , we describe with  FIG. 3 to 5  a particular embodiment of a linking assembly  10  carrying out the connection between the baseplate  210  of the vibration assembly (comprising elements  210 ,  225 ,  260 ,  270  and  280 ) and the footbase  230  (or one of the footbases) of the ground-hugging assembly (comprising elements  220 ,  230 ,  240  and  290 ) which behaves like a lift system. 
     The linking assembly  10  comprises a plurality of flexible straps  30  (five in this embodiment) made with a mainly non-metallic material (polyester material preferentially). 
     Each strap  30  is associated with a coupling device for coupling this strap to the footbase  230  and the baseplate  210 . For the coupling to the footbase  230 , each coupling device comprises two fixing members embodied as two pins, or support axles,  16  fastened (e.g. with nuts) to a fixing bracket  18  which is itself fastened, in particular welded, to the footbase  230 . A bushing can be mounted on the support axle  16  to increase the diameter of the loop. For the coupling to the baseplate  210 , each coupling device comprises a winding axle  14 , like a pin or a larger diameter tubing, fastened to the baseplate  210 . The winding axles  14  of the plurality of coupling devices are fastened, e.g. welded, along one side of the baseplate  210 . 
     Each strap  30  comprises at each end an attachment loop  33 ,  34  for passage of one of the support axles  16 . The link with the baseplate  210  is carried out by a winding of the strap  30  along its length around the winding axle  14 . 
     At least for a winding portion  12 , which winds around the winding axle  14 , the strap  30  is at least doubled in thickness, the two or more thicknesses being preferentially secured to one another. In particular, it is made as a looped band of flexible polymer, possibly with reinforcing non-metallic fibers. 
     If the ground-hugging assembly comprises several footbases (e.g. two, as in the case of Sercel&#39;s vibrator trucks of the “Nomad 65” type), each footbase is connected to the baseplate  210  of the vibration assembly, via a linking assembly  10  as described above (in particular five straps  30  associated with five coupling devices). 
     Each strap  30  has a suitable resistance to traction. In a preferred embodiment, these flat straps  30  of flexible material have the following characteristics:
         polyester material;   working load limit (WLL): 1 t;   width: 30 mm;   utilization factor 1/7 EN1492-1 standard (WXLL/rupture);   five doubled straps on each opposite side of the baseplate  210 : lift capacity=5×2×2×1000=20 000 kg.       

     In a particular embodiment shown in  FIG. 6 , each flexible strap  30  is formed in a closed main loop having: first and second portions  31 ,  32  secured to one another (forming two thicknesses secured to one another and winding around a winding axle  14 ); a third portion  33 , extending from a first end of each of the first and second portions and forming a first attachment loop; and a fourth portion  34 , extending from a second end of each of the first and second portions and forming a second attachment loop. In a particular embodiment, at least for the winding portion  12 , the two thicknesses are glued or sewn to avoid friction. 
     To simplify maintenance in case of breakage/fracture, in particular of the strap, the two support axles  16  are fixed by bolting to the fixing bracket  18 , dismounting of these two support axles  16  making it possible to replace the strap  30 . 
     To limit the traction forces on the straps  30 , the diameter of the winding axle  14  corresponds substantially to the spacing between two support axles  16 . Thus, the recovery force is parallel to the greatest force, i.e. gravity. 
     To preserve the straps  30 , the diameter of the support axles  16  guarantees sufficient radius of curvature of the straps. 
     In a particular embodiment, the vibrator truck has a peak force output equal to or greater than 60,000 lbf (i.e. 264 000 N). 
     In the alternative embodiment of  FIG. 7  (linking assembly  70 ), a single strap  30 ′ is associated with several coupling devices as described above. Each end of the single strap  30 ′ has an attachment loop  33 ,  34  through which passes one of the support axles  16 . In this example, one of the support axles  16  is fastened to the baseplate  210  and the other one is fastened to the footbase  230 . The single strap  30 ′ winds along its length around several winding axle  14  (four in this example), passing alternately from a winding axle  14  fastened to the footbase  230  to a winding axle  14  fastened to the baseplate  210 . 
     In the alternative embodiment of  FIG. 8 , the linking assembly  80  differs from the linking assembly  10  of  FIG. 5  in that, for each coupling device, the two fixing members are embodied as two support axles  16  fastened to a fixing bracket (not shown) which is itself fastened to the baseplate  210 ; and the winding axle  14 ′ is fastened to the footbase  230 . 
     In the alternative embodiment of  FIG. 9  (linking assembly  90 ), for two adjacent coupling devices, one is implemented as in  FIG. 5  and the other as in  FIG. 8 . In other words, the winding axles  14 ,  14 ′ of two adjacent coupling devices are not fastened to the same element (one  14 ′ is fastened to the footbase  230  and the other  14  to the baseplate  210 ). 
     In the alternative embodiment of  FIG. 10  (linking assembly  110 ), the winding axles  14  of the plurality of coupling devices are fastened along two successive sides of the baseplate  210 . In other alternative embodiments, the winding axles can be fastened along two opposite sides of the baseplate  210  or along more that two sides of the baseplate  210 . 
     Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.