Patent Publication Number: US-8981780-B2

Title: Dipole locator using multiple measurement points

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/844,886, filed Jul. 28, 2010, now U.S. Pat. No. 8,497,684, which is a continuation of U.S. patent application Ser. No. 11/382,644, filed May 10, 2006, now U.S. Pat. No. 7,786,731, which claims the benefit of U.S. Provisional Patent Application No. 60/728,066, filed Oct. 19, 2005 and U.S. Provisional Patent Application No. 60/680,780, filed May 13, 2005, the contents of which are incorporated fully herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of locating underground objects, and in particular to locating and tracking a beacon within the field of operation of a horizontal drilling machine. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a receiver system for identifying a location of a magnetic field source. The receiver system comprises a frame, a first antenna assembly supported by the frame, a second antenna assembly supported by the frame, and a processor. Each antenna assembly is adapted to detect the magnetic field from the source. The processor is adapted to receive an antenna signal from each of the antenna assemblies and to determine a location of the source relative to the frame using the antenna signals. 
     The present invention is also directed to method for tracking a below ground source of a magnetic field. The method comprises simultaneously detecting in three dimensions a magnetic field from a source at each of at least two distinct points of a receiver frame. The method further comprises the step of determining a location of the source relative to the receiver frame using the detected field values. 
     The present invention is further directed to a horizontal directional drilling system. The system comprises a drilling machine, a drill string, a downhole tool assembly and a receiver assembly. The drill string is operatively connected to the drilling machine. The downhole tool assembly is supported at a downhole end of the drill string and comprises a magnetic field transmitter. The receiver assembly comprises a frame, at least a first and second antenna assembly, and a processor. The first and second antenna assemblies are supported by the frame and adapted to detect a magnetic field from the magnetic field transmitter. The processor is adapted to receive an antenna signal from each of the antenna assemblies and to determine a location of the magnetic field transmitter relative to the frame using the antenna signals. 
     Further still, the present invention includes a method for drilling a horizontal borehole. The method comprises placing a receiver assembly, comprising a plurality of antenna assemblies, on the ground in proximity of a drill bit, wherein each antenna assembly comprises triaxial antennas and aligning the receiver assembly with a desired bore path. The drill bit is advanced forward without rotation to perform a steering correction in the horizontal plane. An orientation of the drill bit relative to the receiver assembly and a distance of forward advance of the drill bit without rotation are transmitted from the receiver assembly to the operator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a horizontal directional drilling system for drilling a horizontal borehole and a tracking system built in accordance with the present invention. 
         FIG. 2  is a perspective view of a receiver assembly constructed in accordance with the present invention. 
         FIG. 3  is a perspective, partially cut-away view of a support structure for an antenna assembly for use with the present invention. 
         FIG. 4  is a perspective, partially cut-away view of the antenna assembly from  FIG. 3 . 
         FIG. 5  shows an alternative embodiment for an antenna assembly for use with the present invention. 
         FIG. 6  is a block diagram of a portable area monitoring system constructed to detect and process signals emanating from a boring tool. 
         FIG. 7  is a geometric representation of the relationship between the antenna arrangements of a receiver built in accordance with the present invention. 
         FIG. 8  is a geometric representation of the relationship between a transmitter and the antenna arrangements of a receiver built in accordance with the present invention. 
         FIG. 9  is representative visual display for a preferred embodiment of the present invention. 
         FIG. 10  is a graphical representation of total magnetic field readings from a transmitter as detected by a receiver in the y-z plane. 
         FIG. 11  is a graph showing the field readings of  FIG. 9  in the y-z plane. 
         FIG. 12  is an illustration of flux lines radiating from a transmitter, as depicted in the x-y plane. 
         FIG. 13  is a geometrical representation of the relationship between a transmitter and a tilted receiver. 
     
    
    
     BACKGROUND OF THE INVENTION 
     The horizontal directional drilling (HDD) industry traditionally uses walk-over tracking techniques to follow the progress of a bore, to find the surface location immediately above the drill bit, and to determine the depth of the drill bit from that surface location. The primary tracking tools are a subsurface transmitter and a hand-carried surface receiver. The transmitter, located in or very near a boring tool, generally emits a magnetic dipole field created by a single coil dipole antenna. The transmitted dipole field can be used for both location and communication with the above ground receiver. 
     Conventional receivers often contain an arrangement of three antennas mounted in each of the three Cartesian axes. When the antenna arrangement senses the dipole field, the output of each antenna is proportional to the magnitude of the magnetic flux density as detected along the axis of the particular antenna. The signals from the antennas are mathematically resolved to provide information about the relative location of the boring tool. The process of locating the dipole, and thus the boring tool, currently involves two steps: determining its location along the z-axis (fore and aft) and then along the y-axis (left and right). One skilled in the art will appreciate a receiver can locate a transmitter in the fore-aft direction (along the axis) using the amplitude and phase of the transmitter&#39;s generated horizontal and vertical field components as measured in the vertical plane normal to the surface and extending through the transmitter axis (the x-z plane). A receiver can also determine the location of a single transmitter in the left-right directions using the amplitude and phase of the dipole field in the horizontal plane (the y-z plane). However the left-right determination can only be used either in front of or behind the transmitter because there is no y component to the dipole field when the receiver is directly above the transmitter (such that z=0). There is currently no satisfactory method of simultaneously locating the transmitter in both the fore-aft and left-right directions with an antenna arrangement positioned directly over the transmitter. 
     DESCRIPTION OF THE INVENTION 
     With reference now to the drawings in general, and  FIG. 1  in particular, there is shown therein a horizontal directional drilling system (“HDD”) system  10  for use with the present invention.  FIG. 1  illustrates the usefulness of horizontal directional drilling by demonstrating that a borehole  12  can be made without disturbing an above-ground structure, namely a roadway or walkway as denoted by reference numeral  14 . To cut or drill the borehole  12 , a drill string  16  carrying a drill bit  18  is rotationally driven by a rotary drive system  20 . When the HDD system  10  is used for drilling a borehole  12 , monitoring the position of the drill bit  18  is critical to accurate placement of the borehole and subsequently installed utilities. The present invention is directed to a system  22  and method for tracking and monitoring a downhole tool assembly  24  during a horizontal directional drilling operation. 
     The HDD system  10  of the present invention is suitable for near-horizontal subsurface placement of utility services, for example under the roadway  14 , building, river, or other obstacle. The tracking system  22  for use with the HDD system  10  is particularly suited for providing an accurate three-dimensional locate of the downhole tool assembly  24  from any position above ground. The locating and monitoring operation with the present tracking system  22  is advantageous in that it may be accomplished in a single operation. The present invention also permits the position of the downhole tool assembly  24  to be monitored without requiring the tracking system  22  to be moved towards the transmitter  32  or to be placed directly over a transmitter in the downhole tool assembly. These and other advantages associated with the present invention will become apparent from the following description of the preferred embodiments. 
     With continued reference to  FIG. 1 , the HDD system  10  comprises the drilling machine  28  operatively connected by the drill string  16  to the downhole tool assembly  24 . The downhole tool assembly  24  preferably comprises the drill bit  18  or other directional boring tool, and an electronics package  30 . The electronics package  30  comprises a transmitter  32  for emitting a signal through the ground. Preferably the transmitter  32  comprises a dipole antenna that emits a magnetic dipole field. The electronics package  30  may also comprise a plurality of sensors  34  for detecting operational characteristics of the downhole tool assembly  24  and the drill bit  18 . The plurality of sensors  34  may generally comprise sensors such as a roll sensor to sense the roll position of the drill bit  18 , a pitch sensor to sense the pitch of the drill bit, a temperature sensor to sense the temperature in the electronics package  30 , and a voltage sensor to indicate battery status. The information detected by the plurality of sensors  34  is preferably communicated from the downhole tool assembly  24  on the signal transmitted by the transmitter  32  using modulation or other known techniques. 
     With reference now to  FIG. 2 , shown therein is a preferred embodiment of the tracking system  22  of the present invention. The tracking system  22  comprises a receiver assembly  36 . The receiver assembly  36  comprises a frame  38 , a computer processor  40 , and a plurality of antenna arrangements  42  supported by the frame. The processor  40  is supported on the frame  38  and operatively connected to the plurality of antenna arrangements  42 . The frame  38  is preferably of lightweight construction and capable of being carried by an operator using a handle  44 . In a preferred embodiment, the receiver assembly  36  also comprises a visual display  46  and a battery  48  for providing power to the various parts of the receiver assembly. The visual display  46  may be adapted to provide a visual representation of the tracking system  22  relative to the drill bit  18  and other information useful to the operator. The receiver assembly  36  may also comprise a transmitting antenna (not shown) for transmitting information from the receiver assembly to the drilling machine  28  or other remote system (not shown). 
     The antenna arrangements  42  are supported on the frame  38  and separated from each other by a known distance and in known relative positions. One skilled in the art will appreciate the separation and relative position of the antenna arrangements  42  may be selected based on the number of antenna arrangements and antenna design, size, and power. In the preferred embodiment of  FIG. 2 , the plurality of antenna arrangements  42  comprises a first  42   a , a second  42   b , and a third  42   c  antenna arrangement. Preferably, the antenna arrangements  42  are mounted in a plane and at the vertexes of an equilateral triangle. One skilled in the art will appreciate a greater distance or spread between the antennas will provide better resolution and accuracy. A workable compromise between spread and physical size has been found to be a separation distance of at least 18 inches. Other receiver configurations are possible, as long as each antenna arrangement  42  is capable of isolating the magnetic field in each of the Cartesian axes at the point on the frame  38  where the antenna is positioned. For example, the invention contemplates a fourth antenna arrangement that may be supported by the frame  38  at position either above or below the plane formed by the first  42   a , second  42   b , and third  42   c  antenna arrangements. 
     Each of the plurality of antenna arrangements  42  is preferably a tri-axial antenna. More preferably, each antenna arrangement  42  is adapted to measure the total magnetic field at its respective position on the frame  38 . Preferably, each antenna arrangement  42  will comprise three orthogonal antennas which measure the magnetic field along their specific axis of sensitivity. Each of the three orthogonal antenna signals is squared, summed, and then the square root is taken to obtain the total field. This calculation assumes the sensitivities of each antenna are the same and that the center of each antenna is coincident with the other two such that the antenna arrangement is measuring the total field at a single point in space. 
     Referring now to  FIGS. 3 and 4 , there is shown therein the preferred embodiment for an antenna arrangement  42  for use with the present invention. The antenna arrangement  42  comprises a support structure  50  defining three channels  52 . The support structure  50  is preferably formed of lightweight plastic. For ease of construction, the structure  50  may be manufactured in at least two parts that are secured together. The structure  50  is preferably manufactured in such a way that three channels  52  are each dimensionally identical. More preferably, the support structure  50  has a substantially cubical shape and each of the three channels  52  defines a rectangular aperture area having a center point. Most preferably, the channels  52  are mutually orthogonal and oriented so that the center points are coincident. 
     The channels  52  are orthogonally oriented such that a first channel  52   a  is circumvented by a second channel  52   b , and a third channel  52   c  circumvents the first channel and the second channel. A preferred embodiment for such an arrangement comprises an orientation where a long side of the rectangular second channel  52   b  is adjacent to and perpendicular to a short side of the rectangular first channel  52   a , and a diagonal of the rectangular third channel  52   c  is substantially coincident with a plane formed by the rectangular second channel. The size of the antenna  42  can be optimized by designing the channels  52  such that the diagonal of the third channel  52   c  intersects the plane of the second channel  52   b  at an angle of between 0-10 degrees. Most preferably, the diagonal of the third channel  52   c  will intersect the plane of the second channel  52   b  at an angle of approximately 4 degrees. 
     Shown in  FIG. 4 , the antenna arrangement  42  further comprises three antenna coils  54 . The coils  54  are preferably insulated windings of magnet wire. The three coils  54  are separately wound around the structure  50 , one in each of the three channels  52   a ,  52   b , and  52   c , to form three coil loops  54   a ,  54   b , and  54   c . Because of the orientation of the channels  52   a ,  52   b , and  52   c , as previously described, the coils  54   a ,  54   b , and  54   c  do not intersect each other when positioned in the channels. Preferably, the coils  54  comprise approximately 100 turns of magnet wire, though other numbers of turns may be used depending on wire size and antenna sensitivity or other design considerations. Due to the channel configuration, the coil loops  54  all have coincident center points, and their sensitivities are substantially identical. The coil loops  54  also define substantially identical aperture areas and have rounded corners. Since the coils  54  are wound with magnet wire, their resistances are relatively low. Therefore, the antenna  42  can be tuned properly to increase its sensitivity, thus allowing the receiver  36  to detect the magnetic field from greater depths. 
     Applicants&#39; invention also contemplates other embodiments for the antenna arrangement  42 , including use of traditional ferrite rod antennas. For example, though not shown, the antenna arrangement  42  could comprise three ferrite rod antennas in orthogonal relationship. However, the antenna arrangement  42  having coil windings  54  shown in  FIG. 4  has significant advantages over the use of traditional ferrite rod antennas. Ferrite rods greatly enhance the sensitivity of the antenna, thus enabling the receiver to work to deeper depths. However, the ferrite properties are not constant over a temperature range. If a high level of accuracy is required, the drift over the temperature range experienced on work sites is unacceptable. Also, the center of each antenna would obviously not be coincident with the center of the other antennas. This will introduce errors in the total field calculation. 
     Referring now to  FIG. 5 , there is shown therein an alternative embodiment for the antenna arrangement  55  for use with the present invention. As shown in  FIG. 5 , the antenna arrangement  55  comprises three tri-axial antennas made of printed circuit boards  56  (PCBs). Preferably, the PCBs  56  are supported on a mount  58  and configured as a cube. In a cubic configuration, opposite PCBs  56  are connected in series. The PCBs  56  are preferably comprised of many connected layers, allowing the winds to be connected in series to increase the number of turns, and therefore the inductance of the antennas. When configured as a cube, the PCBs  56  antennas can be mounted such that their respective axes are perpendicular and a geometric center of the antenna arrangement  55  will not change as the antenna arrangement is maneuvered. 
     Using PCBs  56  for the antenna arrangement  55  also has significant advantages. The cubic arrangement of the PCBs  56  allows the observation point for calculation of the total field sensed by the antenna arrangement  55  to remain at the geometric center of the antenna. Additionally, as PCBs are manufactured by precision machines, tolerances associated with manually wrapping the loops are reduced. The antennas produced in this fashion are very uniform from one board to the next and less expensive to manufacture. Higher precision measurements will be possible with this configuration. 
     With reference now to  FIG. 6 , shown therein is a block diagram of the preferred embodiment of the receiver assembly  36  of the present invention. The antenna arrangements  42 , as described earlier, measure a change in the magnetic field. A change in the magnetic field sensed will result in a voltage being induced in response to the transmitter&#39;s magnetic field. The voltages from the antennas  42  are sent to filters  60  and amplifiers  62 . Filters  60  eliminate the effects of other signals received by the antennas  42  from local noise sources. Amplifiers  62  increase the signal received by the antennas  42 . An A/D converter  64  is used to convert analog waveform information into digital data. 
     The digital data from the A/D converter  64  is then sent to a central processor  66  (CPU) to calculate the location of the transmitter  32  relative to the receiver assembly  36 . The CPU  66  may comprise a digital signal processor (DSP) and a microcontroller. The CPU  66  decodes the information from the A/D converter  64  and performs calculations to determine the location of the transmitter in a manner yet to be described. The CPU  66  may also discern information transmitted on the magnetic field, to determine the battery status, pitch, roll, and other information about the downhole tool assembly  24 . 
     The receiver assembly  36  may also comprise one or more sensors  68  used to sense operational information about the receiver assembly  36 . For example, one or more accelerometers, or other known inclination and orientation sensors or magnetic compasses, may provide information concerning the roll or tilt of the receiver  36 . Information from the sensors  68  is provided to the A/D converter  64  and to the CPU  66  where the DSP may make calculations to compensate for the receiver  36  not being level. 
     In the preferred embodiment the receiver assembly  36  further comprises a user interface  70  having plurality of buttons, joysticks, and other input devices. The operator can input information for use by the CPU  66  through the user interface  70 . Information entered through the user interface  70  or determined or used by the CPU  66  may be displayed to the operator on a visual display  72  screen. The receiver assembly  36  also comprises a radio antenna  74  for transmitting information from the CPU  66  to a remote unit, such as at the drilling machine  10 . 
     The receiver  36  is preferably powered by a battery assembly  76  and power regulation system  78 . The battery assembly  76  may comprise multiple D-cell sized batteries, though other sources are contemplated, such as rechargeable batteries. The power regulation system  78  may comprise a linear regulator or switch mode regulator to provide power to the various components of the receiver  36 . 
     The receiver assembly  36  of the present invention uses multiple points of measurement, at the plurality of antenna arrangements  42 , to accurately locate the transmitter  32  in three-dimensional (3-D) space. Each antenna arrangement  42  obtains three distinguishable orthogonal components of a magnetic field available at any position. In the preferred embodiment described above, the three antennas  42   a ,  42   b , and  42   c , provide those magnetic field measurements. 
     Referring now to  FIGS. 7 and 8 , shown therein are the relationship of the antenna arrangements  42  to the transmitter  32  and the geometries involved. With three points of measurements from the antennas  42 , the location of the transmitter  32  can be found in 3-D space by the receiver assembly  36  at any point on the ground using the equations below. 
     The Dipole Equations for the Null Field, the field perpendicular to the earth&#39;s surface, and Total Field are: 
                     B   x     =     k   ·         3   ·     z   2       -     r   2         r   5                 (   1   )                 B   y     =     3   ⁢     k   ·       y   ·   z       r   5                   (   2   )                 B   z     =     3   ⁢     k   ·       x   ·   z       r   5                   (   3   )                 B   T     =     k   ·           3   ·     z   2       +     r   2           r   4                 (   4   )               
where r 2 =x 2 +y 2 +z 2 , and k is a calibration constant. These equations assume that the receiver  36  is flat (x 1 =x 2 =x 3 =x) and above the transmitter  32  (x&gt;0). However, one skilled in the art will appreciate the ability to account for tilt of the receiver  36  with information received from the sensors  68  and the pitch of the transmitter  32  with information received from the downhole tool assembly  24 .
 
     Referring to  FIG. 7 , the equations relating each of the points of measurement (at the antennas  42   a ,  42   b , and  42   c ) on the receiver  36  to (x, y, z) are: 
                           y   1     =     y   +         3     3     ·   L   ·     cos   ⁡     (       π   6     +   γ     )                     z   1     =     z   +         3     3     ·   L   ·     sin   ⁡     (       π   6     +   γ     )                         (     4   ⁢   a     )                       y   2     =     y   -         3     3     ·   L   ·     cos   ⁡     (       π   6     -   γ     )                     z   2     =     z   +         3     3     ·   L   ·     sin   ⁡     (       π   6     -   γ     )                         (     4   ⁢   b     )                       y   3     =     y   +         3     3     ·   L   ·     sin   ⁡     (   γ   )                       z   3     ⁢   z     -         3     3     ·   L   ·     cos   ⁡     (   γ   )                       (     4   ⁢   c     )               
Also, it can be seen from  FIG. 8  that
 
               cos   ⁢           ⁢     θ   1       =       z   1       r   1             
or z 1 =r 1 ·cos θ 1 . The same is true for the other points, so in general z i =r i ·cos θ i .
 
     Adjusting for a tilted receiver  36 , the rotated coordinate system gives the following: (note that the  y  axis is unaffected)
 
   z ′=  z   cos  P+  x   sin  P  x ′=−  z   sin  P+  x   cos  P  
 
 z′=z  cos  P+x  sin  P x′=−z  sin  P+x  cos  P  
 
     Solving for B   x   : 
               B       x   _     ′       =       3   ⁢     k   ·         x   ′     ·     z   ′         r   5         ⁢       x   -&gt;     ′       +       k   ·         3   ·     z   ′2       -     r   2         r   5         ⁢         z   _     ′     .               
Plugging in the rotated values and simplifying gives:
 
     
       
         
           
             
               B 
               
                 x 
                 _ 
               
             
             = 
             
               
                 k 
                 · 
                 
                   
                     
                       
                         3 
                         · 
                         
                           x 
                           2 
                         
                         · 
                         sin 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                     
                     + 
                     
                       
                         3 
                         · 
                         x 
                         · 
                         z 
                         · 
                         cos 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                     
                     - 
                     
                       
                         
                           r 
                           2 
                         
                         · 
                         sin 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                     
                   
                   
                     r 
                     5 
                   
                 
               
               ⁢ 
               
                 
                   x 
                   _ 
                 
                 . 
               
             
           
         
       
     
     These equations provide measurable parameters regardless of pitch, and the system of equations can be written as follows: 
     
       
         
           
             
               
                 
                   
                     B 
                     
                       
                         x 
                         _ 
                       
                       , 
                       i 
                     
                   
                   = 
                   
                     k 
                     · 
                     
                       
                         
                           
                             3 
                             · 
                             
                               x 
                               2 
                             
                             · 
                             sin 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                         + 
                         
                           
                             3 
                             · 
                             x 
                             · 
                             
                               z 
                               i 
                             
                             · 
                             cos 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                         - 
                         
                           
                             
                               r 
                               i 
                               2 
                             
                             · 
                             sin 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                       
                       
                         r 
                         i 
                         5 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     
                       3 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       equations 
                     
                     , 
                     
                       i 
                       = 
                       1 
                     
                     , 
                     2 
                     , 
                     3 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     B 
                     
                       T 
                       , 
                       i 
                     
                   
                   = 
                   
                     k 
                     · 
                     
                       
                         
                           
                             3 
                             · 
                             
                               
                                 ( 
                                 
                                   
                                     
                                       z 
                                       i 
                                     
                                     ⁢ 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     P 
                                   
                                   + 
                                   
                                     x 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     P 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                           + 
                           
                             r 
                             i 
                             2 
                           
                         
                       
                       
                         r 
                         i 
                         4 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     
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                       equations 
                     
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     There are now six equations (B   x ,1 , B   x ,2 , B   x ,3 , B T,1 , B T,2 , B T,3 ) and five unknowns (x,y,z,k,γ) and the system can be solved with any number of known methods. One skilled in the art will appreciate that since k is determined from the above equations, there is no calibration required to use this system. 
     The present invention can therefore be used to identify the exact coordinates of the receiver  36  relative to the transmitter  32  using the magnetic field measurements from the plurality of antenna arrangements  42  and the equations above. The present invention can be used to identify the location of the transmitter  32  in 3-D space without any additional movements, as long as the magnetic field from the transmitter can be detected by the plurality of antenna arrangements  42 . Specifically, the location of the transmitter  32  can be determined without movement of the receiver  36  towards the transmitter  32 . The information concerning the location of the transmitter  32  is preferably provided to the operator using the visual display  72 . 
     There is shown in  FIG. 9  a preferred configuration of a screen display  72 . The drill string  16  is shown underground. The x-, y-, and z-coordinates are the distances to the downhole tool assembly  24  from the receiver  36  location. A receiver icon is also on the grid to graphically show the relationship of the receiver  36  to the transmitter  31 . Transmitter  32  temperature, battery status, pitch, roll, yaw, signal strength, signal gain, and signal frequency icons are also shown on the display  72  to provide a graphic and numeric representation of each. Other downhole tool  18  data or operational information could similarly be displayed. This allows the downhole tool assembly  24  position to be monitored and determined without requiring the receiver  36  to be placed directly over the transmitter  32 . All data may be stored in memory or a database to log the history of each bore. Many other functions may be made available thru the main menu such as changing units, calibration mode, alternate two-dimensional view, and demonstrations and help. 
     In an alternative embodiment, the receiver assembly  36  of the present invention can also be used with certain directed steps to take advantage of situations where the transmitter  32  strength or sensitivity of the plurality of antenna arrangements  42  does not permit the 3-D location as described above. In such a case, use of the receiver assembly  36  involves location of a particular spot directly behind the transmitter  32  before pinpointing the location of the transmitter. However, with the multiple measurement points available at the plurality of antenna arrangements  42  of the receiver  36 , the receiver can easily direct an operator to the proper spots to ease determination of the location of the transmitter. The alternative use involves a process of using the visual display  72  to first direct the operator to a position directly behind and oriented in the same direction as the downhole tool assembly  24  and then to a position directly above the downhole tool assembly. 
     In the first step of the alternative embodiment, the operator uses the receiver  36  to find a location where the total magnetic field reading for each of the plurality of antenna arrangements  42  is the same and the receiver is rotationally aligned with the transmitter  32 . This step is preferably accomplished simultaneously, using the display  72  to direct the operator to the desired location. 
     The spot where the magnetic field reading at each antenna arrangement  42   a ,  42   b , and  42   c  is the same is where, from the equations above, B 1T =B 2T =B 3T .  FIGS. 10 and 11  are graphic illustrations of the total magnetic field readings as the receiver  36  is moved within the y-z plane for a constant depth and for a receiver rotationally aligned with the transmitter  32  (so that γ=0). The operator can be directed to the point where the field strengths are the same using the readings from the plurality of antenna arrangements  42  and the following calculations. 
     First, calculate 
                 r   _     i     =         k   /     B   iT       3     .           
Then
 
                 V     1   -   2       =           r   _     1     -       r   _     2       L       ,     
     ⁢       V     1   -   3       =           r   _     1     -       r   _     3       L       ,     
     ⁢   and                 V     2   -   3       =             r   _     2     -       r   _     3       L     .           
And then V y =V 1-2  and
 
               V   z     =           V     2   -   3       ·   cos     ⁢     π   6       +         V     1   -   3       ·   cos     ⁢       π   6     .               
These vectors can be shown in two-dimensional (2-D) space to direct the operator to the spot where the vectors are 0, where B 1T =B 2T =B 3T .
 
     At the same time, the display  72  can be used to direct the operator to rotate the receiver assembly  36  so that the receiver is directionally aligned with the transmitter  32  and, consequently, the downhole tool assembly  24 . One skilled in the art will appreciate that the location of the spot where the magnetic fields are equal at each of the plurality of antenna arrangements  42  (B 1T =B 2T =B 3T ) will be different if the receiver  36  is not aligned with the transmitter  32  (when γ≠0). Therefore the receiver  36  must be rotated properly to ensure the correct spot is found. The receiver assembly  36  will be aligned with the transmitter  32  when the flux line through the antenna assembly  42   c  at the back end of the receiver (the “rear pod”) is along the z-axis. By using the display  72  to show the operator the angle at which the flux impinges the rear pod  42   c , the user can align the receiver  36  with the flux lines and keep it rotated properly. 
     When these steps are followed and the operator is directed to the spot where all conditions are met, then the receiver will be located with y=0 and γ=0. This spot is easily found, requires little computation, and greatly simplifies the location process. The next step in the process is to direct the operator to move the receiver  36  to a position directly above the transmitter  32  to precisely locate the downhole tool assembly  24 . 
     Referring now to  FIG. 12 , there is shown therein a graphical depiction of flux lines radiating from the transmitter  32  in the x—z plane. Assuming the pitch of the receiver  36  is 0, note that the angle α 0 as z 0. Therefore, the receiver  36  preferably displays this angle graphically to the operator, and the operator can move the receiver until this condition is true. At this point, each of the front antenna arrangements  42   a  and  42   b  (the “front pods”) will be located on the line where z=0, and the transmitter  72  located in between and directly below the front pods  42   a  and  42   b.    
     One skilled in the art will appreciate that when the magnetic field is measured at z=0, then 
             r   =         k   /     B   T       3     .           
Since me receiver  36  is located where z=0 if the above steps have been followed, then the geometry shown in  FIG. 13  can be used to calculate the depth, x, of the transmitter  32 . As previously discussed, the receiver  36  may contain sensors  68  to account for tilt of the receiver and enable the calculation of β. Then, as r 1 , r 2 , L, and β are known values, x can be solved for through known geometry. The value for y can also be determined in the event that the receiver  36  has been moved slightly off of the line y=0. The operator can be directed to move the receiver until y=0 in order to be positioned to get a proper depth reading.
 
     The process allows the receiver assembly  36  to be used to locate the downhole tool assembly  24  quickly and accurately, with few steps and little computation. It should also be rioted that the step for finding the spot where the magnetic field strengths in each of the antenna arrangements  42  are equal is only necessary when the operator does not have a relative idea of where the transmitter  32  is located. If the general location of the downhole tool assembly  24  is known, then the operator can use the receiver  36  to find the line where z=0, and then the depth of the transmitter  32 . 
     With the present invention, improved methods for directing and drilling a horizontal directional borehole  12  are also possible. For example, trackers and beacons used for directional drilling generally do not indicate how much the drill bit is moving as an HDD system  10  is used to make steering corrections to redirect the borehole  12 . Currently, steering corrections are dependent on machine operators&#39; expertise. The present invention removes the uncertainty of operators&#39; guesswork. With the present invention, the receiver  36  can indicate at any given point in time the precise relative location of the downhole tool assembly  24  and the drilling bit  18 . 
     In an improved method for boring, the receiver  36  can be set on the ground with a centerline of the receiver directly on the desired path for the borehole  12 . The display  72  can then be used to provide the operator with immediate feedback of the location and heading of the drill bit  18  relative to the desired path. 
     A method for creating a horizontal directional borehole  12  in the earth is also accomplished with the following steps. First, the receiver assembly  36  is placed on the ground in the proximity of the drill bit  18  with the longitudinal display axis of the receiver assembly aligned with the desired bore path  12 . As the drill bit  18  is advanced forward without rotation to perform a steering correction in the horizontal plane, an image of the orientation of the drill bit relative to the receiver  36  can be transmitted from the receiver to the HDD system  10  and its operator. Additionally, the distance of forward advance of the drill bit  18  without rotation can be determined at the receiver  36  and that information also transmitted from the receiver to the FIDD system  10 . Such techniques are useful when boring on-grade boreholes or when desiring to bore to a point where the receiver  36  is positioned. 
     The present invention also contemplates an improved method for communicating information from the downhole tool assembly  24  to the receiver assembly  36 . As is well known in the art, the electronics package  30  in the downhole tool assembly  24  will generally comprise batteries to provide operating power for the transmitter  32  and sensors in the electronics package. However, the need to obtain reasonable operating life from a battery-powered transmitter  32  gives rise to a number of difficult engineering tradeoffs. The transmitter&#39;s  32  maximum operating depth depends on many factors, but power dissipation in the transmitter is a major—if not the dominant—consideration. A transmitter&#39;s  32  operating life is also determined by the battery stack&#39;s energy capacity. Thus, the designer is forced to make a compromise between operating depth, which favors higher operating power and shorter operating life, and operating life, which favors lower power and reduced operating range. These are fundamental design tradeoffs for any battery-powered transmitter  32 . 
     For improved performance, the present invention contemplates an adaptation of a data transmission technique known as Manchester coding. Other data transmission variants may have similar characteristics. Although the invention will be described in terms of Manchester coding, the invention may be used with any data transmission technique meeting similar data signal criteria. 
     Traditional serial digital transmission schemes commonly divide a data stream into small time intervals known as bit cells, data cells, or bit intervals, representing the amount of time needed to convey one bit of binary data. The simplest coding schemes rely on single-level signals during each bit cell. Other coding schemes use somewhat more elaborate waveform constructs for specific reasons. For example, within a very commonly-used family known as NRZ (Non-Return-to-Zero) codes there are either zero or one transition in a bit period. Members of this code family are:
         NRZ-L (-Level), in which a high level represents a “1” and a low level represents a “0”,   NRZ-M (-Mark), in which a “1” is represented by a transition and a “0” by no transition in the bit period,   NRZ-S (-Space), in which a “0” is represented by a transition and a “1” by no transition in the bit period.
 
NRZ-L is seen to be the most common (and intuitive) of the data codes.
       

     This invention disclosed concerns a member of the Biphase code family in which there are at least one but no more than two transitions in a bit period. The particular code of interest is Biphase-L (-Level), in which a “1” or “0” is represented by a level transition in the middle of the bit interval. Biphase-L is commonly known as Manchester or Manchester II code. Manchester II or Biphase-L code occasionally is further subdivided into Bipolar One (logic “0” is defined as a low-to-high or rising edge transition in the middle of the bit period, or Bipolar Zero (a logic “0” is defined as a high-to-low or falling transition in the middle of the bit period. The Bipolar One and Bipolar Two waveforms are logical complements of one another and both are commonly made available by integrated circuit devices which encode and decode Manchester data streams. For simplicity, this disclosure refers to only “Manchester” code, which should be understood to represent all variants of the basic code structure (whether known as Manchester, Manchester II or Biphase-L). It is significant that Manchester code is self-clocking, which is to say data synchronization may be established and maintained using the fact there is a guaranteed transition at the midpoint of each bit cell. 
     The primary advantages attending use of Manchester code in HDD tracking beacons arise from the guaranteed transitions in the signal waveform. Equivalently, the signal waveform will be high for one half of each bit cell and low for the other half of each bit cell. In typical data transmission applications, the high and low signal transactions involve transitions between two different voltage levels. However, in HDD applications this property may be used advantageously in at least two different ways:
         (1) by tuning the beacon transmitter on or off to represent a signal condition (the “1” state) and a no signal condition (the “0” state), respectively, or   (2) by frequency shifting the beacon transmitter frequency in or out of a bandpass filter passband to represent the “1” and “0” states, respectively. In other words, the in-band signal frequency is generated during the high portion of the Manchester waveform and an out-of-band signal frequency is generated during the low portion of the Manchester waveform.
 
For simplicity, let alternative (1) be called Manchester/OOK (Manchester On-Off Keying) and let alternative (2) be called Manchester/FSK (Manchester Frequency Shift Keying).
       

     Manchester/OOK coding is especially desirable. It guarantees the beacon signal will be off half the time data is being transmitted, effectively resulting in a 50% power savings relative to frequency shift keyed (FSK) and phase shift keyed (PSK) data transmissions. Of equal importance, however, is the fact that the received signal amplitude may be simply and accurately averaged over several bit cells while data is being transmitted. This simplifies the software needed to accurately determine depth from transmitted data. 
     Manchester/FSK coding, on the other hand, provides no power savings relative to FSK or PSK transmission, but it does provide greater operational flexibility. This arrangement presumes one or more digital bandpass filters, each identified by different filter coefficients, and the ability to generate a number of different FSK waveforms, also determined by coefficients in software. The bandpass filter response will produce an output very similar to Manchester/OOK coding as the FSK signal moves in and out of the bandpass filter passhand. Although there is no power savings, there is great operational flexibility—the operator may select the operating frequency from a number of different frequency and filter combinations to obtain the combination offering the best overall performance in the presence of local noise or other interference. 
     Various modifications can be made in the design and operation of the present invention without departing from its spirit. Thus, while the principal preferred construction and modes of operation of the invention have been explained in what is now considered to represent its best embodiments, it should be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.