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RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 11/983,979, filed on Nov. 13, 2007, which is a non-provisional application claim benefit of provisional application number 60/858,208 filed on Nov. 10, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates in general to wellbore operations and more specifically to an apparatus and method for jarring wireline deployed tools stuck subsurface in a wellbore. 
       BACKGROUND 
       [0003]    Wellbore tools, such as measurement and sampling tools (e.g., logging tools) are often deployed in the wellbore via a wireline, slick line or electric line. A common problem that occurs in these operations is that the tool gets stuck in the wellbore by running into a restriction in the wellbore called a “squeeze,” an area where the wellbore has collapsed either from formation pressures or from debris sluffing off the wall of the well causing blockage called a “bridge.” When the tool can be freed by jarring the tool in an upward direction because there is no debris or restriction above the tool. When the tool becomes free it can be worked up and down to get the tool through the restriction, this is called “spudding.” 
         [0004]    When logging is completed, the tool can become stuck when it is being pulled out of the wellbore. Sometimes the tool can be jarred upward pulling it through the restriction and sometimes it does not come free because the squeeze or bridge is too large or heavy. 
         [0005]    If the tool becomes permanently stuck the options are to electrically burn off the weak point of the tool or pull off the weak point of the tool and do a fishing trip to retrieve the lost tool. Prior to pulling off or burning the weak point if a downward force could be applied to the tool to move it down out of the restriction the tool could be worked up and down to spud the tool up through the restriction the same way spudding the tool to get it through the restriction while running downhole. 
         [0006]    Therefore, it is a desire of the present invention to provide a method and apparatus for freeing a wireline deployed tool by jarring in a downward direction. 
       SUMMARY OF THE INVENTION 
       [0007]    An example of a wellbore jar includes a first sub-assembly with an upper member secured the lower end of a wireline, a second sub-assembly positioned lower than the first sub-assembly and attached to the first sub-assembly. The second sub-assembly also includes a first housing, a shaft and a first spring, wherein the shaft and the spring are inside the first housing. The wellbore jar also includes a third sub-assembly including a second housing and second spring arrangement, in communication with the second sub-assembly. The wellbore assembly also includes a fourth sub-assembly attached to the third sub-assembly and also attached to a down hole tool. The fourth sub-assembly further including a third housing, and a third spring; wherein the first, second and third string are capable of storing compressive energy. The wellbore assembly also includes an actuator for releasing the stored energy in the first, second and third springs downward onto the tool. 
         [0008]    An example of a method of dislodging a stuck tool downhole is disclosed. The method includes the steps of providing a jarring apparatus above the stuck tool. The jarring apparatus has a first, second, third and fourth sub-assembly in communication with each other. The upper end of the first sub-assembly is attached to a wireline, which imparts an upward force on the first, second and third sub-assemblies by pulling the wireline towards the surface of the well. Compressing a first, and second spring located in the second and third sub-assemblies then releasing the compressed force stored within the second and third sub-assembly downward against the fourth sub-assembly attached to the stuck tool sufficient to dislodge the stuck tool, with an imparting force on the tool attached to the upper end of the first sub-assembly. 
         [0009]    Another example of a wellbore jar for dislodging tools downhole is disclosed. The wellbore jar includes: a first attachment means for attaching to the upper end of the apparatus to a wireline cable. There is also a second attachment means for attaching to the upper end of the tool to the lower end of the apparatus and a spring mechanism within the apparatus for storing compressional force as the length of wireline above the apparatus applies upward force on the apparatus. Also present is an actuating means for rapidly releasing the stored compressional force downward onto the tool lodged downhole. 
         [0010]    The foregoing has outlined some of the features and technical advantages of the present invention in order that a detailed description of an example of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The foregoing and other features and aspects of the wellbore jar will be best understood with reference to the following detailed description of a specific embodiments of the invention, when read in conjunction with the accompanying drawings, wherein: 
           [0012]      FIGS. 1A-1D  illustrate an example of the apparatus in operation; 
           [0013]      FIG. 2A  is a partial cut away view of an example of a first sub-assembly of an apparatus in isolation; 
           [0014]      FIG. 2B  is a partial cut away view of an example of a second sub-assembly of an apparatus in isolation; 
           [0015]      FIG. 2C  is a partial cut away view of an example of a third sub-assembly of an apparatus in isolation; 
           [0016]      FIG. 2D  is a partial cut away view of an example of a fourth sub-assembly of an apparatus in isolation; and 
           [0017]      FIG. 3  is a partial cut away view of an example of the apparatus as it is compressed. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0019]    As used herein, the terms “up” and “down”; “upper” and “lower”; “uphole” and “downhole”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, the terms “up,” “upper,” “uphole,” and other like terms are meant to indicate a position that is closer to the surface along the linear distance of the borehole. It is noted that through the use of directional drilling, a wellbore may not extend straight up and down. Thus these terms describe relative positions along the wellbore. 
         [0020]      FIG. 1A  provides an example of the present invention. Apparatus  10  includes four principal sub-assemblies attached to a tool  11  located in a wellbore  18 . First sub-assembly  12  includes an upper member  14  secured to the lower end of wireline  16 . Second sub-assembly  20  is lower than first sub-assembly  12  and is attached to first sub-assembly  12 . Third sub-assembly  24  is a tube and spring arrangement, in communication with second sub-assembly  20  and the fourth sub-assembly  25 . Fourth sub-assembly  25  is attached to third sub-assembly  24  and is also attached to the down hole tool. Wellbore  18 , as illustrated, is free of any squeezes or impediments. 
         [0021]      FIG. 1B  illustrates tool  11  stuck below obstruction  26 . An end user located topside to the wellbore  18 , would know of the blockage caused by obstruction  26  as the wireline  16  would no longer be moved upward in the wellbore. Apparatus  10  will then be fully extended showing the intersections between sub-assemblies  12 ,  20 , and  24 . During this time none of the springs in sub-assemblies  12 ,  20 , and  24  are constricted and the springs do not store any spring energy. (See  FIG. 2A-D  for spring locations). 
         [0022]      FIG. 1C  illustrates the tension on wireline  16  being removed, whereby the weight of apparatus  10  will cause apparatus  10  to contract due to gravity. When apparatus  10  is contracted the springs located in sub-assemblies  12 ,  20  and  24  will contract storing compression energy. In the fully contracted state, apparatus  10  is primed and its internal springs tightened ( FIGS. 2A-D ). The energy stored in the springs is so great that third sub-assembly  24  will have its internal spring activate. 
         [0023]      FIG. 1D  provides an example of the present invention as shown in  FIG. 1C  except that in  FIG. 1D  third sub-assembly  24  has activated, therein dislodging fourth sub-assembly  25  from obstruction  26  due to a downward force. The weight of apparatus  10  must be greater than force holding apparatus  10  in obstruction  26 , otherwise the discharge of the spring  54  in third sub-assembly  24  will cause the three sub-assemblies  12 ,  20  and  24  above obstruction  26  to move upward instead of forcing fourth sub-assembly  25  downward. 
         [0024]      FIGS. 2A-D  illustrate the sub-assemblies that make up an example of apparatus  10 . 
         [0025]      FIG. 2A  provides an example of first sub-assembly  12  in isolation. First sub-assembly  12  includes of shaft member  28 , cap  30 , housing  32 , spring  34 , threaded tube  36 , split thread collar  38  and shaft cap  40 . Cap  30  is attached to housing  32  such that a portion of cap  30  is housed in housing  32 . The portion of cap  30  that is interior to housing  32  is threadably attached to threaded tube  36 . Also threadably attached to threaded tube  36  is shaft member  28 . A portion of shaft member  28  is inside of housing  32  and a portion is exterior to housing  32 . The interior portion of shaft member  28  is positioned within spring  34 . The portion of housing  32  that is opposite to cap  30  is threaded and joined with split thread collar  38 . Shaft member  28  is also fitted with a shaft cap  40  which is opposite housing  32 . Cap  30  is also constructed to attach to the lower end of wireline  16 . Shaft member  28  of first sub-assembly  12  can be pulled upward to compress an internal spring  34 . 
         [0026]      FIG. 2B  is a cross sectional view of the second sub-assembly  20  in isolation. Second sub-assembly  20  includes tube  42 , housing  44 , spring  46 , cap  48 , tube  50 , and housing  52 . Tube  42  is constructed so as to have shaft member  28  slideably move within housing  44 . In effect, tube  42  acts as a piston wall with shaft member  28  capable of movement inside the piston. Tube  42  is threaded and attached to housing  44 . Interior to housing  44  is another spring  46  which is constructed to fit around shaft member  28 . Spring  46  is also buttressed with cap  48  with an opening. Cap  48  acts as a backstop for spring  46 , but allows for movement of shaft member  28  through the opening. Also threadably attached to housing  44  is housing  52 . Inside of housing  52  and threadably attached to housing  52  is tube  50 . 
         [0027]      FIG. 2C  is a cross sectional view of third sub-assembly  24  in isolation. Third sub-assembly  24  includes a spring  54 , hollow cap  56 , and tube  58 . Spring  54  is located inside of hollow cap  56 , and tube  58 . Tube  58  is threadably joined opposite hollow cap  56  with an additional hollow cap  59  such that hollow cap  59  is a backstop for spring  54 . Although not illustrated, the use of latches associated with tube  58  of third sub-assembly  24  could be implemented to ensure that the springs  34  and  46  remain coiled prior to striking third sub-assembly  24  and releasing the compressed spring energy. 
         [0028]      FIG. 2D  is a cross sectional view of fourth sub-assembly  25  in isolation. Fourth sub-assembly  25  includes a tube  60 , a housing  62 , and a cap  64 . Tube  60  is designed to accommodate tube  58  of third sub-assembly  24  such tube  58  can slide into tube  60  similar to a piston. Fourth sub-assembly  25  also contains cap  64  which can act as the connection point of the lower end of apparatus  10  that connects to the upper end of tool  11 . 
         [0029]      FIG. 3  is a cross sectional view of an example of apparatus  10  of the present invention, shown in a contracted or cocked position. When the tension on wireline  16  is released first sub-assembly  12 , spring  34  between sub-assemblies  12  and  20  is compressed. The weight of sub-assemblies  12  and  20  compress spring  54  between sub-assemblies  20  and  24 . The impact of sub-assemblies  12  and  20  on sub-assembly  24  is sufficient to act as an actuator and to cause spring  54  to expend its stored spring energy and force fourth sub-assembly  25  downward, thereby dislodging fourth sub-assembly  25  from the obstruction  26 . Housings  32  and  62  are constructed to have lips  32   a  and  62   a  respectively, which extend into the interior of the housings  32  and  62 . Tubes  42  and  60  are constructed to have flanges  42   a  and  60   a  respectively which are designed to engage the lips  32   a  and  62   a  respectively so that tubes  42  and  60  will not extend past lips  32   a  and  62   a  when apparatus  10  is fully extended ( FIG. 1B ). 
         [0030]    From the foregoing detailed description of specific examples of the apparatus, it should be apparent that a wellbore drilling system and method that is novel has been disclosed. Although specific examples have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed examples without departing from the spirit and scope of the invention as defined by the appended claims which follow.

Summary:
An example of an apparatus for jarring a wellbore tool from an obstruction includes four sub-assemblies in communication with three springs such that the springs may be compressed forcing the subassemblies to become adjacent to one another. When the springs are actuated, the sub-assembly units spring apart from one another force dislodging the lower most sub-assembly component from the obstruction.