Patent Publication Number: US-6218959-B1

Title: Fail safe downhole signal repeater

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to downhole telemetry and, in particular to, the use of fail safe downhole signal repeaters for communicating signals carrying information between surface equipment and downhole equipment. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with transmitting downhole data to the surface during measurements while drilling (MWD), as an example. It should be noted that the principles of the present invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, testing, completing and production. 
     Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The utilization of MWD with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs. 
     Measurement of parameters such as bit weight, torque, wear and bearing condition in real time provides for a more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques. 
     At present, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface, namely mud pressure pulses, insulated conductors, acoustics and electromagnetic waves. 
     In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system typically transmits at 1 bit per second as the pressure pulse travels up the mud column at or near the velocity of sound in the mud. It has been found, however, that the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow. 
     Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two way communication is possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors which substantially increase the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string. 
     Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. 
     The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. When a toroid is utilized, a primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is formed by the drill pipe. A receiver is connected to the ground at the surface where the electromagnetic data is picked up and recorded. It has been found, however, that in deep or noisy well applications, conventional electromagnetic systems are unable to generate a signal with sufficient intensity to reach the surface. 
     Therefore, a need has arisen for a system that is capable of telemetering real time information in a deep or noisy well between surface equipment and downhole equipment. A need has also arisen for a signal repeater that digitally processes the information to determine whether the signal is intended for that repeater. Further, a need has arisen for a fail safe repeater system that is capable of transmitting information between surface equipment and downhole equipment even in the event of a repeater failure. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein uses fail safe signal repeaters that amplify and process signals carrying information in a system capable of transmitting information between surface equipment and downhole equipment even in the event of a repeater failure. The system and method of the present invention provide for real time communication from downhole equipment to the surface and for the telemetry of information and commands from the surface to downhole tools disposed in a well. 
     The system and method of the present invention utilize at least two repeaters which, for convenience of illustration, will be referred to as first and second repeaters. The first and second repeaters are disposed within a wellbore and receive a first signal carrying information. A memory device within the second repeater stores the information carried in the first signal until a timer device within the second repeater triggers the second repeater to retransmit the information. The timer device will trigger the retransmission of the information, after a predetermined time period, unless the second repeater has detected a third signal carrying the information transmitted by the first repeater. Thus, even if the first repeater is inoperable, the information carried in the first signal is retransmitted by the second repeater. If the first repeater transmits the third signal carrying the information within the predetermined time period and the third signal carrying the information is detected by the second repeater, the second repeater will discard the information stored in the memory device and process the information carried in the third signal. 
     The first and second repeaters of the present invention include electronics packages. The electronics packages transform the first signal into an electrical signal, convert the information carried in the electrical signal from an analog format to a digital format, process the information and convert the information carried in the electrical signal from a digital format to an analog format. The electronics packages also determine whether the first signal is intended for the first or the second repeater. Additionally, the electronics packages determine whether the first signal is carrying the information and whether the information carried in the first signal is accurate. The electronics packages also attenuate noise in the electrical signal to a predetermined voltage, amplify the electrical signal to a predetermined voltage, eliminate noise in the electrical signal in a predetermined frequency range and eliminate the unwanted frequencies above and below the desired frequency. 
     In one embodiment of the present invention, the first and second repeaters may each include an electromagnetic receiver and an electromagnetic transmitter or may include an electromagnetic transceiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas drilling platform operating three fail safe downhole signal repeaters of the present invention; 
     FIGS. 2A-2B are quarter-sectional views of a fail safe downhole signal repeater of the present invention; 
     FIGS. 3A-3B are quarter-sectional views of a fail safe downhole signal repeater of the present invention; 
     FIG. 4A-4B are quarter-sectional views of a fail safe downhole signal repeater of the present invention; 
     FIG. 5 is a schematic illustration of a toroid having primary and secondary windings wrapped therearound for a fail safe downhole signal repeater of the present invention; 
     FIG. 6 is an exploded view of one embodiment of a toroid assembly for use as a receiver in a fail safe downhole signal repeater of the present invention; 
     FIG. 7 is an exploded view of one embodiment of a toroid assembly for use as a transmitter in a fail safe downhole signal repeater of the present invention; 
     FIG. 8 is a perspective view of an annular carrier of an electronics package for a fail safe downhole signal repeater of the present invention; 
     FIG. 9 is a perspective view of an electronics member having a plurality of electronic devices thereon for a fail safe downhole signal repeater of the present invention; 
     FIG. 10 is a perspective view of a battery pack for a fail safe downhole signal repeater of the present invention; and 
     FIG. 11 is a block diagram of a signal processing method used by a fail safe downhole signal repeater of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring to FIG. 1, a plurality of fail safe downhole signal repeaters in use on an offshore oil and gas drilling platform is schematically illustrated and generally designated  10 . A semi-submergible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck of  20  platform  12  to wellhead installation  22  including blowout preventers  24 . Platform  12  has a derrick  26  and a hoisting apparatus  28  for raising and lowering drill string  30 , including drill bit  32  and fail safe downhole signal repeaters  34 ,  35 ,  36 . 
     In a typical drilling operation, drill bit  32  is rotated by drill string  30 , such that drill bit  32  penetrates through the various earth strata, forming wellbore  38 . Measurement of parameters such as bit weight, torque, wear and bearing conditions may be obtained by sensors  40  located in the vicinity of drill bit  32 . Additionally, parameters such as pressure and temperature as well as a variety of other environmental and formation information may be obtained by sensors  40 . The signal generated by sensors  40  may typically be analog, which must be converted to digital data before electromagnetic transmission in the present system. The signal generated by sensors  40  is passed into an electronics package  42  including an analog to digital converter which converts the analog signal to a digital code utilizing “ones” and “zeros” for information transmission. 
     Electronics package  42  may also include electronic devices such as an on/off control, a modulator, a microprocessor, memory and amplifiers. Electronics package  42  is powered by a battery pack which may include a plurality of batteries, such as nickel cadmium or lithium batteries, which are configured to provide proper operating voltage and current. 
     Once the electronics package  42  establishes the frequency, power and phase output of the information, electronics package  42  feeds the information to transmitter  44 . Transmitter  44  may be a direct connect to drill string  30  or may electrically approximate a large transformer. The information is then carried uphole in the form of electromagnetic wave fronts  46  which propagate through the earth. These electromagnetic wave fronts  46  are picked up by receiver  48  of repeater  34  and receiver  49  of repeater  35  located uphole from transmitter  44 . 
     Repeater  34  and repeater  35  are spaced along drill string  30  to receive electromagnetic wave fronts  46  while electromagnetic wave fronts  46  remain strong enough to be readily detected. Receiver  48  of repeater  34  and receiver  49  of repeater  49  may each electrically approximate a large transformer. As electromagnetic wave fronts  46  reach receivers  48 ,  49 , a current is induced in receivers  48 ,  49  that carries the information originally obtained by sensors  40 . 
     The current from receiver  48  is fed to an electronics package  50  that may include a variety of electronic devices such as amplifiers, limiters, filters, a phase lock loop, shift registers and comparators as will be further discussed with reference to FIGS. 9 and 11. Electronics package  50  digitally processes the signal and amplifies the signal to reconstruct the original waveform, compensating for losses and distortion occurring during the transmission of electromagnetic wave fronts  46  through the earth. Electronics package  50  also determines whether the signal was intended for repeater  34  by analyzing the address information carried in the preamble of the signal, as will be explained in more detail with reference to FIG. 11 below. In this case, electromagnetic wave fronts  46  are intended for repeater  34  thus, electronics package  50  forwards the signal to a transmitter  52  that radiates electromagnetic wave fronts  54  into the earth in the manner described with reference to transmitter  44  and electromagnetic wave fronts  46 . 
     Similarly, the current from receiver  49  of repeater  35  is fed to an electronics package  51  that may also include a variety of electronic devices such as amplifiers, limiters, filters, a phase lock loop, a timing device, shift registers and comparators as will be further discussed with reference to FIGS. 9 and 11. Electronics package  51  digitally processes the signal and amplifies the signal to reconstruct the original waveform, compensating for losses and distortion occurring during the transmission of electromagnetic wave fronts  46  through the earth. Electronics package  51  determines whether the signal was intended for repeater  35  by analyzing the address information carried in the preamble of the signal, as will be explained in more detail with reference to FIG. 11 below. In this case, electromagnetic wave fronts  46  are not intended for repeater  35  but are intended for repeater  34 . Because electromagnetic wave fronts  46  are not intended for repeater  35 , electronics package  51  simply processes and stores the information carried in electromagnetic wave fronts  46  but does not immediately forward the signal to transmitter  53 . The signal is forwarded only if repeater  35  does not receive electromagnetic wave fronts  54  from repeater  34  within a specified period of time. If repeater  35  receives electromagnetic wave fronts  54  within the specified period of time, repeater  35  discards the information received in electromagnetic waves fronts  46  and processes the information carried in electromagnetic wave fronts  54  as described above. Alternatively, if repeater  35  does not receive electromagnetic wave fronts  54  within the specified period of time, repeater  35  will forward the signal originally obtained from electromagnetic waves fronts  46  to transmitter  53  that radiates electromagnetic wave fronts  55  into the earth in the manner described with reference to transmitter  44  and electromagnetic wave fronts  46 . 
     As the information continues to be transmitted uphole, fail safe processing is accomplished by each repeater as well as by electromagnetic pickup device  64 . For example, electromagnetic wave fronts  54  are received by receiver  49  of repeater  35  and receiver  56  of repeater  36 . The signal is processed by electronics packages  51  of repeater  35  and by electronics package  58  of repeater  36  as explained above. While electromagnetic wave fronts  54  are intended for repeater  35 , if repeater  35  is unable to retransmit the information via the generation of electromagnetic wave fronts  55  from transmitter  53  within a specified time period, repeater  36  will generate electromagnetic wave fronts  62  from transmitter  60  to continue the process of fail safe transmission of the information originally obtained by sensors  40 . 
     Likewise, electromagnetic wave fronts  55  are received by receiver  56  of repeater  36  as well as by electromagnetic pickup device  64  located on sea floor  16 . Electromagnetic pickup device  64  may sense either the electric field or the magnetic field of electromagnetic wave front  55  using electric field sensors  66  or a magnetic field sensor  68  or both. The signal is processed by electronics packages  58  of repeater  36  and by electromagnetic pickup device  64  in the manner explained above. While electromagnetic wave fronts  55  are intended for repeater  36 , if repeater  36  is unable to retransmit the information via the generation of electromagnetic wave fronts  62  from transmitter  60  within a specified time period, electromagnetic pickup device  64  will fire the information received in electromagnetic wave fronts  55  to the surface via wire  70  that is connected to buoy  72  and wire  74  that is connected to a processor on platform  12 . Upon reaching platform  12 , the information originally obtained by sensors  40  is further processed making any necessary calculations and error corrections such that the information may be displayed in a usable format. 
     Alternatively, when repeater  36  does generate electromagnetic wave fronts  62  from transmitter  60  within a specified time period, electromagnetic pickup device  64  discards the information received from electromagnetic wave fronts  55  and processes the information received from electromagnetic wave fronts  62 . Electromagnetic pickup device  64  then fires the information received in electromagnetic wave fronts  62  to the surface via wire  70  that is connected to buoy  72  and wire  74  that is connected to a processor on platform  12 . Upon reaching platform  12 , the information originally obtained by sensors  40  is further processed making any necessary calculations and error corrections such that the information may be displayed in a usable format. 
     In this manner, the fail safe downhole repeaters of the present invention are able to transmit information at a great distance between the surface and a downhole location even if a failure occurs in the transmission of information by any repeater, such as repeaters  34 ,  35 ,  36 . The system of the present invention will therefore avoid the high cost of tripping drill string  30  out of wellbore  38  to repair the communication system in the event of a repeater failure. Similarly, the use of the fail safe downhole repeater system of the present invention during production of fluids from formation  14  will eliminate the need to bring out a rig to repair the communication system due to a repeater failure. 
     Even though FIG. 1 depicts three repeaters  34 ,  35 ,  36 , it should be noted by one skilled in the art that the number of repeaters located within drill string  30  will be determined by the depth of wellbore  38 , the noise level in wellbore  38  and the characteristics of the earth&#39;s strata adjacent to wellbore  38  in that electromagnetic waves suffer from attenuation with increasing distance from their source at a rate that is dependent upon the composition characteristics of the transmission medium and the frequency of transmission. For example, repeaters  34 ,  35 ,  36  may be positioned between 2,000 and 4,000 feet apart. Thus, if wellbore  38  is 15,000 feet deep, between three and seven repeaters would be desirable. 
     Even though FIG. 1 depicts repeaters  34 ,  35 ,  36  and electromagnetic pickup device  64  in an offshore environment, it should be understood by one skilled in the art that repeaters  34 ,  35 ,  36  and electromagnetic pickup device  64  are equally well-suited for operation in an onshore environment. In fact, in an onshore environment, electromagnetic pickup device  64  would be placed directly on the land. Alternatively, a receiver such as receivers  48 ,  49 ,  56  could be used at the surface to pick up the electromagnetic wave fronts for processing at the surface. 
     Additionally, while FIG. 1 has been described with reference to transmitting information uphole during a measurement while drilling operation, it should be understood by one skilled in the art that repeaters  34 ,  35 ,  36  and electromagnetic pickup device  64  may be used in conjunction with the transmission of information downhole from surface equipment to downhole tools to perform a variety of functions such as opening and closing a downhole tester valve or controlling a downhole choke. 
     Further, even though FIG. 1 has been described with reference to one way communication from the vicinity of drill bit  32  to platform  12 , it should be understood by one skilled in the art that the principles of the present invention are applicable to two way communication. For example, a surface installation may be used to request downhole pressure, temperature, or flow rate information from formation  14  by sending electromagnetic wave fronts downhole using electromagnetic pickup device  64  as an electromagnetic transmitter and retransmitting the request using repeaters  34 ,  35 ,  36  as described above. Sensors, such as sensors  40 , located near formation  14  receive this request and obtain the appropriate information which would then be returned to the surface via electromagnetic wave fronts which would again be retransmitted as described above with reference to repeaters  34 ,  35 ,  36 . As such, the phrase “between surface equipment and downhole equipment” as used herein encompasses the transmission of information from surface equipment downhole, from downhole equipment uphole or for two way communication. 
     Even though FIG. 1 has been described with reference to communication using electromagnetic waves, it should been understood by those of skill in the art that the principles of the present invention are equally well-suited for use with other communication systems including, but not limited to, acoustic repeaters, electromagnetic-to-acoustic repeaters, acoustic-to-electromagnetic repeaters as well as repeaters that retransmit both electromagnetic and acoustic signals. 
     Representatively illustrated in FIGS. 2A-2B is one embodiment of a fail safe downhole signal repeater  76  of the present invention. For convenience of illustration, FIGS. 2A-2B depict repeater  76  in a quarter sectional view. Repeater  76  has a box end  78  and a pin end  80  such that repeater  76  is threadably adaptable to drill string  30 . Repeater  76  has an outer housing  82  and a mandrel  84  having a full bore so that when repeater  76  is interconnected with drill string  30 , fluids may be circulated therethrough and therearound. Specifically, during a drilling operation, drilling mud is circulated through drill string  30  inside mandrel  84  of repeater  76  to ports formed through drill bit  32  and up the annulus formed between drill string  30  and wellbore  38  exteriorly of housing  82  of repeater  76 . Housing  82  and mandrel  84  thereby protect the operable components of repeater  76  from drilling mud or other fluids disposed within wellbore  38  and within drill string  30 . 
     Housing  82  of repeater  76  includes an axially extending generally tubular upper connecter  86  which has box end  78  formed therein. Upper connecter  86  may be threadably and sealably connected to drill string  30  for conveyance into wellbore  38 . 
     An axially extending generally tubular intermediate housing member  88  is threadably and sealably connected to upper connecter  86 . An axially extending generally tubular lower housing member  90  is threadably and sealably connected to intermediate housing member  88 . Collectively, upper connecter  86 , intermediate housing member  88  and lower housing member  90  form upper subassembly  92 . Upper subassembly  92  is electrically connected to the section of drill string  30  above repeater  76 . 
     An axially extending generally tubular isolation subassembly  94  is securably and sealably coupled to lower housing member  90 . Disposed between isolation subassembly  94  and lower housing member  90  is a dielectric layer  96  that provides electric isolation between lower housing member  90  and isolation subassembly  94 . Dielectric layer  96  is composed of a dielectric material, such as teflon, chosen for its dielectric properties and capably of withstanding compression loads without extruding. 
     An axially extending generally tubular lower connecter  98  is securably and sealably coupled to isolation subassembly  94 . Disposed between lower connecter  98  and isolation subassembly  94  is a dielectric layer  100  that electrically isolates lower connecter  98  from isolation subassembly  94 . Lower connecter  98  is adapted to threadably and sealably connect to drill string  30  and is electrically connected to the portion of drill string  30  below repeater  76 . 
     Isolation subassembly  94  provides a discontinuity in the electrical connection between lower connecter  98  and upper subassembly  92  of repeater  76 , thereby providing a discontinuity in the electrical connection between the portion of drill string  30  below repeater  76  and the portion of drill string  30  above repeater  76 . 
     It should be apparent to those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward, etc. are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. It is to be understood that repeater  76  may be operated in vertical, horizontal, inverted or inclined orientations without deviating from the principles of the present invention. 
     Mandrel  84  includes axially extending generally tubular upper mandrel section  102  and axially extending generally tubular lower mandrel section  104 . Upper mandrel section  102  is partially disposed and sealing configured within upper connecter  86 . A dielectric member  106  electrically isolates upper mandrel section  102  from upper connecter  86 . The outer surface of upper mandrel section  102  has a dielectric layer disposed thereon. Dielectric layer  108  may be, for example, a teflon layer. Together, dielectric layer  108  and dielectric member  106  serve to electrically isolate upper connecter  86  from upper mandrel section  102 . 
     Between upper mandrel section  102  and lower mandrel section  104  is a dielectric member  110  that, along with dielectric layer  108 , serves to electrically isolate upper mandrel section  102  from lower mandrel section  104 . Between lower mandrel section  104  and lower housing member  90  is a dielectric member  112 . On the outer surface of lower mandrel section  104  is a dielectric layer  114  which, along with dielectric member  112 , provides for electric isolation of lower mandrel section  104  from lower housing number  90 . Dielectric layer  114  also provides for electric isolation between lower mandrel section  104  and isolation subassembly  94  as well as between lower mandrel section  104  and lower connecter  98 . Lower end  116  of lower mandrel section  104  is disposed within lower connecter  98  and is in electrical communication with lower connecter  98 . 
     Intermediate housing member  88  of outer housing  82  and upper mandrel section  102  of mandrel  84  define annular area  118 . A receiver  120 , an electronics package  122  and a transmitter  124  are disposed within annular area  118 . In operation, receiver  1 receives an electromagnetic input signal carrying information which is transformed into an electrical signal that is passed onto electronics package  122  via electrical conductor  126 , as will be more fully described with reference to FIG.  4 . Electronics package  122  processes and amplifies the electrical signal, as will be more fully discussed with reference to FIG.  11 . The electrical signal is then fed to transmitter  124  via electrical conductor  128 , as will be more fully described with reference to FIG.  4 . Transmitter  124  transforms the electrical signal into an electromagnetic output signal carrying information that is radiated into the earth. 
     Representatively illustrated in FIGS. 3A-3B is another embodiment of a fail safe downhole signal repeater  130  of the present invention. For convenience of illustration, FIGS. 3A-3B depicted repeater  130  in a quarter sectional view. Repeater  130  has a box end  132  and a pin end  134  such that repeater  130  is threadably adaptable to drill string  30 . Repeater  130  has an outer housing  136  and a mandrel  138  such that repeater  130  may be interconnected with drill string  30  providing a circulation path for fluids therethrough and therearound. Housing  136  and mandrel  138  thereby protect the operable components of repeater  130  from drilling mud or other fluids disposed within wellbore  38  and within drill string  30 . 
     Housing  136  of repeater  130  includes an axially extending generally tubular upper connecter  140  which has box end  132  formed therein. Upper connecter  140  may be threadably and sealably connected to drill string  30  for conveyance into wellbore  38 . 
     An axially extending generally tubular intermediate housing member  142  is threadably and sealably connected to upper connecter  140 . An axially extending generally tubular lower housing member  144  is threadably and sealably connected to intermediate housing member  142 . Collectively, upper connecter  140 , intermediate housing member  142  and lower housing member  144  form upper subassembly  146 . Upper subassembly  146  is electrically connected to the section of drill string  30  above repeater  130 . 
     An axially extending generally tubular isolation subassembly  148  is securably and sealably coupled to lower housing member  144 . Disposed between isolation subassembly  148  and lower housing member  144  is a dielectric layer  150  that provides electric isolation between lower housing member  144  and isolation subassembly  148 . Dielectric layer  150  is composed of a dielectric material chosen for its dielectric properties and capably of withstanding compression loads without extruding. 
     An axially extending generally tubular lower connecter  152  is securably and sealably coupled to isolation subassembly  148 . Disposed between lower connecter  152  and isolation subassembly  148  is a dielectric layer  154  that electrically isolates lower connecter  152  from isolation subassembly  148 . Lower connecter  152  is adapted to threadably and sealably connect to drill string  30  and is electrically connected to the portion of drill string  30  below repeater  130 . 
     Isolation subassembly  148  provides a discontinuity in the electrical connection between lower connecter  152  and upper subassembly  146  of repeater  130 , thereby providing a discontinuity in the electrical connection between the portion of drill string  30  below repeater  130  and the portion of drill string  30  above repeater  130 . 
     Mandrel  138  includes axially extending generally tubular upper mandrel section  156  and axially extending generally tubular lower mandrel section  158 . Upper mandrel section  156  is partially disposed and sealing configured within upper connecter  140 . A dielectric member  160  electrically isolates upper mandrel section  156  and upper connecter  140 . The outer surface of upper mandrel section  156  has a dielectric layer disposed thereon. Dielectric layer  162  may be, for example, a teflon layer. Together, dielectric layer  162  and dielectric member  160  service to electrically isolate upper connecter  140  from upper mandrel section  156 . 
     Between upper mandrel section  156  and lower mandrel section  158  is a dielectric member  164  that, along with dielectric layer  162 , serves to electrically isolate upper mandrel section  156  from lower mandrel section  158 . Between lower mandrel section  158  and lower housing member  144  is a dielectric member  166 . On the outer surface of lower mandrel section  158  is a dielectric layer  168  which, along with dielectric member  166 , provides for electric isolation of lower mandrel section  158  with lower housing number  144 . Dielectric layer  168  also provides for electric isolation between lower mandrel section  158  and isolation subassembly  148  as well as between lower mandrel section  158  and lower connecter  152 . Lower end  170  of lower mandrel section  158  is disposed within lower connecter  152  and is in electrical communication with lower connecter  152 . 
     Intermediate housing member  142  of outer housing  136  and upper mandrel section  156  of mandrel  138  define annular area  172 . A transceiver  174  and an electronics package  176  are disposed within annular area  172 . In operation, transceiver  174  receives an electromagnetic input signal carrying information which is transformed into an electrical signal that is passed onto electronics package  176  via electrical conductor  178 . Electronics package  176  processes and amplifies the electrical signal which is fed back to transceiver  174  via electrical conductor  178 . Transceiver  174  transforms the electrical signal into an electromagnetic output signal that is radiated into the earth carrying information. 
     Representatively illustrated in FIGS. 4A-4B is another embodiment of a fail safe downhole signal repeater  330  of the present invention. For convenience of illustration, FIGS. 4A-4B depicted repeater  330  in a quarter sectional view. Repeater  330  has a box end  332  and a pin end  334  such that repeater  330  is threadably adaptable to drill string  30 . Repeater  330  has an outer housing  336  and a mandrel  338  such that repeater  330  may be interconnected with drill string  30  providing a circulation path for fluids therethrough and therearound. Housing  336  and mandrel  338  thereby protect the operable components of repeater  330  from drilling mud or other fluids disposed within wellbore  38  and within drill string  30 . 
     Housing  336  of repeater  330  includes an axially extending generally tubular upper connecter  340  which has box end  332  formed therein. Upper connecter  340  may be threadably and sealably connected to drill string  30  for conveyance into wellbore  38 . 
     An axially extending generally tubular intermediate housing member  342  is threadably and sealably connected to upper connecter  340 . An axially extending generally tubular lower housing member  344  is threadably and sealably connected to intermediate housing member  342 . Collectively, upper connecter  340 , intermediate housing member  342  and lower housing member  344  form upper subassembly  346 . Upper subassembly  346  is electrically connected to the section of drill string  30  above repeater  330 . 
     An axially extending generally tubular isolation subassembly  348  is securably and sealably coupled to lower housing member  344 . Disposed between isolation subassembly  348  and lower housing member  344  is a dielectric layer  350  that provides electric isolation between lower housing member  344  and isolation subassembly  348 . Dielectric layer  350  is composed of a dielectric material chosen for its dielectric properties and capably of withstanding compression loads without extruding. 
     An axially extending generally tubular lower connecter  352  is securably and sealably coupled to isolation subassembly  348 . Disposed between lower connecter  352  and isolation subassembly  348  is a dielectric layer  354  that electrically isolates lower connecter  352  from isolation subassembly  348 . Lower connecter  352  is adapted to threadably and sealably connect to drill string  30  and is electrically connected to the portion of drill string  30  below repeater  330 . 
     Isolation subassembly  348  provides a discontinuity in the electrical connection between lower connecter  352  and upper subassembly  346  of repeater  330 , thereby providing a discontinuity in the electrical connection between the portion of drill string  30  below repeater  330  and the portion of drill string  30  above repeater  330 . 
     Mandrel  338  includes axially extending generally tubular upper mandrel section  356  and axially extending generally tubular lower mandrel section  358 . Upper mandrel section  356  is partially disposed and sealing configured within upper connecter  340 . A dielectric member  360  electrically isolates upper mandrel section  356  and upper connecter  340 . The outer surface of upper mandrel section  356  has a dielectric layer disposed thereon. Dielectric layer  362  may be, for example, a teflon layer. Together, dielectric layer  362  and dielectric member  360  service to electrically isolate upper connecter  340  from upper mandrel section  356 . 
     Between upper mandrel section  356  and lower mandrel section  358  is a dielectric member  364  that, along with dielectric layer  362 , serves to electrically isolate upper mandrel section  356  from lower mandrel section  358 . Between lower mandrel section  358  and lower housing member  344  is a dielectric member  366 . On the outer surface of lower mandrel section  358  is a dielectric layer  368  which, along with dielectric member  366 , provides for electric isolation of lower mandrel section  358  with lower housing number  344 . Dielectric layer  368  also provides for electric isolation between lower mandrel section  358  and isolation subassembly  348  as well as between lower mandrel section  358  and lower connecter  352 . Lower end  370  of lower mandrel section  358  is disposed within lower connecter  352  and is in electrical communication with lower connecter  352 . 
     Intermediate housing member  342  of outer housing  336  and upper mandrel section  356  of mandrel  338  define annular area  372 . A receiver  374  and an electronics package  376  are disposed within annular area  372 . In operation, receiver  374  receives an electromagnetic input signal carrying information which is transformed into an electrical signal that is passed onto electronics package  376  via electrical conductor  378 . Electronics package  376  processes and amplifies the electrical signal. An output voltage is then applied between intermediate housing member  342  and lower mandrel section  358 , which is electrically isolated from intermediate housing member  342  and electrically connected to lower connector  352 , via terminal  380  on intermediate housing member  342  and terminal  382  on lower mandrel section  358 . The voltage applied between intermediate housing member  342  and lower connector  352  generates the electromagnetic output signal that is radiated into the earth carrying information. 
     Referring now to FIG. 5, a schematic illustration of a toroid is depicted and generally designated  180 . Toroid  180  includes magnetically permeable annular core  182 , a plurality of electrical conductor windings  184  and a plurality of electrical conductor windings  186 . Windings  184  and windings  186  are each wrapped around annular core  182 . Collectively, annular core  182 , windings  184  and windings  186  serve to approximate an electrical transformer wherein either windings  184  or windings  186  may serve as the primary or the secondary of the transformer. 
     In one embodiment, the ratio of primary windings to secondary windings is 2:1. For example, the primary windings may include 100 turns around annular core  182  while the secondary windings may include 50 turns around annular core  182 . In another embodiment, the ratio of secondary windings to primary windings is 4:1. For example, primary windings may include 10 turns around annular core  182  while secondary windings may include 40 turns around annular core  182 . It will be apparent to those skilled in the art that the ratio of primary windings to secondary windings as well as the specific number of turns around annular core  182  will vary based upon factors such as the diameter and height of annular core  182 , the desired voltage, current and frequency characteristics associated with the primary windings and secondary windings and the desired magnetic flux density generated by the primary windings and secondary windings. 
     Toroid  180  of the present invention may serve as the receivers and transmitters as described with reference to FIGS. 1,  2  and  4  such as receivers  48 ,  49 ,  56 ,  120 ,  374  and transmitters  44 ,  52 ,  53 ,  60  and  124 . Toroid  180  of the present invention may also serve as the transceiver  174  as described with reference to FIG.  3 . The following description of the orientation of windings  184  and windings  186  will therefore be applicable to all such receivers, transmitters and transceivers. 
     With reference to FIGS. 2 and 5, windings  184  have a first end  188  and a second end  190 . First end  188  of windings  184  is electrically connected to electronics package  122 . When toroid  180  serves as receiver  120 , windings  184  serve as the secondary wherein first end  188  of windings  184  feeds electronics package  122  with an electrical signal via electrical conductor  126 . The electrical signal is processed by electronics package  122  as will be further described with reference to FIG. 11 below. When toroid  180  serves as transmitter  124 , windings  184  serve as the primary wherein first end  188  of windings  184 , receives an electrical signal from electronics package  122  via electrical conductor  128 . Second end  190  of windings  184  is electrically connected to upper subassembly  92  of outer housing  82  which serves as a ground. 
     Windings  186  of toroid  180  have a first end  192  and a second end  194 . First end  192  of windings  186  is electrically connected to upper subassembly  92  of outer housing  82 . Second end  194  of windings  186  is electrically connected to lower connecter  98  of outer housing  82 . First end  192  of windings  186  is thereby separated from second end  192  of windings  186  by isolations subassembly  94  which prevents a short between first end  192  and second end  194  of windings  186 . 
     When toroid  180  serves as receiver  120 , electromagnetic wave fronts, such as electromagnetic wave fronts  46  induce a current in windings  186 , which serve as the primary. The current induced in windings  186  induces a current in windings  184 , the secondary, which feeds electronics package  122  as described above. When toroid  180  serves as transmitter  124 , the current supplied from electronics package  122  feeds windings  184 , the primary, such that a current is induced in windings  186 , the secondary. The current in windings  186  induces an axial current on drill string  30 , thereby producing electromagnetic waves. 
     Due to the ratio of primary windings to secondary windings, when toroid  180  serves as receiver  120 , the signal carried by the current induced in the primary windings is increased in the secondary windings. Similarly, when toroid  180  serves as transmitter  124 , the current in the primary windings is increased in the secondary windings. 
     Referring now to FIG. 6, an exploded view of a toroid assembly  226  is depicted. Toroid assembly  226  may be designed to serve, for example, as receiver  120  of FIG.  2 . Toroid assembly  226  includes a magnetically permeable core  228 , an upper winding cap  230 , a lower winding cap  232 , an upper protective plate  234  and a lower protective plate  236 . Winding caps  230 ,  232  and protective plates  234 ,  236  are formed from a dielectric material such as fiberglass or phenolic. Windings  238  are wrapped around core  228  and winding caps  230 ,  232  by inserting windings  238  into a plurality of slots  240  which, along with the dielectric material, prevent electrical shorts between the turns of winding  238 . For illustrative purposes, only one set of winding, windings  238 , have been depicted. It will be apparent to those skilled in the art that, in operation, a primary and a secondary set of windings will be utilized by toroid assembly  226 . 
     FIG. 7 depicts an exploded view of toroid assembly  242  which may serve, for example, as transmitter  124  of FIG.  2 . Toroid assembly  242  includes four magnetically permeable cores  244 ,  246 ,  248  and  250  between an upper winding cap  252  and a lower winding cap  254 . An upper protective plate  256  and a lower protective plate  258  are disposed respectively above and below upper winding cap  252  and lower winding cap  254 . In operation, primary and secondary windings (not pictured) are wrapped around cores  244 ,  246 ,  248  and  250  as well as upper winding cap  252  and lower winding cap  254  through a plurality of slots  260 . 
     As is apparent from FIGS. 6 and 7, the number of magnetically permeable cores such as core  228  and cores  244 ,  246 ,  248  and  250  may be varied, dependent upon the required length for the toroid as well as whether the toroid serves as a receiver, such as toroid assembly  226 , or a transmitter, such as toroid assembly  242 . In addition, as will be known by those skilled in the art, the number of cores will be dependent upon the diameter of the cores as well as the desired voltage, current and frequency carried by the primary windings and the secondary windings, such as windings  238 . 
     Turning next to FIGS. 8,  9  and  10  collectively and with reference to FIG. 2, therein is depicted the components of electronics package  122  of the present invention. Electronics package  122  includes an annular carrier  196 , an electronics member  198  and one or more battery packs  200 . Annular carrier  196  is disposed between outer housing  82  and mandrel  84 . Annular carrier  196  includes a plurality of axial openings  202  for receiving either electronics member  198  or battery packs  200 . 
     Even though FIG. 8 depicts four axial openings  202 , it should be understood by one skilled in the art that the number of axial openings in annular carrier  196  may be varied. Specifically, the number of axial openings  202  will be dependent upon the number of battery packs  200  which will be required for a specific implementation of downhole signal repeater  76  of the present invention. 
     Electronics member  198  is insertable into an axial opening  202  of annular carrier  196 . Electronics member  198  receives an electrical signal from first end  188  of windings  184  when toroid  180  serves as receiver  120 . Electronics member  198  includes a plurality of electronic devices such as limiter  204 , preamplifier  206 , notch filter  208 , bandpass filters  210 , phase lock loop  212 , timing devices  214 , shift registers  216 , comparators  218 , parity check  220 , storage devices  222 , and amplifier  224 . The operation of these electronic devices will be more full discussed with reference to FIG.  11 . 
     Battery packs  200  are insertable into axial openings  202  of axial carrier  196 . Battery packs  200 , which includes batteries such as nickel cadmium batteries or lithium batteries, are configured to provide the proper operating voltage and current to the electronic devices of electronics member  198  and to toroid  180 . 
     Even though FIGS. 8-10 have described electronics package  122  with reference to annular carrier  196 , it should be understood by one skilled in the art that a variety of configurations may be used for the construction of electronics package  122 . For example, electronics package  122  may be positioned concentrically within mandrel  84  using several stabilizers and having a narrow, elongated shape such that a minimum resistance will be created by electronics package  122  to the flow of fluids within drill string  30 . 
     Turning now to FIG.  11  and with reference to FIG. 1, one embodiment of the method for processing the electrical signal within a fail safe downhole repeater, such as repeaters  34 ,  35 ,  36 , is described. The method  264  utilizes a plurality of electronic devices such as those described with reference to FIG.  9 . Method  264  provides for digital processing of the information carried in the electrical signal that is generated by receiver  266 . Limiter  268  receives the electrical signal from receiver  266 . Limiter  268  may include a pair of diodes for attenuating the noise in the electrical signal to a predetermined range, such as between about 0.3 and 0.8 volts. The electrical signal is then passed to amplifier  270  which may amplify the electrical signal to a predetermined voltage suitable of circuit logic, such as five volts. The electrical signal is then passed through a notch filter  272  to shunt noise at a predetermined frequency, such as 60 hertz which is a typical frequency for noise in an offshore application in the United States whereas a European application may have a 50 hertz notch filter. The electrical signal then enters a bandpass filter  274  to eliminate unwanted frequencies above and below the desired frequency to recreate a signal having the original frequency, for example, two hertz. 
     The electrical signal is then fed through a phase lock loop  276  that is controlled by a precision clock  278  to assure that the electrical signal which passes through bandpass filter  234  has the proper frequency and is not simply noise. As the electrical signal will include a certain amount of carrier frequency, phase lock loop  276  is able to verify that the received signal is, in fact, a signal carrying information to be retransmitted. The electrical signal then enters a series of shift registers that perform a variety of error checking features. 
     Sync check  280  reads, for example, the first six bits of the information carried in the electrical signal. These first six bits are compared with six bits that are stored in comparator  282  to determine whether the electrical signal is carrying the type of information intended for a repeater such as repeaters  34 ,  35 ,  36  of FIG.  1 . For example, the first six bits in the preamble to the information carried in electromagnetic wave fronts  46  must carry the code stored in comparator  282  in order for the electrical signal to pass through sync check  280 . Each of the repeaters of the present invention, such as repeaters  34 ,  35 ,  36 , will require the same code in comparator  282 . 
     If the first six bits in the preamble correspond with that in comparator  282 , the electrical signal passes to a repeater identification check  284 . Identification check  284  determines whether the information received by a specific repeater is intended for that repeater. The comparator  286  of repeater  34  will require a specific binary code while comparator  286  of repeater  35  will require a different binary code. For example, because electromagnetic wave fronts  46  are intended for repeater  34 , the preamble information carried by electromagnetic wave fronts  46  will correspond with the binary code stored in comparator  286  of repeater  34 . As explained above, however, repeater  35  is disposed within wellbore  38  within the range of electromagnetic wave fronts  46 . Repeater  35  will, therefore, receive electromagnetic wave fronts  46  and determine that electromagnetic wave fronts  46  were not intended for repeater  35 . Identification check  284 , however, will recognize that electromagnetic wave fronts  46  were intended for repeater  34  by matching the binary code in comparator  287  and will process the signal as described below thus, providing a fail safe method for transmitting information between surface equipment and downhole equipment. 
     After passing through identification check  284 , the electrical signal is shifted into a data register  288  which is in communication with a parity check  290  to analyze the information carried in the electrical signal for errors and to assure that noise has not infiltrated and abrogated the data stream by checking the parity of the data stream. If no errors are detected, the electrical signal is shifted into one or more storage registers  292 . Storage registers  292  receive the entire sequence of information and either pass the electrical signal directly into power amplifier  294 , if the signal was intended for that repeater, or will store the information for a specified period of time determined by timer  293 , if the signal was not intended for that repeater. For example, since electromagnetic wave fronts  46  are intended for repeater  34 , the electrical signal will be passed directly into power amplifier  294  of repeater  34  and to transmitter  296 . Transmitter  296  transforms the electrical signal into an electromagnetic signal, such as electromagnetic wave fronts  54 , which are radiated into the earth to be picked up by repeater  35  and repeater  36  of FIG.  1 . 
     Alternatively, since electromagnetic wave fronts  46  are not intended for repeater  35 , the information will be stored by storage registers  292  of repeater  35  for a specified period of time determined by timer  293 . As explained above, if repeater  35  receives electromagnetic wave fronts  54  within the time specified by timer  293 , the information received and stored by repeater  35  from electromagnetic wave fronts  46  is discarded by repeater  35 . If electromagnetic wave fronts  54  are not received by repeater  35  within the time specified by timer  293 , the information carried in electromagnetic wave fronts  46  that was received by repeater  35  is passed into power amplifier  294  of repeater  35  and to transmitter  296  that generates electromagnetic wave fronts  55  which propagate to repeater  36  and electromagnetic pickup device  64 . 
     Even though FIG. 11 has described sync check  280 , identification check  284 , data register  288  and storage register  292  as shift registers, it should be apparent to those skilled in the art that alternate electronic devices may be used for error checking and storage including, but not limited to, random access memory, read only memory, erasable programmable read only memory and a microprocessor. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.