Patent Publication Number: US-2013249705-A1

Title: Casing collar locator with wireless telemetry support

Description:
BACKGROUND 
     After a wellbore has been drilled, the wellbore is often cased by inserting lengths of steel pipe (“casing sections”) connected end-to-end into the wellbore. Threaded exterior rings called couplings or collars are typically used to connect adjacent ends of the casing sections at casing joints. The result is a “casing string”, i.e., a series of casing sections with connecting collars that extends from the surface to a bottom of the wellbore. The casing string is then cemented in place to complete the casing operation. 
     After a wellbore is cased, the casing is often perforated to provide access to a desired formation, e.g., to enable formation fluids to enter the well bore. Such perforating operations require the ability to position a tool at a particular and known position in the well. One method for determining the position of the perforating tool is to count the number of collars that the tool passes as it is lowered into the wellbore. As the length of each of the steel casing sections of the casing string is known, correctly counting a number of collars or joints traversed by a device as the device is lowered into a well enables an accurate determination of a depth or location of the tool in the well. Such counting can be accomplished with a casing collar locator (“CCL”), an instrument that may be attached to the perforating tool and suspended in the wellbore with a wireline. A wireline is an armored cable having one or more electrical conductors to facilitate the transfer of power and communications signals between the surface electronics and the downhole tools. Such cables can be tens of thousands of feet long and subject to extraneous electrical noise interference and crosstalk. In certain applications, the detection signals from conventional casing collar locators and/or data signals from wireline logging tools may not be reliably communicated via the wireline. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accordingly, there are disclosed in the drawings and the following description specific embodiments of downhole systems and methods for casing collar location with combined communications support for other downhole instruments. In the drawings: 
         FIG. 1  shows an illustrative wireline tool system including a casing collar locator (CCL) tool; 
         FIG. 2  shows a first illustrative CCL tool embodiment; 
         FIG. 3  is an illustrative coil response to a passing casing collar; 
         FIG. 4  shows an illustrative optical interface for the CCL tool; 
         FIG. 5A  shows a second illustrative CCL tool embodiment; 
         FIG. 5B  is a top view of an illustrative ferrite “star”; 
         FIG. 6  shows a third illustrative CCL tool embodiment; 
         FIG. 7  shows a fourth illustrative CCL tool embodiment; 
         FIG. 8  shows an illustrative interface schematic for bi-directional communication; and 
         FIG. 9  is a flowchart of an illustrative telemetry method. 
     
    
    
     It should be understood, however, that the specific embodiments given in the drawings and detailed description thereof do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims. 
     DETAILED DESCRIPTION 
     Turning now to the figures,  FIG. 1  provides a side elevation view of a well  10  with an illustrative wireline tool system  14  including a sonde  12  suspended in the well  10  by a fiber optic cable  18  having one or more optical fiber(s)  20 . The well  10  is cased with a casing string  16  having casing sections  30 A and  30 B connected end-to-end by a collar  32 . As is typical, the casing sections  30  of the casing string  16  and the collars connecting the casing sections  30  (e.g., the collar  32 ) are made of steel, an iron alloy, and hence it exhibits a fairly high magnetic permeability and a relatively low magnetic reluctance. In other words, the casing string material conveys magnetic field lines much more readily than air and most other materials. 
     The illustrated sonde  12  houses a casing collar locator (CCL) tool  22  and two logging tools  24  and  26 . A surface unit  28  is coupled to the sonde  12  via the fiber optic cable  18  and configured to receive optical signals from the sonde  12  via the optical fiber(s)  20 . In the embodiment of  FIG. 1 , the CCL tool  22  is configured to generate an electrical “location” signal when passing a collar of the casing string  16 , to convert the electrical location signal into an optical location signal, and to transmit the optical location signal to the surface unit  28  via the optical fiber(s)  20  of the fiber optic cable  18 . As described in more detail below, the CCL tool  22  is also configured to receive electromagnetic telemetry signals (e.g., from the logging tools  24  and  26 ), to convert the electromagnetic telemetry signals into optical telemetry signals, and to transmit the optical telemetry signals along with the optical location signal to the surface unit  28  via the optical fiber(s)  20  of the fiber optic cable  18 . 
     In the embodiment of  FIG. 1 , the CCL tool  22  includes an optical interface  34  coupled to the optical fiber(s)  20 , and a sensor  36  coupled to the optical interface  34 . The sensor  36  produces an electrical signal in response to magnetic field changes attributable to passing collars (e.g., the collar  32 ) in the casing string  16 . In some embodiments, the CCL tool  22  includes one or more permanent magnet(s) producing a magnetic field that changes when the CCL tool  22  passes a collar, and the sensor  36  includes a coil of wire (i.e., a coil) positioned in the magnetic field to detect such changes. As the CCL tool  22  passes a collar, the resultant change in the strength of the magnetic field passing through the coil causes an electrical voltage to be induced between the ends of the coil (in accordance with Faraday&#39;s Law of Induction). This induced electrical signal is the electrical “location” signal referred to above. In other embodiments, the sensor  36  may include, for example, a magnetometer or a Hall-effect sensor. 
     The logging tools  24  and  26  are configured to gather information regarding a formation property or a physical condition downhole. For example, the logging tools  24  and  26  may be configured to gather information about the casing string  16  and/or the well  10 , such as electrical properties (e.g., resistivity and/or conductivity at one or more frequencies), sonic properties, active and/or passive nuclear measurements, dimensional measurements, borehole fluid sampling, and/or pressure and temperature measurements. The logging tools  24  and  26  generate electromagnetic telemetry signals conveying gathered information. 
     For example, in the embodiment of  FIG. 1 , the logging tool  24  produces a modulated magnetic field  38  such that the magnetic field  38  conveys information gathered by the logging tool  24 . In one implementation, logging tool  24  may produce the magnetic field  38  such that the magnetic field has a magnitude and direction that varies sinusoidally, and has a base frequency, phase, and amplitude. The logging tool  24  varies or modulates the base frequency, the phase, or the amplitude of the magnetic field  38  dependent upon the information to be transmitted. Similarly, the logging tool  26  produces a modulated magnetic field  40  such that the magnetic field  40  conveys information gathered by the logging tool  26 . The modulation can be performed in digital or analog fashion, and with an appropriate multiplexing scheme (e.g., time division or frequency division), the modulation scheme can be determined independently by each tool. 
     The strengths of the modulated magnetic fields  38  and  40  produced by the respective logging tools  24  and  26  are chosen to ensure that sensor  36  produces responds to changes in the magnetic fields  38  and  40  with electrical signals that correspond to the electromagnetic telemetry signals produced by the respective logging tools  24  and  26 . As a result, the combined electrical signal produced by the sensor  36  includes the electrical location signal, attributable to passing collars in the casing string  16 , and electrical telemetry signals attributable to the electromagnetic telemetry signals transmitted by the logging tools  24  and  26 . 
     The optical interface  34  of the CCL tool  22  includes a light source controlled or modulated by the electrical signal received from the sensor  36 , thereby producing an optical signal. The light source may include, for example, an incandescent lamp, an arc lamp, an LED, a semiconductor laser, or a super-luminescent diode. The optical signal produced by the optical interface  34  includes a optical location signal produced in response to the electrical location signal, and optical telemetry signals produced in response to the electromagnetic telemetry signals from the logging tools  24  and  26 . The optical interface  34  transmits the optical signal to the surface unit  28  via the optical fiber(s)  20  of the fiber optic cable  18 . The surface unit  28  processes the optical signal received via the optical fiber(s)  20  to obtain a casing collar locator signal and telemetry signals (i.e., transmitted information) from the logging tools  24  and  26 . 
     In at least some embodiments, the surface unit  28  includes a photodetector that receives the optical signal and converts it into an electrical signal (e.g., a voltage or a current) dependent on a magnitude of the optical signal. The photodetector may be or include, for example, a photodiode, a photoresistor, a charge-coupled device, or a photomultiplier tube. 
     In some embodiments, the resultant electrical signal spans a frequency range, and the casing collar locator signal occupies a first portion of the frequency range. The modulated magnetic field  38  produced by the logging tool  24  occupies a second portion of the frequency range, and the modulated magnetic field  40  produced by the logging tool  26  occupies a third portion of the frequency range. The surface unit  28  recovers the casing collar locator signal from the first portion of the frequency range, the telemetry signal from the logging tool  24  from the second portion of the frequency range, and the telemetry signal from the logging tool  26  from the third portion of the frequency range. 
     In the embodiment of  FIG. 1 , the fiber optic cable  18  preferably also includes armor to add mechanical strength and/or to protect the cable from shearing and abrasion. Some of the optical fiber(s)  20  may be used for power transmission, communication with other tools, and redundancy. The fiber optic cable  18  may, in some cases, also include electrical conductors if desired. The fiber optic cable  18  spools to and from a winch  42  as the sonde  12  is conveyed through the casing string  16 . The reserve portion of the fiber optic cable  18  is wound around a drum of the winch  42 , and the fiber optic cable  18  having been dispensed or unspooled from the drum supports the sonde  12  as it is conveyed through the casing string  16 . 
     In the illustrated embodiment, the winch  42  includes an optical slip ring  44  that enables the drum of the winch  42  to rotate while making an optical connection between the optical fiber(s)  20  and corresponding fixed port(s) of the slip ring  44 . The surface unit  28  is connected to the port(s) of the slip ring  44  to send and/or receive optical signals via the optical fiber(s)  20 . In other embodiments, the winch  42  includes an electrical slip ring  44  to send and/or receive electrical signals from the surface unit  28  and an electro-optical interface that translates the signals from the optical fiber  20  for communication via the slip ring  44  and vice versa. 
     In certain alternative embodiments, the logging tool  26  does not communicate directly with CCL tool  22 , but rather communicates indirectly via logging tool  24  using the magnetic field  40 , another form of wireless communication, or one or more wired connections. The logging tool  26  may provide gathered information to the logging tool  24 , and the logging tool  24  may modulate the magnetic field  38  to produce an electromagnetic telemetry signal that conveys information gathered by both the logging tool  24  and the logging tool  26 . 
       FIG. 2  provides a more detailed version of a first illustrative CCL tool embodiment. In the embodiment of  FIG. 2 , the CCL tool  22  includes a pair of opposed permanent magnets  50 A and  50 B and a wire coil  52  having multiple windings, the coil  52  serving as the sensor  36  of  FIG. 1 . The coil  52  is positioned between the magnets  50 A and  50 B to detect changes in the magnetic field produced by magnets  50 A,  50 B. In the embodiment of  FIG. 2 , each of the magnets  50 A and  50 B is cylindrical and has a central axis. The magnets  50 A and  50 B are positioned on opposite sides of the coil  52  such that their central axes are colinear, and the north magnetic poles of the magnets  50 A and  50 B are adjacent one another and the coil  52 . A central axis of the coil  52  is colinear with the central axes of the magnets  50 A and  50 B. The coil  52  has two ends coupled to the optical interface  34 . 
     The magnet  50 A produces a magnetic field  56 A that passes or “cuts” through the windings of the coil  52 , and the magnet  50 B produces a magnetic field  56 B that also cuts through the windings of the coil  52 . The magnet  50 A and the adjacent walls of the casing string  16  form a first magnetic circuit through which most of the magnetic field  56 A passes. Similarly, the magnetic field  56 B passes through a second magnetic circuit including the magnet  50 B and the adjacent walls of the casing string  16 . The intensities of the magnetic fields  56 A and  56 B depend on the sums of the magnetic reluctances of the elements in each of the magnetic circuits. 
     Any change in the intensities of the magnetic field  56 A and/or the magnetic field  56 B cutting through the coil  52  causes an electrical voltage to be induced between the two ends of the coil  52  in accordance with Faraday&#39;s Law of Induction. As the sonde  12  of  FIG. 2  passes through a casing section of the casing string  16  (e.g., the casing section  30 A), the intensities of the magnetic fields  56 A and  56 B cutting through the coil  52  remain substantially the same, and no appreciable electrical voltage is induced between the two ends of the coil  52 . On the other hand, as the sonde  12  passes by a collar (e.g., the collar  32 ), the magnetic reluctance of the casing string  16  changes, causing the intensities of the magnetic fields  56 A and  56 B cutting through the coil  52  to change in turn, and an electrical voltage to be induced between the two ends of the coil  52 .  FIG. 3  is an illustrative graph of the electrical voltage that might be produced between the two ends of the coil  52  as the sonde  12  passes by collar  32 . This signal is the location signal produced by the CCL tool  22  as described above. 
     In the embodiment of  FIG. 2 , the sonde  12  also includes a second wire coil  58  coupled to the logging tool  24 . The logging tool  24  drives coil  58  with an electrical telemetry signal that conveys gathered information. In response to the electrical telemetry signal, the coil  58  produces a modulated magnetic field (e.g., the modulated magnetic field  38  of  FIG. 1 ) that couples with coil  52  to convey the information gathered by the logging tool  24 . The logging tool  26  may include a similar coil, and may produce a similar modulated magnetic field (e.g., the modulated magnetic field  40  of  FIG. 1 ) to convey its gathered information. Alternatively, the logging tool  26  may transmit gathered information to the logging tool  24 , and the logging tool  24  may modulate the magnetic field produced by the coil  58  such that the modulated magnetic field conveys information gathered by both the logging tool  24  and the logging tool  26 . 
     As shown in  FIG. 2 , the coil  58  is positioned near the permanent magnet  50 B such that the modulated magnetic field produced by the coil  58  affects or perturbs the magnetic field  56 B produced by the magnet  50 B, and the change in the magnetic field  56 B causes a change in the magnetic field  56 A produced by the magnet  50 A. As a result, the intensities of the magnetic fields  56 A and  56 B cutting through the coil  52  are changed, and an electrical voltage is induced between the two ends of the coil  52 . The electrical signal produced by the coil  52  thus includes the electrical location signal, attributable to passing collars (e.g., the collar  32 ) in the casing string  16 , and the electrical telemetry signal attributable to the electromagnetic telemetry signal transmitted by the logging tool  24 . 
     In other embodiments, the CCL tool  22  may include a single permanent magnet producing a magnetic field that changes in response to passing a collar in the casing string. Suitable single magnet embodiments are shown and described in co-pending U.S. patent application Ser. No. 13/226,578 entitled “OPTICAL CASING COLLAR LOCATOR SYSTEMS AND METHODS” and filed Sep. 7, 2011, incorporated herein by reference in its entirety. 
       FIG. 4  is a diagram of an illustrative embodiment of the optical interface  34  of  FIG. 2 . In the embodiment of  FIG. 4 , the optical interface  34  includes a voltage source  70 , a resistor  72 , a light source  74 , and a pair of Zener diodes  76 A and  76 B. The light source  74  includes a light emitting diode (LED)  78 . The voltage source  70 , the resistor  72 , the LED  78 , and the coil  52  (see  FIG. 2 ) are connected in series, forming a series circuit. The voltage source  70  is a direct current (DC) voltage source having two terminals, and one of the two terminals of the voltage source  70  is connected to one end of the coil  52  (see  FIG. 2 ). In the embodiment of  FIG. 4 , the LED  78  has two terminals, one of which is connected to the other of the two ends of the coil  52 . The resistor  72  is connected between the voltage source  70  and the LED  78 , and limits a flow of electrical current through the LED  78 . 
     The voltage source  70  produces a DC bias voltage that at least partially forward-biases the LED  78 , improving the responsiveness of the light source  74 . The voltage source  70  may be or include, for example, a chemical battery, a fuel cell, a nuclear battery, an ultra-capacitor, or a photovoltaic cell. In some embodiments, the voltage source  70  produces a DC bias voltage that causes an electrical current to flow through the series circuit including the voltage source  70 , the resistor  72 , the LED  78 , and the coil  52  (see  FIG. 2 ), and the current flow through the LED  78  causes the LED  78  to produce light. A lens  80  directs at least some of the light produced by the LED  78  into an end of the optical fiber(s)  20  (see  FIG. 2 ) to form the optical signal, labeled ‘ 82 ’ in  FIG. 4 . The optical signal  82  propagates along the optical fiber(s)  20  to the surface unit  28  (see  FIG. 1 ). The surface unit  28  processes the optical signal  82  to obtain the casing collar locator signal and telemetry signals (i.e., transmitted information) from the logging tools  24  and  26 . 
     Changes in the strengths of the magnetic fields  56 A and  56 B induce positive and negative voltage pulses between the ends of the coil  52  (see  FIG. 2 ). Within the series circuit including the voltage source  70 , the resistor  72 , the LED  78 , and the coil  52 , the voltage pulses produced between the ends of the coil  52  are summed with the DC bias voltage produced by the voltage source  70 . In some embodiments, a positive voltage pulse produced between the ends of the coil  52  causes a voltage across the LED  78  to increase, and the resultant increase in current flow through the LED  78  causes the LED  78  to produce more light (i.e., light with a greater intensity). Similarly, a negative voltage pulse produced between the ends of the coil  52  causes the voltage across the LED  78  to decrease, and the resultant decrease in the current flow through the LED  78  causes the LED  78  to produce less light (i.e., light with a lesser intensity). In these embodiments, the DC bias voltage produced by the voltage source  70  causes the optical signal  82  produced by the optical interface  34  to have an intensity that is proportional to a magnitude of an electrical signal produced between the ends of the coil  52 . 
     The Zener diodes  76 A and  76 B are connected in series with opposed orientations as shown in  FIG. 4 , and the series combination is connected between the two terminals of the LED  78  to protect the LED  78  from excessive forward and reverse voltages. In other embodiments, the light source  74  may be or include, for example, an incandescent lamp, an arc lamp, a semiconductor laser, or a super-luminescent diode. In other embodiments, the DC bias voltage produced by the voltage source  70  may match a forward voltage threshold of one or more diodes in series with the light source  74 . 
       FIG. 5A  is a diagram of another embodiment of the sonde  12  of  FIG. 2 . In the embodiment of  FIG. 5A , a ferrite “star”  90 A replaces the coil  52  positioned between the magnets  50 A and  50 B.  FIG. 5B  shows a top view of the ferrite star  90 A of  FIG. 5A . Referring to  FIG. 5B , the ferrite star  90 A has four azimuthally-distributed legs  92 A,  92 B,  92 C, and  92 D projecting radially outward from a central hub  94 . A wire coil is positioned around each of the legs (coils  96 A- 96 D), each coil being individually coupled to the optical interface  34  as indicated in  FIG. 5A . The ferrite star  90 A is made of a ferromagnetic material, and the legs concentrate the magnetic fields  56 A and  56 B produced by the magnets  50 A and  50 B (see  FIG. 2 ) into azimuthal lobes that cut through the windings of the corresponding coils  96 A- 96 D, thereby providing azimuthal sensitivity to the measurements by any given coil. Any change in the intensity of the magnetic field  56 A and/or the magnetic field  56 B cutting through one of the coils  96 A- 96 D causes an electrical voltage to be induced between the two ends of the coil. 
     In the embodiment of  FIG. 5A , each of the four coils  96 A- 96 D produces an electrical casing collar locator signal, and the optical interface  34  produces four corresponding optical casing collar locator signals. The optical interface  34  may, for example, produce the four corresponding optical casing collar locator signals using different wavelengths of light such that each of the optical signals occupies a different portion of an optical frequency range. The surface unit  28  may recover the four separate electrical casing collar locator signals from the respective portions of the optical frequency range. 
     As the sonde  12  of  FIG. 5A  passes through the casing string  16 , the sonde  12  can move laterally within the casing string  16 . As the sonde  12  passes through a casing section (e.g., the casing section  30 A) of the casing string  16 , the intensities of the magnetic fields  56 A and  56 B cutting through the coils  96 A- 96 D change with a changing distance between the coils  96 A- 96 D and an inner surface of the casing string  16 . The relative amplitudes of the respective electrical location signals will vary in a pattern that can be used to determine the sonde&#39;s lateral position within the casing. As the sonde  12  passes by a collar (e.g., the collar  32 ), the magnetic reluctance of the casing string  16  changes, causing the intensities of the magnetic fields  56 A and  56 B cutting through the coils  96 A- 96 D to change, and inducing electrical voltages between the ends of the coils  96 A- 96 D. The coils  96 A- 96 D closest to the inner wall of the casing string  16  expectedly produce electrical voltages having the greatest magnitudes, and the coils  96 A- 96 D farthest from to the inner wall of the casing string  16  expectedly produce electrical voltages having the smallest magnitudes. 
     In the embodiment of  FIG. 5A , the logging tool  24  has a ferrite star  90 B similar to the ferrite star  90 A, and the logging tool  26  has a ferrite star  90 C similar to the ferrite star  90 A. The ferrite star  90 B has four legs  92 E,  92 F,  92 G, and  92 H projecting radially outward from a central hub, and coils  96 E- 96 H are positioned around the respective legs  92 E- 92 H. The ferrite star  90 C has four legs  92 I,  92 J,  92 K, and  92 L projecting radially outward from a central hub, and coils  96 I- 96 L are positioned around the respective legs  92 I- 92 L. The central hubs of the ferrite stars  90 A,  90 B, and  90 C have central axes that are collinear, and corresponding legs of the ferrite stars  90 A,  90 B, and  90 C are aligned along the collinear central axes such that the strengths of the magnetic couplings between the corresponding legs are relatively strong. The corresponding legs are:  92 A,  92 E, and  92 I;  92 B,  92 F, and  92 J;  92 C,  92 G, and  92 K; and  92 D,  92 H, and  92 L, and the corresponding coils are:  96 A,  96 E, and  96 I;  96 B,  96 F, and  96 J;  96 C,  96 G, and  96 K; and  96 D,  96 H, and  96 L. 
     The logging tool  24  drives an electrical telemetry signal that conveys gathered information on at least one of the coils  96 E- 96 H. In response to the electrical telemetry signal, at least one of the coils  96 E- 96 H produces a modulated magnetic field conveying information gathered by the logging tool  24 . The modulated magnetic field produced by the at least one of the coils  96 E- 96 H cuts through a corresponding at least one of the coils  96 A- 96 D of the CCL tool  22 , and an electrical voltage is induced between the ends of the corresponding at least one of the coils  96 A- 96 D. The electrical signal produced by the corresponding at least one of the coils  96 A- 96 D thus includes the electrical location signal, attributable to passing collars (e.g., the collar  32 ) in the casing string  16 , and the electrical telemetry signal attributable to the electromagnetic telemetry signal transmitted by the logging tool  24 . The logging tool  26  transmits an the electromagnetic telemetry signal to the CCL tool  22  in a similar manner. In some embodiments, different corresponding coils are assigned to the logging tools  24  and  26  for the transmission of gathered information. 
     The coils  96 E- 96 H of the logging tool  24 , and the coils  96 I- 96 L of the logging tool  26  may be coupled together in appropriate polarities to achieve one of several orthogonal transmission modes. The four-coil embodiments can support the monopole mode, X-dipole mode, Y-dipole mode, and quadrupole mode, as four orthogonal signaling modes. In other words, representing the relative magnitude and polarity of the signals on coils A, B, C, D in  FIG. 5B  as a vector [A, B, C, D], the four orthogonal signaling modes could be [1, 1, 1, 1], [1, 0, −1, 0], [0, 1, 0, −1], and [1, −1, 1, −1]. Upon reception by an azimuthally-aligned set of coils, the coil signals would be combined with the appropriate magnitudes and polarities to extract the signals sent via the chosen modes. More information on orthogonal transmission modes can be found in “Multiconductor Transmission Line Analysis”, by Sidney Frankel, Artech House Inc., 1977, “Analysis of Multiconductor Transmission Lines (Wiley Series in Microwave and Optical Engineering), Clayton R. Paul, 1994, and in U.S. Pat. No. 3,603,923 dated Sep. 10, 1968 by Nulligan. 
     The orthogonal transmission modes can be used to support simultaneous half duplex and/or full duplex communication between the CCL tool  22  and multiple logging tools  24 ,  26 . That is, the logging tools  24  and  26  may use different ones of the orthogonal transmission modes to communicate the gathered information to the CCL tool  22 . The orthogonal transmission mode selected for each tool may be configurable and may, for example, be set when the sonde is assembled. 
       FIG. 6  shows an alternative embodiment of the CCL tool  22 . In the embodiment of  FIG. 6 , the coil  52  is positioned between the magnets  50 A and  50 B as in  FIG. 2  and described above. Four communication coils  110 A,  110 B,  110 C, and  110 D surround the coil  52  such that central axes of the coils  110 A- 110 D and extend radially from the central axis of the coil  52 . The coils  110 A- 110 D are azimuthally distributed about the central axis of the coil  52 , similar to the coils of  FIG. 5A . The optical interface  34  measures the responses of each of the coils and communicates them to the surface. Coil  52  responds to passing collars to provide a location signal as described previously, and may further respond to telemetry signals from other logging tools. The communications coils  110 A,  110 B,  110 C, and  110 D respond to other component of the magnetic field, providing additional degrees of freedom for providing orthogonal transmission modes that would support simultaneous communications with multiple logging tools. (Of course, time or frequency multiplexing could also or alternatively be employed for this purpose.) The logging tools  24  and  26  would have communication coils similar to communication coils  110 A- 110 D. 
       FIG. 7  shows another alternative embodiment of the CCL tool  22 . In the embodiment of  FIG. 7 , the coil  52  is positioned between the magnets  50 A and  50 B as shown in  FIG. 2  and described above. A hollow, cylindrical form  120  made of a non-magnetic material is positioned about the magnet  50 B. The magnet  50 B and the form  120  are coaxial, and in the embodiment of  FIG. 7  the form  120  extends a length of the magnet  50 B. Four communication coils  122 A,  122 B,  122 C, and  122 D are wound about the form  120  at equal distances along the form&#39;s perimeter (at equal angles about a central axis of the form  120 ). As with the communication coils of  FIG. 6 , each coil is coupled to the optical interface to respond to different components of the magnetic field and thereby provide additional degrees of freedom for supporting additional signal transmission modes. The logging tools  24 ,  26  would have similarly oriented communication coils for optimal coupling. 
       FIG. 8  shows an illustrative wireline tool system  14  that supports full-duplex communications. In the embodiment of  FIG. 8 , the CCL tool  22  includes the coil  52  and the communication coils  122 A- 122 D shown in  FIG. 7  and described above. Logging tool  24  includes a set of communication coils  122 E- 122 H similar to coils  122 A- 122 D. Corresponding coils are:  122 A and  122 E,  122 B and  122 F,  122 C and  122 G, and  122 D and  122 H. Magnetic couplings between corresponding coils is relatively strong. 
     In the embodiment of  FIG. 8 , the surface unit  28  includes an optical interface  132  coupled between a digital signal processor (DSP)  130  and the optical fiber(s)  20 . The optical interface  132  includes an optical transmitter  134  and an optical receiver  136 , both coupled to the DSP  130  and the optical fiber(s)  20 . The optical interface  34  of the CCL tool  22  includes an optical receiver  138 , an optical transmitter  140  for telemetry signals, and an optical transmitter  142  for a location signal. The logging tool  24  includes a receiver  146 , a transmitter  148 , and communication electronics  150 . Each of the optical transmitters  134 ,  140 , and  142  includes a light source (e.g., an incandescent lamp, an arc lamp, an LED, a semiconductor laser, and/or a super-luminescent diode). Each of the optical receivers  136  and  138  includes at least one photodetector (e.g., a photodiode, a photoresistor, a charge-coupled device, and/or a photomultiplier tube). 
     In the embodiment of  FIG. 8 , the coils  122 A- 122 D and the coils  122 E- 122 H are configured and operated to achieve a full duplex dipole transmission mode. One end of the coil  122 A is connected to one end of the coil  122 C such that electrical voltages induced between the ends of the coils  122 A and  122 C add together (reinforce one another), and the sum of the voltages is present between the other “free” ends of the coils  122 A and  122 C. Ends of the coils  122 B and  122 D,  122 E and  122 G, and  122 F and  122 H are connected similarly. 
     An “upgoing” transmission of the location signal from the CCL tool  22  to the DSP  130  will now be described. As described above, the coil  52  produces the location signal when the sonde  12  including the CCL tool  22  passes a collar in the casing string  16  (see  FIG. 1 ). As indicated in  FIG. 8 , the ends of the coil  52  are coupled to an input of the optical transmitter  142 . An output of the optical transmitter  142  is coupled to the optical fiber(s)  20  via a splitter. The optical transmitter  142  receives the electrical location signal from the coil  52  at the input, and drives an optical signal conveying the location signal from the coil  52  on the optical fiber(s)  20 . 
     An input of the optical receiver  136  in the optical interface  132  of the surface unit  28  is coupled to the optical fiber(s)  20  via a splitter. The optical receiver  136  receives the optical signal conveying the location signal from the CCL tool  22  at the input, and produces an electrical signal conveying the location signal at an output. The DSP  130  is coupled to the output of the optical receiver  136 , and receives the electrical signal conveying the location signal from the optical receiver  136 . 
     A “downgoing” communication path from the surface unit  28  to the logging tool  24  will now be described. The DSP  130  generates an electrical control signal, and provides the electrical control signal to the optical transmitter  134 . The optical transmitter  134  receives the electrical control signal at an input. An output of the optical transmitter  134  is coupled to the optical fiber(s)  20  via the splitter. The optical transmitter  134  drives an optical signal conveying the control signal from DSP  130  on the optical fiber(s)  20 . 
     The free ends of the coils  122 B and  122 D are coupled to an output of the optical receiver  138 . An input of the optical transmitter  140  is coupled to the optical fiber(s)  20  via the splitter. The optical receiver  138  receives the optical signal conveying the control signal from the DSP  130 , and drives an electrical signal conveying the control signal from the DSP  130  on the coils  122 B and  122 D at the output. In response to the electrical signal from the optical receiver  138 , the coils  122 B and  122 D of the CCL tool  22  produce a changing magnetic field (i.e., an electromagnetic signal) conveying the control signal from the DSP  130 . The corresponding coils  122 F and  122 H of the logging tool  24  receive the electromagnetic signal conveying the control signal from the DSP  130 , and an electrical signal conveying the control signal from the DSP  130  is provided to an input of the receiver  146 . The receiver  146  receives the electrical signal conveying the control signal from the DSP  130  at the input, equalizes it, and provides it to the logging tool&#39;s communications electronics  150 . As indicated in  FIG. 8 , the communication electronics  150  of the logging tool  24  may be coupled to other logging tools via a wireless or wired communication link to relay the control information. 
     An “upgoing” communication path from the logging tool  24  to the surface unit  28  will now be described. The communication electronics  150  of the logging tool  24  is coupled to an input of the transmitter  148 . The communication electronics  150  produces an electrical signal conveying information (e.g., an electrical telemetry signal conveying gathered data), and provides the electrical signal to the transmitter  148 . The transmitter  148  receives the electrical signal at the input, and drives the communication coils  122 E and  122 G accordingly. The resulting electromagnetic signal induces a response in communications coils  122 A and  122 C, which are coupled to an input of the optical transmitter  140  in the CCL tool. An output of the optical transmitter  140  is coupled to the optical fiber(s)  20  via the splitter. The optical transmitter  140  receives the electrical signal conveying the information from the logging tool  24  at the input, and drives an optical signal conveying the information from the logging tool  24  on the optical fiber(s)  20 . 
     In the surface unit  28 , the optical receiver  136  receives the optical signal conveying the information from the logging tool  24  at the input, and produces an electrical signal conveying the information from the logging tool  24  at an output. The DSP  130  is coupled to the output of the optical receiver  136 , and receives the electrical signal conveying the information from the logging tool  24 . 
       FIG. 9  is a flowchart of an illustrative telemetry method  160  that may be carried out by a wireline tool system (e.g., the wireline tool system  14  of  FIG. 1 ). As represented by block  162 , the method includes generating an electromagnetic telemetry signal with a first downhole logging tool (e.g., the logging tool  24  of  FIGS. 1 ,  2 ,  5 A, or  8 ). The method further includes converting the electromagnetic telemetry signal into an electrical telemetry signal with a sensing coil (e.g., the coil  52  of  FIGS. 2 ,  6 , and  7 , or one of the coils  92 A- 92 D of  FIGS. 5A-5B ) in a casing collar locator (e.g., the casing collar locator  22  of  FIGS. 2 ,  5 A,  6 , or  7 ), as represented by block  164 . The electrical telemetry signal is then transformed into a light signal where the light signal includes a casing collar location signal, as represented by block  166 . The light signal is then sent along an optical fiber (e.g., one of the optical fiber(s)  20  of  FIGS. 1 ,  2 ,  5 A, or  8 ), as represented by block  168 . Optionally, the received light signal from the optical fiber may be converted into a digitized signal, as represented by block  170 . Optionally, the digitized signal may be processed to extract the casing collar location signal and the telemetry signal, as represented by block  172 . 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. The foregoing description discloses a wireline embodiment for explanatory purposes, but the principles are equally applicable to, e.g., a tubing-conveyed sonde with an optical fiber providing communications between the sonde and the surface. In addition or alternatively to sensing communications signals from other logging tools in the sonde, the disclosed CCL tool can be employed for communications with other downhole tools, e.g., permanent sensors or downhole actuators. While the sonde is in proximity to such tools, the foregoing principles can be employed for communications between the surface and those tools. It is intended that the following claims be interpreted to embrace all such variations and modifications.