Abstract:
A method and apparatus for collecting data downhole in a well bore, even during drilling operations, are disclosed. The apparatus generally comprises an antenna and some associated electronic circuitry. The antenna includes a plurality of arrayed transceiver elements and the electronic circuitry steers transmission or reception through the antenna by controlling the application of power to the array elements. In operation, a transceiver unit containing such an antenna is positioned proximate a remote sensor placed into a formation. An electromagnetic signal is then steered to communicate with the remote sensor over a wireless link.

Description:
[0001]    This application claims priority of provisional U.S. application No. 60/289,667 filed May 9, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to the drilling of wells, such as for the production of petroleum products, and, more particularly, the acquisition of subsurface formation data such as formation pressure, formation permeability and the like.  
           [0004]    2. Description of the Related Art  
           [0005]    In oil well description services, one part of the standard formation evaluation characteristics is concerned with the reservoir pressure and the permeability of the reservoir rock. Present day operations obtain these characteristics either through wireline logging via a “formation tester” tool or through drill stem tests. Both types of measurements are available in “open-hole” or “cased-hole” applications, and require a supplemental “trip”. A trip typically involves removing the drill string from the well bore, running a formation tester into the well bore to acquire the formation data, and, after retrieving the formation tester, running the drill string back into the well bore for further drilling. Because “tripping the well” uses significant amounts of expensive rig time, it is typically done under circumstances where the formation data is absolutely needed, during a drill bit change, or when the drill string is being removed for some other drilling related reason.  
           [0006]    On the other hand, during well drilling activities, the availability of reservoir formation data on a “real time” basis is a valuable asset. Real time formation pressure obtained while drilling will allow a drilling engineer or driller to make decisions concerning changes in drilling mud weight and composition as well as penetration characteristics at a much earlier time to thus promote the selected aspects of drilling. The availability of real time reservoir formation data is also desirable to enable precision control of drill bit weight in relation to formation pressure changes and changes in permeability so that the drilling operation can be carried out with greater efficiency.  
           [0007]    It is therefore desirable to acquire various formation data from a subsurface zone of interest while the drill string is present within the well bore. This eliminates or minimizes the need for tripping the well solely to run formation testers into the well bore to identify formation characteristics such as pressure, temperature, permeability, etc. One such technique is disclosed in U.S. Pat. No. 6,028,534, issued to Schlumberger Technology Corporation on Feb. 22, 2000, as assignee of the named inventors Ciglenec, et al. This patent is commonly assigned herewith. In this technique, a remote sensor containing sensor instrumentation and associated electronics is deployed into a formation. The remote sensor also contains an antenna and a battery to communicate with a host antenna on the drill collar while the drill string is in the well bore. Once deployed, the remote sensor measures one or more of the formation&#39;s characteristics. After the measurement is complete, the data is stored in the remote sensor. A wireless communication channel is subsequently established between the remote sensor and the drill collar for data transfer.  
           [0008]    The data transfer will typically occur, at least some of the time, during drilling operations. The drill string in which the drill collar is installed will both rotate and translate during drilling operations. However, the remote sensor, by virtue of its deployment into the formation, will neither translate nor rotate in any significant sense. Thus, during the data transfer, there may be rotational as well as translational movement of the collar antenna with respect to the remote antenna. Consequently, there frequently are two main aspects in regards to electromagnetic coupling between the collar antenna and the remote antenna—locating the remote sensor, and maintaining the communication channel for the entire data transfer once the remote sensor has been found.  
           [0009]    The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention includes a method and apparatus for collecting data downhole in a well bore, even during drilling operations. The apparatus generally comprises an antenna and some associated electronic circuitry. The antenna includes a plurality of arrayed transceiver elements and the electronic circuitry steers transmission or reception through the antenna by controlling the application of power to the array elements. In operation, a transceiver unit containing such an antenna is positioned proximate a remote sensor placed into a formation. An electromagnetic signal is then steered to communicate with the remote sensor over a wireless link. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:  
         [0012]    [0012]FIG. 1 is a diagram of a drill collar positioned in a borehole and equipped with a steerable transceiver unit in accordance with the present invention;  
         [0013]    [0013]FIG. 2 is a schematic illustration of the steerable transceiver unit of the drill collar of FIG. 1 showing a hydraulically energized system for emplacing a remote sensor from the borehole into a selected subsurface formation;  
         [0014]    [0014]FIG. 3 schematically diagrams the electronic circuitry of the steerable transceiver unit of the drill collar of FIG. 1 for receiving data signals from and transmitting signals to the remote sensor;  
         [0015]    [0015]FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D illustrate alternative implementations of the arrayed antenna of the steerable transceiver unit of FIG. 3;  
         [0016]    [0016]FIG. 5A and FIG. 5B illustrate in electronic block diagrams the electronics employed to steer the transmission from and reception by the transceiver of FIG. 3 in accordance with the present invention in two different implementations;  
         [0017]    [0017]FIG. 5C illustrates in an electronic block diagram a portion of the electronic controls in the embodiment of FIG. 5B;  
         [0018]    [0018]FIG. 6 is an electronic block diagram schematically illustrating the electronics of a remote sensor;  
         [0019]    [0019]FIG. 7 is a block diagram conceptually illustrating operation of the steerable transceiver unit in conjunction with the remote sensor in accordance with the present invention; and  
         [0020]    [0020]FIG. 8 graphs the measured voltage received by the remote sensor upon the introduction as a function of coil location in one particular implementation.  
     
    
       [0021]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0023]    [0023]FIG. 1 depicts one particular embodiment of a drill collar  100 . The drill collar  100  comprises but one component of a drill string (not otherwise shown) for drilling a well bore  105 . The drill collar  100  is provided with a sonde section  110  including a power cartridge  200 , shown in FIG. 2, incorporating the transmitter/receiver circuitry  300  of FIG. 3. As shown in FIG. 2, the drill collar  100  includes a pressure gauge  205  whose pressure sensor  210  is exposed to borehole pressure in the well bore  105  via a drill collar passage  215 . The pressure gauge  205  senses ambient pressure at the depth of a selected subsurface formation and is used to verify pressure calibration of remote sensors. Electronic signals (not shown) representing ambient well bore pressure are transmitted via the pressure gauge  205  to the circuitry of the power cartridge  200 . The power cartridge  200  then performs a pressure calibration of a remote sensor  115 , shown best in FIG. 1, being deployed at that particular well bore depth.  
         [0024]    The drill collar  100  is also provided with one or more remote sensor receptacles  120 , also shown in FIG. 1. Each sensor receptacle  120  contains a remote sensor  115  for positioning within a selected subsurface formation of interest intersected by the well bore  105 . As will be discussed further below, the remote sensor  115  is positioned, in this particular embodiment, while the well bore  105  is being drilled. Note, however, that the remote sensor  115  may be previously emplaced and used in conjunction with the steerable transceiver unit of the present invention. In such embodiments, efforts will typically need to be made to identify the location of the remote sensor  115 , as is discussed more fully below.  
         [0025]    The remote sensors  115  are encapsulated “intelligent” sensors that are moved from the drill collar  100  to a position within the formation surrounding the well bore  105 . The remote sensors  115  sense formation characteristics such as pressure, temperature, rock permeability, porosity, conductivity, and dielectric constant, among others. The remote sensors  115  are appropriately encapsulated in a sensor housing of sufficient structural integrity to withstand damage during movement from the drill collar  100  into laterally embedded relation with the subsurface formation surrounding the well bore  105 .  
         [0026]    [0026]FIG. 1 illustrates a single remote sensor  115  embedded in a formation in a roughly perpendicular orientation relative to the well bore  105  and, hence, the drill collar  100 . Those skilled in the art having the benefit of this disclosure will appreciate that such lateral embedding movement need not be perpendicular to the well bore  105 , but may be accomplished through numerous angles of attack into the desired formation position. Sensor deployment can be achieved utilizing one or more of the following: (1) drilling into the borehole wall  125  and placing the remote sensor  115  into the formation; (2) punching/pressing the encapsulated remote sensors  115  into the formation with a hydraulic press or other mechanical penetration assembly; or (3) shooting the remote sensors  115  into the formation by utilizing propellant charges. Any of these techniques are suitable, depending on the implementation. For instance, although the illustrated embodiment uses a hydraulic mechanism (discussed more fully below), an alternative embodiment emplaces the remote sensor  115  ballistically.  
         [0027]    [0027]FIG. 2 illustrates a hydraulically energized ram  220  employed for this purpose in the illustrated embodiment. The ram  220  deploys the remote sensor  115  and causes its penetration into the subsurface formation to a sufficient position outwardly from the borehole  130  of the well bore  105  so that it can sense selected characteristics of the formation. For sensor deployment, the drill collar  100  is provided with an internal cylindrical bore  222  within which is positioned a piston element  225  having the ram  220  disposed in driving relation with the encapsulated remote intelligent sensor  115 . The piston element  225  is exposed to hydraulic pressure communicated to a piston chamber  230  from a hydraulic system  235  via a hydraulic supply passage  240 . The hydraulic system  235  is selectively activated by the power cartridge  200  so that the remote sensor  115  can be calibrated with respect to ambient borehole pressure at formation depth, as described above. The remote sensor  115  can then be moved from the receptacle  120  into the formation beyond the borehole wall  125  so that formation pressure characteristics will be free from borehole effects.  
         [0028]    Referring now to FIG. 3, the power cartridge  200  of the drill collar  100  includes a transceiver unit  305  driven by a transceiver power drive  310  (e.g., a power amplifier) at a frequency determined by an oscillator  315 . The transceiver unit  305  will receive signals that will be transmitted to the sonde section  110  of the drill collar  100  by the remote sensor  115  as will be explained hereinbelow. Note that the 2:1 ratio is not necessary to the practice of the invention, and that other ratios may be employed. The transceiver unit  305  includes an arrayed antenna  325  and one or more transceivers  330 , depending on the implementation, which are also discussed more fully below.  
         [0029]    [0029]FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D illustrate different implementations of the arrayed antenna  325  of the power cartridge  200 . Each of these implementations employs multiple transceiver elements  400 . In the implementation of FIG. 4A, each transceiver element  400  comprises a coil  405  wound upon a ferrite core  410  in a groove  415  in the ferrite core  410 . FIG. 4B illustrates an implementation wherein multiple coils  420  are arrayed upon a flexible insulating board  425  that can be wrapped around the interior of the drill collar  100 . Power is supplied to each coil  420  by a respective feed terminal  430 . The coils  420  may be of any shape known to the art. The embodiment of FIG. 4C is much the same as the embodiment of FIG. 4B, except that the circular coils  420  are replaced byspiral coils  435 . Note that the coils  420 ,  435  may be replaced by coils of virtually any shape or type in alternative embodiments. FIG. 4D illustrates an implementation wherein multiple slot antennae  440  are arrayed in a metal sheet  445 . The metal sheet  445  may be conformal to the drill collar  100 . Note that each of these implementations may be generically referred to as arrayed transceiver elements including (i.e., the coils  405 , the coils  420 , the coils  435 , and the slot antennae  440 , respectively) since they can all be used in both transmitting and receiving signals. Note also that various aspects of the embodiments in FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D may be combined in some embodiments, such as that shown in FIG. 5B and discussed below.  
         [0030]    The array elements  400  may be configured in series, in parallel, or in a combination of in series and in parallel, depending on the implementation. This configuration can be hardwired or controlled by the transceiver power drive  310 , as discussed further below in connection with FIG. 5A, FIG. 5B. The implementations of the transmitter/receiver elements illustrated in FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D are highly scalable in the axial direction. However, certain difficulties arise in exciting an axially long antenna including several coils or slots. In particular, the power requirement can become a limiting factor. This difficulty can be overcome by exciting only a subset of coils or slots at a given time in applications requiring large axial coverage. This design constraint can be realized in a manner discussed more fully below in connection with FIG. 5A, FIG. 5B.  
         [0031]    Note that the configuration of the array antenna  325  affects the generation and propagation of the electromagnetic field. Consider the embodiment of FIG. 4A, which will produce an axisymmetric electromagnetic field. Assuming that each coil has the same amount of current (this is not necessary to the practice of the invention, but is being considered only to explain the method), the objective is to find the direction p k  of current in each coil and the corresponding location d k  that maximize the functional, J(p,d), which is a measure of homogeneity and amplitude of magnetic field  
                 J        (     p   ,   d     )       =         C   1            ∫   v                     B   0          (     r   ,   z     )            2                        v           +       C   2          (       C   3     -       ∫   v              (         ∂            B   0          (     r   ,   z     )                             ∂   z       )     2                        v           )           ,           (   1   )                               
  p:={p   0   , p   1   , . . . , p   N   };p   k ∈ {−1,1}   
           d:={d   0   , d   1   , . . . , d   N   };d   k ∈         , 
         [0032]    where the first term on the right-hand side of Eq. (1) maximizes the mean value of the field and the second term inside the bracket ensures uniformity along the axial direction. From Eq. (1) it is clear that while the distance (d) between coils  405  is a continuous variable, the direction of current (p) flowing through them can assume only two values—positive or negative.  
         [0033]    As a result, conventional optimization techniques are either ineffective or inefficient. One particular implementation uses a genetic algorithm employing a random search method and works with discreet variables for optimization. Commercially available genetic algorithms, such as the GEATbx genetic and evolutionary algorithm toolbox for use with MATLAB™, are suitable. Positive constants C 1  and C 2  are selected to give appropriate weight to each term. Since the genetic algorithm maximizes the objective function, a sufficiently large positive constant C 3  is chosen to make the term in the bracket a positive quantity. These constants are empirically selected, primarily based on the requirements of electronic circuits and the signal to noise ratio. For instance, there is a lower limit on the signal level that can be detected by the receiver. Furthermore, there is also a limit on the ability of an electronic circuit to withstand the fluctuation in signal level when the tool moves. These criteria give an idea of the constants that can be used in the formulation. Generally speaking, the values for these constants should range between about 1 and 10, inclusive. In Eq. (1) the number of the coils  405 , configured in series in the illustrated embodiment, is represented by N. The space v represents the region over which the spatial variation of the field is optimized.  
         [0034]    During the optimization process, for each parameter set (p, d), the magnetic field is computed at every point in the space v. This involves first generating a base field for a single coil  405  and then using superposition to compute the total field for the entire arrayed antenna  325 . Although superposition may not hold exactly, it is a reasonable assumption for low-frequency applications if the space v is not too close to the drill collar  100 . The total field computation by superposition greatly reduces the computation time. Once an “optimal” set (p, d) is obtained, a finite element method (“FEM”) analysis of the whole arrayed antenna  325  is performed. The embodiment illustrated in FIG. 4A achieves a uniform field whose magnitude depends largely on the availability of power, which is quite restricted for the environment in which well-logging tools usually operate. Note that a well-logging tool often operates at a typical depth of about 5 kilometers and has to withstand high temperature (around 175° C.) and high pressure (typically 20,000 psi) environment.  
         [0035]    Consider now the embodiment of FIG. 4B, the objective function in Eq. (1) is modified to 
           J ( p,d )= C   3 −∫ v   |B   0 ( r,z )− {tilde over (B)}   0 ( r,z )| 2   dv,   (2)  
         [0036]    where {tilde over (B)} 0 (r,z) is the desired spatial variation of the field. These fields may be normalized for ease in computation. As mentioned before, the spatial variation of the electromagnetic field may be required to change during the data transfer process to keep bullet and collar antenna in constant communication. As a result there will be several sets of {tilde over (B)} 0 (r,z) and hence, (p,d), the direction of current and position of a coil. These can be computed and stored for real-time application.  
         [0037]    [0037]FIG. 5A and FIG. 5B illustrate how the antenna array elements  500  can be excited individually to produce a desired spatial distribution of electromagnetic fields. Furthermore, these spatial distributions can be changed in real time to keep the collar  100  and the antenna  605  of the remote sensor  115  in constant communication when there is a relative motion between the two antennas. In FIG. 5A, the antenna array  505  is axisymmetric, e.g., such as the embodiment of FIG. 4A discussed above. For this case, only axial controllability of the arrayed antenna  505  is required. That is, given the relative position of the drill collar  100  and the antenna  605  of the remote sensor  115 , an appropriate subset of the array elements  500  may be energized using an array  510  of switchable elements, such as switches,  515 . FIG. 5B illustrates the general case for driving the array elements  500 . Here, both angular as well as axial controllability of excitation can be achieved. Note that, in FIG. 5A, the array elements  500  are configured in series. However, in both FIG. 5A and FIG. 5B, the individual array elements can be connected together in series or in parallel or a combination thereof, with some (slight) modification to the embodiment of FIG. 5A  
         [0038]    More particularly, and referring first to FIG. 5A, the array elements  500  produce an axisymmetric electromagnetic field, and so only axial control is needed, as was mentioned above. Note that the array elements  500  are coils wound upon a ferrite core (not shown) as in the embodiment of FIG. 4A. The switches  515  provide this control by controlling power to the individual array elements  500 . The switches  515  are, in turn, controlled by electronic control logic  520 . Once the control logic  520  determines the pattern in which it wishes to vary the field, it opens and closes the switches  515  to affect the variations. This determination may be made in real-time, or may be predetermined. For instance, if the primary concern is providing adequate power to all of the array elements  500 , the switches  515  may be operated to provide power to them sequentially in series. If the concern is to obtain data from multiple remote sensors  115  through some, but not all, of the array elements, additional selectivity can be shown in which array elements  500  receive power.  
         [0039]    Turning now to FIG. 5B, the embodiment shown therein includes the array elements  500 , the array  510  of switches  515 , and the control logic  520  of the embodiment in FIG. 5A. The embodiment of FIG. 5B exerts axial control over the electromagnetic field in the same manner as described above for the embodiment of FIG. 5A. However, the embodiment of FIG. 5B also includes several layers of array elements  525 . The array elements  525  may be coils formed in a sheet  530  conformed to the internal surface of the drill collar  100 , as in the embodiments of FIG. 4B and FIG. 4C discussed above. Power to the array elements  525  is controlled by a second array  535  of switches  540 , whose operation is controlled by the control logic  545 . Note that in some embodiments, the functionality of the control logic  545  can be combined with that of the control logic  520  to create a single control logic block. Once the control logic  545  determines the pattern in which it wishes to vary the field, it opens and closes the switches  540  to effect the variations. This determination may be made in real-time, or may be predetermined, as in the case of axial variations. Note that these particular embodiments employ a single transceiver  550  for axial control and a plurality of transceivers  555 , one for each level of array elements  525 , for angular control.  
         [0040]    Thus, in the embodiment of FIG. 5B, the antenna  560  consists of an array of magnetic loop antennas  500 ,  525 , each of which is an “array element” as was discussed above. The loops  525  provide a field mainly in a radial or angular orientation and the loops  500  provide a field mainly in an axial direction. The loops  500 ,  525  can be configured by the switches  515 ,  540  thought the control logic  520 ,  545 . In this manner, virtually any combination of loops  500 ,  525  can operate in series or in parallel, be switched on or off, or operate with reversed polarity. The antenna  560  can therefore be configured to maximize a field in a particular direction and will be more sensitive to receive a field from that particular direction.  
         [0041]    Note again that, for the axial loops  500 , there is only the one transceiver  550 , but that for the radial loops  525  there are several transceivers  555 . Depending on the position of the collar antenna  560  relative to the remote sensor  115 , the radial loops  525  are driven with a different signal amplitude. To measure the relative position of the collar antenna  560  to the antenna  606  (shown in FIG. 6) of the remote sensor  115 , the antenna  605  of the remote sensor  115  sends short tones whenever the collar antenna  560  stops transmitting. The collar transceivers  555  detect these tones. As shown in FIG. 5C, each collar transceiver  555  consists of a transmitter  565 , a receiver  570 , and a duplexer  575 . The transceivers  555  detect the tones through the receivers  570  via the arrayed antenna  560  (shown in FIG. 5B) and the duplexer  575 , which are then forwarded to the processor  580 . The processor  580  is a digital signal processor (“DSP”) in the illustrated embodiment.  
         [0042]    The processor  580  then calculates the position of the remote sensor  115  using a triangulation technique. This position information is used for a proper switch selection in an initial configuration phase and later for calculating the amplitude of the different radial loops  525 . The transmitter  565  consists of a power amplifier. The power amplifier  585  is driven from a programmable oscillator (not shown). The supply voltage (also not shown) of each power amplifier  585  is also programmable. The output amplitude of each transceiver  555  is programmed via the supply voltage. For better efficiency, the supply voltage is generated with programmable switching supplies. The output amplitude of the transmitters  565  is varied via the supply voltage by pulse width modulation (“PWM”) to steer the signal in the right direction by a superposition fields of different amplitudes generated by several separately driven transmitter loops.  
         [0043]    With reference to FIG. 6, the electronic circuitry of the remote “smart sensor”  115  is shown by a block diagram generally at  600  and includes at least one transmitter/receiver coil  605  or RF antenna, with the receiver thereof providing an output  610  from a detector  615  to a controller circuit  620 . The controller circuit  620  is provided with one of its controlling outputs  625  being fed to a pressure gauge  630  so that gauge output signals will be conducted to an analog-to-digital converter (“ADC”)/memory  635 , which receives signals from the pressure gauge via a conductor  640  and also receives control signals from the controller circuit  620  via a conductor  645 . A battery  650  is provided within the remote sensor circuitry  600  and is coupled with the various circuitry components of the sensor  115  by power conductors  655 ,  670  and  675 . A memory output  680  of the ADC/memory circuit  635  is fed to a receiver coil control circuit  685 . The receiver coil control circuit  685  functions as a driver circuit via conductor  690  for transmitter/receiver coil  605  to transmit data to the transmitter/receiver circuitry  300 .  
         [0044]    Throughout the complete transmission sequence, the transceiver unit  305 , shown in FIG. 3, is also used as a receiver. When the amplitude of the received signal is at a maximum, the remote sensor  115  is located in close proximity for optimum transmission between the drill collar  100  and the remote sensor  115 .  
         [0045]    Turning to FIG. 7, in operation, once the remote sensor  115  is emplaced, it begins collecting data. In one particular embodiment, the remote sensor  115  includes a timer that periodically initiates a power up of the electronic circuitry  600  (shown in FIG. 6). The remote sensor  115  then acquires data, stores it in the ADC/memory  635 , and goes back to sleep. When the arrayed antenna  325  is aligned with the antenna  605  of the remote sensor  115 , the collar transmitter  700 , which contains a power amplifier (not shown), sends a wakeup tone to the remote sensor  115  through the arrayed antenna  325 . The wakeup tone is transmitted at a frequency close to the resonant frequency of the remote sensor  115 . The remote sensor  115  receives the tone through its antenna  605  if the arrayed antenna  325  is close enough, detects the received signal through the receiver wakeup electronics  705 , and wakes up if the signal is of the right frequency. The remote sensor  115  then sends an acknowledge signal to the collar transmitter  700  and waits to receive a command.  
         [0046]    When awakened by the collar transmitter  700 , the remote sensor  115  is capable of receiving and executing a number of commands, such as acquire data, transmit data, memory read, and memory write. Most commonly, the collar transmitter  700  will instruct the remote sensor  115  to transmit data. The remote sensor  115  transmits measurement data from the transmitter  710  through the antenna  605  to the transmitter/receiver circuitry  300  and goes back to sleep. The receiver  715  in the transmitter/receiver circuitry  300  amplifies, demodulates and decodes the data. A duplexer  720  in the collar electronics protects the receiver  715  in the collar  100 . The arrayed antenna  325  in the collar  100  is tuned in resonance to the transmit frequency of the remote sensor  115 . The transmitter/receiver circuitry  300  also contains, in addition to the resonance frequency tuning circuit  725 , the array of switches (shown in FIG. 5A, FIG. 5B) for the selection of the active antenna array elements  400  and their polarity.  
         [0047]    More particularly, the drill collar  100  is positioned in close proximity of the remote sensor  115 . In some implementations, the drill collar  100  is actually used to emplace the remote sensor  115 , in which case the drill collar  100  will be proximate the remote sensor  115 . If the remote sensor  115  was previously emplaced, its location may be determined from records regarding its emplacement. As a last resort, the transceiver unit  305  can be used to locate the remote sensor  115  by bobbing the drill collar  100  in the well bore  105 . An electromagnetic wave is transmitted from the transmitter/receiver circuitry  300  in the drill collar  100  to ‘switch on’ the remote sensor  115  and to induce the remote sensor  115  to send back an identifying coded signal. This “handshaking” process can be used to identify the location of the remote sensor  115 , since the receipt of the handshaking signal from the remote sensor  115  will indicate the drill collar  100  is positioned sufficiently proximate to the location of the remote sensor  115 .  
         [0048]    The location of the remote sensor  115  must then be tracked once the location is identified. Communication between the drill collar  100  and the remote sensor  115  will typically occur during drilling operations, although this is not necessary to the practice of the invention. There typically will therefore be some degree of translational and rotational movement of the transceiver unit  305  relative to the remote sensor  115 , and this movement should be tracked. This can be accomplished by identifying the array elements  400 , or groups of array elements  400 , receiving the handshaking signal from the remote sensor  115  over time, assuming the array elements  400  are arrayed both axially and angularly. From this information, the relative positions of the transceiver  305  elements and the remote sensor  115  can be extrapolated. Once the relative positions are extrapolated, control logic (e.g., the control logic  520 ,  545  in FIG. 5A and FIG. 5B) can determine how it wishes to vary the electromagnetic field generated by the transceiver unit  305  to maintain continuous contact between the drill collar  100  and the remote sensor(s)  115 .  
         [0049]    The distance from the well bore  105  that the remote sensor  115  is emplaced into the formation should also be determined. One approach to this determination is to triangulate from the phase differences of the handshaking signal as received at three or more of the array elements  400 . Note that the distance determination should follow the location identification.  
         [0050]    One advantage to identifying the location and emplacement distance of the remote sensor  115  and tracking its location is that it permits the transceiver unit to focus the electromagnetic field that it generates in the direction of the remote sensor  115 . This advantage will not be appreciated in embodiments where the arrayed antenna  315  generates an axisymmetric field, however, since the field is—by definition—axisymmetric. Thus, the realization of this advantage is not necessary to the practice of the invention. This is true even in embodiments where the generated electromagnetic field is not axisymmetric. Note, however, that in embodiments where this is performed, it is performed in real time and can be implemented during drilling operations.  
         [0051]    The handshaking process initiates the electronics of the remote sensor  115  to go into the acquisition and transmission mode, and pressure data and other data representing selected formation characteristics, as well as the sensor&#39;s identification code, are obtained at the level of the remote sensor  115 . Note that, in some embodiments, the remote sensor  115  might continuously acquire data even while in a sleep state, such that it will enter a transmission mode only, on awaking. At the same time pressure gauge data (pressure and temperature) and other selected formation characteristics are acquired and the electronics of the remote sensor  115  convert the data into one or more serial digital signals. This digital signal or signals, as the case may be, is transmitted from the remote sensor  115  back to the drill collar  100  via the transmitter/receiver circuitry  300 . This is achieved by synchronizing and coding each individual bit of data into a specific time sequence. Data acquisition and transmission, or at least transmission (depending on the embodiment), is terminated after stable pressure and temperature readings have been obtained and successfully transmitted to the on-board circuitry of the drill collar  100 .  
         [0052]    Whenever the sequence above is initiated, the transmitter/receiver circuitry  300  located within the drill collar  100  is powered by the transceiver power drive  310 . An electromagnetic wave is transmitted from the drill collar  100  at a frequency determined by the oscillator  315 . The frequency can be selected within the range from 100 KHz up to 500 MHz. As soon as the remote sensor  115  comes within the zone of influence of the transmitter/receiver circuitry  300 , the receiver coil  605  located within the remote sensor  115  will radiate back an electromagnetic wave at twice the original frequency by means of the receiver coil control circuit  685  and the coil  605 .  
         [0053]    In one particular implementation of the embodiment in FIG. 4A, an eight-turn coil was designed. A base field was generated using a commercially available FEM code. The FEM code was validated by solving a simpler geometry using an analytical solution for coil response in a cylindrically layered medium. The field was optimized along a line extending from −10 to 10 inches in the axial direction (z) at y=7 inches. The orientation and position of each coil are shown in FIG. 8 which also gives measured voltage received by the antenna when 1 milliampere current is passed through a coil inside the formation with conductivity σ F =5.55 S/m (ρ=0.18 Ω-m). The plot indicates good uniformity of the voltage (and hence, the magnetic field) in the optimization region.  
         [0054]    This concludes the detailed description of particular embodiments. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and that such variations are considered within the scope of the invention as claimed. Accordingly, the protection sought herein is as set forth in the claims below.