Abstract:
A retrievable tool for steering through an earth formation includes a first tool assembly and a tilted antenna attached to the first tool assembly. The tool also includes a second tool assembly attached to the first tool assembly and a tilted antenna attached to the second tool assembly. The first tool assembly attaches to the second tool assembly so that the antennas are tilted in predetermined directions. The tilted antennas are transmitter antennas or receiver antennas. Each tool assembly is a tubular cylinder with a longitudinal axis running the length of the cylinder, wherein the tubular cylinder has two ends, each end including a rotational attachment mechanism. The tool assemblies attach to each other through their rotational attachment mechanisms. The rotational attachment mechanism may be a screw-on mechanism, press-fit mechanism, or welded mechanism.

Description:
[0001]    The present application is a divisional of U.S. patent application Ser. No. 12/306,267 (Allowed), filed Dec. 23, 2008 and titled “Modular Geosteering Tool Assembly” by inventors Michael S. Bittar, Clive D. Menezes and Martin D. Paulk, which claims priority to PCT App. PCT/US2007/015806, filed Jul. 11, 2007 and titled “Modular Geosteering Tool Assembly” by inventors Michael S. Bittar, Clive D. Menezes and Martin D. Paulk, which claims priority to provisional U.S. Pat. App. 60/806,981, filed Jul. 11, 2006 and titled “Modular Geosteering Tool Assembly” by inventors Michael S. Bittar, Clive D. Menezes and Martin D. Paulk. 
     
    
     BACKGROUND 
       [0002]    The gathering of downhole information has been done by the oil well industry for many years. Modern petroleum drilling and production operations demand a great quantity of information relating to the parameters and conditions downhole. Such information typically includes the location and orientation of the wellbore and drilling assembly, earth formation properties, and drilling environment parameters downhole. The collection of information relating to formation properties and conditions downhole is commonly referred to as “logging”, and can be performed during the drilling process itself. 
         [0003]    Various measurement tools exist for use in wireline logging and logging while drilling. One such tool is the resistivity tool, which includes one or more antennas for transmitting an electromagnetic signal into the formation and one or more antennas for receiving a formation response. When operated at low frequencies, the resistivity tool may be called an “induction” tool, and at high frequencies it may be called an electromagnetic wave propagation tool. Though the physical phenomena that dominate the measurement may vary with frequency, the operating principles for the tool are consistent. In some cases, the amplitude and/or the phase of the receive signals are compared to the amplitude and/or phase of the transmit signals to measure the formation resistivity. In other cases, the amplitude and/or phase of the receive signals are compared to each other to measure the formation resistivity. 
         [0004]    In certain situations, such as when drilling through formations in which the formation boundaries extend vertically, or when drilling from an off-shore platform, it is desirable to drill wells at an angle with respect to bed boundaries in the strata. This is often termed “horizontal” drilling. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. This can be difficult since formations may dip or divert. Thus, while attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. 
         [0005]    As the rotating bit approaches the bed boundary, the bed boundary will be on one side of the bit axis, i.e. in one azimuthal range with respect to the bit axis. Conventional resistivity tools are not azimuthally sensitive and hence they do not enable the detection and avoidance of approaching bed boundaries. Moreover, conventional resistivity tools are manufactured as a single unit, and hence they cannot be readily customized as new measurement or boundary detection techniques are discovered and refined. Rather, new tools must be manufactured as different hardware configurations are discovered to be useful. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the ensuing detailed description, reference will be made to the accompanying drawings in which: 
           [0007]      FIG. 1  shows a logging while drilling environment; 
           [0008]      FIG. 2  shows an illustrative base module in the form of a logging while drilling (LWD) resistivity tool; 
           [0009]      FIG. 3  shows coordinates for defining the orientation of a tilted antenna; 
           [0010]      FIGS. 4A-4E  show illustrative extension modules for a modular geosteering tool assembly; 
           [0011]      FIG. 5  shows an illustrative modular geosteering tool assembly; 
           [0012]      FIG. 6  shows another illustrative modular geosteering tool assembly; 
           [0013]      FIG. 7  shows a third modular geosteering tool assembly with a different tool interposed between modules; 
           [0014]      FIG. 8  shows illustrative electronics for base and extension modules; 
           [0015]      FIG. 9  shows an illustrative multi-tap antenna schematic; 
           [0016]      FIG. 10A  shows a detail view of a modular geosteering tool assembly during manufacture; 
           [0017]      FIGS. 10B-10D  show components of an illustrative tilted antenna module embodiment; 
           [0018]      FIGS. 11A-11E  show components of a second illustrative tilted antenna module embodiment; and 
           [0019]      FIG. 12  is a flow diagram of an illustrative logging method. 
       
    
    
       [0020]    While the disclosed inventions are susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the inventions to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims. 
       Notation and Nomenclature 
       [0021]    Certain terms are used throughout the following description and claims to refer to particular system components and configurations. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or a direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. In addition, the term “attached” is intended to mean either an indirect or a direct physical connection. Thus, if a first device attaches to a second device, that connection may be through a direct physical connection, or through an indirect physical connection via other devices and connections. 
       DETAILED DESCRIPTION 
       [0022]    The issues identified in the background above are at least partly addressed by the methods and tool assemblies disclosed herein. In some method and tool assembly embodiments, an extension module is coupled directly or indirectly to a base module, which in some cases may have the antenna configuration of an existing commercial resistivity logging while drilling (LWD) tool. The extension module operates cooperatively with the base module to enable the detection of azimuthal variations in formation resistivity. Geosteering signals can be derived from the azimuthal variations to enable steering relative to bed boundaries. A set of various extension module types is made available to enable custom configuration of the tool assembly. Other tools or tubulars may be located between the extension module and the base module, thereby enabling deep measurement configurations to be provided without unduly lengthening the tool string. 
         [0023]    Turning now to the figures,  FIG. 1  shows a well during drilling operations. A drilling platform  2  is equipped with a derrick  4  that supports a hoist  6 . Drilling of oil and gas wells is carried out by a string of drill pipes connected together by “tool” joints  7  so as to form a drill string  8 . The hoist  6  suspends a kelly  10  that lowers the drill string  8  through rotary table  12 . Connected to the lower end of the drill string  8  is a drill bit  14 . The bit  14  is rotated and drilling accomplished by rotating the drill string  8 , by use of a downhole motor near the drill bit, or by both methods. 
         [0024]    Drilling fluid, termed “mud”, is pumped by mud recirculation equipment  16  through supply pipe  18 , through drilling kelly  10 , and down through the drill string  8  at high pressures and volumes to emerge through nozzles or jets in the drill bit  14 . The mud then travels back up the hole via the annulus formed between the exterior of the drill string  8  and the borehole wall  20 , through a blowout preventer, and into a mud pit  24  on the surface. On the surface, the drilling mud is cleaned and then recirculated by recirculation equipment  16 . 
         [0025]    For logging while drilling (LWD), downhole sensors  26  are located in the drillstring  8  near the drill bit  14 . Sensors  26  include directional instrumentation and a modular resistivity tool with tilted antennas for detecting bed boundaries. The directional instrumentation measures the inclination angle, the horizontal angle, and the rotational angle (a.k.a. “tool face angle”) of the LWD tools. As is commonly defined in the art, the inclination angle is the deviation from vertically downward, the horizontal angle is the angle in a horizontal plane from true North, and the tool face angle is the orientation (rotational about the tool axis) angle from the high side of the well bore. In some embodiments, directional measurements are made as follows: a three axis accelerometer measures the earth&#39;s gravitational field vector relative to the tool axis and a point on the circumference of the tool called the “tool face scribe line”. (The tool face scribe line is drawn on the tool surface as a line parallel to the tool axis.) From this measurement, the inclination and tool face angle of the LWD tool can be determined. Additionally, a three axis magnetometer measures the earth&#39;s magnetic field vector in a similar manner. From the combined magnetometer and accelerometer data, the horizontal angle of the LWD tool can be determined. In addition, a gyroscope or other form of inertial sensor may be incorporated to perform position measurements and further refine the orientation measurements. 
         [0026]    In a some embodiments, downhole sensors  26  are coupled to a telemetry transmitter  28  that transmits telemetry signals by modulating the resistance to mud flow in drill string  8 . A telemetry receiver  30  is coupled to the kelly  10  to receive transmitted telemetry signals. Other telemetry transmission techniques are well known and may be used. The receiver  30  communicates the telemetry to a surface installation (not shown) that processes and stores the measurements. The surface installation typically includes a computer system of some kind, e.g. a desktop computer, that may be used to inform the driller of the relative position and distance between the drill bit and nearby bed boundaries. 
         [0027]    The drill bit  14  is shown penetrating a formation having a series of layered beds  34  dipping at an angle. A first (x,y,z) coordinate system associated with the sensors  26  is shown, and a second coordinate system (x″,y″,z″) associated with the beds  32  is shown. The bed coordinate system has the z″ axis perpendicular to the bedding plane, has the y″ axis in a horizontal plane, and has the x″ axis pointing “downhill”. The angle between the z-axes of the two coordinate systems is referred to as the “dip” and is shown in  FIG. 1  as the angle β. 
         [0028]    Referring now to  FIG. 2 , an illustrative base module  102  is shown in the form of a resistivity tool. The base module  102  is provided with one or more regions  106  of reduced diameter. A wire coil  104  is placed in the region  106  and spaced away from the surface of  102  by a constant distance. To mechanically support and protect the coil  104 , a non-conductive filler material (not shown) such as epoxy, rubber, fiberglass, or ceramics may be used in the reduced diameter regions  106 . The transmitter and receiver coils may comprise as little as one loop of wire, although more loops may provide additional signal power. The distance between the coils and the tool surface is preferably in the range from 1/16 inch to ¾ inch, but may be larger. 
         [0029]    In the tool embodiment of  FIG. 2 , coils  104  and  108  are transmitter coils, and coils  110  and  112  are receiving coils. In operation, a transmitter coil  104  transmits an interrogating electromagnetic signal which propagates through the well bore and into the surrounding formation. Signals from the formation reach receiver coils  110 ,  112 , inducing a signal voltage that is detected and measured to determine an amplitude attenuation and phase shift between coils  110  and  112 . The measurement is repeated using transmitter  108 . From the measured attenuation and phase shifts, the resistivity of the formation can be estimated using conventional techniques. 
         [0030]    However, base module  102  lacks any azimuthal sensitivity, making it difficult to determine the direction of any approaching bed boundaries. Accordingly, it is desirable to tilt one or more of the antennas.  FIG. 3  shows an antenna that lies within a plane having a normal vector at an angle of θ with the tool axis and at an azimuth of α with respect to the tool face scribe line. When θ equals zero, the antenna is said to be coaxial, and when θ is greater than zero the antenna is said to be tilted. 
         [0031]    Though the illustrative base module  102  does not include a tilted antenna, other base module configurations are contemplated. For example, the base module may include one or more tilted antennas to provide azimuthal sensitivity. It may include as little as one antenna (for transmitting or for receiving), or on the other extreme, it may be a fully self-contained geosteering and resistivity logging tool. When an extension module is employed, at least one antenna in the base module is expected to be employed for transmitting to a receiver on the extension module or receiving from a transmitter on the extension module. In this fashion, the extension module extends the functionality of the base module. 
         [0032]      FIGS. 4A-4E  illustrate various extension modules that may be added to a base module such as tool  102  ( FIG. 2 ) to provide that tool with azimuthal sensitivity or other enhancements such as deeper resistivity measurements. In some alternative embodiments, these modules can also serve as base modules, enabling these modules to be mixed and matched to form a completely customized logging tool as needed for new logging techniques or geosteering techniques that are developed. As discussed further below, these modules may be provided with electronics that allow them to operate each antenna as a transmitter or a receiver. In some embodiments, a one-line communications bus (with the tool body acting as the ground) is provided to enable power transfer and digital communications between modules. In some system embodiments, a separate power and control module (not shown here) is provided to coordinate the operations of the various tool modules and to collect (and perhaps process) the measurements of those modules operating as receivers. 
         [0033]    The resistivity tool modules have an attachment mechanism that enables each module to be coupled to other modules. In some embodiments, the attachment mechanism may be a threaded pin and box mechanism as shown in  FIGS. 4A-4E . In some other embodiments of the invention, the attachment means may be a screw-on mechanism, a press-fit mechanism, a weld, or some other attachment means that allows tool assemblies to be attached to other tool assemblies with controlled azimuthal alignments. 
         [0034]      FIG. 4A  shows an extension module  402  having a coaxial antenna  404 .  FIG. 4B  shows an extension module  406  having an angled recess  408  containing a tilted antenna  410 , thereby enabling azimuthally-sensitive resistivity measurements. Titled antenna  410  (and the recess  408 ) are preferably set at an angle of θ=45°.  FIG. 4C  shows an extension module  412  having two angled recesses  414 ,  418  with respective tilted antennas  416  and  420 . Providing multiple antennas in a single module may enable tighter spacing requirements to be satisfied and may enable more accurate differential measurements to be performed. 
         [0035]      FIG. 4D  shows an extension module  422  with a recess  424  and tilted antenna  426  at an azimuth 180° away from that of the antenna in  FIG. 4B . Extension module  422  may be designed to couple with the other modules in a manner that ensures this distinct alignment of antenna  426  relative to any other antennas such as those antennas in  FIGS. 4B-4C . Alternatively, the extension modules may be provided with a coupling mechanism that enables the antennas to be fixed at any desired azimuthal alignment, thereby making modules  406  and  422  equivalent. As yet another alternative, a multi-axial antenna module  428  may be provided as shown in  FIG. 4E  to enable virtual steering of the antenna alignment. Virtual steering involves the combination of measurements made by or with the different antennas  430 ,  432 , and  434 , to construct the measurement that would have been made by or with an antenna oriented at an arbitrary angle and azimuth. 
         [0036]    As described above, each tool module includes a recess around the external circumference of the tubular. An antenna is disposed within the recess in the tubular tool assembly, leaving no radial profile to hinder the placement of the tool string within the borehole. In some alternative embodiments, the antenna may be wound on a non-recessed segment of the tubular if desired, perhaps between protective wear bands. 
         [0037]      FIG. 5  shows the base module  102  of  FIG. 2 , coupled to an extension module  406  having a tilted antenna to enable azimuthally sensitive resistivity measurements that can be used to provide geosteering with respect to nearby bed boundaries. Details of suitable methods for determining distance and direction to nearby bed boundaries may be found in, e.g., U.S. Pat. No. 7,019,528, “Electromagnetic wave resistivity tool having a tilted antenna for geosteering within a desired payzone”, to Michael Bittar; and co-pending U.S. patent application Ser. No. ______ (Atty Dkt 1391-681.01), “Tool for Azimuthal Resistivity Measurement and Bed Boundary Detection”, also to Michael Bittar. 
         [0038]      FIG. 6  shows a modular resistivity/geosteering tool assembly made up of modules from  FIGS. 4A-4E . As may be readily perceived, the use of modules enables the ready construction of custom resistivity tools that can best exploit new logging and geosteering methods. Moreover, as antennas or electronics become damaged, the individual modules can be economically repaired or replaced, prolonging the useful life of the tool. 
         [0039]    Even more significant is the possibility of interspersing resistivity tool modules with other instruments or tubulars as shown in  FIG. 7 . In the assembly of  FIG. 7 , a tool such as a geosteering mechanism or other logging instrument  702  is positioned between resistivity tool modules. Such an arrangement enables deep resistivity measurements without requiring that the resistivity tool itself be excessively long. Moreover, this ability may enable portions of the resistivity tool to be located much closer to the drill bit, enabling earlier detection of approaching bed boundaries. 
         [0040]    In at least some embodiments, tool  702  is a stabilizer having adjustable blades in accordance with the disclosure in commonly assigned U.S. Pat. Nos. 5,318,137 and 5,318,138, the teachings of which are incorporated by reference herein. As disclosed in these patents, the inclination of the bottomhole assembly can be changed by selectively varying the extension of the stabilizer blades. As one skilled in the art will immediately recognize, the course of the drill bit also can be changed in accordance with other techniques, such as by selectively turning on or off a downhole motor, adjusting the angle of bend in a bent motor housing, or changing the weight on bit of the system. 
         [0041]    In some embodiments, the modular resistivity tool may be assembled in the field, e.g., at the well-site. Different tool assemblies may be created with different amounts of rotation of each tool module relative to other tool modules about the longitudinal axis. The capability to reconfigure an existing tool string allows collection of more data about the formation surrounding the borehole. Thus, more robust and sophisticated resistivity graphs for steering the drilling apparatus in the proper direction may be determined. The use of tool assemblies described above for the geosteering tool increases modularity, reliability, and reduces the cost of manufacturing, maintenance, design, reuse and replacement. 
         [0042]      FIG. 8  shows a block diagram of an illustrative embodiment for the electronics of the base and extension modules. When assembled, the various modules are coupled via a one-wire tool bus  802 . In some embodiments, a cable is run through the bore of the tools and manually attached to terminal blocks inside the tool modules as the tool is assembled. In some alternative embodiments, the tool bus cable passes through an open or closed channel in the tool wall and is attached to contacts or inductive couplers at each end of the module. As the modules are connected together, these contacts or inductive couplers are placed in electrical communication due to the geometry of the connection. For example, in a threaded box-and-pin connector arrangement, the box connector may include a conductive male pin held in place on the central axis by one or more supports from the internal wall of the module. A matching female jack may be similarly held in place on the central axis of the pin connector and positioned to make electrical contact with the male pin when the threaded connection is tight. An O-ring arrangement may be provided to keep the electrical connection dry during drilling operations. In systems requiring an empty bore, the electrical connector may be modified to be an annular connection in which a circularly-symmetric blade abuts a circular socket, again with an O-ring arrangement to keep the electrical connection dry. Other suitable electrical-and-mechanical connectors are known and may be employed. 
         [0043]    In the embodiments illustrated by  FIG. 8 , the tool bus  802  is inductively coupled to the module electronics via a transformer  804 . A power supply  806  extracts alternating current (AC) power from the tool bus and conditions the power for use by the other portions of the electronics. Bi-directional communication with the other modules is carried out by a modem  808  under control of controller  810 . Controller  810  operates in accordance with firmware and software stored in memory  812  to coordinate operations with other modules and to control a transmitter  814  and receiver  816  for each antenna  818 . When transmitting an electromagnetic signal into the formation, the controller provides a synchronization signal via the tool bus to the other modules. When operating as a receiver, the controller receives the synchronization pulse and begins digitizing and storing the received signal(s) in memory for later communication to the power and control module. 
         [0044]      FIG. 9  is an illustrative schematic of antenna  818 . Antenna  818  includes multiple coils of wire surrounding a central core  905 . Leads  910 ,  915 ,  920 ,  925  are attached to different coils to enable the transmitter or receiver electronics to change the number of effective turns in the coil. When an alternating current is applied to coil  818 , an electromagnetic field is produced. Conversely, an alternating electromagnetic field in the vicinity of antenna  818  induces a voltage at the leads. In this manner, antenna  818  may be used as to transmit or receive electromagnetic waves. 
         [0045]      FIG. 10A  shows a detail view of two partially assembled modules  402  and  412 . A hatch  1008  for the transmitter/receiver electronics of antenna  406  in module  402  can be seen, but the antenna itself cannot be seen in this view because it is protected by a layer of interleaved bands  1010  and  1012 . Bands  1012  are steel wear bars to protect the antenna from damage. To avoid having the steel wear bars  1012  suppress the antenna signal, they are oriented perpendicular to the plane of the antenna and interleaved with bands of insulating material  1010 . 
         [0046]    Antennas  416  and  420  of module  412  are shown supported in their respective recesses  414  and  418  by support blocks  1002  and  1004 . The space around the antennas will be filled with a support material and a protective structure will be placed over the antennas to provide wear resistance. Hatches  1006  for the transmitter/receiver electronics of antennas  416  and  420  are also visible. 
         [0047]      FIG. 10B  shows a first embodiment of a protective structure to be placed over the tilted antennas. The protective structure is a sleeve  1013  consisting of a tubular body  1014  having a pattern of windows  1016  arranged so as to be aligned with one or more tilted antennas. In some embodiments, the windows are substantially rectangular, with the edges nearest the antenna oriented generally perpendicular to the plane of the antenna. Mounting holes  1018  may be provided as a means to secure the cover to the tool body. Cover  1013  is made of materials that act as a rigid shell to protect the antennas. The tubular body  1014  may be formed a conductive or non-conductive material, and in at least some embodiments the tubular body consists of non-magnetic steel. Tubular body  1014  may be hard faced with, for example, tungsten carbide. Tubular body  1014  has open ends so that it can be slipped on and off the module body while allowing the module to be attached to other modules at either end. The shape, thickness, diameter, and length of tubular body  1014  may vary from one application to the next. The number of windows may vary from one application to the next, and the dimensions, spacing, and other characteristics of each window or each set of windows may vary from one application to the next. 
         [0048]    Mounting holes  1018  may be used to affix cover  1013  to the module body. As such, matching holes may be formed in the module and screws or other known means may be used to join cover  1013  to the module body. Such means may be in addition to a pressure fit, weld or other supplemental method of retaining cover  1013  in place. 
         [0049]      FIGS. 10C-10D  show two views of the protective covering  1013  in place on module  412 . For explanatory purposes, the covering  1013  is shown as a semi-transparent material to enable visualization of the relationship between the antennas  416 ,  420  and the windows  1016  cut into the protective covering  1102 . It is expected that covering  1013  will comprise steel or some other electrically conductive metal. Accordingly, windows  1016  are cut with edges perpendicular to the antennas  416 ,  420  to prevent induced currents in the protective covering  1013  from suppressing the antenna signal. 
         [0050]      FIG. 10C  shows a side view of the protective cover  1013  in place on tool module  412 . Tilted recesses  414 ,  418  and antennae  416 ,  420  underlie the patterns of windows  1016 . When properly mated, windows  1016  are aligned above and perpendicular to antennae  416 ,  418  around the circumference of module  412 .  FIG. 10C  further illustrates that in some embodiments antennae  416 ,  420  are tilted 45 degrees from the tool axis. 
         [0051]      FIG. 10D  shows a bottom view of the protective cover  1013  in place on tool module  412 . The bottom view illustrates an additional view of tilted recesses, tilted antennae, and windows arranged perpendicular to antennae  416 ,  420  around the circumference of module  412 . In  FIGS. 10C and 10D , hatches  1006  in tool module  412  are shown. A hermetically sealed cavity beneath each hatch contains electronics for transmitting and receiving signals via the corresponding antenna  416 ,  420 . The volume of recesses  414 ,  418  and the windows  1016  and other areas may be filled and sealed to prevent penetration of drilling fluid and other material. Suitable methods may include those described in U.S. Pat. No. 5,563,512. However, the sealant preferably does not substantially degrade the ability of windows  1016  to pass radiated and reflected energy. 
         [0052]    As an alternative to employing protective covering  1013 , the tilted antennas may be protected using interleaved wear bands  1012  like those shown in  FIG. 10A .  FIG. 11A  shows a resistivity tool  500  having a module  505  with a tilted recess  510  having a tilted antenna  515 . The recess has shoulders  525  for supporting the interleaved band structure  550  shown in  FIG. 11B . The structure comprises an insulating material  555  containing steel wear bars  560  oriented generally across the width of the structure. The insulating material  555  prevents the flow of currents that would suppress the antenna signal. 
         [0053]      FIG. 11C  shows a side view of another alternative cover  572  having a pattern of windows that aligns with the tilted antenna. Cover  572  comprises a band  574  having windows  576 . Cover  572  is supported by shoulders  525 , and perhaps additionally any antenna supports. Like windows  1016 , windows  576  are preferably aligned with and perpendicular to an antenna, in this case, antenna  515 . The materials used to form cover  572  and the dimensions of the cover and windows may vary from one implementation to the next as previously mentioned with regard to cover  1013  and windows  1016 . Likewise, windows  576  and other areas may be sealed to prevent penetration of drilling fluid and other material by any known method. Cover  572  may be affixed to segment  500  by any known method(s) of attachment, e.g., screws, compression, clamp(s). A gasket may be affixed to cover  572  or shoulders  525 . 
         [0054]      FIG. 11D  shows a front view of the cover  572 . The cover  572  may be cut from a flat sheet of steel and formed into a (tilted) cylindrical shape. After it has been fitted in the recess, a weld can be made along seam  582  to secure the cover in place. Tabs  578  may be provided to prevent rotation of the cover, and notches  580  may be provided to fit around access covers, securing hardware, or other tool elements. Note that window shapes need not be uniform in shape or size as indicated by window  584 . 
         [0055]      FIG. 11E  shows the cover  572  in place on a partially assembled logging tool to illustrate the relationship between the antenna  515  and the windows. Within a machined recess  588  are an electronics cavity  590  and various threaded holes for securing the electronics and a hatch. A matching recess  586  with additional threaded holes allows the hatch to be secured (beneath cover  572 ) across the width of the antenna recess, providing a wireway between the antenna and the electronics if desired. In practice the antenna will not be visible as the elliptical recess and the cover windows will be filled with some insulating material to support and protect the antenna. 
         [0056]    Once assembled, inserted in the borehole, and powered on, the resistivity/geosteering tool assembly fires its various transmitters in turn and collects measurements from each receiver. In some embodiments the base module includes orientation and position tracking hardware, while in other embodiments the base module accesses orientation and position information provided by another module. In still other embodiments, the base module forwards relevant measurements to another tool having access to position and orientation information. Although the following description of  FIG. 12  proceeds with the assumption that the base module performs the described actions, these actions may alternatively be carried out by one or other components of the system. 
         [0057]    In block  1202 , the expansion modules are coupled to the base module. In some embodiments, the expansion modules are simply threaded into the bottom hole assembly or tool string with the base module, and electrical contacts in the connectors establish the tool bus connection. Other suitable communication techniques are known and may be used. 
         [0058]    In block  1204 , the base module identifies each of the extension modules to which it is coupled. Each extension module preferably includes a preprogrammed unique identifier, along with some indication of the module type (e.g., transmitter, receiver, antenna orientation, and single or differential configuration) and version number to enable this identification process to be performed automatically by the base module. However, custom configuration or programming by a field engineer can also be used as a method for setting up the tool. 
         [0059]    Once the base module has completed the identification process, it initiates a clock synchronization procedure in block  1206 . To ensure measurement accuracy, the synchronization process may be repeated or refined before each measurement. In some embodiments, each module has its own high-accuracy clock and the base module merely determines the relative clock offset for each module using a request &amp; response process. For further refinement, the base module may also determine and track the rate of change of each clock offset. 
         [0060]    In block  1208 , the base module establishes the measurement parameters and communicates them to the relevant expansion modules. For example, the measurement parameters may specify the transmitter antenna, the desired frequency and power setting, and the desired firing time. (The desired firing time may be specified using a special trigger signal on the bus.) Where pulse signals are employed, the shape and duration of the pulse may also be specified. 
         [0061]    In block  1210 , the transmitter fires and the receivers measure phase and attenuation. These measurements are made relative to any one of several possible references. The phase may be measured relative to the individual clocks, relative to the phase of the transmit signal, or relative to the phase of a receive signal from another antenna. Similarly, the attenuation may be measured relative to a calibration value, relative to the specified transmit power setting or relative to the amplitude of a receive signal from another antenna. The base module communicates with each of the extension modules to collect the receiver measurements. Where an extension module transmitted the signal, an actual time of transmission may also be collected if that module measured it. 
         [0062]    In block  1212 , the base module determines the tool orientation and processes the phase and attenuation measurements accordingly. In some embodiments, the tool rotates as it collects measurements. The measurements are sorted into azimuthal bins and combined with other measurements from that bin. Measurement error can be reduced by combining measurements in this fashion. The base module processes the measurements to determine azimuthal and radial dependence of the measurements, and may further generate a geosteering signal by taking the difference between measurements at opposite orientations or between the measurements for a given bin and the average of all bins. 
         [0063]    In block  1214 , the base module optionally compresses the data before storing it in internal memory and/or providing the data to the telemetry transmitter to be communicated to the surface. In block  1216 , the base module determines if logging should continue, and if so, the operations repeat beginning with block  1206 . 
         [0064]    Although the foregoing description has focused on the use of azimuthally sensitive resistivity measurements to enable geosteering relative to bed boundaries, such measurements can also be used to provide additional well bores generally parallel to one or more existing well bores. The existing well bores may be filled with a fluid having a resistivity quite different from the surrounding formations. As the new well bore is drilled, the azimuthally sensitive resistivity tool enables the detection of direction and distance to the existing well bores. The accurate placement of generally parallel well bores enables the use of such techniques as steam-assisted gravity drainage (SAGD), in which steam is pumped from a first well bore into a formation to heat the formation, thereby increasing the fluidity of hydrocarbons. A second well bore then drains these hydrocarbons from the reservoir, significantly improving the reservoir&#39;s yield. 
         [0065]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. For example, it is expected that the disclosed tool construction methods may be employed in wireline tools as well as logging while drilling tools. In logging while drilling, the drill string may be wired or unwired drill pipe or coiled tubing. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.