Background of the Invention l. Field of the Invention The present invention relates generally to an apparatus and method for measuring formation parameters by transmitting and receiving electromagnetic signals by means disposed in recesses in a tubular housing member and including means for reducing the capacitive coupling of noise from conducting elements located adjacent the recess and housing means. More particularly, a method and apparatus is disclosed employing electrostatic shielding of the antennas and ground loop isolation to reduce noise and minimum spacing of antennas from conductive elements adjacent the recesses and inductive current coupling means to enhance the signal level at the receiving means whereby the system is capable of use in a drill string to make measurements while drilling.
2. Description of the Background It is desirable for many reasons to transmit electrical signals through the earth as a propagating medium, and receive the signals at a location spaced from the transmitter. Such a signal propagation system is, for example, used both for the determination of various parameters associated with the prop-grating medium and for communication purposes. These systems are often used in the investigation of the environment surrounding a Barlow, and in particular, the surrounding earth formations.
25 Various types of Barlow logging systems are available to per-form these investigations. A class of these systems utilize electromagnetic field phenomena to obtain data from the environ-mint surrounding the Barlow.
One type of electromagnetic logging is electrode logging I which utilizes an electric field in the surrounding formation to produce a measure of the conductivity of the formation. A con-ductile mud is necessary for proper use of this system, thus non-during the system inoperative with oil base muds. Inductive logging is another type of electromagnetic logging which uses a I
~23S~9 magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second mug-netic field which induces current in receiving coils positioned in the Barlow. The induced current in the receiving coil or oils is proportional to the secondary current flow in the formation and thus is directly proportional to the conductivity or inversely pro-portion Al to the resistivity of the surrounding formation. Electron magnetic wave propagation affords still another logging system for investigating the environment around a Barlow and is the subject of the present invention.
An electromagnetic logging system of the wave propagation type is disclosed in Gould et at, U.S. Patent No. 3,551,797. This patent discloses a wire line system having a transmitter and no-severs for measuring formation parameters, and utilizing phase comparison and amplitude. However, the Gould wire line system is not usable in a measuring while drilling (hereinafter, Mud configuration. The Gould patent discloses a noninductive Sunday of insufficient strength to operate in a drill string kirk-terraced by a mass of steel and more particularly drill dollars in the vicinity of the drill bit and measurement apparatus. U.S.
Patent Nos. 4,107,597 and 4,185,238 also show electromagnetic wave propagation systems for use in wire line apparatus. U.S. Patent No.
4,107,597 describes the wire line Sunday as being constructed of a non-conductive material which is customary in such devices in order to aooommcdat~ the use of electromagnetic transmitting and receiving apparatus. The U.S. Patent No. 3,079,550 shows an induction logging system for measuring similar formation parameters, utilizing lower frequencies and requiring a conductive mud in the Barlow.
Both the electrode and induction systems, heretofore the prim many methods for measuring formation resistivity, as well as thewireline systems using wave propagation, have certain drawbacks, particularly for application in a MUD configuration. An elect 123~ 79 trove system requires insulation of the drill string from the several transmitting and receiving electrodes in the system.
This normally requires a special insulation coating to be applied over the steel drill string in the vicinity of the electrodes.
This coating is expensive to maintain and is of questionable reliability. An induction logging system normally operates at 20 KHz and requires large diameter coils to obtain the necessary coupling. In a MUD configuration, inductive logging coils must be mounted in or about a drill collar in a drill string and that portion of the collar must be non-conductive. Non-conductive collars are difficult to build while maintaining the structural integrity and strength necessary to their use in a drill string.
In this regard thicker collar walls and improved mechanical strength characteristics are obtainable in a collar by reducing the size of the coils. In order, however, to achieve the necessary coupling between spaced coils which are small, the operating frequency of the system must be increased. As the ire-quench is raised from 20 KHz, wave propagation begins and stank dart induction is no longer effective. The wire line electromag-netic wave propagation devices described above do not use a Sunday assembly having sufficient structural integrity and strength to be incorporated in a drill string or noise reduction and signal enhancement techniques which enable reliable performance in an MUD environment.
The unsuitability of the above systems for incorporation into a drill string to measure formation parameters using electron magnetic signals in a measurement while drilling configuration is clear. The electrode system discussed above is limited to use with electrically conducting, water base muds. Induction logging systems utilize large coil configurations which require too much space on a drill collar to maintain the strength and fluid come monkeyshine characteristics described above. Smaller coils no-quite the use of higher frequencies to insure proper coupling be-tweet coils, and the higher frequencies propagate in the format lion. Thus, standard induction measurements cannot be utilized in MUD systems. The lack of structural integrity and ineffec-live signal reception are problems associated with these systems. The art has long sought a means of overcoming these disadvantages useful in providing an effective apparatus using electromagnetic signals to measure formation parameters while drilling.
Summary of the Invention The present invention is directed to a method and appear-tusk for reducing the coupling of noise from conducting elements near the receiving means and enhancing electromagnetic signal reception suitable for measuring formation parameters about a drill hole and particularly useful for making such measurements while drilling by incorporation of the apparatus in the drill string. The apparatus of the present invention is character-iced by a tubular, electrically conductive housing means threaded at each end for threaded engagement in a drill string above the drill bit and including a longitudinal passage suit-able for conveying drilling fluids there through; means for transmitting electromagnetic energy into the formation sun-rounding said housing means, said transmitting means insulated from but disposed about the exterior of said housing means;
first receiving means for receiving electromagnetic energy from said formation, said first receiving means longitudinally spaced a first distance from said transmitting means and insulated from but disposed about the exterior of said housing means; first shield means substantially surrounding said first receiving means for electrically, but not magnetically, shielding said first receiving means by reducing the coupling of the electric field component of the received signal, including noise, to said first receiving means, said first shield means comprising a pair of first conductive shield tubes substantially surrounding said first receiving means and within which said first receiving means is disposed, said first tubes each having a first end, said first ends spaced from one another with a first gap to prevent eddy currents therein, each tube having a second end, said second ends Jo ~23~ 9 -pa-joined to a first conductive case at a pair of opposed open-ins in the side walls of said first case through which said first receiving means extends, said first shield tubes and said first conductive case insulated from said housing means;
means for electrically connecting said first shield tubes and said first conductive case to ground potential while insulate in them from said conductive housing to reduce electrical noise from ground loop currents therein; and first inductive coupling means mounted within said first conductive case for inductively coupling the signal from said first receiving means to an electrical conductor leading to signal handling means.
The present invention provides a new and improved Barlow logging system for use in a MUD configuration and utilizing electromagnetic wave propagation while maintaining the motion-teal strength and fluid flow characteristics of the drill string.
A presently preferred embodiment of the invention includes a tubular housing member having the essential mechanical and strength characteristics of a drill collar and which is likely to be capable of conducting an electrical current. The transmitting, receiving and noise reducing means of the present system are arranged on or about the drill collar member. One or more recesses are formed in the wall of the drill collar to house the transmitting and receiving components, leaving a full longitudinal path through the drill collar to thereby permit unobstructed flow of the drilling fluids through the pipe string. The transmitting and receiving antennas are preferably arranged in a generally circular array about the exterior of the tubular ~Z3S~79 member. Preferably the receiving antenna comprises a low impel dance antenna electrically insulated from the tubular member.
Most preferably, an antenna having an impedance of less than about lo ohms, such as a single loop antenna, is disposed annum S laxly about the tubular member. Transmitting and receiving antennas of the system are arranged in one or more recesses formed in the peripheral surface of the drill collar.
Molded sleeves of insulative and wear resistant material encapsulate the antennas in the recesses and fill the spaces bet-wren the outer surface profile of the collar and the coils. Two receiving antennas are preferably employed and are located on the same axial side of the transmitting antenna. More preferably, the longitudinal distance between each pair of antennas, in-eluding transmitting and receiving antennas, is different.
The capacitive coupling of noise from conducting elements located adjacent the recess means and housing means, e.g., from a conducting drilling fluid, the drill string or a conducting housing is reduced, preferably minimized, by use of a low impel dance receiver antenna, electrostatic but not magnetic shielding of the antenna, ground loop current isolation and inductive current coupling of the received signal from the antenna to the data handling means. Preferably, the low impedance antenna is substantially shielded, i.e., about ninety percent or more shielded.
The inductive current coupling means is preferably an impedance matching towardly ferrite core transformer and compare-sues a secondary coil used to pick up the signal from the receiver antenna. The transformer coil is wound on a ring of low magnetic permeability, preferably ferrite, which in turn encircles an annular antenna loop about the collar. Each secondary trays-former coil couples its respective antenna coil to a receiver circuit. Transmitter and receiver circuits as well as power supplies are arranged in the wall of the collar.
In an exemplary system employing the present invention, a . .
~Z3S~79 transmitting means and two spaced receiving means with associated circuitry detect phase change in a wave propagated through the formation surrounding the Barlow between the spaced receiver antennas. The required comparison circuitry is disposed in the wall of the tubular housing member in this system. The present apparatus is also capable of measuring amplitude ratios as well as phase changes relative to spaced receiver antennas.
The method of the present invention comprises transmitting and receiving electromagnetic signals with a device as described above and preferably incorporated into a drill string for measurement while drilling. The method further comprises reducing the capacitive coupling of noise into the system and enhancing the signal at the receiving means with the shielded, low impedance antenna and inductive current coupling means disclosed.
The method and apparatus of the present invention have many advantages. Capacitive coupling of signals transmitted along the tubular housing member or through adjacent conducting eye-mints, e.g., the drill string or the drilling mud, is reduced, thus enhancing reception of the signal from the surrounding for-motion. A device having sufficient structural integrity and strength for incorporation into a drill string is provided.
Accordingly, the method and apparatus of the present invention make possible the measurement of formation parameters using electromagnetic signals in a measurement while drilling con-figuration. These and other meritorious features and advantages of the present invention will be more fully appreciated from the following detailed description and claims.
Brief Description of the Drawings Other features and intended advantages of the invention will be more readily apparent by reference to the following detailed description in connection with the accompanying drawings where-in:
Fig. l is a schematic view of a drill string in a Barlow 1235~79 with a section of drill collar incorporating a measurement while drilling logging system in accord with one embodiment of the pro-sent invention included in the drill string; also illustrated is a schematic analysis of wave propagation in a surrounding for-motion viewed relative to the transmitting and receiving means spacing on the collar;
Figs. PA and 2B combined provide a schematic side elevation Al view in cross-section of a drill collar including a logging system in accord with one embodiment of the present invention, Fig. PA illustrating the top portion of the apparatus and Fig. 2B
the bottom portion;
Figs. 2C, ED and YE, respectively, illustrate, in more detail, schematic side elevation Al views of the transmitting antenna, the first receiving antenna and the second receiving antenna;
Fig. 3 is a schematic illustrative cross-sectional view taken along line 3-3 of Fig. PA, illustrating a transmitter antenna coil in accord with the present invention;
Fig. 4 is a schematic illustrative cross-sectional view taken in the direction of line 4 of Fig. 2B, showing a portion of the I shielding means and a coupling for connecting a receiving antenna coil to an associated circuit;
Fig. 5 is a schematic illustrative partial cross-sectional view of a receiver antenna coil, shield and coupling means in accord with the present invention;
Fig. 6 is a schematic view of an alternative embodiment of a logging system in accord with the present invention; and Fig. 7 is a schematic block diagram of a circuit for pro-cussing a signal received in accord with the present invention;
While the invention will be described in connection with a I presently preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit of the ~,~
g invention as defined in the appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In drilling wells, the hourly cost of a drilling rig is very large and, therefore, it is desirable to minimize the time during which the equipment is not drilling due to mechanical failure of the drill string. Thus, maintenance of the mechanical integrity of the drill string is of primary concern in any measuring while drilling (MUD) logging system.
By definition, an MUD system seeks to provide realtime measurement of formation parameters and, therefore, measuring instrumentation should be positioned in the drill string near the bit, an extremely hostile physical environment. During drilling operations there are two characteristics of the drill string which are of prime importance: the integrity of the longitudinal conduit through the string for the flow of drilling fluids, and the physical strength of the components of the string to resist failure under stress. MUD logging systems include very delicate instrumentation which must be designed to function reliably and very precisely as an integral part of the mass of steel hardware required to survive the tremendous heat, pressures and stresses near a working drill bit. MUD instrumentation must be placed in steel drill collars which form part of the drill string near the bit. Such collars are basically cylinders of solid steel having an axial tubular conduit for the passage of circulating drilling fluids down the string to the bit. Such an instrumentation carrier provides many design constraints on both the packaging and the function of MUD systems. This is especially true in the case of the system of the present invention where the instrument station is a radio frequency transmitter and receiver which must broadcast and receive and measure low-power electromagnetic signals in a very noisy electrical environment. Transmit and receive antennas are part of the present invention and are required to be mounted very close to a highly conductive mass of solid steel. These design constraints require a careful con-~Z3~179 side ration of all the factors which affect the function of an electromagnetic logging tool to produce an accurate measurement of the formation parameters of interest.
There are three formation parameters which affect an electron magnetic wave in an earth formation, whether the wave commune-gates from one point to another by induction or by propagation.
These three formation parameters are the conductivity (resistivity) the magnetic susceptibility and dielectric constant. Conductivity provides an indication of the energy absorbing characteristics of the medium, while magnetic suscep-tabulate and dielectric constant each give a measure of the energy storing capacity of a material. Conductivity, which is` a commonly measured parameter in well logging operations, exhibits wide variations in value for earth materials and strongly affects all electromagnetic waves. The magnetic susceptibility of most earth materials has relatively little effect on electromagnetic waves. The dielectric constant shows considerable variation in earth formations and has a large influence on high frequency pro-pagating waves.
Amplitude and phase are the two fundamental characteristics of a wave. As the wave passes, or propagates, through a medium the amplitude and phase change. In a Wilbur, the wave begins at some source, or transmitting point, and radiates away from that point by propagation, as indicated in Fig. l. As the wave passes one receiver point the wave has a certain amplitude and phase character which reflects the effects of the Wilbur and the formation through which the wave traveled. As the wave passes a second receiver point the amplitude and phase are generally changed, reflecting the effects of that same Wilbur, the same formation and the additional formation through which the wave passed. By comparing the amplitude and phase of the wave as it passes the two receivers, propagation changes due to the for-motion may be studied. The difference in amplitude and phase between the two received wave signals can be measured and related Jo 'I-' `
I. ., .
~Z35179 to the wave propagation parameters, i.e., travel time and cite-nation.
The slower a wave travels in a medium, the greater will be the measured phase difference between the two spaced receiver points. Thus, phase difference can be measured and related to propagation velocity, i.e., the larger the measured phase dip-furriness the slower the wave is traveling. The multiple graphs in Fig. l schematically illustrate an electromagnetic wave as a function of time as the wave is propagated from a transmitter antenna T, and is received by a receiver antenna R2 relatively near to the transmitter antenna, and by a receiver antenna R
relatively far from the transmitter antenna. The wave is received at the far receiver antenna Al a time delay "D" after reception at the near receiver antenna R2.
From the foregoing, it can be seen that different media affect a propagating wave through two mechanisms. One such mechanism is travel time and the other is attenuation. Since the dielectric constant and the conductivity are the only parameters of the media that affect wave propagation, they should be deter-I mixable by a measurement of phase difference (travel time amplitude ratio (attenuation), or both.
One of the major parameters in the design of an electromagnet tic logging system is the frequency of operation. The higher the frequency of electromagnetic energy propagating in a medium, the more the signal is affected by the dielectric constant of the medium rather than the conductivity of the medium. Because of the well-developed relationship between formation lithology and conductivity, it is this parameter which is of the most general interest to the drilling community. Thus, a frequency is I selected to produce data closely correlated to the formation con-ductility, however, many of the aspects of the present invention could be included in a dielectric logging tool if desired. In the present preferred embodiment, a frequency has been used so that the signal is still predominantly influenced by the conduct ^ ::
I. . .
12351~9 tivity of the formation and not very strongly affected by dielectric constant variations.
Another aspect of frequency selection is that the lower the frequency of operation, the closer are the correlations between received amplitude and phase variations and the conductivity of the formation. However, the lower the frequency, the longer the wave-length and the greater are the effects of other physical factors such as the spacing between the transmitting and receiving antennas and any conductive surfaces at ground potent trial. A factor, d/A , where d is the distance between an antenna and a conductive mass, such as a steel drill collar, and is the wave-length of the transmitted wave, is very important.
The lower the frequency, the larger the wave-length and, there-fore, the smaller the d/A value. The smaller the d/A ratio the more power there is required in order to transmit sufficient energy into the formation to produce a measurable received signal due to increased coupling of the signal into the conductive mass and the signal degrading losses produced thereby. Power to operate instrumentation systems Donnelly in an MUD environment is at a premium and efficiency of operation is a major consideration of design. Thus, frequencies high enough to produce acceptable do ratios for acceptable power consumption levels are required.
The other aspect of ensuring a minimum do ratio is that d, the minimum distance between the antennas and the steel drill collar, involve system packaging factors which directly affect the structural reliability of the drill string. While the anion-nay could be mounted outside the periphery of a drill collar and protected against damage from interaction with the Barlow by stabilizers, this is not practical. In order to mount these come pennants within the peripheral borders of a collar, a recess must be formed in the outer surface of the collar. These recesses must be deep enough to surround the antenna for physical protection and still provide a sufficient spacing, d, between the antenna and the nearest surface of the conductive steel collar .
;
, ~2351~79 for an acceptable do ratio. However, the depth of the recess ( also affects the physical strength of the collar and, therefore, the structural reliability of the drill string. In a drill collar, there is both a twisting torque force component as well as a bending moment force which requires great strength to withstand. Cutting recesses and grooves into the surface of a collar invariably affects its strength. Thus, for a given diameter drill collar both the axial length and radial depth of the antenna receiving recesses in the collar must be carefully selected.
The environment within which the present MUD electromagnetic logging system must operate is extremely noisy from an electrical standpoint. Other electrical equipment operating Donnelly ire-quaintly produces spurious- signals which have a strong electric component and a weak magnetic field component. The system of the present invention is configured to reduce the effect of noise from outside sources by two techniques: The use of low impedance antennas which are more sensitive to magnetic field components and less sensitive to electric fields, and the use of highly efficient electrostatic shielding means. In the preferred embo-dominate a single turn small loop antenna lying in a plane normal to the axis of the drill collar is used to simplify the matte-mattes of interpreting the received signal, however, other con-figurations of low impedance antennas could be employed. Single turn loop antennas are used to eliminate inter winding capacitance and reduce stray capacitance coupling between the antennas and the drill collar. The loops are also electrostatically shielded by a cylindrical shield open at the electrical center of the loop and antenna couplings are shielded by conductive cases. The electrostatic shield is also electrically isolated from the body of the drill collar to minimize ground currents in the shielding.
Fig. 1 shows schematically drill string 11 positioned in Barlow 12 traversing earth formations 13. The drill string includes sections of drill pipe and, at the lower end of the 123~7~
string, drill collars to provide weight to the system. A MUD
logging apparatus in accord with the present invention and designed to make the measurements discussed above is housed in drill collar or logging sub 14 or the like which is illustrated positioned in the drill string above drill bit 16. The logging sub 14 is illustrated having longitudinal passage 17 for pro-voiding drilling fluid or mud flow through the system and access to the bottom of the drill string. Such access is vital for a number of reasons which are known to those skilled in the art and, therefore, will not be discussed here. Power supply and electronic circuit components for operating the system are housed in circumferential compartments 18 in the wall of sub 14 arranged about the passage 17. Transmitter antenna 19 and receiver anion-nay 21 and 22 having an annular configuration are illustrated arranged about collar 14 and spaced apart to provide the come prison of travel time and attenuation as described above.
By comparison of the arrangement of components of the MUD
logging sub 14 with the graph of Fig. 1, it will be appreciated that an electromagnetic wave from the transmitter antenna 19, corresponding to transmitter antenna T, penetrates surrounding formations 13. The wave propagated within the formation in the vicinity of sub 14 is first received at receiver antenna 22 which corresponds to the near receiver antenna R2 spaced relatively closer along the sub to transmitter antenna 19 than receiver ; 25 antenna Al. The propagated wave is received later by a delay time "D" at receiver antenna 21 which corresponds to the far receiver antenna Al, spaced relatively farther along the sub from transmitter antenna 19.
Referring next to Figs. PA and 2B of the drawings, sub 14 is schematically illustrated in greater detail. The compartment 18 is located within a recessed bore 31 and houses a power supply and data handling means such as the electronic components for the transmitter and receiver circuits as well as other circuitry, if desired.
: ;
.
~2351~79 In Figs. PA and 2B and the details of 2C, ED and YE, MUD
apparatus is illustrated with transmitter antenna 19 just below the electronic compartments 18, and with receiver antenna 21, which corresponds to the far receiver Al positioned toward the bottom of sub 14.
Beginning at the lower end of recessed bore 31, as viewed in Fig. PA, passageways 34 and 35 provide openings to pass electric eel wires 36 and 37 from the transmitter and receiver circuitry in 18 to transmitter antenna 19 and receiver antennas 21 and 22, respectively.
Transmitter antenna 19 coil consists of an antenna coil 119 positioned in an annular recess 38 which is formed in the outer surface of drill collar 14. Receiver antennas 21 and 22 consists of coils 121 and 122 located in elongate annular recess 39 in the outer surface of drill collar 14 below and axially spaced from first recess 38. A durable, electrical insulator material 23 encapsulates the transmitter and receiver antenna coils to pro-vise electrical insulation of the coils, to protect the coils from wear and to maintain the hydrostatic integrity of the drilling fluid system. One example of a useful insulator material 23 is nitrite rubber. Generally, coils 119, 121 and 122 and the additional components described below located within the recesses 38 and 39 are constructed and arranged to allow the depths of the recesses to be minimized while completely accom-modeling the coils and other components within the outer surface profile of the drill collar 14 and having the antenna coils spaced a minimum distance from the nearest surface of the collar.
Thus, the structural strength characteristics of drill collar 14 are preserved since relatively little material is removed from the wall of the collar to form recesses 38 and 39.
The construction of transmitter antenna coil 119 and its mounting in recess 38 may be appreciated by reference to Figs.
PA, 2C and 3. The coil 119 is made up of one or more wire win-dings 42 which are wound about the collar in recess 38. Coil 119 ~2~35179 is of low impedance, e.g., four or less turns 42 of multi filament insulated wire. The coil 119 may be mounted in recess 38 by various means, such as suspension within an encapsulation material 23 filling recess 38 or by being positioned about an annular insulative sleeve received into recess 38. In any case a minimum space "d" between the wire windings 42 of the transmitter coil 119 and the bottom of recess 38 is maintained to preserve the signal strength of the transmitted signal. As discussed above, unless a minimum value of d/A is maintained for a par titular operating frequency too much of the signal will be lost into the conductive steel collar to obtain a measurable signal level in the formation for practical power input values.
Windings 42 are enclosed within copper tube 43 which serves as an electrostatic shield. See Figs. 3 and 2C. Electrostatic shield 43 is split in two halves midway along its accurate length, with the halves being mechanically connected by insular live connector 44.
A copper coated steel case 46 is also mounted in the recess 38 and has openings in each side thereof to receive and electric gaily connect the ends of shield 43 and enclosed coil windings 42. Copper coated steel case 46 is electrically isolated from collar 14. A direct connection (not illustrated) between the ends of windings 42 and lead wires 36 extending from the transmitter circuitry is made in case 46 so that the lead lines continue a loop as the windings. Copper coated steel tube 47 extends from the end of passageway 34 to an opening in the top of case 46 to provide a conduit for lead wires 36. Molded insular live material 23 fills recess 38 and encapsulates case 46, tube 47, shield 43 and coil 42 to provide a protective and electric I gaily insulating cover for transmitter antenna 19 and to maintain the hydrostatic integrity of the drilling system.
Tube 47 is in mechanical and electrical contact with the steel body of drill collar 14 at its upper end toward passageway 34. However, tube 47 is isolated from electrical contact with : .
~23~79 case 46 by an electrically insulating annular spacer 4B. Thus, transmitter lead lines 36 are electrostatically shielded by tube 47 and collar 14, which is electrically grounded at the transmitter circuitry, thereby grounding the tube.
Transmitter lead lines 36 may comprise a pair of coaxial cables, with the center lead of the cables continuing as wire windings 42 of transmitter antenna coil 119. The shield lead (not illustrated) of each cable, grounded at the transmitter air-quoter, is in electrical contact with each of tube shield halves 43 but is insulated from drill collar 14. Thus, windings 42 are also electrostatically shielded by case 46 and tube 43, which are at ground potential.
The transmitter antenna shield 43 aids in reducing the generation of noise in the transmitted signal while the split in tube shield 43 prevents circulating eddy currents from flowing about the tube which would interfere with the magnetic field of the propagating electromagnetic wave. Also, since grounded tube 47 is insulated from direct contact with grounded case 46, no ground currents are able to be generated in the shielding of lead wires 36 and coil windings 42. Therefore, transmitter antenna 19, including lead wires 36 and coil windings 42, is electricity-tidally shielded, but is not magnetically shielded.
- Both the receiver antenna coils 121 and 122 are mounted about the body of drill collar 14 in the same way and in a manner semi-z5 far to that of transmitter coil 119.
Annular recess 39 is sized in depth to preserve the strength of the drill collar 14 and to accommodate the receiver antenna coils and other components within the outer surface profile of the collar while having the antenna coils spaced the minimum I distance "d" from the nearest conductive surface of the collar.
To enhance the sensitivity of the receiver antenna coils 121 and 122 to magnetic field components and reduce their sensitivity to the noisy electric field components low-impedance antenna coils are preferred. Although single turn loop antennas are used lZ35179 in the present system to reduce inter winding capacitance and capacitive coupling of noise into the antenna, other types of low impedance antennas could be employed. Also, axially symmetrical loops simplify the mathematics of interpreting the received signals but with proper analysis techniques other configurations could be used.
The receiver coil 122 is enclosed within a copper tube 55 which serves as an electrostatic shield, as shown in Figs. 2B, 4 and 5. The shield 55 is split in two halves midway along its arcuate length with the halves being mechanically connected by an insulative connector 56. As in the case of transmitter coil 119 and conductive case 46, a copper coated steel case 57 is mounted in but electrically isolated from the annular recess 39 and has openings in each side thereof to receive the ends of shield 55 and enclosed coil 122. Mounted within the case 57 is a trays-former for inductively coupling the receiver antenna coil 122 to the receiver circuitry.
To maximize the energy coupled from the receiver antenna to the receiver input circuitry, a high efficiency transformer such as a ferrite towardly transformer is preferred. AS shown in Figs. 4 and 5, a ring 58 of low magnetic permeability material, which may be a ferromagnetic material such as ferrite or powdered iron, is positioned about the receiver coil 121 and wound with a towardly coil 59. The output of the coil 59 couples the received signal from the antenna coil 121 to the transmission line con-netted to the receiver circuit components (not shown). The impel dance of the coil 59 is selected to match the impedances between the antenna coil 121 and the receiver input for maximum signal transfer. The coil 121 impedance may be on the order of a few ohms while the transmission line leading to the receiver circuit may be on the order of 50 ohms.
Receiver lead wires 37 include two coaxial cables extending from the receiver circuitry through a sub passageway 35 and a copper coated steel shielding tube 49 to connector case 57. As .
~23~7~
in the case of transmitter antenna coil 119, the outer shielding leads of coaxial cables 37, which are grounded at the receiver circuitry, are electrically connected to conductive case 57 and, therefore, to tube shield halves 55. The center leads of receiver coaxial cables 37 are connected to opposite ends of towardly coil 59. As illustrated in Fig. 4, shielding tube 49 is mechanically connected to case 57 but is electrically isolated therefrom by insulating spacer 60. As in the case of shielding tube 47, shielding tube 49 is mechanically and electrically con-netted to the wall of drill collar 14 at the upper end of tube 49 toward the bottom of recess 31. Consequently, the receiver antenna leads and antenna coil 122 and 121 are electrostatically shielded by separately grounded tubing shield 49 and 67 enclosing lead cables 37 and the combination of cases 57 and tube halves 55, enclosing antenna coils 122 and 121, respectively. The split in copper shielding tube halves 55 prevents eddy currents from circulating about the tube which would interfere with reception of magnetic field components of the propagating waves by coils 122 and 121. Further, since grounded shield tubes 49 and 67 (enclosing lead cables for receive antenna coil 121) are ins-fated from direct contact with grounded cases 57, no ground currents can be generated in the shielding of lead wires 37. The receiver antenna coils 121 and 122 and their coupling components are carefully electrostatically shielded from noise, while remaining exposed to the magnetic field components of the electromagnetic waves generated by transmitter antenna 19 and propagated through the surrounding formations 13.
Electromagnetic waves generated by transmitter antenna 19 and 30~ propagated through surrounding formations 13 are incident on receiver antennas 22 and 21, whereby the varying magnetic fields of the waves interact with coils 122 and 121 of respective receiver antennas 22 and 21 to induce electric currents in the windings. Low impedance antennas, such as the relatively large wire, single loop preferred construction of receiver coils 122 . .
~Z35~79 and 121 exhibit minimized impedance to such magnetic field induced current flow while remaining less susceptible to noise due to varying electric fields. The current flow in receiver coil 122 generates electromagnetic fields in the plane of the corresponding ferritomagnetic ring 58 and towardly coupling coil 59, thus providing a highly efficient inductive transformer coupling of the towardly coil to the antenna coil. Similarly, a transformer towardly coil is coupled to antenna coil 121.
Voltages varying in accord with the field variations of the electromagnetic wave incident on the receiver antenna coil 121 are thus generated across the output of its coupling coil 59, and transmitted therefrom to receiver input circuitry. Similarly, voltages are generated due to electromagnetic waves incident upon receive antenna coil 122. The pair of received voltages are amplified and converted to lower frequencies for ease of handling in the electronics section.
In one preferred embodiment of the system of the invention, a drill collar having an outside diameter of 7 inches was formed with cylindrical annular recesses having a diameter of 5.75 inches. Antenna coils having an inner diameter of 6.2 inches were arranged in the recesses to produce a minimum distance bet-wren antenna and drill collar of about 0.22 inches. This system performed well Donnelly at an operating frequency of about 2 MHz.
Next referring to Fig. 6 of the drawings, an alternative arrangement is illustrated for the transmitting and receiving antenna coils. The upper portions of sub 80, housing circuitry ; component and power supply section 83, are similar to those of sub 14 set out with respect to Fig. PA. Transmitter antenna 30 coil 84 and receiver antenna coils 85 and 86, however, are illustrated in eccentric longitudinal recesses 81 and 82, respect lively, on the outer wall surface of drill collar 80. Each coil 84, 85 and 86 typically includes a plurality of turns wound about a longitudinal core snot illustrated) and arranged parallel to :
.
the longitudinal axis of drill collar 80. The cores and coils are made as small as possible to limit the depth of recesses 81 and 82 needed to house and protect the coil assemblies. The coils are spaced from the steel body of drill collar 80 a minimum 5 distance "d" to preserve an adequate d/A ratio for efficient operation and covered with an encapsulating medium 87 to allow magnetic fields to encircle the coils, to protect the coils and to maintain the hydrostatic integrity of the drilling fluid system. Appropriate electrostatic shielding (not illustrated) is also provided in the system, preferably as described above with respect to Figs. PA and 2B.
Referring now to Fig. 7 of the drawings, a schematic circuit diagram is illustrated for providing a system to make formation resistivity measurements using the apparatus heretofore described. Such data handling means are included in the electron nits section in drill collars 14 or 80, although the embodiment of Figs. PA and 2B will be referred to hereafter. The output of the circuit shown in Fig. 7 may be recorded in the electronics section or may be Ted by a telemetry system (not illustrated) to the surface for concurrent processing and readout at the surface.
In any event, the transmitter is operated by power supply 24 at a frequency preferably in the range of 500 KHz to 10 MHz to provide an electromagnetic wave for output from the transmitter antenna coil lit. This wave is propagated through the environment, including earth formations 13 surrounding the Barlow, to receiver antenna coils 122 and 121 of receivers 22 and 21, respectively, located at spaced longitudinal distances on the collar. The receiver antenna coils are coupled to the respective electronic receiver components which are driven by local oscilla-ion 90 operating at a frequency within several KHz of the transmitted frequency to generate lower frequency output signals which are more easily handled. For example, transmitter 19 is illustrated operating at 2 MHz and oscillator 90 is operating at 1.998 MHz. The outputs from receivers 22 and 21 are then fed to ,::.
:' ' :
lZ~5~79 a phase comparator 91 and/or amplitude measuring circuits 92 and 93 which are in turn coupled to a ratio circuit 94 to measure signal phase shift and attenuator all in accordance with the teaching of the aforementioned prior art patents.
yin the operation of the system heretofore described, the signal emanates from a source, transmitter antenna 19, and ire-vets away from it. The steel of drill collar 14 has a high per-mobility and is located to the inside of the source so that the steel only causes small effects on the signal as long as a mini-mum distance d from antenna to drill collar is maintained so that an adequate do ratio is maintained. The signal that does travel along the steel is highly attenuated, so that the signal received at the receiver antenna coils must come from the region outside collar 14, i.e. earth formations 13 as long as the receiver antennas coils 121 and 122 are spaced the distance "d" from the surface of the collar.
The foregoing description of the invention has been directed primarily to a particular preferred embodiment in accordance with the requirements of the patent statutes and for purposes of explanation and illustration. It will be apparent, however, to those skilled in the art that many modifications and changes in the specifically described and illustrated apparatus and method may be made without departing from the scope and spirit of the invention. For example, while the disclosure of the system has Boone described primarily with regard to resistivity measurements, particular coil configurations, and specific frequencies and ire-quench ranges, it may be appreciated from the present descrip-Sheehan and illustrations that other measurements, coil configurations, frequencies and the like could be used without deporting from the present invention in its broadest aspects.
Therefore, the invention is not restricted to the particular form of construction illustrated and described, but covers all modify-cations which may fall within the scope of the following claims.
It is Applicants' intention in the following claims to cover ~Z35~
such modifications and variations as fall within the true spirit and scope of the invention.