Patent Publication Number: US-2005120576-A1

Title: Device to measure axial displacement in a borehole

Description:
RELATION TO PRIOR APPLICATIONS AND FEDERAL RIGHTS  
      This application claims priority of U.S. Provisional Patent Application No. 60/527,255 filed Dec. 6, 2003. 
    
    
      The invention was supported in part by Grant No. 20-201-XXXX-0919-206-2002030 from the national Science Foundation. The U.S. government has certain rights to practice or have practiced on its behalf the claimed technology. 
    
    
     FIELD OF THE INVENTION  
      The invention is in the general field of technical and scientific equipment used in field studies in the earth sciences. More specifically it is a device to measure with a high degree of precision axial displacement in a borehole wherein the axial displacement occurs in response to the removal or injection of a fluid, or the dissolution of a mineral. Specifically, the patent is directed to a unique removable borehole extensometer and to a device to detect minute changes in rock structures comprising an axial support system and elements of a borehole extensometer.  
     BACKGROUND OF THE INVENTION  
      Extensometers have been used to measure movement in naturally occurring rock structures, in coal mine roofs, and in foundations. Such changes are of basic scientific interest and of practical significance. Monitoring minute changes in naturally occurring fractures provides important information concerning the flow of ground water and potential transport of pollutants as well as the geological impact of either the extraction or injection of fluids into boreholes on fracture stability.  
      U.S. Pat. No. 5,929,341 issued to Bawden, et al on Jul. 27, 1999 describes and claims a device that indirectly measures stability of rock strata by measuring stress exerted on support cables positioned to support otherwise unstable material. The device finds particular application in the mining industry in which blocks of ore of a maximum dimension are removed potentially weakening remaining rock or with mining operations where the rock is inherently weak or fractured. The &#39;341 invention addresses cable geometry and various systems to anchor the cable, such that elongation of the cable accurately reflects stress and the movement of rock.  
      U.S. Pat. No. 6,311,564 issued to Martin, et al. on Nov. 6, 2001 describes and claims an apparatus to provide support for a structure (i.e. rocks) and for measuring stress on the apparatus from the structure. The apparatus comprising an elongated center wire, several stress measuring devices, such as wire or other strain gauges positioned along the wire, a forming material encasing the center wire, several non-center wires extending longitudinally from the center wire and wound around the length of the center wire, stress measuring devices, and forming material, and a device to collect data. The apparatus is useful in measuring stress in the roof structure of a coal or similar, underground mine, or rock mass.  
      Capelle, et al. in U.S. Pat. No. 4,719,803 describe and claim improvements in a borehole extensometer. Compared with ten existing borehole extensometers, the &#39;803 improvements eliminate the requirement of a surface reference head and the borehole extensometer is capable of providing in borehole displacement measurements.  
      U.S. Pat. No. 5,585,555 issued to McRea on Dec. 17, 1996 describes and claims a multiple position, recoverable borehole strainmeter. The device includes two or more anchors with releasable pistons that engaged the walls of the borehole to mount the strainmeter in the borehole. A relative displacement sensor senses changes in the relative displacement between adjacent anchors thereby measuring mass displacement axially along the borehole between the anchors. The pistons are independently, gas operated.  
      U.S. Pat. No. 5,629,480 issued to Herget on May 13, 1997 describes and claims an extensometer for use in a borehole. The device comprises a combination of linear motion transducers located with daisywheel anchors.  
      U.S. Pat. No. 4,607,435 issued to Boisen on Aug. 26, 1986 claims a temperature-compensated borehole extensometer. The device compensates for temperature effects on sensing rods by use of an element with materials of disparate linear coefficients of expansion.  
     SUMMARY OF THE INVENTION  
      A purpose of the invention is a device capable of detecting and measuring displacement in boreholes caused by very small movements occurring in fractures. A further purpose is a device capable of being easily removed from a borehole in which it is positioned by retracting anchoring means. A still further purpose is a device capable of compensating for the effects of temperature on the expansion/contraction of the equipment, hence on the accuracy of measurements.  
      These and other purposes are achieved by a device with two major components: an axial support that holds all measuring equipment and provides the structure by which the device is lowered into and extracted from a borehole, and a group of elements directly or indirectly connected to the axial support and that in structure and function combine to measure very small movements in rocks; these elements include at least one pair of anchor units each member of the pair having a fixed point, a deployable face, and an actuator that the force to secure the anchor, a proximal reference rod and a distil reference rod, each of which is physically connected to one of the two anchor units, a temperature compensating means to the proximal reference rod and supporting the displacement transducer which is in contact with the distil reference rod, a registration element that sets the benchmark distance between the anchor units and the critical space between the proximal and distil reference rods by insertion of a deployable/extractable element; in addition, these and other purposes of the invention are further achieved by a borehole extensometer that is readily removed from a borehole that comprising a central support rod on which are positioned two registration units each associated with an individual anchor unit that is deployable and retractable and has a mechanical locking device, a temperature compensating unit, and a displacement transducer. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  describes a device to measure changes in rock structures comprising an axial frame unit and elements of an extensometer.  
       FIG. 2  illustrates details of anchor units to secure elements of an extensometer in a borehole.  
       FIG. 3  illustrates details of the frame release units.  
       FIG. 4  provides details of the registration element in relation to the reference rods.  
       FIG. 5  provides details of the temperature compensation means for the device with an axial frame unit.  
       FIG. 6A  illustrates a borehole extensometer with a central support rod and mechanically deployable anchor legs.  
       FIG. 6B  provides details of the registration unit for an extensometer with a central support rod.  
       FIG. 7  illustrates details of the actuator and deployable legs.  
       FIG. 8  provides details of the ratchet/lock system to secure deployable legs.  
       FIG. 9  illustrates the major components of the temperature compensator used with an extensometer having a central support rod and deployable legs. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      Functionally, the borehole extensometer measures displacement in rock structures by sensing minute (micrometers) movement of the rocks on opposite sides of a naturally occurring fracture, or set of fractures. Anchors are firmly attached on opposite sides of the structure (fractured zone), and a device is positioned between the anchors to detect any change in the distance between the anchor points. When the anchors are secured and temperature effects on measuring equipment are accounted for, only movement of the rock structure between the anchors will cause displacement of the measuring device.  
      The registration device set the initial position of the anchors in the borehole and establishes an appropriate distance between functional components of the linear variable differential transformer. The leg deployment capabilities also allow retraction of the legs and thus removal of the extensometer from the borehole.  
     EXAMPLE 1  
      A device to measure minute displacements in rock structures is described in reference to  FIG. 1 . Functionally, the device  101  comprises two major components: an axial frame  103  and six units of an extensometer: proximal and distil anchor units  104 A and  104 B, respectively, at least one pair of reference rods  107 A and  107 B, a temperature compensating means  111 , a displacement transducer  109 , a proximal  119 A and a distil  119 B frame release unit, and a reference rod registration element  113 .  
      The axial frame  103  has a proximal (upper) end  106 A and a distil (lower) end  106 B. The proximal end  106 A of the axial frame  103  is releasably connected to the proximal anchor unit  104 A by means of the proximal release unit  119 A, and the distil end  106 B of the axial frame  103  is releasably connected to the distil anchor unit  104 B. In this manner, the axial frame supports all units of the extensometer until the proximal anchor unit  104 A and distil anchor unit  104 B are deployed and securely positioned in the borehole as further described below.  
      One member  107 A of the pair of reference rods is firmly attached to the proximal anchor unit  104 A and extends vertically downward towards the distil anchor unit  104 B. The second member  107 B of the pair of reference rods is similarly attached to the distil anchor unit  104 B and extends vertically upward towards the proximal anchor unit  104 B.  
      When operationally deployed in a borehole, the extensometer detects changes in the dimensions  128  of the fracture aperture  127  in a rock surface as a direct change of the space  122  detected by the displacement transducer  109 . To detect minute changes, the initial, or bench mark space  122 , must be established. This is accomplished by the registration element  113 . A deployable registration pin  115  passes through a precisely positioned and aligned opening in the proximal reference rod  107 A, to which the registration element is attached, into a precisely aligned opening in the distil reference rod  107 B. The registration element  113  comprises a cylinder  114  capable of being pressurized and alternately pressurized into which the deployable registration pin  115  is positioned, a piston to deploy the pin into position connecting and precisely aligning the proximal and distil reference rods, and essential fittings as described in detail in  FIG. 4 .  
      The registration element  114  also serves as a locking mechanism to fix the position of the reference rods  107  during transport and positioning of the extensometer. Once the deployable registration pin  115  has been disengaged, the reference rods are completely free to move relative to each other.  
      To ensure maximum accuracy and detection of minute displacements of the rock, in addition to the critical registration of reference rods, temperature induced variation in the length of the proximal  107 A, and distil  107 B reference rods that could affect the critical measuring space  122  are detected by and compensated for by the displacement transducer  109  operating functionally in association with the temperature compensating means  111 . The temperature compensating means  111  comprises a metallic expansion element  121  (preferably a brass rod) connected to the proximal reference rod  107 A at its distil tip  120  by mechanical means, preferably a screw. The metallic expansion element  121  supports the displacement transducer  109  and is in functional communication with it to measure the space  122 . The metallic expansion element  121  is fabricated from material with a larger thermal expansion coefficient than the material from which the reference rods  107 A and  107 B are fabricated. The metal expansion element is shorter in length than the reference rod to which it is attached. The differences in length combined with the differences in expansion coefficient allow for precise compensation for temperature induced changes in the reference rods, thus in the critical measuring space  122 . As one skilled in the art recognizes, in this manner when the extensometer is deployed and the deployable registration pin retracted, only movement of the rock will cause movements to be sensed by the displacement transducer.  
      A metallic plunger rod  123  is mechanically connected to the upper face of the distil reference rod  125  preferably by threaded means, and the metallic plunger rod  123  moves freely in its functional relation with the displacement transducer  109 . With the deployable registration pin  115  retracted, changes in the space  122  detected by the displacement transducer  109  as a function of the relative position of the metallic plunger rod  123  reflect displacements the rock, not temperature effects.  
      The proximal anchor unit  104 A and distil anchor unit  104 B are comparable in structure and function. Thus the following description of the proximal anchor unit  104 A is fully applicable to the distil anchor unit  104 B.  
      The proximal anchor unit  104 A comprises three functional elements: a fixed anchor point  118 A, an anchor actuator  105 A, and a deployable anchor face  117 A. Corresponding parts for the distil anchor until  104 B are  118 B,  105 B, and  117 B, respectively. The anchor actuator applies pressure (up to 2000 PSI) to extend the deployable anchor face  117 A outward against the borehole wall  110 B. Deployment of the deployable anchor face  117 A and  117 B and the resultant force exerted by them against the wall  110 B of the borehole forces the fixed points  118 A and  118 B to contact and anchor to the opposite side of the borehole  110 A. Pressure to extend the deployable anchor face  117 A and  117 B may be provided through the anchor actuators  105 A and  105 B, respectively, by pneumatic means or by hydraulic means. When deployed and secured in position, the anchor units  104 A and  104 B support the extensometer and simultaneously release the axial frame from the extensometer by disengaging the proximal frame release unit  119 A and distil frame release unit  119 B.  
      Functionally, the device  101  is connected by the axial support  103  to an external mechanical device (not illustrated) that lowers the device into the borehole  102  to a predetermined depth and supports the axial frame unit  103 . The anchor actuators  105 A and  105 B are activated and the deployable anchor face  117 A and  117 B and fixed anchor points  118 A and  118 B secure the extensometer in position. The deployable registration pin  115  holds the reference rods  107 A and  107 B in a pre-designated spacing  122  with respect to the temperature compensating means  111 . The functional elements are disengaged from the axial frame  103  by activation of the proximal and distil release units  119 A and  119 B respectively. The deployable registration pin  115  is retracted into the registration element  113 , and changes in the space  122  must be due to changes in the fracture  127  as detected and recorded by the displacement transducer  109 .  
      The following dimensions and materials are examples of acceptable ranges not limitations on the invention.  
      The axial frame is manufactured from aluminum to support the elements of the extensometer. Maximum length of the entire device is approximately 12 feet (4 meters), and the width established by the diameter of the borehole (hence of the extension of anchor elements) ranges from a minimum of 2 inches (5 cm) to a practical, but not technical limitation of 36 inches (93 cm).  
      The displacement transducer is commercially available (for example. Macro Sensors, Pensaukenn, N.J.) and reference rods are made from Invar (Carpenter direct, Reading, Pa.). Reference rods jointly are up to 12 feet (4.0 m), with each rod ranging from 4 to 5 feet (about 1.8 m). Rods are generally 0.5 inch (1.3 cm) in diameter. Other rods and plungers are preferably stainless steel; the metallic expansion unit may be aluminum or brass. The fixed anchor points are carbide.  
      Structurally and functionally, the proximal anchor unit  104  and the distil anchor unit  104 B are the same. A single anchor unit  104  representing either or both is illustrated in  FIG. 2 . Numbers indicating parts previously identified and described in  FIG. 1  are retained, but letters designating “proximal” or “distil” distinction are omitted.  
      In  FIG. 2 , the anchor unit  104  comprises the fixed anchor point  118 , the deployable anchor face  117 , and the anchor actuator  105 . The anchor actuator comprises a cylinder housing  201  that encases a cylinder plunger  202  with a pressure input value  203  and alternate pressure valve  204 . Reference rod  107  is physically connected to the base region  207  of the fixed anchor point  118 . The cylinder plunger  202  passes through the axial support  103 ; as illustrated, the frame release unit  119  is released, and the deployable anchor face  117  is pressed against or into the borehole wall  110 . The fixed anchor point  118  is embedded into the borehole wall at a position opposite the deployable anchor face  117 .  
      The cylinder is pressurized by introducing fluid (liquid or air) under pressure (up to 2000 PSI) via the pressure input valve  203 . Pressure causes the cylinder plunger  202  to move outward, in direction of arrow  206 . Pressure and resulting movement cause fixed anchor point  118  to be embedded in borehole wall and deployable face  117  to be pressed tightly to, or imbedded in the borehole wall. An optional coil spring  208  holds the cylinder plunger  202  in the deployed position, rather than continued pressure application. The deployable anchor faces are retracted by reversing the cylinder pressure via pressure release valve  204 . The optional spring  208  is mechanically compressed, and the device may be removed from the borehole. As the deployable anchor faces are retracted, the frame release unit reengages to secure the axial frame and extensometer as illustrated in  FIG. 3 .  
      Structurally and functionally, the proximal and distil frame release units,  119 A and  119 B, respectively, are the same. A single frame release unit,  119  representing either or both frame release units is illustrated in  FIG. 3 . Numbers indicating parts identified and described in  FIG. 1  and  FIG. 2  are retained, but the letters designating “proximal” or “distil” distinctions are omitted.  
      The frame release unit functions in response to pressurizing the cylinder of the anchor actuator  105 . The frame release unit  119  comprises a beveled opening  301  in the axial frame  103  member, and a securing cone  305  attached to the cylinder plunger  202 , with the deployable anchor face  117  connected to the distil surface  306  of the securing cone  305 .  
      The beveled surface  308  of the opening  301  slopes inward at a constant angle from the exterior surface  302  of the axial frame  103  member to the interior surface  303 . The securing cone  305  is beveled  309  at an angle complimentary to the slope  308  of the beveled opening  301 . The maximum diameter of the securing cone  307  is nominally equal to, or greater than the diameter of the beveled opening on the exterior surface  302  or the axial frame  103  member.  
      As illustrated in  FIG. 3 , the deployable anchor face  117  is extended (deployed) and the securing cone  305  is disengaged from the axial frame by extension of the cylinder plunger  202  in response to pressurizing the anchor actuator  105 . In this configuration, the reference rods and all associated parts of the extensometer are freed from support by the axial frame and supported in the borehole by the anchor units ( 104 A ad  104 B of  FIG. 1 ). When pressure in the anchor unit  105  is reversed, the cylinder plunger  202  retracts, and the securing cone  305  reengages the axial frame  103  member, thereby reconnecting the extensometer to the axial frame unit.  
      As illustrated in  FIG. 4 , the registration element  113  comprises a registration rod actuator  401  mounted on a registration rod housing  402  comprising a cylinder  403 , that when pressurized via valve  411  causes the registration rod plunger  406  to move the registration pin  115  through the opening  407  in the proximal reference rod  107 A and to engage a precisely aligned opening  408  in the distil reference rod  107 B, thereby aligning the distil reference rod  107 B with the proximal reference rod  107 A in relation to space  122  ( FIG. 1 ). The registration pin  115  supports the reference rods  107 A and  107 B when the pin  115  is fully deployed. The reference rod plunger  406  moves forward in the direction of arrow  410  when pneumatic pressure is introduced via value  411 . Pressure is applied via value  412  and the registration pin  115  retracted. Bearings  413  in the reference rod housing  402  allow the proximal reference rod  107 A to move freely into alignment with opening  408  in the distil reference rod  107 B to effect the essential registration of reference rods  107 A and  107 B with respect to space  122  ( FIG. 1 ).  
      The reference rods  107  are locked in position relative to each other when the registration pin  115  is engaged in the opening in the distil reference rod  107 B. Retracting the registration pin  115  so that it is contained within the proximal reference rod  407  completely decouples the proximal  107 A and distil reference rods  107 B so they are free to move axially.  
      The reference rods  107 A and B are round, except where they overlap at the registration element  113  in  FIG. 4 . The overlap is achieved by machining the rods so they are semi-circular in cross-section over a distance of approximately 4 inches (10 cm) in  FIG. 4 . The overlapping region of the reference rods is enclosed in two sleeve bearings  413  so the rods remained aligned, but are free to move relative to each other along their axes.  
      The correct functioning of the registration element requires that the opening in the proximal rod  407  never becomes misaligned with the opening in the distil rod  408  by more than half the diameter of the registration pin  115 . The travel of the reference rods is limited to ensure correct functioning of the registration element. The travel is limited by a rectangular protuberance  415  machined into the end of the proximal reference rod and a rectangular slot  414  machined into the distil rod. The width of the slot  414  is 0.1 inch (2.5 mm) wider than the width of the protuberance  415 . The relative motion of the proximal and distil rods is limited when the face of the rectangular slot engages the face of the rectangular protuberance.  
      Details of the temperature compensation means  111  and its relation to the displacement transducers are shown in  FIG. 5 . The general structures illustrated in  FIG. 5  are also applicable to the extensometer illustrated in Example 2.  
     EXAMPLE 2  
       FIG. 6A  illustrates a borehole extensometer  601  that incorporates the reference rods, temperature compensation means, and registration means of Example 1 and that incorporates a central support rod  603  in lieu of the axial support frame of Example 1 and at least two, extendable/retractable, mechanically lockable legs that anchor and support the functionally positioned extensometer.  
      The borehole extensometer of  FIG. 1 , like the device of Example 1, is capable of detecting very small changes in displacement caused by minute increases or decreases in the aperture of a fracture of a rock structure. The borehole extensometer  601  comprises a central support rod  603 , preferably stainless steel of various lengths and diameter. Lengths of 8 feet (2.75 meters) and diameter of 0.75 inches (2.0 cm) are appropriate, but not limitations. The support rod  603  has a top end  698  and a bottom end  699 . The following description is from the top end  698  towards the bottom end  699 .  
      An upper registration unit comprised of an upper cylinder station  605 A and upper slideable element  605 B is positioned on the support rod  603 . An upper anchor  607  is positioned between the upper cylinder station  605 A and an upper slideable element  605 B. An cylinder  606  threads on to an cylinder station and rod  620  connects the piston of the cylinder  606  to the upper slide unit  605 B. Pressure applied to the upper surface  622 A of the plunger rod forces the upper cylinder  605 A apart from the upper slideable element  605 B and pressure on the lower surface  622 B brings these structures together. Movement of the upper cylinder  605 A station and upper slideable element  605 B serves to allow control of the position and orientation of the upper anchor  607 .  
      The upper anchor  607  comprises deployable legs  613  and a pneumatic or hydraulic powered deployment means with latch capabilities and retraction capability (see  FIG. 7 ).  
      The temperature compensator  609  and linear varying differential transducer  611  are positioned immediately below and in contact with the bottom surface of the upper anchor  607 . A spring loaded, plunger  690  that is part of the linear varying differential transducer  611  is positioned below the lower surface  689  of the temperature compensator, and that surface  689  is separated from the upper surface  688  of the lower anchor  617 , by a space  615 . In operation, the sensor contact  690  is in physical contact with the upper surface  689  of the lower anchor  617 . The structure and functions of the lower registration slider  619 B, air cylinder station  619 A, and anchor legs are as described above for corresponding structures.  
      Details of the registration are shown in  FIG. 6B . The upper air cylinder station  605 A is physically linked to the slideable cylinder  605 B by a plunger-like rod  620 . Pressurized air entering the air cylinder  606  at the upper fitting  616 A exerts downward pressure on the plunger-like rod  620 , and pressure entering at the lower fitting  616 B exerts upward pressure on the plunger-like rod  620  which is physically attached to the slideable unit  605 B at a point  623 .  
      Maximum travel of the upper air cylinder station  605 A and slide unit  605 B is limited by stops  614 A and  614 B positioned on the support rod  603 . Controlled movement of the upper air cylinder station  605 A and slideable unit  605 B in response to injection of air through fittings  616 A and  616 B allow positioning of the anchor  607 .  
       FIG. 7  illustrates the actuator  702  with deployable legs  713  and powered by air pressure from an air cylinder  703  air pressure from the air cylinder  703  exerts pressure on a moveable wedge  704 . The moveable wedge, when moving downward, exerts outward pressure on leg  613  thereby deploying leg  613 . Tooth surfaces  707  on the actuator engage a pistol latch device  705  to hold deployed leg in position when air pressure is released. A plunger device  708  is connected to the moveable wedge  704 . Air pressure can be introduced at first point  710 A above the plunger device  708  and causes the device to move downward, thereby deploying the legs  613  and causing the latch  705  to engage the toothed surface  707  of the leg  613  and holding the leg securely in the deployed position. Air pressure introduced at a second point  710 B below the plunger device  708  causes the wedge to move upward, releasing the latch, and thereby retracting the legs  613 .  
       FIG. 8  illustrates details of the ratchet/lock system  801  used in deploying the legs  113 . The plunger device  708  contacts the wedge  704  and is also connected to a block  802  that engages the first end  880  of the lock arm  803  that is pivoting mounted  805  to allow the second end  806  of the lock arm  803  to engage the tooth structure  707  of the actuator. Downward pressure forces the legs  113  outward in a deployed configuration, upward pressure forces the wedge  704  upward, releasing the lock and thereby allowing the legs  613  to be retracted and the extensometer  601  to be moved in or removed from the borehole.  
       FIG. 9  illustrates the major components of the temperature compensator  909 . At least one invar rod  902  is connected by a first end  904  to the bottom  903  of the anchor  607  by screw-thread means. A second end  905  of the invar rod  902  is bolted  907  to the floor  906  of the brass expansion tube  908 . The brass expansion tube  908  encases a portion of the invar rod. Heating or cooling cause the invar rod  902  and brass expansion tube  908  to expand to different degrees, but absolute differences are effectively the same owing to difference in length and material properties between the invar rod  902  and brass expansion tube  908 .  
      Specific terms, devices, and descriptions are used for purposes of illustration, not limitations of the invention. In addition, one skilled in the art recognizes that various elements of different embodiments can be interchanged to yield still more embodiment, all of which are anticipated in the scope and intent of the invention. Consequently, the appended claims should be accorded the widest-scope of interpretation, and not be limited by the specific term, devices, and descriptions herein.