Abstract:
Apparatus and methods for establishing electrical communication between an instrument subsection disposed below a mud motor and an electronics sonde disposed above the mud motor in a drill string conveyed borehole logging system. Electrical communication is established via at least one conductor disposed within the mud motor and connecting the instrument sub section to a link disposed between the mud motor and the electronics sonde. The link can be embodied as a current coupling link, a magnetic coupling ling, an electromagnetic telemetry ling and a direct electrical contact link. Two way data transfer is established in all link embodiments. Power transfer is also established in all but the electromagnetic telemetry link.

Description:
This invention is related to measurements made while drilling a well borehole, and more particularly toward methodology for transferring data between the surface of the earth and sensors or other instrumentation disposed below a mud motor in a drill string. 
   BACKGROUND OF THE INVENTION 
   Borehole geophysics encompasses a wide range of parametric borehole measurements. Included are measurements of chemical and physical properties of earth formations penetrated by the borehole, as well as properties of the borehole and material therein. Measurements are also made to determine the path of the borehole. These measurements can be made during drilling and used to steer the drilling operation, or after drilling for use in planning additional well locations. 
   Borehole instruments or “tools” comprise one or more sensors that are used to measure “logs” of parameters of interest as a function of depth within the borehole. These tools and their corresponding sensors typically fall into two categories. The first category is “wireline” tools wherein a “logging” tool is conveyed along a borehole after the borehole has been drilled. Conveyance is provided by a wireline with one end attached to the tool and a second end attached to a winch assembly at the surface of the earth. The second category is logging-while-drilling (LWD) or measurement-while-drilling (MWD) tools, wherein the logging tool is an element of a bottom hole assembly. The bottom hole assembly is conveyed along the borehole by a drill string, and measurements are made with the tool while the borehole is being drilled. 
   A drill string typically comprises a tubular which is terminated at a lower end by a drill bit, and terminated at an upper end at the surface of the earth by a “drilling rig” which comprises draw works and other apparatus used to control the drill string in advancing the borehole. The drilling rig also comprises pumps that circulate drilling fluid or drilling “mud” downward through the tubular drill string. The drilling mud exits through opening in the drill bit, and returns to the surface of the earth via the annulus defined by the wall of the borehole and the outer surface of the drill string. A mud motor is often disposed above the drill bit. Mud flowing through a rotor-stator element of the mud motor imparts torque to the bit thereby rotating the bit and advancing the borehole. The circulating drilling mud performs other functions that are known in the art. These functions including providing a means for removing drill bit cutting from the borehole, controlling pressure within the borehole, and cooling the drill bit. 
   In LWD/MWD systems, it is typically advantageous to place the one or more sensors, which are responsive to parameters of interest, as near to the drill bit as possible. Close proximity to the drill bit provides measurements that most closely represent the environment in which the drill bit resides. Sensor responses are transferred to a downhole telemetry unit, which is typically disposed within a drill collar. Sensor responses are then telemetered uphole and typically to the surface of the earth via a variety of telemetry systems such as mud pulse, electromagnetic and acoustic systems. Conversely, information can be transferred from the surface through an uphole telemetry unit and received by the downhole telemetry unit. This “down-link” information can be used to control the sensors, or to control the direction in which the borehole is being advanced. 
   If a mud motor is not disposed within the bottom hole assembly of the drill string, sensors and other borehole equipment are typically “hard wired” to the downhole telemetry unit using one or more electrical conductors. If a mud motor is disposed in the bottom hole assembly, the rotational nature of the mud motor presents obstacles to sensor hard wiring, since the sensors rotate with respect to the downhole telemetry unit. Several technical and operational options are, however, available. 
   A first option is to dispose the sensors and related power supplies above the mud motor. The major advantage is that the sensors do not rotate and can be hard wired to the downhole telemetry unit without interference of the mud motor. A major disadvantage is, however, that the sensors are displaces a significant axial distance from the drill bit thereby yielding responses not representative of the current position of the drill bit. This can be especially detrimental in geosteering systems, as discussed later herein. 
   A second option is to dispose the sensors immediately above the drill bit and below the mud motor. The major advantage is that sensors are disposed near the drill bit. A major disadvantage is that communication between the non rotating downhole telemetry unit and the rotating sensors and other equipment must span the mud motor. The issue of power to the sensors and other related equipment must also be addressed. Short range electromagnetic telemetry systems, known as “short-hop” systems in the art, are used to telemeter data across the mud motor and between the downhole telemetry unit and the one or more sensors. Sensor power supplies must be located below the mud motor. This methodology adds cost and operational complexity to the bottom hole assembly, increases power consumption, and can be adversely affected by electromagnetic properties of the borehole and the formation in the vicinity of the bottom hole assembly. 
   A third option is to dispose the one or more sensors below the mud motor and to hard wire the sensors to the top of the mud motor using one or more conductors disposed within rotating elements of the mud motor. A preferably two-way transmission link is then established between the top of the mud motor and the downhole telemetry unit. U.S. Pat. No. 5,725,061 discloses a plurality of conductors disposed within rotating elements of a mud motor, wherein the conductors are used to connect sensors below the mud motor to a downhole telemetry unit above the motor. In one embodiment, electrical connection between rotating and non rotating elements is obtained by axially aligned contact connectors at the top of the mud motor. This type of connector is known in the art as a “wet connector” and is used to establish a direct contact electrical communication link. In another embodiment, an electrical communication link is obtained using an axially aligned, non-contacting split transformer. The rotating and non rotating elements are magnetically coupled using this embodiment thereby providing the desired communication link. 
   SUMMARY OF THE INVENTION 
   This disclosure is directed toward LWD/MWD systems in which a mud motor is incorporated within the bottom hole assembly. More specifically, the disclosure sets forth apparatus and methods for establishing electrical communication between elements, such as sensors, disposed below the mud motor and a downhole telemetry unit disposed above the mud motor. 
   The bottom hole assembly terminates the lower end of a drill string. The drill string can comprise joints of drill pipe or coiled tubing. The lower or “downhole” end of the bottom hole assembly is terminated by a drill bit. An instrument subsection or “sub” comprising one or more sensors, required sensor control circuitry, and optionally a processor and a source of electrical power, is disposed immediately above the drill bit. The elements of the instrument sub are preferably disposed within the wall of the instrument sub so as not to impede the flow of drilling mud. The upper end of the instrument sub is operationally connected to a lower end of a mud motor. One or more electrical conductors pass from the instrument sub and through the mud motor and terminated at a motor connector assembly at the top of the mud motor. The mud motor is operationally connected to the electronics sub comprising an electronics sonde. This connection is made by electrically linking the motor connector assembly to a downhole telemetry connector assembly disposed preferably within an electronics sub. The electronics sonde element of the electronics sub can further comprise the downhole telemetry unit, power supplies, additional sensors, processors and control electronics. Alternately, some of these elements can be mounted in the wall of the electronics sub. 
   Several embodiments can be used to obtain the desired electrical communication link between the mud motor connector and the downhole telemetry connector assembly. As stated previously, this link connects sensors and circuitry in the instrument package with uphole elements typically disposed at the surface of the earth. 
   In one embodiment, a communication link is established between the mud motor connector and the downhole telemetry connector assemblies using an electromagnetic transceiver link. The axial extent of this transceiver link system is much less than a communications link between the instrument sub, and across the mud motor, to the telemetry sub, commonly referred to as a “short hop” in the industry. This, in turn, conserves power and is mush less affected by electromagnetic properties of the borehole environs. The transceiver communication link can be embodied as two-way data communication link. The transceiver link is not suitable for transmitting power downward to the sensor sub. 
   In another embodiment, a flex shaft is used to mechanically connect the rotor element of the mud motor to the lower end of the electronics sub. The flex shaft is used to compensate for this misalignment, with the upper end of the flex shaft being received along the major axis of the electronics sub. Stated another way, the flex shaft compensates, at the electronics sub, for any axial movement of the rotor while rotating. The one or more wires passing through the interior of the rotor are electrically connected to a lower toroid disposed around and affixed to the flex shaft. The lower toroid rotates with the rotor. An upper toroid is disposed around the flex shaft in the immediate vicinity of the lower toroid. Both the upper and lower toroids are hermetically sealed preferably within an electronics sonde. The upper toroid is fixed with respect to the non rotating electronics sonde thereby allowing the flex shaft to rotate within the upper toroid. Upper and lower toroids are current coupled through the flex shaft as a center conductor thereby establishing the desired two-way data link and power transfer link between the sensors below the mud motor and the downhole telemetry unit above the mud motor. The upper toroid is hard wired to the downhole telemetry element. 
   In still another embodiment, the flex shaft arrangement discussed above is again used. The upper, non rotating toroid is again disposed around the flex shaft as discussed previously. In this embodiment, the lower toroid is electrically connected to conductors passing through the rotor and is disposed near the bottom of the flex shaft and near the top of the mud motor. The lower toroid is hermetically sealed within the mud motor. The upper toroid is hermetically sealed within the electronics sub. The two-way data link and power transfer link is again established via current coupling by the relative rotation of the lower and upper toroids, with the flex shaft functioning as a center conductor. 
   In yet another embodiment, the conductors are electrically connected to axially displaced rings at or near the top of the flex shaft. The rings, which rotate with the stator and the flex shaft, are contacted by non rotating electrical contacting means such as brushes. The brushes are electrically connected to the downhole telemetry element within the electronics sonde of the telemetry sub. Other suitable non rotating electrical contacting means may be used such as conducting spring tabs, conducting bearings and the like. The desired communication link is thereby established between the mud motor and the electronics sub by direct electrical contact. This embodiment also permits two way data transfer, and also allows power to be transmitted from above the mud motor to elements below the mud motor. Power can also be transmitted downward through the mud motor to the instrument sub. 
   In still another embodiment, a lower and an upper magnetic dipole are used to establish a magnetic coupling link. The flex shaft used in previous embodiments is not required. This link is not suitable for the transfer of power. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects the present invention are obtained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
       FIG. 1  is a conceptual illustration of the major elements of the invention disposed in a well borehole; 
       FIG. 2  illustrates in more detail the elements of the bottom hole assembly of the invention; 
       FIG. 3  is a conceptual illustration of an electromagnetic transceiver link between the mud motor and electronics sonde of the bottom hole assembly; 
       FIG. 4  illustrates a data link embodiment that is based upon current coupling of sensors below a mud motor and a downhole telemetry unit above the mud motor; 
       FIG. 5  illustrates another data link embodiment that is based upon current coupling of sensors below a mud motor and a downhole telemetry unit above the mud motor; 
       FIG. 6  illustrates a data link using direct electrical contacts rather than current coupling; 
       FIG. 7  illustrates a data link using magnetic coupling; 
       FIG. 8  shows a borehole drilled by the bottom hole assembly and penetrating an oil bearing formation and bounded by non oil bearing formation; 
       FIG. 9  shows a log obtained from gamma ray and inclinometer sensors within said bottom hole assembly; and 
       FIG. 10  illustrates a pair of steam assisted gravity drainage (SAG-D) wells drilled using the geosteering and other features of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   This section of the disclosure will present an overview of the system, details of link embodiments, and an illustration the use of the system to determine one or more parameters of interest. 
   Overview of the System 
     FIG. 1  is a conceptual illustration of the major elements of the invention disposed in a well borehole  26  penetrating earth formation  24 . A bottom hole assembly, designated as a whole by the numeral  10 , comprises an instrument subsection or “sub”  12 , a mud motor  16 , and an electronics sub  18 . The instrument sub  12  is terminated at a lower end by a drill bit  14  and operationally connected at an upper end to a lower end of a mud motor  16 . The upper end of the mud motor  16  is operationally connected to a lower end of an electronics sub  18 . The upper end of the electronics sub  18  is operationally connected to a drill string  22  by means of a connector head  20 . The drill string  22  terminates at an upper end at a rotary drilling rig that is well known in the art and indicated conceptually at  30 . The drilling rig  30  cooperates with surface equipment  32  which typically comprises an uphole telemetry unit (not shown), means for determining depth of the drill bit  14  in the borehole  26  (not shown), and a surface processor (not shown) for combining sensor response from one or more sensors in the bottom hole assembly  10  with corresponding depth to form a “log” of one or more parameters of interest. Data are transfer between the electronics sub  18  and the uphole telemetry unit by telemetry systems known in the art including mud pulse, acoustic, and electromagnetic systems. This two-way data transfer is illustrated conceptually by the arrows  25 . 
   It is noted that the drill string  22  can be replaced with coiled tubing, and the drilling rig  30  replaced with a coiled tubing injector/extractor unit. Telemetry can incorporate conductors inside or disposed in the wall of the coiled tubing. 
     FIG. 2  illustrates in more detail the elements of the bottom hole assembly  10 . The drill bit  14  (see  FIG. 1 ), which is received by the instrument bit box  36 , is not shown. Moving upward through the elements of the bottom hole assembly  10 , the instrument sub  12  comprises at least one sensor  40  and an electronics package  42  to control the at least one sensor  40 . A power supply  38 , such as a battery, powers the at least one sensor  40  and electronics package  42  in embodiments in which power can not be supplied by from sources above the mud motor  16 . The electronics package  42  typically comprise electronics to control the one or more sensors  40 , and a processor which processes, preprocesses, and conditions sensor response data for telemetering. The at least one sensor  40  and electronics package  42  are electrically connected to a lower terminus  44  of one or more conductors  46  that extend upward through the bottom hole assembly  10 . These conductors can be single strands of wire, twisted pairs, shielded multiconductor cable, coaxial cable and the like. Alternately, the conductors  46  can be optical fiber, with the instrument sub  12  comprising suitable elements (not shown) for convert electrical sensor response signals to corresponding optical signals. The one or more sensors  40  can be essentially any type of sensing or measuring device used in geophysical borehole measurements. These sensor types include, but are not limited to, gamma radiation detectors, neutron detectors, inclinometers, accelerometers, acoustic sensors, electromagnetic sensors, pressure sensors, and the like. An example of a log generated by a gamma ray detector and a measure of bottom hole assembly inclination will be presented in a subsequent section of this disclosure. When possible, elements of the instrument sub  12  are mounted within the sub wall so as not to impede the flow of drilling mud downward through the bottom hole assembly  10 . 
   Still referring to  FIG. 2 , the instrument sub  12  is connected to a drive shaft  48 , which is supported within the bearing section of the mud motor  16  by radial bearings  50  and  54 , and by an axial bearing  52 . The drive shaft  48  is connected to a rotor  58  by a driver flex shaft  56  that transmits power from the rotor  58  to the drive shaft  48 . The driver flex shaft  56  is disposed in a bend section  57  of the mud motor thereby allowing the direction of the drilling to be controlled. The rotor  58  is rotated within a stator  60  by the action of the downward flowing drilling mud. The upper end of the rotor  58  terminates at a mud motor connector  62 . Conductors  46 , that extend from the lower terminus  44  through the drive shaft  48  and driver flex shaft  56  and rotor  58 , terminate at an upper terminus  66  within the mud motor connector  62 . The upper terminus  66 , like the lower terminus  44  and conductors  46 , rotate. 
   Again referring to  FIG. 2 , an electronics sonde or insert  19  is disposed within the electronics sub  18 .  FIG. 2  is conceptual and not to scale. The outside diameter of the electronics sonde  19  is sufficiently smaller than the inside diameter of the electronics sub  18  to form an annulus suitable for mud flow. This annulus is clearly shown at  21  in  FIGS. 3-6 . The mud motor connector  62  rotatably couples the mud motor  16  to the electronics sub  18  and to the electronics sonde  19  therein through a downhole telemetry connector  64 . Mud flows through both the mud motor connector  62  and the downhole telemetry connector  64 . The downhole telemetry connector  64  comprises a telemetry terminus  70  that is electrically connected to elements within the electronics sonde  19 . These elements include a downhole telemetry unit  72 , optionally a power supply  74 , and optionally one or more additional sensors  76  of the types previously listed for the one or more instrument sub sensors  40 . The electronics sub  18  and electronics sonde  19  are operationally connected to the drill string  22  through the connector  20 , and two-way data transfer between the surface telemetry unit (not shown) and the downhole telemetry unit  72  is illustrated conceptually, as in  FIG. 1 , by the arrow  25 . 
   Once again referring to  FIG. 2 , a link between the rotating terminus  68  and the non rotating terminus  70  is illustrated by the broken line  68 . The following section will detail several embodiments of this link, which allows response of sensors  40  disposed on the downhole side of the mud motor  16  to be transmitted to the surface of the earth thereby allowing the sensors to be disposed in close axial proximity to the drill bit  14 . 
   It is noted that some embodiments do not use a mud motor connector  62  and a downhole telemetry connector  64 , with the corresponding terminuses  66  and  70 . Other embodiments use variations of the arrangement shown in  FIG. 2 . The discussion of each linking embodiment will include details of the link connections. 
   Link Embodiments 
   In the context of this disclosure, the term “operational coupling” comprises data transfer, power transfer, or both data and power transfer. 
   An electromagnetic transceiver link between the mud motor  60  and electronics sonde  19  is shown conceptually in  FIG. 3 . The conductor  46 , shown here as a twisted pair of wires, is again disposed within the rotor  58  and terminates at the terminus  66  within the mud motor connector  62 . The terminus is hard wired to a lower transceiver  80  disposed within the mud motor connector  62 . As in  FIG. 2 , the mud motor connector  62  is rotatably attached to the downhole telemetry connector  64 , which is attached to the lower end of the electronics sub  18 . The downhole telemetry connector  64  contains an upper transceiver  82  hard wired to the terminus  70 . The downhole telemetry unit  72  disposed within the electronics sonde  19  is hard wired to the terminus  70 . Data are transmitted to and from the downhole telemetry unit  72  and the surface, as indicated conceptually with the arrow  25 . The transceiver link, two-way electromagnetic data link between the upper and lower transceivers  82  and  84 , respectively, is indicated conceptually by the broken line  68 . As stated previously, elements within the downhole telemetry connector  64  and the mud motor connector  62  are disposed to allow drilling mud to flow through. It should be noted that power can also be transmitted to elements within the instrument sub, or alternatively these elements must be powered by a source  38  (see  FIG. 2 ) such as a battery. 
     FIG. 4  illustrates a data link embodiment that is based upon current coupling of sensors below the mud motor and the downhole telemetry unit above the mud motor. Elements and functions of this embodiment will be discussed beginning at the bottom of the illustration. As in the previous embodiment, the conductors  46  leading from the instrument sub  12  are shown as a twisted pair disposed within the rotor  58 . The conductors pass through feed throughs  66 A and  66 B, that are somewhat analogous to the terminus structure  66  shown in  FIGS. 2 and 3 . The conductors  46  terminate at a lower toroid  92  that surrounds and rotates with a flex shaft  90 . The lower toroid is hermetically sealed from the mud flow by a sealing means such as a rubber boot  99 . As stated previously, the flex shaft essentially compensates for axial movement of the rotor, when rotating, with respect to the electronics sub. 
   Still referring to  FIG. 4 , the flex shaft extends  90  upward through a pressure housing  97  through a sealing element  96 , and is supported by a radial bearing  98  that provides a conductive path to the electronics sonde housing  19 . An upper toroid  94  surrounds the upper end of the flex shaft  90 . The upper toroid  94  is stationary with respect to the rotating flex shaft  90 . Leads from the upper toroid  94  pass through feed throughs  70 A and  70 B (which are roughly analogous to the terminus  70  in  FIGS. 2 and 3 ) and connect to the downhole telemetry unit  72  disposed in the electronics sonde  19 . Data and/or power are transmitted to and from the downhole telemetry unit  72  as illustrated conceptually by the arrow  25 . 
   Again referring to  FIG. 4 , the upper and lower toroids  94  and  92  rotate with respect to one another thereby forming a current coupling via the flex shaft  90  functioning as a center conductor. It should be understood that, within the context of this disclosure, relative rotation of the upper and lower toroids  92  and  94  also comprises the previously discussed axial movement component of the lower toroid with respect to the upper toroid. The upper end of the flex shaft  90  is electrically connected through the radial bearings  98  to casing of the mud motor  60 , which is electrically connected to the rotor  58  through the axial bearings  52  (see  FIG. 2 ), which electrically connected to the lower end of the flex shaft  90  thereby completing the conduction circuit. An upward data link is obtained by applying a data current signal, such as a response of a sensor  40  (see  FIG. 2 ), to the lower toroid  92 . A corresponding data current signal is induced in the upper toroid  94 , via the previously described current loop, and telemetered to the surface via the downhole telemetry unit  72 . Conversely, data can be transmitted to the instrument sub  12  from the surface. This “down-link” data are telemetered from the surface telemetry unit contained in the surface equipment  32  to the downhole telemetry unit  72 , converted within the electronics sonde  19  to a current and applied to the upper toroid  94 . A corresponding current induced in the lower toroid  92  that is carried to the instrument sub via the conductors  46 . The two-way current coupled link is shown conceptually by the broken lines  68 . The current link may also be used to transfer power from a source contained in the downhole telemetry unit  72  to the instrument sub  12  in  FIG. 2   
   As mentioned previously, the mud motor connector, downhole telemetry connector, and terminus structure shown in  FIG. 4  has been modified in the link embodiment. Axial elements within by the broken line  62 A are roughly analogous to mud motor connector and associated terminus. Axial elements within the broken line  64 A are roughly analogous to the downhole telemetry connector and associated terminus. 
     FIG. 5  illustrates another embodiment of a data link that is based upon current coupling of sensors below the mud motor and the downhole telemetry unit above the mud motor. Elements and functions of this embodiment will again be discussed beginning at the bottom of the illustration. The lower end of the flex shaft  90  is attached to the rotor  58  by means of a flange  49 , and the upper end of the flex shaft  90  extends through a seal  106  and into the electronics sonde  19 . Conductors  46  leading from the instrument sub  12  are again shown as a twisted pair disposed within the rotor  58  and the flex shaft  90 . The conductors pass through feed through  114  in the wall of the flex shaft  90  and are attach to a lower toroid  92  that surrounds and rotates with a flex shaft  90 . A lower electrical conducting radial bearing  108  supports the flex shaft below the lower toroid  92 . 
   Still referring to  FIG. 5 , the flex shaft  90  extends upward through an upper toroid  94 , which is fixed with respect to the electronics sonde  19 . The upper toroid  94  is supported by an electrical conducting upper radial bearing  110  disposed above the upper toroid  94 . The upper toroid  94  is stationary with respect to the rotating flex shaft  90 . Leads from the upper toroid  94  pass through feed throughs  70 A and  70 B and connect to the downhole telemetry unit  72  disposed in the electronics sonde  19 . Data are transmitted to and from the downhole telemetry unit  72  as illustrated conceptually by the arrow  25 . Note that the upper and lower toroids  94  and  92 , and the upper and lower bearings  110  and  108 , are all disposed within the electronics sonde  19 . 
   Again referring to  FIG. 5 , the upper and lower toroids  94  and  92  rotate with respect to one another thereby forming a current coupling via the flex shaft  90  that functions as a center conductor. The upper end of the flex shaft  90  is electrically connected through the upper radial bearings  110  to housing of the electronics sonde  19 , which is electrically connected to the flex shaft  90  through the lower radial bearing  108 , which electrically connected to the lower end of the flex shaft  90  thereby completing the conduction circuit. As in the previous embodiment, an upward data link is obtained by applying a data current signal, such as a response of a sensor  40  (see  FIG. 2 ), to the lower toroid  92 . A corresponding data current signal is induced in the upper toroid  94 , via the previously described current loop, and telemetered to the surface via the downhole telemetry unit  72 . Conversely, data can be transmitted to the instrument sub from the surface. The data are telemetered to the downhole telemetry unit  72 , converted within the electronics sonde  19  to a current and applied to the upper toroid  94 . A corresponding current induced in the lower toroid  92 , which is carried to the instrument sub via the conductors  46 . The two-way current coupled link is again shown conceptually by the broken lines  68 . 
     FIG. 6  illustrates a data link using direct electrical contacts rather than current coupling. The lower end of the flex shaft  90  is attached to the rotor  58  by means of a flange  49 , and the upper end of the flex shaft  90  extends through a seal  120  and into a pressure housing  122 . Conductors  46  leading from the instrument sub  12  are once again shown as a twisted pair disposed within the rotor  58  and the flex shaft  90 . The conductors are terminated at a lower and upper conductor rings  128  and  126 , respectively. The upper and lower conductor rings are electrically insulated from one another and from the flex shaft  90 , and rotate with the flex shaft. The flex shaft  90  is supported by a radial bearing  124  disposed below the lower conducting ring  128 . It has been previously noted that the number of conductors can vary. A conductor ring is provided for each conductor. 
   Still referring to  FIG. 6 , the upper and lower conductor rings  126  and  128  are electrically contacted by upper and lower brushes  129  and  130  that are fixed with respect to the electronics sonde  19 . Leads from the from the upper and lower brushes  129  and  130  pass through feed throughs  134  and  132 , respectively, and electrically connect with the downhole telemetry unit  72  disposed within the electronics sonde  19 . Data are transmitted to and from the downhole telemetry unit  72  as illustrated conceptually by the arrow  25 . As stated above, the number of conductors can vary. A conductor ring and a cooperating brush are provided for each conductor. 
     FIG. 7  illustrates still another embodiment of a data link that is based upon magnetic coupling of sensors below the mud motor and the downhole telemetry unit  72  above the mud motor. A lower and an upper magnetic dipole, represented as a whole by  220  and  210 , respectively, are used to establish the link. The flex shaft used in previous embodiments has been eliminated. Elements and functions of this embodiment will again be discussed beginning at the bottom of the illustration. The lower dipole  220  is attached to the rotor  58 , and comprises a ferrite element  204  surrounding a steel mandrel  200 . Wires  218  are wound around the circumference of the ferrite element  205  and connect through feed through  212  to conductors  46  emerging from the rotor  58 . 
   Still referring to  FIG. 7 , the upper dipole  210  is attached to the electronic sonde  19 , and comprises a ferrite element  205  surrounding a steel mandrel  202 . Wires  221  are wound around the circumference of the ferrite element  205  and connect through feed throughs  222  to the downhole telemetry unit  72  disposed in the electronics sonde  19 . Data are transmitted to and from the downhole telemetry unit  72  as illustrated conceptually by the arrow  25 . 
   Again referring to  FIG. 7 , the upper and lower dipoles  210  and  220  rotate with respect to one another thereby forming a magnetic coupling illustrated conceptually by the broken curves  230 . The magnetic filed generated by the lower dipole  220  is indicative of the response of elements of the instrument sub  12 , such responses of a sensor  40  (see  FIG. 2 ). This magnetic field induces a corresponding data current signal is in the upper dipole  210 , which is typically telemetered to the surface via the downhole telemetry unit  72 . Conversely, data can be transmitted to the instrument sub  12  from the surface via the same magnetic link. The link illustrated in  FIG. 7  is not suitable for the transfer of power. 
   Applications 
   Two MWD/LWD geophysical steering applications of the system are illustrated to emphasize the importance of disposing the instrument sub  12  as near as possible to the drill bit  14 . It is again emphasized that the system is not limited to geosteering applications, but can be used in virtually any LWD/MWD application with one or more sensors disposed in the instrument sub  12 . In applications where the axial displacement between sensors and the drill bit is not critical, additional sensors can be disposed within the electronics sonde  19  or in the wall of the electronics sub  18 . These applications include, but are not limited to, LWD type measurements made when the drill string is tripped. 
   For purposes of geosteering illustration, it will be assumed that the one or more sensors  40  in the instrument sub  12  comprise a gamma ray detector and an inclinometer. Using the response of these two sensors, the position of the bottom hole assembly  10  in one earth formation can be determined with respect to adjacent formations. Gamma radiation and inclinometer data are telemetered to the surface in real time using previously discussed methodology thereby allowing the path of the advancing borehole to be adjusted based upon this information. Some processing of the sensor responses can be made in one or more processors disposed within elements of the bottom hole assembly  10  where the information is decoded by appropriate data acquisition software. 
     FIG. 8  shows a borehole  26  penetrating several earth formations. As shown, the bottom hole assembly  10 , operationally attached to the drill string  22 , is advancing the borehole  26  in an oil bearing formation  140 . The objective of the drilling operation is to advance the borehole  26  within the oil bearing formation  140 , as shown, thereby maximizing hydrocarbon production from this formation. As illustrated in  FIG. 8 , the oil bearing formation  142  is relatively thin, and bounded by “floor” and “ceiling” formations  144  and  142  at bed boundaries  152  and  143 , respectively. Natural gamma radiation levels in oil bearing formations are typically low. Oil bearing formations are typically bounded by shales, which exhibit high natural gamma ray activity. For purposes of illustration, it will be assumed that the oil bearing formation  140  is low in gamma ray activity, and the bounding “floor” and “ceiling” formations  144  and  142 , respectively, that are shales exhibiting relatively high levels of natural gamma radiation. 
     FIG. 9  is a “log” of a measure of natural gamma ray intensity (ordinate), depicted as the solid curve  160 , as a function of depth (abscissa) along the borehole  26 . The broken curve  166  of  FIG. 9  illustrates a log of the inclination bottom hole assembly  10 , as measured by the inclinometer sensor, as a function of depth. Downward vertical is arbitrarily denoted as −180 degrees, and horizontal is denoted as 0 degrees. As will be discussed below, this log information is telemetered in real time to the surface thereby allowing drilling direction changes to be made quickly in order to remain within the target formation. 
   Referring to both  FIGS. 8 and 9 , the borehole is within the ceiling shale formation  142  at a depth  149 , and the borehole  26  is near vertical. This is represented on the log of  FIG. 9  at depth  149 A as a maximum gamma radiation reading and an inclinometer reading of about −180 degrees. As the borehole enters the oil bearing formation  140  as indicated by a decrease in gamma radiation, the borehole is diverged from the vertical by the driller in order to remain within this target formation. At  150  of  FIG. 8 , it can be seen that the borehole is near the center of the formation  140 , and the inclination is about −90 degrees. This location is reflected in at depth  150 A in the log of  FIG. 9  by minimum gamma radiation intensity and an inclination of approximately −90 degrees. Between  150  and  152  of  FIG. 8 , it can be seen that the borehole is approaching the bed boundary  152  of the floor formation  144  by the driller. The gamma ray detector senses the close proximity of the formation, and is reflected as an increase in gamma radiation at a depth  152 A of the  FIG. 9  log. This alerts the driller-that the borehole is approaching floor formation, and the drilling direction must be altered to near horizontal so that the bottom hole assembly  10  remains within the target zone  140 . The broken curve  166  indicates at  152 A that the borehole is near horizontal. As seen in  FIG. 8 , the borehole  26  is essentially horizontal between  152  and  154 , but is approaching the bed boundary  143  of the ceiling formation  142 . This is sensed by the gamma ray detector and is reflected in an increase in gamma radiation that reaches a maximum at depth  154 A. This increase is observed in real time by the driller. As a result of this real time observation, the drilling direction is adjusted downward between  153  and  154  until a decrease in gamma radiation below depth  154 A indicates that the bottom hole assembly  10  is once again being directed toward the center of the target formation. This change in inclination is reflected In  FIG. 9  by the broken curve  166  at a depth between  153 A and  154 A. 
   To summarize, the system can be embodied to steer the drilling operation and thereby maintain the advancing borehole within a target formation. In this application, where directional changes are made based upon sensor responses, it is of great importance to dispose the sensors as close as possible to the drill bit. As an example, if the sensor sub were disposed above the mud motor, the floor formation  144  could be penetrated at  152  before the driller would receive an indication of such on the gamma ray log  160 . The present system permits sensors to be disposed as close a two feet from the drill bit. 
   The drill bit-sensor arrangement of the invention is also very useful in the drilling of steam assisted gravity drainage (SAG-D) wells. SAG-D wells are usually drilled in pairs, as illustrated in  FIG. 10 . The drilling system and cooperating bottom hole assembly  10  are typically used to drill the curve and lateral sections of the first well borehole  26 A. Using the geosteering methodology discussed above, this borehole is drilled within the oil bearing formation  140  but near the bed boundary  141  of the floor formation  144 . Once the borehole  26 A is completed, a magnetic ranging tool  165  is disposed within the borehole  26 A. The second well borehole  26 B drilled with a magnet sensor as one of the sensors  40  used in the sensor sub  12  (see  FIG. 2 ) of the bottom hole assembly  10 . The magnetic sensor responds to the location of the magnetic ranging tool  165  in borehole  26 A and is, therefore, used to determine the proximity of the borehole  26 B relative to the borehole  26 A. The borehole pairs are typically drilled within close proximity to one another, with tight tolerances in the drilling plan, in order to optimize the oil recovery from the target formation  140 . Steam is pumped into the upper borehole  26 B, which heats oil in the target formation  140  causing the viscosity to decrease. The low viscous oil then migrates downward toward the lower borehole  26 A where it is collected and pumped to the surface. 
   To summarize, the effective drilling SAG-D wells require sensors to be disposed as close as possible to the drill bit in order to meet the tight tolerances of the drilling plan. 
   One skilled in the art will appreciate that the present invention can be practiced by other that the described embodiments, which are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.