Abstract:
A technique is described for releasing a borehole seismic acquisition tool from a stuck position within a borehole. The borehole seismic acquisition tool includes a tool body having at least one sensor package. The borehole seismic acquisition tool further includes an anchoring arm, and anchoring arm linkage adapted to connect the anchoring arm to the tool body. The anchoring arm linkage includes a weakpoint mechanism adapted to break in response to a minimum threshold amount of breaking force being applied upon the weakpoint mechanism. The anchoring arm includes a first end and a second end. The second end of the anchoring arm is pivotally attached to the anchoring arm linkage. The first end of the anchoring arm includes a tip portion which defines a slot. The anchoring arm also includes a movable cam inserted into the slot and removably secured to the first end via a first connector pin inserted through an aperture in the cam. According to at least one implementation, the cam is at least partially rotatable about the first connector pin. An exposed portion of the cam is engaged with the surface of borehole wall. When a failure of retraction of the anchoring arm is detected, the cam may be caused to rotate against the borehole wall surface in response. The rotation of the cam against the borehole wall surface causes a force to be exerted upon the weakpoint mechanism, which, in turn, causes the weakpoint mechanism to break. The breaking of the weakpoint mechanism results in a release of the borehole seismic acquisition tool from the stuck position. According to a specific embodiment, rotation of the cam against the borehole wall surface may be caused by a pulling force applied to a top portion of the borehole seismic acquisition tool.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Seismic shuttle logging tools are generally known for use in performing vertical seismic profile (VSP) surveys from boreholes to evaluate the surrounding underground formations. An example of a multi-shuttle seismic array is illustrated in  FIG. 1  of the drawings. As illustrated in  FIG. 1 , a multi-shuttle seismic array  100  may comprise a number of seismic logging tools linked together by means of a cable and logged through a borehole while seismic signals are generated at the surface. In the example of  FIG. 1 , the logging cable  102  (which may range in length, for example, from 4 to 46 km) connects to a logging head  104  which may include an Electrically Controlled Release Device (ECRD) and a Versatile Seismic Imager (VSI) cartridge  106 . The multi-shuttle seismic array may also include a VSI interconnect cable  108 , and a plurality of seismic logging shuttles  112  which may function as seismic receivers. At each measurement level, each shuttle is anchored to the wall of a borehole during a measurement operation by means of an anchoring arm. VSP survey techniques typically require multiple measurements to be taken at different levels or depths in a borehole. As a result, the tool anchoring mechanisms must be released to allow the tools to be repositioned. After the tools have been repositioned at the next measurement level, the tool anchoring mechanisms are then re-engaged before new measurements are performed.  
         [0002]      FIG. 2  shows a detailed view of a conventional anchoring mechanism  200 . The mechanism includes a permanent magnet (or electromagnet) brake  212 , motor  214  and reducer arrangement  216  housed in the shuttle body  210 . The output drive from the reducer  216  connects through a joint  218  and bearing  220  to a ball screw  222 . The ball screw  222  drives a nut  224 . The end of the screw  222  projects into the hollow end of a push rod  226  and the end of the nut  224  engages the outer surface of the push rod  226  through a clutch mechanism  228  which is described in more detail below. The end of the push rod  226  is connected to a link  230  through which it drives the anchoring arm of the shuttle (shown partially at  227 ).  
         [0003]     The inner end of the push rod  226  is formed into a base section  232  which fits inside the nut  224 . The base section  232  also has extensions outside the nut  224  to provide a connection to a potentiometer (or a linear variable differential transformer “LVDT”)  234  which acts as position sensor for the push rod  226  and so can be used for a caliper measurement in the borehole using the anchoring arm. A compression spring  236  is located around the motor/ball screw mechanism inside the shuttle body and acts on the base section  232  so as to normally urge the push rod  226  and hence the arm, outwards. The extension of the push rod  226  under by the spring  236  is limited by the position of the nut  224  on the screw  222  such that operating the motor  214  to move the nut  224  causes the push rod  226  to move out due to the spring  236  or be pulled in by the action of the nut  224 .  
         [0004]     Extension of the push rod  226  by the spring  236  is limited by either the arm contacting the borehole wall or by the base section  232  reaching the stops  238  positioned in the body (fully extended). Once the arm contacts the borehole wall, the nut  224  moves over the push rod  226  to activate the clutch  228  such that the screw  222  and nut  224  drives the push rod  226  directly and forces it against the borehole wall to anchor the shuttle. To release the arm, the motor is reversed and the screw  222  retracts the nut  224 , releasing the clutch  228 . The arm is then only held against the borehole wall by the spring  236  and so can move in or out as the shuttle is dragged up to a different position in the well. It is not necessary to retract the arm completely. If it is desired to retract the arm completely, the reverse motor drive is continued and the nut  224  is retracted along the screw  222  until it contacts the base section  232  of the push rod  226  which it then pulls back against the action of the spring  236  to retract the push rod  226  and thus the anchor arm. When the arm is fully retracted, the motor stalls and this is detected to find the fully retracted/closed position of the arm. The output from the potentiometer  234  can also be used to detect the arm in its fully retracted position. A detailed description of conventional anchoring mechanisms such as that illustrated in  FIG. 2  is described in U.S. Pat. No. 6,315,075 to Nakajima, incorporated herein by reference in its entirety for all purposes.  
         [0005]     Recently, the size of borehole seismic arrays has been increasing to more than 40 shuttles (at costs exceeding US$1,000,000). As a result, there is growing concern about losing assets (e.g., multi-shuttle seismic arrays) in a borehole. More importantly, the loss of a potentially important hole section and the cost of rig time may be an even greater concern if a seismic array retrieval operation (commonly referred to as a “fishing operation”) is ineffective.  
         [0006]     For example, in one undesirable scenario, a multi-shuttle seismic array may become stuck in a borehole due to a failure in retracting the seismic shuttle anchoring arms. In order to handle such scenarios, a mechanical weakpoint is typically built into the anchoring arm linkage and designed to break under a certain pulling force. The breaking of the anchoring arm linkage allows the arm a few inches of travel so that the shuttle can be freed. However, this final, forceful freeing procedure may not be successful in cased holes where lack of friction does not allow sufficient force to be transferred to the anchor-arm weakpoints.  
         [0007]     Accordingly, it will be appreciated that there exists a continual need to improve multi-shuttle seismic array designs in order to provide improved seismic logging capabilities, performance, and reliability.  
       SUMMARY OF THE INVENTION  
       [0008]     Various aspects of the present invention are directed to an anchoring arm for use with a borehole seismic acquisition tool which includes a tool body. The anchoring arm includes a first end adapted for contacting a borehole wall surface. The first end includes a tip portion which defines a slot. The anchoring arm also includes a movable cam inserted into the slot and removably secured to the first end via a first connector pin inserted through an aperture in the cam. According to at least one implementation, the cam is at least partially rotatable about the first connector pin. In one embodiment, the cam is designed to rotate in a substantially elliptical motion about the first connector pin. In this way, the radius of the cam may increase as the cam is rotated about the first connector pin. Further, according to at least one embodiment, when the cam is placed in contact with the surface of a borehole wall, rotation of the cam against the borehole wall surface may cause a displacement of the first end of the anchoring arm away from the borehole wall surface, thereby causing a force to be exerted upon the first end of the anchoring arm.  
         [0009]     An alternate embodiment of the present invention is directed to a borehole seismic acquisition tool. The tool includes a tool body having at least one sensor package, an anchoring arm, and anchoring arm linkage adapted to connect the anchoring arm to the tool body. The anchoring arm linkage includes a weakpoint mechanism adapted to break in response to a minimum threshold amount of breaking force being applied to the weakpoint mechanism. The anchoring arm includes a first end and a second end, the second end being pivotally attached to the anchoring arm linkage, and the first end being adapted for contacting a surface of the borehole wall. According to at least one implementation, the first end of the anchoring arm includes a tip portion which defines a slot. The anchoring arm also includes a movable cam inserted into the slot and removably secured to the first end via a first connector pin inserted through an aperture in the cam. According to at least one implementation, the cam is at least partially rotatable about the first connector pin. In one embodiment, the cam is designed to rotate in a substantially elliptical motion about the first connector pin. In this way, the radius of the cam may increase as the cam is rotated about the first connector pin. Further, according to at least one embodiment, when the cam is placed in contact with the surface of a borehole wall, rotation of the cam against the borehole wall surface may cause a displacement of the first end of the anchoring arm away from the borehole wall surface, thereby causing a force to be exerted upon the weakpoint mechanism. In at lest one embodiment, the force exerted upon the weakpoint mechanism may be at least equal to the minimum threshold amount of breaking force.  
         [0010]     A different aspect of the present invention is directed to a method for releasing a borehole seismic acquisition tool from a stuck position within a borehole. The borehole seismic acquisition tool includes a tool body having at least one sensor package. The borehole seismic acquisition tool further includes an anchoring arm, and anchoring arm linkage adapted to connect the anchoring arm to the tool body. The anchoring arm linkage includes a weakpoint mechanism adapted to break in response to a minimum threshold amount of breaking force being applied upon the weakpoint mechanism. The anchoring arm includes a first end and a second end. The second end of the anchoring arm is pivotally attached to the anchoring arm linkage. The first end of the anchoring arm includes a tip portion which defines a slot. The anchoring arm also includes a movable cam inserted into the slot and removably secured to the first end via a first connector pin inserted through an aperture in the cam. According to at least one implementation, the cam is at least partially rotatable about the first connector pin. An exposed portion of the cam is engaged with the surface of borehole wall. When a failure of retraction of the anchoring arm is detected, the cam may be caused to rotate against the borehole wall surface in response. The rotation of the cam against the borehole wall surface causes a force to be exerted upon the weakpoint mechanism, which, in turn, causes the weakpoint mechanism to break. The breaking of the weakpoint mechanism results in a release of the borehole seismic acquisition tool from the stuck position. According to a specific embodiment, rotation of the cam against the borehole wall surface may be caused by a pulling force applied to a top portion of the borehole seismic acquisition tool.  
         [0011]     Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows an example of a multi-shuttle seismic array  100 .  
         [0013]      FIG. 2  shows a detailed view of a conventional anchoring mechanism  200 .  
         [0014]      FIG. 3  shows a portion of VSI shuttle  300  which may be used for implementing various aspects of the present invention.  
         [0015]     FIGS.  4 A-C illustrate one technique for implementing an anchor arm weakpoint mechanism in a VSI shuttle.  
         [0016]     FIGS.  5 A-F illustrate different views of a modified anchor arm in accordance with a specific embodiment of the present invention.  
         [0017]     accordance with a specific embodiment of the present invention.  
         [0018]      FIG. 6A  shows a perspective view of the anchor arm linkage weakpoint mechanism of  FIG. 4B .  
         [0019]      FIG. 6B -C show perspective views of different embodiments of anchor arm linkage weakpoint mechanisms of the present invention.  
         [0020]      FIG. 7  shows various design features of a cam design in accordance with a specific embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The present invention describes different techniques which may be used to improve multi-shuttle seismic array designs in order to provide improved seismic logging capabilities, performance, and reliability. One aspect of the present invention is directed to an improved anchor arm for a seismic logging tool such as, for example, a VSI shuttle.  
         [0022]      FIG. 3  shows a portion of VSI shuttle  300  which may be used for implementing various aspects of the present invention. In the example of  FIG. 3 , VSI shuttle portion  300  is shown to be anchored to a wall portion ( 318   a ,  318   b ) of a borehole using anchor arm  330 . According to at least one embodiment, VSI shuttle portion  300  may be similar in design to the VSI shuttle  200  of  FIG. 2 . In the embodiment of  FIG. 3 , the anchoring mechanism for shuttle portion  300  includes gear drive motor  304 , ball screw  306 , ball nut  308 , push rod  313 , anchor arm linkage  314 , pivoting anchor arm  320 , standoff portions  316 , etc.  
         [0023]     It will be appreciated that situations may occur when one or more VSI shuttles of a multi-shuttle seismic array become stuck in a borehole due to failures in the shuttles&#39; anchoring systems. In the event that an anchor arm cannot be closed properly, a pulling force may be exerted from the surface in an attempt to break an anchor arm weakpoint mechanism. In the embodiment of  FIG. 3 , the anchor arm weakpoint mechanism is incorporated into anchor arm linkage  314 , which is described in greater detail in FIGS.  4 A-C of the drawings.  
         [0024]     FIGS.  4 A-C illustrate one technique for implementing an anchor arm weakpoint mechanism in a VSI shuttle. As illustrated in  FIG. 4A , a force F 1  is exerted upon push rod  404  to thereby engage anchor arm  420  against borehole wall portion  402 . The anchor arm linkage  406  includes a weakpoint mechanism which is designed to break when a sufficient force F 2  is exerted upon the end ( 420   a ) of the anchor arm which is in contact with the borehole wall.  FIG. 4B  shows a detailed view of linkage portion  400  of  FIG. 4A . As shown in  FIG. 4B , one end of link  406  is connected to push rod  404  via connector pin  408   a , and the other end of link  406  is connected to anchor arm  420  via connector pin  408   b . Weakpoints have been incorporated into the link body at or near connector pin  408   b , resulting in a peninsula of link material  411  surrounded by a void or aperture  409 . The anchor arm linkage weakpoint is illustrated in greater detail in  FIG. 6A  of the drawings.  
         [0025]      FIG. 6A  shows a perspective view of the anchor arm linkage weakpoint mechanism of  FIG. 4B . As illustrated in  FIG. 6A , the anchor arm linkage weakpoint mechanism  600  includes a link body portion  612  and at least one connector pin  604 . For purposes of simplification, other components of the anchor arm linkage are not shown. Weakpoints have been incorporated into the link body portion by removing material as shown at locations  605   a  and  605   b . As a result, the structural integrity of the link body material near connector pin apertures  603  have been weakened to allow a break to occur in this region with the application of a sufficient amount of force.  
         [0026]     Thus, returning to  FIG. 4B , when a sufficient amount of force F 2  is exerted upon anchor arm  420 , a force F 4  is exerted upon connector pin  408   b  and link body portion  411 . When the force F 4  is sufficient (e.g., about 2000 kgf tensional force), the weekend portion  411  of the link body will break away, resulting in the diagram shown in  FIG. 4C . As shown in  FIG. 4C , the breaking of the region  411  of the link body allows the end portion ( 420   b ) of the anchor arm (which is connected to connector pin  408   b ) to move freely within cavity portion  409 . As anchor arm end  420   b  moves in the direction indicated by arrow B, the other anchor arm end  420 A moves in the direction indicated by arrow A. In this way, the breaking of the weakpoint mechanism provides the anchor arm with a few additional inches of travel. This additional range of motion of anchor arm  420  is intended to help release the VSI shuttle from a stuck position within the borehole.  
         [0027]     Referring to  FIG. 3 , it will be appreciated that the transfer of force to the anchor arm linkage weakpoint may depend upon various factors including, for example, the frictional force (e.g., F 1 ) being created at the contact between the anchor arm tip  320   a  and the borehole wall. Such frictional force creates rotational torque (M 1 ) around the anchor arm pivot  311  and a counterforce (Fw) on the anchor arm linkage weakpoint mechanism. While this criterion may be relatively easily satisfied in open-hole conditions, a low-friction condition may exist inside of casing allowing the anchor arm to slip as it is pulled. For example, the wall surface inside of casing may be smooth and even lubricated by drilling fluid, causing the anchor arm to slip without creating sufficient force to break the weakpoint. In such cases, the anchor arm weakpoint may not break without some design consideration to facilitate frictional force and efficient force transfer to the weakpoint. According to at least one implementation, a special arm tip design mechanism may be employed to facilitate the gathering of frictional force, and to give a mechanical advantage to break the weakpoint. This as illustrated, for example, in FIGS.  5 A-D of the drawings.  
         [0028]     FIGS.  5 A-F illustrate different views of a modified anchor arm in accordance with a specific embodiment of the present invention.  
         [0029]      FIG. 5A  shows an exploded view of a modified anchor arm  500  in accordance with a specific embodiment of the present invention. The anchor arm  500  includes a body portion  501  having two ends, namely  501   a  and  501   b . One end  501   b  is designed to attach to the anchor arm linkage (not shown) of a seismic logging tool such as, for example, a VSI shuttle. The other end  501   a  of the anchor arm is designed for contact with the borehole wall, which may or may not include an inner casing. As illustrated in  FIG. 5A , the end portion  501   a  of the anchor arm includes a slot  511  for receiving a removable cam  510 . According to one implementation, the cam  510  may be movably attached to the anchor arm body  501  by inserting cam  510  into slot  511  and securing using locking pin  504 , for example, by aligning apertures  502   c  and  510   a  and inserting locking pin  504  into the apertures.  
         [0030]     In the embodiment of  FIG. 5A , cam  510  has been designed to have a substantially elliptical rotation. According to at least one embodiment, the modified anchor arm  500  may be designed such that, during normal seismic logging operations, cam  510  prevented from rotating until a pulling force (e.g., from the surface) creates enough initial tangential force to start an outward rotation of the cam. In this way, the cam mechanism is able to intensify friction at the borehole wall (which may or may not include an inner casing) as it rotates outward. Moreover, the rotation of the cam further enhances the grip on the borehole wall since an increasing inward force is created that works against the anchoring force, thereby ensuring that the cam mechanism rotates further outward without slipping. Rotation of the cam  510  also directly increases mechanical stress at the anchor arm linkage weakpoint since displacement of the anchor arm away from the borehole wall increases as the cam continues its elliptical rotation. Eventually, the displacement of the anchor arm will be sufficient to cause a break in the anchor arm linkage weakpoint.  
         [0031]     FIGS.  5 B-F show different a perspective views of the end portion  501   a  of the modified anchor arm  500  in accordance with a specific embodiment of the present invention. As illustrated in  FIG. 5B , the cam  510  is movably secured within the slot  511  using locking pin  504 . According to at least one embodiment, the cam  510  may rotate within the slot  511  about the axis of locking pin  504 . As explained in greater detail below, rotation of the cam  510  may occur, for example, in situations where a frictional force is created at the contact point between cam  510  and a borehole wall in an attempt to break the anchor arm linkage weakpoint.  
         [0032]     According to at least one implementation, the modified anchor arm of the present invention may include a cam protector mechanism, such as that illustrated, for example, in  FIGS. 5B and 5C  of the drawings. As illustrated in the embodiments of  FIGS. 5B and 5C , the cam protector mechanism includes cam protectors  507   a  and  507   b  which may be pivotally attached to anchor arm  501  via connector pins (e.g.,  507   c ,  507   d ). According to a specific implementation, the protectors  507   a  and  507   b  provide protection for the cam  510 , for example, in order to reduce wear of the cam surface during normal seismic logging operations. In the examples of  FIGS. 5B and 5C , a separate cam protector is located on each side of the cam  510 . However, in alternate embodiments (not shown), a single cam protector may be used.  
         [0033]     As illustrated in the embodiments of  FIGS. 5B and 5C , the modified anchor arm may also include board springs  508   a ,  508   b  which are used to exert a spring force on cam protectors  507   a ,  507   b  in order to control movement of the cam protectors  507   a ,  507   b . It will be appreciated that the board springs  508   a ,  508   b  depicted in  FIGS. 5B and 5C  have been exaggerated in order to show details of the board springs  508   a ,  508   b  and cam protectors  507   a ,  507   b . In a preferred implementation,  508   a ,  508   b  may be configured or designed to maintain contact with cam covers  507   a ,  507   b , as illustrated, for example, in  FIG. 5D .  
         [0034]     According to a specific embodiment, board springs  508   a ,  508   b  may each exert a spring force on a respective cam cover  508   a ,  507   b . In one implementation, the spring force of board springs  508   a ,  508   b  is greater than the opening force of the anchor arm, but is weaker than the actuation force or anchoring force (e.g., the force needed to anchor a VSI shuttle against the borehole wall). According to a specific implementation, the modified anchor arm components are designed to allow the cam protectors to make contact with the borehole wall during dragging-up operations (e.g., when the multi-shuttle array is repositioned within the borehole). In this situation, the cam protectors prevent the cam from touching or engaging with the borehole wall. However, when an anchoring force is applied to the modified anchor arm (e.g., when the modified anchor arm is engaged to anchor the VSI shuttle against the borehole wall), the anchoring force will overcome the spring force of the board springs  508   a ,  508   b , resulting in movement of the cam protectors  507   a ,  507   b  towards board springs  508   a ,  508   b . The movement of the cam protectors causes the cam  510  to become exposed and make contact with the borehole wall.  
         [0035]     According to a specific implementation, the surface of the cam protectors may coated with hard material such as, for example, tungsten carbide. The coating can preferably be chosen additionally to provide a substantially high level of friction with the borehole wall. Suitable examples include sprayed-on tungsten carbide having relatively large particle size. Particle sizes comparable to that found on 50 grit sandpaper has been found to be suitable for some applications, although smaller particle sizes, such as found on 80 grit sandpaper may be useful under some circumstances.  
         [0036]     According to at least one embodiment, the cam  510  is preferably designed to roll against a borehole wall by a pulling force (e.g., from the surface) exerted upon the VSI shuttle. Additionally, the rotation of the cam is caused by the friction between the surface of the cam and the surface of the borehole wall. The rotational force exerted on the cam may depend at least in part upon on the friction coefficient of the cam material and borehole wall surface material.  
         [0037]     In at least one implementation, a temporary stop mechanism may be employed to prevent rotation of the cam  510  during normal seismic logging operations. For example, as illustrated in the embodiments of FIGS.  5 D-F, spring  506 , hook  505  and connector pin  509  may be utilized to prevent cam  510  from rotating until a desired threshold amount of force has been exerted upon cam  510  sufficient to cause cam  510  to change the direction of the rotational torque (Ms) Thus, for example, as illustrated in  FIG. 5E , the spring force (Fs) from spring  506  and the relative alignment of pivot pins  504 ,  509  and  512  causes a rotational torque (Ms) to be exerted on the cam  510  in a counterclockwise direction, which causes the cam  510  to be positioned in its “normal” position (e.g., during normal operations).  
         [0038]     According to a specific embodiment, when a given amount of force is exerted upon the cam  510  (such as, for example, when an upward pulling force is exerted upon an anchored shuttle in a borehole), the cam  510  will begin to rotate in a clockwise direction, which changes the relative alignment of pivot pins  504 ,  509  and  512 . When the rotation of the cam exceeds the “dead point” (i.e., the point at which all pivot pins  504 ,  509  and  512  are aligned in a substantially straight line), the rotational torque (Ms) caused by the spring force (Fs) changes direction to thereby cause the cam  510  to rotate into its “fully extended” position, as illustrated, for example, in  FIG. 5F  of the drawings. It will be appreciated that this change in the direction of the rotational torque facilitates the cam in further rotational movement, and helps to prevent the cam  510  from slipping against the borehole wall surface.  
         [0039]     According to a specific implementation, the cam  510  may be designed to rotate in a substantially elliptical manner such that its radius increases as it rotates. In at least one implementation, the amount of rotation permitted for cam  510  may be limited in order to prevent over rotation of the cam in a direction which will cause its radius to decrease. As the cam  510  begins to rotate (e.g., in a clockwise direction) from its normal or rest position (as shown, for example, in  FIG. 5E ), the anchor arm  501  will begin being displaced away from the borehole wall by a distance which may be represented as AD. The value of AD may increase as the cam is rotated, for example, from its normal position (e.g.,  FIG. 5E ) to its fully extended position (e.g.,  FIG. 5F ). It will be appreciated that the threshold breaking force necessary for causing the breaking of the anchor arm weakpoint mechanism may be dependent upon both the actuation force (anchoring force) and the force exerted upon the anchor arm by the AD movement of the anchor arm as caused by the cam rotation. Accordingly, taking into account all the different forces contributing to the threshold breaking force, in at least one embodiment the anchor arm weakpoint mechanism may be configured or designed to break well before the full extension of the cam is reached. In specific embodiments where a connector pin is used as the anchor arm weakpoint mechanism (as described, for example, in  FIGS. 6B and 6C ), the threshold breaking force necessary for causing the breaking of the connector pin may be related to its shear strength.  
         [0040]      FIG. 5D  shows a side view illustrating how end portion  501   a  of the modified anchor arm may engage borehole wall, in accordance with a specific embodiment of the present invention. In its fully extended position (as shown, for example, in  FIG. 5D ), edge portion  510   a  of the cam  510  engages the borehole wall surface  560 . It will be appreciated that the embodiment of  FIG. 5D  represents an extreme case where the cam  510  has been rotated into a maximum extension position. According to at least one embodiment, however, the anchor arm weakpoint mechanism may be configured or designed to fail or break at some intermediated point which occurs after the cam  510  has begun rotating, but before the cam  510  has reached its maximum extension position. According to different embodiments, the different sized cams may be configured or designed to provide a range of ΔD values which, for example, may range from may range from 3-30 mm (as measured, for example, from the cam&#39;s normal position to its fully extended position). In some implementations, a ΔD range of 8-13 mm has been found to be appropriate, and in at least one application a cam providing a ΔD range of 11 mm has been found to be appropriate. It is noted that the additional force on anchor arm  501  created by the rotation of cam  510  is a mechanically more efficient way to exert leverage on the anchor arm linkage weakpoint, as compared to conventional anchor arm designs.  
         [0041]     It will be appreciated that different implementations of the cam design may be used in the modified anchor arm technique of the present invention, depending upon a variety of factors such as, for example, borehole diameter, anchor arm length, anchor arm design, anchor arm strength, cam hardness, cam surface, frictional force characteristics of the borehole wall. For example, as illustrated in  FIG. 5D , the design of cam  510  is asymmetrical, and may include a combination of flat edges and/or curved edges.  
         [0042]     An additional design consideration of the cam relates to the rate of increase in the cam&#39;s radius as a function of its angular rotation. This concept is illustrated in  FIG. 7  of the drawings. As illustrated in  FIG. 7 , the radius (r) of cam portion  700  may vary as a function of angular rotation (a) as the cam is rotated about the point represented at O. According to one implementation, the cam may be designed to conform to the following criteria:  
         [0043]     O: center of cam;  
         [0044]     C 1 : center of beginning circular arc,  
         [0045]     C 2 : center of ending circular arc,  
           α     D   1       =     π   4       ,       
 
         [0046]     the angel of D 1 ,  
           α     D   2       =       19   ⁢           ⁢   π     20       ,       
 
         [0047]     the angel of D 2 ,  
           for   ⁢     :     ⁢     π   4       &lt;   α   &lt;       19   ⁢           ⁢   π     20       ,     
     ⁢       r   ⁡     (   α   )       =     11   ⁢           ⁢   exp   ⁢           ⁢     (     0.2   ⁢     (     α   -     π   4       )       )         ,       
 
x D     1   =11e 0.2(α     D1     −π/4) cos (α D     1   ) 
 
y D     1   =11 e   0.2(α     D1     −π/4) sin (α D     1   ) 
 
x D     2   =11e 0.2(α     D2     −π/4) cos (α D     2   ) 
 
y D     2   =11e 0.2(α     D2     −π/4) sin (α D     2   )
 
         [0048]     According to a specific embodiment, the lines C 1 D 1  and C 2 D 2  are perpendicular to the cam profile. In this embodiment, their inclinations may be calculated as follows:  
         m       D   1     ,     D   2         =       -         0.2   ⁢           ⁢   cos   ⁢           ⁢   α     -     sin   ⁢           ⁢   α           0.2   ⁢           ⁢   sin   ⁢           ⁢   α     +     cos   ⁢           ⁢   α           ⁢          α   =     α       D   1     ,     α     D   2                     
 
         [0049]     As illustrated in  FIG. 7 , the lines with the above inclinations, which pass through D 1  and D 2 , cross the x-axis at C 1  and C 2  respectively. As a result we obtain: 
 
 x   C     1   =x D     1   −y D     1     /m   D     1   =−3.889087 
 
 x   C     2     =x   D     2     −y   D     2     /m   D     2   =−9.648644 
 
         [0050]     And we will have: 
 
 R   1 =|C 1   D   1 |=14.022304 
 
 R   2 =|C 2   D   2 |=7.696361 
 
θ 1 =arcsin( y   D     1     /R   1 )=0.588003 rad 
 
θ 2 =arcsin( y   D     2     /R   2 )=0.354475 rad 
 
         [0051]     Additionally, according to a specific embodiment, the coordinates of the points on beginning and ending circular arcs are obtained as follows:  
         [0052]     The points on the beginning arc (0&lt;θ b &lt;θ 1 ) 
 
 x=R   1  cos θ b   +x   C     1   =14.022304 cos θ b −3.889087 
 
 y=R   1  sin θ b =14.022304 sin θ b  
 
         [0053]     The points on the ending arc (0&lt;θ e &lt;θ 2 ): 
 
 x=−R   2  cos θ e   +x   C     2   =−7.696361 cos θ e −9.648644 
 
 y=R   2  sin θ e =7.696361 sin θ e  
 
         [0054]     Other considerations for the modified anchor arm (which may include aspects of the inventive cam design) may include one or more of the following criteria: 
        the modified anchor arm design may be configured or designed to be compatible with conventional seismic logging tools;     the modified anchor arm design may be configured or designed to be compatible with conventional seismic logging techniques;     the modified anchor arm design may be configured or designed to be compatible with conventional seismic tool rescue techniques;     the modified anchor arm design may be configured or designed to be easily adapted to interface with conventional seismic logging tools;     the modified anchor arm design may be configured or designed to create suitable anchoring forces for conventional seismic logging operations;     the modified anchor arm design may be configured or designed to minimize the time required for breaking the anchor arm linkage weakpoint;     the modified anchor arm design may be configured or designed to not affect the acoustic characteristics of the seismic logging tool;     the modified anchor arm design may be configured or designed to not significantly affect the diameter and/or weight of the seismic logging tool;     the modified anchor arm design may be configured or designed to help protect the cam from external environmental factors.        
 
         [0064]     Additionally, other design features may be incorporated into the cam design, for example, in order to prolong the operational life of the cam. Such design features may include, for example, using relatively hard materials to form the cam in order to prevent premature wear; modifying the cam surface with texturing in order to improve the frictional contact with the borehole wall; modifying the shape of the cam surface (e.g., using a convex shape or a concave shape) to improve and/or maintain frictional contact with the borehole wall, etc.  
         [0065]     It will be appreciated that, in alternate embodiments of the present invention (not shown), conventional VSI shuttle anchor arms may be modified to include the cam mechanism described herein. Such modifications will generally be known to one having ordinary skill in the art.  
         [0066]     Other Embodiments of the Anchor Arm Weakpoint Linkage  
         [0067]     It will be appreciated that there are a variety of different implementations of the anchor arm linkage weakpoint mechanism which may be used in the VSI shuttle of the present invention. For example, one alternate embodiment of the anchor arm linkage weakpoint mechanism is illustrated in  FIG. 6B  of the drawings.  
         [0068]      FIG. 6B  shows a perspective view of an alternate embodiment of an anchor arm linkage weakpoint mechanism in accordance with a specific embodiment of the present invention. As illustrated in  FIG. 6B , anchor arm linkage  650  includes linkage body portion  662 , and at least one connector pin  654 . For purposes of simplification, other components of the anchor arm linkage are not shown. In the embodiment of  FIG. 6B , the weakpoint mechanism of the anchor arm linkage is incorporated into the connector pin  654  (rather than the linkage body as shown in  FIG. 6A ) which is used to secure the anchor arm (not shown) to the anchor arm linkage. More specifically, according to one implementation, weakpoints  659  are incorporated into connector pin  654  to allow the connector pin to break (at the weakpoints  659 ) when a specified amount of force is exerted upon the connector pin. According to different embodiments, implementation of weakpoints  659  may be achieved, for example by using weaker material at weakpoints  659 ; by removing connector pin material at weakpoints  659 ; and/or by other structural weakening techniques. According to a specific implementation, the modified connector pin  654  may be designed such that the positions of weakpoints  659  are located within the inner linkage cavity  663  when the connector pin is inserted thru apertures  653  (as shown in phantom in  FIG. 6B ). Thus, for example, in the implementation of  FIG. 6B , the distance D 3  (which represents the position of weakpoint  659  relative to the end of the connector pin) is preferably greater than distance D 1  (which represents the thickness of the link body portion). Additionally, according to a specific embodiment, the distance D 2  (which represents the distance between the two weakpoints) is preferably greater than the width of the anchor arm end which connects to the anchor arm linkage.  
         [0069]      FIG. 6C  shows a perspective view of how the weakpoint mechanism of the present invention may be used in conjunction with anchor arm  501 , in accordance with a specific embodiment of the present invention. As illustrated in  FIG. 6C , anchor arm may include two pivot points  676  and  678 . According to a specific embodiment, during deployment of the seismic logging tool in the field, pivot point  676  is connected to linkage  650  using connector pin  674 . The other pivot point  658  is connected to the body or housing of the tool (not shown) using connector pin  671 . According to different implementations, a modified connector pin which includes the weakpoint mechanism of the present invention (as described, for example, in  FIG. 6B ) may be utilized for connector pin  671 , and/or may be utilized for connector pin  674 , and/or may be utilized for connector pin  673 . As illustrated in  FIG. 6C , the modified connector pin which includes the weakpoint mechanism of the present invention is utilized only for connector pin  671  (and not for connector pins  673  and  674 ) because connector pin  671  is easier to service and/or replace when broken than connector pins  673  and  674 .  
         [0070]     It will be appreciated that the anchor arm linkage weakpoint mechanism implementations of  FIGS. 6B and 6C  provide a number of advantages over other anchor arm linkage weakpoint mechanism designs such as that shown in  FIG. 6A . For example, the modified connector pin of the present invention which includes the weakpoint mechanism of the present invention is simpler in design and therefore less costly to manufacture. Additionally, replacement of the weakpoint mechanism of the present invention (e.g., modified connector pin  654  with integrated weakpoints) is less expensive than replacement of the weakpoint mechanism of  FIG. 6A  (i.e., anchor arm link body  612 ). Also, replacement of a broken connector pin may be performed more quickly than replacement of the anchor arm linkage.  
         [0071]     According to different embodiments, the anchor arm linkage weakpoint mechanism characteristics may vary with borehole diameter. For example, the characteristics of the anchor arm linkage weakpoint mechanism may be designed to take into account that it may be more difficult to break the anchor arm linkage weakpoints in relatively smaller diameter boreholes. Additionally, as stated previously, the characteristics of the anchor arm linkage weakpoint mechanism may also depend upon the amount of frictional force being created at the contact between the anchor arm tip and the borehole wall. The characteristics of the anchor arm linkage weakpoint mechanism may also depend upon characteristics of other complements of the multi-shuttle seismic array. For example, according to at least one implementation, the force required to break the anchor arm linkage weakpoint mechanism preferably does not exceed the force required for breaking of the VSI interconnect cable.  
       INDUSTRIAL APPLICABILITY  
       [0072]     The present invention finds application in the field of borehole logging tools, particularly seismic multi-shuttle logging tools which can be used to evaluate the formations surrounding boreholes such as are drilled for the extraction of hydrocarbons or geothermal energy.  
         [0073]     Although several preferred embodiments of this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of spirit of the invention as defined in the appended claims.