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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to well logging tools, and more particularly to a continuous obstruction monitor installed between the drill string and the logging tool which utilizes drilling fluid to indicate when the logging tool has encountered an obstruction and allows for transporting and setting of the logging tool, circulation through the monitor housing, and reciprocation of the drill pipe while continuously monitoring for borehole obstructions. 
     2. Brief Description of the Prior Art 
     Systems which transport well logging tools on the drill string are known in the art. Conventional systems incorporate a cable and wet connect transport assembly or locomotive which is forced down the borehole by mud pressure within the drill string to make the electrical connection to the logging tool. A side door sub in the drill string allows the cable to be run along the outside of the pipe for some distance downhole. The side door sub permits the cable to enter the pipe and seals it to prevent loss of drilling mud pressure. In the logging operation, with the tool at the total depth and electrical connection from the logging tool to the surface, drill pipe is removed one joint at a time while the cable is spooled in at the same rate. The present invention is used in conjunction with conventional wet connect cable transport assemblies or locomotives, side door subs, logging tools, and utilizes state of the art logging procedures. 
     The most common method of transporting the logging tool is to mount the logging tool on the drill string enclosed in a protective sleeve with openings to allow sensors to contact the borehole from within the sleeve. Distorted data curves sometimes occur due to the proximity and interference from the sleeve. The only indication of an obstruction in this type of system is when the drill string stops which often causes damage to the logging tool or the sleeve. Attempts at circulating the obstruction away using the openings in the sleeve is limited because the circulation is above the obstruction. 
     Another common method of transporting the logging tool is to mount the logging tool on the drill pipe with a shock absorbing device to protect the tool while monitoring obstructions electrically with a tension device downhole. This eliminates sleeve interference, but because of side door sub cable limitations, satisfactory electrical connection is not always possible. A well with a 2,500 foot kick out depth (point at which bore begins to deviate from vertical) and 12,500 feet in total depth would leave 10,000 feet of hole to traverse without a weight indicator. If an obstruction is encountered, circulation can be made at the top of the tool string but not at the obstruction location at the bottom of the string. 
     Another transport method is one which utilizes protective sleeves mounted on the drill string with the logging tool retained by a complex latch mechanism. This system uses a two piece wet connect cable transport assembly which separates after being latched to the logging tool. Releasing the latch mechanism allows the logging tool to be lowered from the protective sleeve for the logging operation. The complexity of the latching system makes it prone to numerous failures. Because most of the drilling fluid escapes at the latch assembly, only a small amount of circulating fluid is directed to the obstruction. The logging tool may also be blocked in the sleeve by debris from the obstruction building up in the sleeve. 
     There are several patents which disclose apparatus for monitoring or measuring pressure while drilling and supporting sensing devices in well logging tools. 
     Conley et al, U.S. Pat. No. 3,965,978 discloses a subsurface transient pressure indicator suitable for for shutting off fluid flow through tubing at a subsurface location in a completed well containing a packer in the tubing-casing string annulus. The device has a seating nipple forming a part of the tubing string positioned at the downhole location at which it is desired to shut off fluid flow through the tubing. The nipple has an upwardly facing shoulder, and a main plug member having a downwardly facing seating area is adapted to seat in the seating nipple. The main plug member is connected to the surface of the well by a wireline or cable. A port extends through the main plug below the point of seal with the nipple to a second seating area and a second port from the second seating area to a point above the point of seal. A pressure measuring means communicates with the tubing space below the point of seal. A relief plug is adapted to seat in the second seating area of the main plug. Pressure is equalized in the tubing string above and below the device by pulling upwardly on the wireline to pull the relief plug body away from the second seating area. 
     This device measures transient pressure in a completed well and is quite different than the present invention which is a transport system for well logging tools which is transported on drill pipe and functions as a continuous obstruction monitor independent of cable connections. 
     Claycomb, U.S. Pat. No. 4,184,545 discloses a measuring and transmitting apparatus comprising an elongate tubular member functioning as a drill collar and adapted to to be placed in a drill string while drilling a well. The apparatus utilizes a mud driven motor, while the mud flows therethrough functioning as a drill collar and out the bottom. The motor creates electric power which is used to operate transducers forming measurements which are encoded. The encoding portion of the equipment utilizes an oil reservoir, a mud driven pump, a valve which dumps the oil back to the reservoir if there is no signal and which otherwise delivers it to a spool valve which drives a modulated signal generator. The signal generator modulated the back pressure in the mud flow by restricting the mud flow, thereby forming a signal which is sensed in the mud flow path. This device modulates mud flow through it and must have a surface mud pump attached to function. 
     Garney, U.S. Pat. No. 4,359,900 discloses an elongated well logging instrument having a lever latch and positioning mechanism attached to a carriage and a snorkel and barrel assembly. One end of a pressure sensing device is mounted on the carriage and the snorkel is affixed to the opposite end. The latch and positioning mechanism allows the assembly to be moved until the snorkel enters the barrel and retains it in place. The mechanism is then secured in place. The mechanism contains an indicator which is visible if the mechanism is not in a position to be secured. Electrical signals are passed from the sensing device, through the snorkel to an electrical connector at the end of the barrel and through a cable to the surface. 
     Peppers et al, U.S. Pat. No. 4,483,394 discloses a hydraulic power unit for for a downhole instrument of a measurement while drilling system. The hydraulic unit is enclosed within a drill collar that is connected to a drill string of a rotary drilling rig. An outer body sleeve is rigidly mounted in the instrument drill collar, and a fixed inner sleeve is connected to the outer body sleeve in a concentric relation. A longitudinally movable plunger assembly is supported between the outer sleeve and inner sleeve and movable in a telescopic fashion between an extended position and a retracted position to provide hydraulic power fluid for the operation of hydraulically powered circuits of the instrument. 
     The prior art in general, and these patents in particular, do not disclose the present invention of a continuous obstruction monitor installed between the drill string and the logging tool which utilizes drilling fluid to indicate when the logging tool has encountered an obstruction and allows for transporting and setting of the logging tool, circulation through the monitor housing, and reciprocation of the drill pipe while continuously monitoring for borehole obstructions. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide a continuous obstruction monitor for well logging tools which may be used in conjunction with conventional wet connect cable transport assemblies, side door subs, and utilizes state of the art logging procedures. 
     Another object of this invention is to provide a continuous obstruction monitor for well logging tools which utilizes the displacement of drilling fluid to indicate when the logging tool has encountered an obstruction and requires no cable or pump connections to accomplish this end. 
     Another object of this invention is to provide a continuous obstruction monitor for well logging tools which allows the logging tool to retract within an outer housing when an obstruction is encountered and allows drilling fluid to be circulated directly at the location of the obstruction. 
     Another object of this invention is to provide a continuous obstruction monitor which allows for transporting and setting of the logging tool, circulation through the monitor housing, and reciprocation of the drill pipe while continuously monitoring for borehole obstructions. 
     Another object of this invention is to provide a continuous obstruction monitor which is simple in construction and operation, economical to manufacture, reliable, and rugged and durable in use. 
     Other objects of the invention will become apparent from time to time throughout the specification and claims as hereinafter related. 
     The above noted objects and other objects of the invention are accomplished by a continuous obstruction monitor for well logging drill strings including a well logging instrument at the forward end thereof which transports the logging instrument through an earth borehole while continuously monitoring for obstructions. An outer tubular housing is connected at its upper end to the drill string and a piston therein is connected to the upper portion of the logging instrument. The housing has a variable volume greater than the volume of an equal length of the drill string. The piston is electrically connected to the surface. A latch retains the piston in the lower portion of the housing, and releases it when an obstruction is encountered. The outer housing moves downward relative to the piston to a point where a sealing surface at the mid portion of the housing shuts off fluid flow below the piston and causing the drilling fluid to rise within drill string to be detected at the surface. Further movement of the housing positions the piston in the upper portion thereof to open a fluid flow path through the drill string bypassing the piston to exit the housing to circulate drilling fluid close to the borehole obstruction. In its uppermost position, the piston seals the drill string preventing a through pipe blowout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a, 1b, and 1c taken together constitute a longitudinal section showing the details of a preferred continuous obstruction monitor for well logging tools. 
     FIG. 1d is a horizontal cross section taken along line 1d--1d of FIG. 1b showing the longitudinal slots of the bypass sleeves of the tool. 
     FIG. 2 is a longitudinal cross section showing the details of a modified lower portion of the outer housing of the continuous obstruction monitor for well logging tools. 
     FIG. 3 is an enlarged cross section of the wet connect latch assembly of the tool. 
     FIG. 4 is a schematic elevation of a wet connect cable transport assembly used with the tool. 
     FIG. 5 is a schematic elevation of a visual float assembly used with the tool. 
     FIG. 6 is a schematic elevation of the assembled continuous obstruction monitor with various piston positions shown in dotted line. 
     FIG. 7 is a schematic elevation of the assembled continuous obstruction monitor having a modified lower portion. 
     FIGS. 8, 9, and 10 illustrate an alternate expansible cylindrical ring which surrounds the reduced diameter portion of the piston body. 
     FIG. 11 is a vertical cross section of an alternate wet connect latch mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings by numerals of reference, there is shown in FIGS. 1a-1c and schematically in FIG. 6, a preferred continuous obstruction monitor assembly 10, hereinafter referred to as &#34;COM&#34;. An outer housing 11 comprises a series of bypass sleeves 12 each of which has external threads 13 at each end and a circumferential groove 14 disposed on the exterior surface inwardly of the threads. Annular seals 15 are provided on the sleeves 12 at each end intermediate the threads 13 and grooves 14 for sealing the threaded connection. Bypass sleeves 12 have a plurality of circumferentially spaced longitudinal slots 16 formed on the interior surface, as shown in FIG. 1d. 
     The bypas sleeves 12 are connected together at each end by a series of collars 17 having internal threads 18 and a central reduced internal diameter forming a shoulder 19 with longitudinal slots 20 corresponding to the sleeve slots 16. The collars 17 are threadedly hand tightened onto the sleeves 12 and with the slots 16 and 20 in longitudinal axial alignment are locked onto the sleeves by set screws 21 which are received within the grooves 14 of the sleeves 12. 
     A blowout prevention sleeve 22 hereinafter &#34;BOP&#34; sleeve is connected at its lower end to the uppermost bypass sleeve 12. The BOP sleeve 22 has external threads 23 at each end and a circumferential groove 24 disposed on the exterior surface inwardly of the threads. Annular seals 25 on the exterior surface of the BOP sleeve 22 at each end intermediate the threads 23 and grooves 24 seal the threaded connection. The BOP sleeve 22 is shorter in length than bypass sleeves 12 and has a smooth internal diameter 26. Another collar 17 is installed and locked (as previously described) on the upper threaded end of the BOP sleeve 22. 
     A crossover sub 27 having an externally threaded lower portion 28 is threaded and locked (as previously described) into the upper portion of the upper collar 17. The upper portion 29 of the crossover sub 27 has a smaller O.D. than the lower portion 28 and has internal threads 30 extending inwardly from the top to a smaller I.D. bore 31. The bottom of sub 27 is tapered inwardly downward forming a convex conical sealing surface 32. An annular seal 33 fits the outer circumference of the lower portion 28 above the threads. The lower threaded portion 34 of a conventional drill pipe 35 is secured in the threads 30. 
     The lower intermediate portion of outer housing 11 of the COM assembly 10 comprises a series of sealing sleeves 36 installed beneath the bypass sleeves 12. Each sealing sleeve 36 has external threads 37 at each end and a circumferential groove 38 on the exterior surface inwardly of the threads. Annular seals 39 on the sealing sleeves 36 at each end intermediate the threads 37 and grooves 38 seal the threaded connection. The internal surface 40 of each sealing sleeve 36 is smooth and substantially the same I.D. as the bypass sleeves 12. The sealing sleeves 36 are connected together at each end by a series of collars 17 threadedly hand tightened onto the sleeves 36 and locked by set screws 21 fitting the grooves 38. 
     A latch collar 41 threadedly on the bottom portion of the lower sealing sleeve 36 has internal threads 42 at each end, a reduced diameter bore defining a shoulder 43 immediately below the top internal threads, an internal annular groove 44 immediately below the shoulder 43, and a plurality of circumferentially spaced longitudinal slots 45 formed on the interior surface below the annular groove 44. Shoulder 43 has circumferentially spaced longitudinal slots 46 corresponding to the lower slots 45. the latch collar 41 is threadedly hand tightened onto the lowermost sealing sleeve 36 and locked onto the sleeve by set screws 21 which are received within the grooves 38. 
     A transport sleeve 47 with threads 48 at each end and a groove 49 inward of the threads is installed beneath the latch collar 41. Annular seals 50 on the sleeve 47 at each end intermediate the threads 48 and grooves 49 seal the threaded connection. The upper external threads 48 are secured in the lower internal threads 42 of the latch collar 41. 
     The transport sleeve 47 is hand tightened into the latch collar 41 and locked by set screws 21 which fit the grooves 49. A plurality of circumferentially spaced longitudinal slots 51 on the interior surface of the transport sleeve 47 and a plurality of circumferentially spaced apertures 52 extend through the side wall of the transport sleeve for passage of drilling mud from the interior of the sleeve into the borehole. 
     A retaining ring 53 with internal threads 54 is installed on the lower external threads 48 of the transport sleeve 47. The internal threads 54 of the retaining ring 53 extend inwardly from the top to a reduced diameter bore 55 defining an inwardly and downwardly tapered shoulder 56. 
     Optionally, as shown schematically in FIG. 7, the housing 11 may be adapted to be connected to other sizes of pipe or casing and for use with large diameter logging instruments. As shown in FIG. 2, a modified retaining ring 58 with internal threads 59 is installed on the lower threads 48 of the transport sleeve 47. The internal threads 59 of the retaining ring 58 extend inwardly from the top and terminate in a reduced diameter bore 60 to define an inwardly and downwardly tapered shoulder 61. 
     The exterior bottom portion of the retaining ring 58 has an annular seal 62 and exterior threads 63 which receive the upper interior threads 64 of a crossover adapter coller 65. In the example illustrated, the bottom portion of the modified retaining ring 58 has tapered casing threads 66, allowing conventional sections of casing 67 connected by conventional casing collars 68 to be installed below the transport sleeve 47. A thread protector ring 69 may be installed on the threaded bottom end of the lowermost section of casing to protect the threads. 
     A floating piston 70 is reciprocally contained within the outer housing 11. The piston 70 has a cylindrical body 71 with a central bore 72 and internal threads 73 in the upper portion with a shoulder 74 therebetween. The interior bottom portion has a counterbore 75 extending from the flat bottom surface 76. Circulation ports 77 extend angularly from the exterior of the piston body 71 to the central bore 72 for circulating drilling mud through the piston to the outside. 
     The piston body 71 has a reduced O.D. portion 78 with a shoulder 79 above the ports 77 and a circumferential groove 80 near the upper end. The upper portion of the piston body 71 has a reduced O.D. forming a camming surface 78A between the upper reduced diameter and the groove 80. A compression spring 81 surrounds the reduced O.D. portion 78 and abuts the shoulder 79. A cylindrical ring 82 is supported on the top of the compression spring 81. A series of spaced tapered apertures 83 extend through the side wall of the ring 82. 
     A series of ball members 84 are movably contained in the apertures 83 between the camming surface 78A and the internal diameter of the ring 82. The diameter of the ball members is greater than the slots 45 of the latch collar 41 so that they are prevented from entering the slots. The tapered apertures 83 prevent the ball members 84 from falling out of the ring while allowing them to rotate and travel radially inward and outward following the profile of the camming surface 78A and groove 80 as the ring 82 travels along the longitudinal axis of the piston body 71. 
     The compression spring 81 is of sufficient strength to require 2,500 pounds of force to move the piston from the latched position with the ball members 84 in groove 44 of the latch collar 41 to the unlatched position with the ball members retracted into the groove 80 of the piston 70. 
     The upper end of a cylindrical wet connector housing 85 is received within the counterbore 75 and locked into the bottom of the piston body 71 below the circulation ports 77 by set screws 86. The lower portion of the wet connector housing 85 extends outwardly from the piston body 71. The lower end of the wet connector housing 85 is connected by conventional connection means to the top portion of a conventional well logging tool 87. 
     A seal assembly 88 has resilient seal members 89 interposed between a series of thin spacer discs 90 and surrounds the lower portion of the wet connector housing 85. A bottom spacer disc 91 is disposed at the bottom of the lowermost seal member 89. Sufficient clearance is provided between the circumference of the housing 85 and the internal diameter of the seal members 89 and the spacer discs 90 to allow the seal assembly 88 to travel longitudinally on the housing 85. 
     A thrust bearing 92 is installed on the housing 85 and a thin retaining disc 93 is installed therebelow. A retaining plate 94 is installed on the housing 85 below the retaining disc 93 and carries a rotary bearing 95 within a counterbore 96 at its upper end. The bottom surface 97 of the retaining plate 94 is beveled at an angle corresponding to the tapered shoulder 56 of the retaining ring 53 to be supported thereon. 
     A cylindrical stop sleeve 98 is installed on the housing 85 below the retaining plate 94 and secured thereon by set screws 99 to limit downward travel of the seal assembly 88 and retaining plate 94. The lower end of the wet connector housing 85 below the stop sleeve 98 is of sufficient diameter to freely pass through the bore 55 of the retaining ring 53. 
     The male element of a wet connector 101 is secured by conventional means within the upper portion of housing 85 and the connecting portion 102 extends upwardly a distance from the housing 85 centrally within the piston bore 72. The bottom of the male element is connected by wire leads 103 to the logging tool 87 in a conventional manner. 
     A guide cone 104 having a reduced diameter bottom portion with external threads 105 is threadedly received and secured in the threaded top portion of the piston body 71. The O.D. of the top portion of the guide cone 104 is sized to clear the I.D. of the sleeves and collars of the outer housing 11. The top surface of the guide cone 104 tapers angularly downward and inward to a central bore 106 of the same I.D. as the bore 72 of piston body 71. The tapered top surface of the guide cone forms a concave conical seating surface 107 corresponding to the convex seating surface 32 of the crossover sub 27 at the top of the housing 11. The conical seating surface 107 serves as a guide for receiving the female element 108 of the wet connector and forms a metal to metal seal with the convex surface 32 when the piston 70 acts as a through pipe blowout preventer (described hereinafter). 
     A set connect latch assembly 109 is secured within an aperture 110 through the side wall of the upper portion of the guide cone 104. The latch assembly 109 is shown in greater detail in FIG. 3. The latch assembly 109 comprises a split housing 111 divided into two generally recessed mating halves and joined by transverse pins 112. 
     Each half of the housing 111 has an upper horizontal slot 113 which slidably receives an elongated shear pin 114 having notches 115 on the bottom surface. The rear end of the shear pin 114 has a vertical slot 116 perpendicular to the notched bottom. The slot 116 receives the upper end of a drive arm 117, the lower end of which is pivotally connected to the housing 111 by pivot pin 118. 
     A horizontal bore 119 extends inwardly from one side of the housing 111 just above the pivot pin 118. A compression spring 120 is received within the bore 119 to bias the drive arm 117, and push the shear pin 114 toward the central bore of the piston body. 
     An angular bore 121 and counterbore 122 extend downwardly from the inward side of the housing 111. A cylindrical latch member 123 having a rounded top surface 124 is slidably received within the angular bore 121. The latch member 123 has a disc shaped back end 125 which is slidably received within the counterbore 122. A compression spring 126 is received within the lower counterbore 122 in contact with the disc shaped back end 125 of the latch member 123. 
     A rod member 127 having a slotted bottom is pivotally connected to the latch member 123 and extends generally vertical therefrom between two pins 112. The top end of the rod 127 is beveled 128 corresponding to the notches 115 of the shear pin 114 to be received therein for retaining the shear pin against outward movement. In this manner, as the latch member 123 moves angularly inward and outward, the rod 127 will move vertically to engage or disengage the notches of the shear pin 114. 
     A backing plate 130 is installed at the back end of the housing 111 and secured by screws 131 to the outer surface of the guide cone 104 to cover outer side of the housing 111 and retain the spring 120 within the bore 119. 
     In operation, the bottom of the wet connect cable transport assembly 132 (FIG. 4) entering the central bore 72 of the piston body 71 will force the latch member 123 inwardly pulling the rod 127 down and release the shear pin 114 to be driven outwardly by the drive arm 117 and be received in the latching groove 135 on the female connector element 108. 
     A locomotive or wet connect cable transport assembly 132 is shown schematically in FIG. 4. The assembly is conventional in the art and comprises a series of connected tubing joints 133 having drive cups 134 installed in a spaced relation along its length. It should be understood that at least one upper drive cup must remain in the drill pipe 35 for effective connection of the wet connector elements 101 and 108. 
     The internal diameter of the assembly 132 is sufficient to allow passage of cable and rope sockets therethrough (also conventional in the art). The female element 108 of the wet connector is carried at the bottom of the assembly 132. A latching groove 135 is provided on the circumference of the female element 108. The latching groove 135 is located such that the shear pin 114 will be received therein when the female element 108 is properly connected with the male element 101 of the wet connector within the bore 71 of piston 70. 
     FIG. 5 illustrates a visual float indicator 136. The indicator 136 comprises two sections of tubing 136A threadedly connected with appropriate seals and sealed at each end to form an elongated bouyant tubular member approximately 20 feet in length. A detent 137 is pivotally connected near the top of the float assembly 136 and biased by a compression spring 138 to extend outward. A threaded rod 139 is secured to the bottom of the lowermost tubing joint and receives a number of weights 140 to selectively adjust the bouyancy of the assembly. A lock nut 141 retains the weights 140 on the rod 139. 
     The float indicator 136 floats in the drilling mud within the uppermost section of drill pipe 35 at the surface of the rig floor and visually indicates that the piston 70 is displacing drilling mud before any mud is spilled on the rig floor. Detent 137 will be biased outward by the spring 138 if the float 136 is displaced above the drill pipe 35. The outwardly extended detent 137 also aids in removal of the float 136 for pump installation. It should be understood that the float indicator may be operatively connected to various alarm devices. 
     FIGS. 8, 9, and 10 illustrate an alternate expansible cylindrical ring 200 which surrounds the reduced diameter portion 78 of the piston body 71 and abuts the compression spring 81. The expansible ring 200 is divided into cylindrical segments 201, each having an inwardly extending horizontal slot 202 at one and a flat mating tongue 203 at the other end. The tongue 203 of one segment is slidably received within the slot 202 of the adjacent segment. A second guide slot 204 perpendicular to the horizontal slot 202 extends angularly inward from the slotted end of each segment. Pins 205 extend vertically through each tongue 203 and are slidably received within the guide slots 204. 
     An oval-shaped roller member 206 is rotatably pinned within each segment 202 such that the outer circumference extends outwardly of the inside and outside diameters of the segments. The rollers 206 rotate and the segments 201 travel radially inward and outward following the profile of the camming surface 78A and groove 80 as the ring 82 travels along the longitudinal axis of the piston body 71. 
     The compression spring 81 is of sufficient strength to require 2,500 pounds of force to move the piston from the latched position with the roller members 206 in the groove 44 of the latch collar 41 to the unlatched position with the roller members retracted into the groove 80 of the piston 70. 
     FIG. 11 is a vertical cross sectional view of an alternate wet connect latch mechanism 309. The wet connect latch assembly 309 is secured within an aperture 110 through the side wall of the upper portion of the guide cone 104. The latch assembly 309 comprises a housing 311 having an upper horizontal bore 312 and counterbore 313, and a vertically spaced lower horizontal bore 314 and counterbore 315 extending therethrough. A vertical bore 316 extends between the bores 312 and 314. An elongated shear pin 317 having a series of notches 318 along the lower side is slidably received within the upper bore 312 and counterbore 313. A flat circular disc 319 is slidably received within the upper counterbore 313 in contact with the inward end of the shear pin 317. A compression spring 320 is received within the upper counterbore 313 in contact with the disc 319. 
     A cylindrical latch member 321 having an upwardly facing beveled top surface 322 is slidably received within the lower bore 314. The latch member 321 has a disc shaped back end 323 which is slidably received within the lower counterbore 315. A compression spring 324 is received within the lower counterbore 315 in contact with the disc shaped back end 323 of the latch member 321. 
     A longitudinal slot 325 extends transversely through the latch member 321. The slot 325 extends angularly upward relative to the longitudinal axis of the latch member 321 from a point near its front end and terminates near the disc shaped back end 323. A rod member 326 having a notched top end 327 corresponding to the notches 318 of the shear pin 317 and a yoke shaped bottom end 328 is received within the vertical bore 316 and attached by pin 329 to the slot 325 of the latch member 321. In this manner, as the latch member 321 moves horizontally, the rod 326 will move vertically to engage or disengage the notches 318 and 327. 
     A backing plate 330 is installed at the back end of the housing 311 and secured by screws 331 to the outer surface of the guide cone 304 to cover the counterbores 313 and 315 and retain the springs 320 and 324 therein. 
     OPERATION 
     The continuous obstruction monitor (hereinafter referred to as &#34;COM&#34;) is assembled in two sections for ease in raising to the rig floor. The bottom section is set in slips and connected to the upper section. The complete assembly is raised up and out of the way. The piston 70 is contained within the transport sleeve 47 supported by the retaining ring 53 at the bottom of the outer housing with the wet connect housing 85 extending outwardly therefrom. In the transport position, the circulation ports 77 of the piston 70 are adjacent the apertures 52 of the transport sleeve 47, and the ball members 84 are retained in the annular groove 44 of the latch collar 41 by the piston camming surface 78a. 
     The conventional well logging tool 87 is assembled and suspended over the borehole on the make-up plate. The lower end of the wet connect housing 85 is connected to the upper end of the logging tool 87. 
     Verification is made that the borehole is full of drilling mud and that drilling mud is not being lost in the formation. It is essential that the borehole be full of drilling mud for proper operation of the COM and the prevention of blowouts. 
     The assembled logging tool 85 and COM assembly 10 is lowered into the borehole and the lowermost drill pipe 35 is connected to the crossover sub 27 at the top of the COM assembly. 
     With the top of the drill pipe at the rig floor, the visual float indicator 136 (FIG. 5) is inserted thereinto and the bouyancy of the float indicator properly adjusted by addition or subtraction of weights 140 so that the top of the float indicator is just below the rig floor. The float indicator may be removed, if desired, by filling the pipe with water. 
     The assembly is again lowered down the borehole. At this point, the float indicator 136 is continuously observed. If an obstruction is encountered by the logging tool 85, the indicator 136 will protrude out of the top of the drill pipe. Continued downward movement after an obstruction is encountered will cause drilling mud to be displaced out of the top of the drill pipe. If an obstruction is indicated by the float indicator, detent 137 on the side of the indicator will spring out preventing the float from going back down into the drill pipe. 
     The drill pipe is raised to the stabbing board and the float indicator is removed. By noting how far the pipe stand was in the borehole when the obstruction was encountered, calculations are made to determine how many drill pipe joints should be removed to install the kelly hose. The kelly hose is installed, and the drill pipe and kelly assembly is lowered into the borehole. Drilling mud displaced by the COM assembly is allowed to flow out through the kelly hose. As the obstruction is once again encountered, the drill pipe is raised slightly, and circulation is begun. Circulation should begin slowly, working through the obstruction. 
     Once the obstruction has been cleared, the drilling mud flowing past the piston will carry it back down to the transport sleeve at the bottom of the COM assembly. The kelly is removed, and the drill pipe and COM assembly continues to be lowered in the borehole. 
     The kelly is disconnected and removed, and the COM assembly is brought back up through the borehole in the logging mode as logging operations are performed in the conventional manner. If stops are desired, the COM allows circulation and pipe movement within the transport sleeve. In the event of a blowout, the COM piston will be blown to the top of the housing to seal on the conical bottom of the crossover sub. 
     Referring now to FIGS. 1a-1c and 6, the movement of the piston 70 within the outer housing 11 of the COM assembly 10 will be explained. As shown schematically in FIG. 6, the outer housing 11 may be divided into several longitudinally spaced sections. The lowermost section T extends from the retaining ring 53 to the top of the latch collar 41 and represents the transport mode. In this position, the piston 70 is supported at the bottom of the transport sleeve 47 while being transported down the borehole. 
     The D section represents the detection mode and extends upwardly from the top of the latch collar 41 to the bottom of the lowermost bypass sleeve 12, and includes the series of sealing sleeves 36. The piston 70 moves up into this area when the logging tool 87 encounters an obstruction. It should be remembered that an upward force of 2,500 pounds is required for the piston to pass through the latch, and only a very slight force to pass downwardly through the latch. The piston seal assembly 88 seals on the internal surfaces of the sealing sleeves 36 displacing drilling mud at the top of the drill pipe 35 to raise the visual float indicator 136. 
     The B section represents the bypass mode and extends upwardly from the top of the uppermost sealing sleeve 36 to the bottom of the blowout prevention sleeve 22, and includes the series of bypass sleeves 12. The piston 70 moves up into this area after the logging tool 87 has encountered an obstruction. The longitudinal slots 16 on the internal diameter of the bypass sleeves 12 permits drilling mud to bypass the piston and allow circulation to the bottom of the outer housing for clearing the obstruction. 
     The S section represents the safety or blowout prevention mode and extends from the top of the uppermost bypass sleeve 12 to the bottom of the crossover sub 27, and includes the blowout prevention sleeve 22. The piston 70 moves up into this area when the borehole pressure exceeds the weight of the fluid column. The concave top surface 107 of the piston 70 will seal on the convex conical bottom surface 32 of the crossover sub 27 and prevent a through pipe blowout. 
     Referring now to FIGS. 1a-1c and 7, the housing may be modified by removing a slotted bypass sleeve 12 from the upper assembly and adding one below the lowermost sealing sleeve 36 and compensating for the increased length by the addition of the modified retaining ring 58 and casing 67 below the transport sleeve 47. This modification allows tranporting and setting of the logging tool, circulation through the housing, and reciprocation of the drill pipe, all within the confines of the retaining ring 53 and the latch collar 41. This mode of operation, represented by R, is referred to as the reciprocation mode. 
     Having illustrated a preferred embodiment of the apparatus wherein the outer housing is connected to the drill string and the piston to the logging instrument, it should be understood that the outer housing could be connected to the logging instrument and the piston connected to the drill string without departing from the scope of the invention. 
     While this invention has been described fully and completely with special emphasis upon a preferred embodiment, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

Summary:
A continuous obstruction monitor for well logging drill strings including a well logging instrument at the forward end thereof transports the logging instrument through an earth borehole while continuously monitoring for obstructions. An outer tubular housing is connected at its upper end to the drill string and a piston therein is connected to the upper portion of the logging instrument. The housing has a variable volume greater than the volume of an equal length of the drill string. The piston is electrically connected to the surface. A latch retains the piston in the lower portion of the housing, and releases it when an obstruction is encountered. The outer housing moves downward relative to the piston to a point where a sealing surface at the mid portion of the housing shuts off fluid flow below the piston and causing the drilling fluid to rise within drill string to be detected at the surface. Further movement of the housing positions the piston in the upper portion thereof to open a fluid flow path through the drill string bypassing the piston to exit the housing to circulate drilling fluid close to the borehole obstruction. In its uppermost position, the piston seals the drill string preventing a through pipe blowout.