SPECIFICATIONAperture antenna system for measurement of formation paametersThe invention relates to the measurement of formation parameters while drilling in a borehole, and more particularly, to the measurement of formation dip through the utilization of an aperture antenna.
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 propagating medium and for communications purposes.
These systems are often used in the investigation of the environment surrounding a bore hole, and in particular, the surrounding earth formations. Various types of borehole logging systems are available to perform these investigations. A class of these systems utilize electromagnetic field phenomena to obtain data from the environment surrounding the borehole.
One type of electromagnetic logging is electrode logging which utilizes an electric field in the surrounding formation to produce and measure the conductivity of the formation. A conductive mud is necessary for proper use of this system, thus rendering the system inoperative with oil base muds. Inductive logging is another type of electromagnetic logging which uses a magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second magnetic field which induces current in receiving coils positioned in the borehole.
The induced current in the receiving coil or coils is proportional to the secondary current flow in the formation and thus is directly proportional to the conductivity or inversely proportioned to the resistivity of the surrounding formation. Electromagnetic wave propagation affords still another logging system for investigating the environments around a borehole and is related to the subject of the present invention. An exemplary electromagnetic logging system of the wave propagation type, is disclosed in Gouilloud et al., U.S.
patent No. 3,551,797 . This patent discloses a wire line system having a transmitter and a pair of receivers spaced axially along the borehole. The transmitter propagates electromagnetic energy in a field which is uniformly azimuthally distributed around the borehole and the signal received by the two axially spaced receiving antennas is analyzed for amplitude and phase differences as indicia of conductivity of the formations surrounding the borehole.
Parameters which are extremely useful in the evaluation of the nature of the subterranean formations penetrated by a borehole is that of formation thickness and dip. The dip of a geological strata is, generally speaking, the angle of the plane within which the boundary of a formation of one particular type and the boundary of the contiguous formation makes with a horizontal reference plane. More particularly, the dip of a formation is evaluated by determining the angle with which the walls of a bore hole intersect the plane of the boundary of a formation. The problem of the determination of formation dip evolves in to one of locating three points within a depositional layer with reference to a horizontal plane such that the plane is defined by the three points and the angle of dip is the angle between this plane and the horizontal plane.
Prior art wire line instrumentation which has been used to evaluate formation strata orientation must make three measurements. The first is a measurement of dip of the formation relative to the bore hole and this has been done by recording three electric logs, preferably azmuthally spaced and oriented in the borehole and, in particular, by including three identical sets of electrodes circumferentially spaced around a tool at 120 and all positioned on the same plane perpendicular to the axis of the tool. The second measurement is a measurement of the direction and angle of inclination of the borehole and the third is a measurement of the orientation of the axis of the borehole relative to magnetic north.
Borehole inclination and azimuth are conventionally measured by accelerometers and/or magnetometers which use the gravitional and magnetic forces of the earth as frames of reference. Formation dip, however, requires the measurement of some charateristic of the formation surrounding the borehole with sufficient accuracy to determine where along the length of the borehole a formation of one composition ends and another begins.
One type of prior art dip meter uses three spontaneous potential (SP) curves to obtain the dip of a formation relative to the borehole axis. These devices require electrodes to be pressed against the side walls of the borehole and measure minute electrical currents, or potentials in the formations with respect to a surface reference potential. Another technique for measuring formation parameters to obtain dip information is shown in U.S. Patent No.
3,388,323 which discloses a wire line instrument for performing induction logging by transmitting electromagnetic energy having a frequency on the order of 10 KHz into the borehole wall and measuring the electrical conductivity and magnetic susceptibility of the formation.
In U.S. Patent No. 4,383,220 there is disclosed a microwave electromagnetic borehole dip meter which comprises a wire line tool having a plurality of radially extending arms each of which carries a pad which is biased into close engagement with the borehole wall. Each pad carries a microwave transmitting antenna and a microwave receiving antenna. Microwave energy having a frequency on the order of 1-3 GHz is propagated into the formation by the transmitting antennas and the energy received by the receiving antennas is taken as indicative of characteristics of the formation such that the relative variations along the borehole from the three azimuthally spaced sensors defines the dip of the formation.
Each of the prior art systems for measuring formation parameters such as conductive electrodes for measuring spontaneous potential, induction systems for measuring conductivity and susceptibility, as well as wire line systems using microwave wave propagation have certain drawbacks, particularly for application in a measuring-while-drilling environment. To incorporate measuring instruments into a borehole drilling string an electrode system requires insulation of the steel drill collar from the several transmitting and receiving electrodes in the system. This normally requires special insulation coating to be applied over the steel drill string in the vicinity of the electrodes which is expensive and of questionable reliability.Induction logging systems must normally operate at a frequency on the order of tens of KHz's and, therefore, require large diameter coils in order to obtain the necessary electromagnetic coupling with the formation. In an MWD configuration, inductive logging coils must be mounted in or about the drill collar in the 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 the drill string. And, if the size of the coils is reduced, the operating frequency of the system must be increased in order to obtain the necessary coupling between the spaced coils. For an antenna of fixed dimensions, as the operating frequency is raised from tens of kilohertz, wave propagation becomes the predetermined energy transfer mechanism and standard induction measurements with conventional antenna systems are no longer effective.The dipmeters employing wire line microwave propagation devices, as described above, do not use a sound assembly having sufficient structural integrity and strength to be incorporated in a drill string and, further, require radially extending appendages to engage the borehole walls and enable measurement of the needed parameters which are. of course, totally impractical for measurements in an MWD environment.
In the use of antenna structures within borehole measurement instruments, virtually all are concerned with the generation of electromagnetic fields to be propagated into the surrounding medium. The subject matter of the present invention however teaches the utilization of a slot antenna of fluid dimensions operating at a sufficiently low frequency to use the inductive field component but a sufficiently high frequency to permit focusing of the energy in a highly directional fashion to investigate the nature of formations surrounding a borehole, namely, the r.f. region which is above the induction logging frequencies but below the microwave logging frequencies.
Such measurements afford the drilling operator with resistivity/conductivity values signalling changing formations and, therefore, bed dip. The essence of a measurement-whiledrilling system is to provide a drilling operator with a continuous flow of useful data which are indicative of conditions within the borehole. Thus, a drilling operator can anticipate and avoid potentially dangerous situations as well as be aware of critical data necessary to evaluate the formations through which the bore hole is passing. The antenna structures used in the system of the present invention are readily adaptable for inclusion in measuring-while-drilling logging systems.
The system of the present invention overcomes the difficulties of the prior art by providing an apertured antenna system incorporated into a drill collar structure which is capable of generating electromagnetic energy having an intense H field component which is normal to the edges of the aperture for highly directional induction of the field into the adjacent borehole formation for evaluation of the characteristics of that formation.
SUMMARY OF THE INVENTIONThis invention is directed to a system for measuring the characteristics of sub-surface earth formations penetrated by a borehole.
The system includes means for generating electromagnetic energy which contains a strong H field in the near field and then highly focusing that field into an azimuthally narrow configuration and directing the field in to the formation to obtain information related to the nature of the formation in a particular azimuthal direction. A borehole tool is provided that can be incorporated in to the drill string of a drilling apparatus and includes three aperture antenna assemblies azimuthally distributed equally around the longitudinal axis of the tool. The tool also includes means for receiving a signal with similar aperture antenna assemblies to evaluate the nature of the formation.
The invention includes a method and apparatus for measuring a charateristic of an earth formation adjacent a borehole by positioning a first aperture antenna in the borehole so that the walls of the aperture lie in a plane which extends parallel to the axis of the borehole.
An electromagnetic field is generated having a strong H component in the near field and having a frequency where the transverse limi tations of the aperture antenna are small with respect to a quarter wavelength. The electromagnetic field is divided through the aperture antenna to define a narrow non-propogated, induced field component for penetration of the borehole wall for a substantial distance into the surrounding formation. An electromagnetic signal is received which is responsive to the penetration of the formation by the electromagnetic energy directed through the aperture antenna as a measure of the characteristic of the formation in a particularly azimuthal direction.
The invention includes a method and apparatus for measuring a characteristic of an earth formation adjacent a borehole by positioning a second aperture antenna in the borehole having side walls parallel to the first aperture antenna and receiving the electromagnetic signal which passes from the formation back through the second aperture antenna as a measure of the charateristic of the formation. Further, the invention contemplates the use of a pair of spaced apart receiving aperture antennas and a measurement of phase difference of the electromagnetic signal received by the two antenna as a measure of the characteristic of the formation in a particular azimuthal direction.In addition, the invention includes a plurality of pairs of transmitting and receiving aperture antennas spaced azimuthally from one another to simultaneously measure formation characteristics in different azimuthal directions.
The invention further includes a method and apparatus for measuring a charateristic of an earth formation adjacent a borehole wherein the sidewalls of the aperture antenna are rectangular with the longer dimension parallel to the axis of the borehole and where the frequency of the electromagnetic field is on the order of 2 MHz and the dimensions of the aperture antenna are in the range of 0.5-8 inches.
In another aspect the invention comprises a system for measuring the dip and bed thickness of an earth formation penetrated by a borehole in a measuring-while-drilling environment which includes a conductive cylindrical section of drill collar having a plurality of first cavities formed in the body of said section of drill collar at circumferentially spaced intervals around said section.The drill collar also has a plurality of first aperture antennas formed in the outer wall of said section and in communication with respective ones of said first cavities along with means mounted in each of the first cavities for generating an electromagnetic field, the field having a strong H field component and being at a frequency at which the wavelength is long with respect to the dimensions of the first aperture antennas to direct a narrowly defined lobe of induced electromagnetic energy through the aperture antennas into the walls of the borehole for a substantial distance.
The drill collar also has a plurality of second cavities formed in the body of the section of drill collar in axial alignment with but axially spaced from respective ones of said first cavities and a plurality of second aperture antennas formed in the outer wall of the section and in communication with respective ones of said second cavities. The system includes means mounted in each of the second cavities for receiving electromagnetic energy from the formation surrounding the borehole in response to the energy directed through the first aperture antennas into the formation and means responsive to the energy received by the receiving means for measuring borehole formation parameters along a length of borehole and determining dip and bed thickness of formations penetrated by the borehole.
BRIEF DESCRIPTION OF THE DRAWINGSFor more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:Figure 1 is a diagrammatic, side-elevational, cross-sectional view of a borehole drilling operation illustrating the electromagnetic survey of formations penetrated by the borehole;Figure 2 is an enlarged, fragmentary, illustrative, perspective view of a drill pipe having an aperture antenna formed therein for the generation of an electromagnetic field;Figure 3 is a diagrammatic, schematic of one method of exciting an aperture antenna for the generation electromagnetic energy having of H field normal to the edges of the aperture;;Figure 4 is a top plan view of an aperture antenna array mounted in a drill collar and an illustration of the induced field pattern emitted from each aperture; andFigure 5 is an illustrative, partially cut-away, longitudinal cross-section view of a measuring-while-drilling tool for determining the dip of sub-surface formations along a borehole incorporating the aperture antenna system of the present invention.
DETAILED DESCRIPTION OF THE INVENTIONReferring first to Figure 1, here is shown a drilling rig 11 disposed atop a bore hole 1 2.
One embodiment of a system 10 for measuring the dip of subterranean formations while drilling is carried by a sub 14 comprising a portion of drill collar 1 5 and disposed within the bore hole 1 2. The system 10 is provided for a measurement of parameters related to the geological composition of strata surrounding the bore hole and, hence, the dip of formations through which the bore hole passes in a measuring while drilling mode.
Still referring to Figure 1, a drill bit 22 is disposed at the lower end of drill string 1 8 and carves the bore hole 1 2 out of the earth formations 24 while drilling mud 26 is pumped from the well head 28. Metal surface casing 29 is shown to be positioned in the bore hole 12 above the drill bit 22 for maintaining the integrity of the bore hole 12 near the surface. As described below, the present invention permits accurate measurement of dip angle of formations through which the borehole passes in a measuring-whiledrilling configuration. The annulus 1 6 between the drill string 1 8 and the borehole wall 20 creates a theoretically closed return mud flow path. Mud is pumped from the wellhead 28 by a pumping system 30 through mud supply line 31 coupled with the drill string 18.
Drilling mud is, in this manner, forced down the central axial passageway of the drill string 18 and egresses at the drill bit 22 for carrying cuttings comprising the drilled sections of earth, rock and related matter upwardly from the drill bit to the surface. A conduit 32 is supplied at the wellhead for channeling the mud from the borehole 1 8 to a mud pit 34.
The drilling mud is typically handled and treated by various apparatus (not shown) such as outgassing units and circulation tanks for maintaining select viscosity and consistency of the mud. The system contained in sub 14 enables the controlled direction of R.F. electromagnetic energy having a strong H field component in the near field into the formation surrounding the borehole and enables the measurement of the nature and composition of the surrounding formation during the pumping of drilling fluid through the drill string.
Continuing to refer to FIG. 1, there is shown a sub 14 and drill collar 1 5 comprising a portion of the system 10 of the present invention in a downhole environment. The system 10 is constructed to generate a series of signals for telemetry to the wellhead or for downhole recording of information indicative of the composition of the adjacent formations and the dip angle of the formation strata with respect to the axis of the borehole. This information is obtained from electromagnetic antenna and equipment disposed in the sub 14 as will be described in more detail below.
The requisite telemetry and analysis systems are deemed to be of conventional design and not specifically set forth or addressed herein.
The method and apparatus for formation parameter evaluation is, however, described in detail below and this is subject to the present invention.
The central principle of the system of the present invention incorporates a structure conventionally called a slot antenna. Slot antennas are usually built so that L = N A/2. That is, the length of the antenna is an integral multiple of a half wavelength of the electromagnetic radiation to be carried by the antenna. In addition, a principal concern is usually the power that is radiated and propagated to the far field by such antenna devices.
However, in measuring-while-drilling applications, it is totally impractical to design antenna structures employing dimensions which are precisely half wavelengths in length in the desired operating environment. That is, since a downhole measuring tool is operated in earth formations where the constitutive parameters of the transmission medium can change which changes the wavelength and, hence, the distance which constitutes a half wavelength can also vary widely. Thus, it is impossible to operate a slot antenna at resonance in the various and changing environmental transmission media.
What is necessary in order to effectively perform azimuthally definitive measurements with electromagnetic radiation is that an aperture antenna be constructed which produces a highly directional and strong H field in the near field environment. As used herein the term strong H field is an expression which implies that the vector magnitude of the H field of the electromagnetic radiation is large relative to the vector magnitude of the E field at some specified distance away from the antenna. Similarly, strong local E field is an expression which implies the vector magnitude of the E field is large with respect to the vector magnitude of the H field.A strong localH field is desired because it is generally understood that antennas which produce strong local H fields are far more effective at inducing currents at points distant from the borehole without being overly sensitive to borehole geometries and local constitutive conditions than antennas which produce strong local E fields.
The ability to induce currents in earth formation is essential to being able to subsequently measure a signal created by these induced currents which is proportional to the constituent parameters of the formation. In addition, being able to induce currents at some distance from the borehole, past mud cake regions and invaded zones is a necessary part of determining the constitutive parameters of the unperturbed formations. Thus, the proposal of the present invention includes a method for exciting an aperture antenna in such a way that it will produce a strong localH field and thus will be effective at inducing currents in a distant formation.
As show illustratively in FIG. 2, an aperture which could be used in such a measurement system could be quite general in shape and does not necessarily have to comprise a particular configuration slot. As shown in FIG. 2, a wall of a drill collar 41 has formed therein an opening 42 the outer parameters of which form an irregular encircling periphery 43.
Within the body of the drill pipe 41 an electromagnetic field is excited so as to create an H field in the direction of arrow 44 normal to the edges of the aperture 43 of the aperture 42. With the H field so oriented a current will be induced to flow around the edges of the aperture and energy will be radiated into the external environment in a direction normal to the edges of the aperture 43.
Referring further to illustration shown inFIG. 3, a rectangular aperture 45 having generally linear sidewalls 46 is formed in the walls of a drill collar 47. Immediately adjacent the aperture 45 to the interior of the drill collar is a loop antenna which may comprise one or more turns of wire 48 and is driven byR.F. energy so that currents flow in the loop antenna in the direction of arrow 49 so as to produce a strong local H field in the direction of arrow 50. To create the desired radially directed H field, it is preferred that the slot or aperture 45 be excited by R.F. energy having a frequency where the dimensions of the sidewalls 46 of the aperture 45 are small with respect to the wavelength of the energy.
Thus, it can be seen that the slot or aperture antenna taught by the present invention is not operated at or near its resonant frequency so as to propagate energy into the surrounding formation, but rather, considerably below its resonant frequency so as to create an R.F.
induction field having a strong H component in the near field which is directed radially away from the aperture 45.
As further illustrated in FIG. 4, the aperture or slot antennas used in the present invention are constructed in such a way and the frequency selected in such a manner that energy is directed into the formation in a very selective manner. As shown, an illustrative drill collar 51 has a plurality of slots 52 formed at 90 angles to one another and all lying in a common plane transverse to the central axis of the drill collar 51. The antenna apertures or slots 52 are excited with R.F. energy so as to provide a collumating or focusing effect in the horizontal plane illustrated by the near field pattern 53 shown extending into the formation 54.It should also be understood that a similar vertical focusing effect would be necessary in order to give the type of vertical resolution necessary to generate high quality formation logs from which the dip of formations can be determined. The several apertures 52 formed around the circumferential periphery of the drill collar 51 enables several measurements to be made simultaneously.
However, it should also be understood that a single aperture or aperture array could be used to permit measurements in a plurality of different directions and used in conjunction with an azimuthal position monitoring device to enable the determination of formation dip.
It should be clear that a sequence of measurements of azimuthal information is repeated as the drill string moves down the borehole through the fonmation in order to obtain all the information necessary to determine the dip of the formation as well as the bed thickness.
It should also be noted that the slot or aperture antenna used at subresonant frequencies may also be arranged in transmitting pairs as in other configurations of slotted arrays which shape the pattern of energy being directed into the borehole.
Certainly the relationship between electrical parameters and physical constitution of formations adjacent a borehole is known so as to be able to determine the dip of a formation bed from definitively measured set of electrical parameters. So what is essential to the present invention is rather the manner in which the formation electrical parameters are evaluated so as to be able to readily determine the axial and azimuthal location within the borehole at which those parameters change as an indication of the transition region between one formation and the next contiguous formation.
Referring now to FIG. 5, there is shown a partially cutaway diagramatic cross-sectional view of the sub 14 which carries the system constructed in accordance with the teachings of the present invention. Sub 14 preferably comprises a drill collar which is coupled between sections of drill pipe comprising the drill string 1 8 positioned in the borehole 1 2.
Borehole 1 2 penetrates the earth formations 24 which may comprise, for purposes of illustration, a strata of shale 61 and a strata of clay 62 having a boundry therebetween illustratively illustrated at 63. The function of the method and system of the present invention may be to determine the points along the borehole wall where the boundary layer 63 intersects the borehole 1 2 so as to evaluate the angle of the dip of the formation boundary layer. It is also to be understood that the presentation of the relationship between layers 61 and 62 and the boundary layer 63 is purely illustrative for purposes of description.
In actuality, instead of the sharp definition of the region shown, the transition of geological construction would be much more gradual.
The sub 14 comprises a cylindrical steel body 71 having an axial passageway 72 formed therethrough. The bore 72 permits the flow of drilling fluids down the drill string and out the drill bit 22 where it flows back to the surface through the annular region 1 6 between the sides of the sub 14 and the walls of the borehole 1 2. The annular region 1 6 is thus filled with conductive drilling fluids.
Formed near one end of the cylindrical drill collar 71 adjacent the bore 72 is a stepped region 73 which forms a instrumentation compartment 74. The compartment 74 is sealed against the penetration of drilling fluids by a generally cylindrical insert 75 having an inner diameter the same as the inner diameter of the axial passageway 72 and a radially flared upper end portion 76 to close the instrument compartment 74.
The instrument compartment 74 contains both radio frequency transmitters and radio frequency receivers as well as power supplies and either recording instruments and/or instruments for preparing data for telemetry to the surface by means not shown.
The embodiment of the present invention illustratively illustrated in FIG. 5 includes three pairs of aperture or slot antennas mounted at 120 azimuthal spacing from one another. Transmitter slots 81 are rectangular in configuration with the long direction extending parallel to the axis of the drill collar.
The transmitter slots may, for example, may be on the order of one-half inch wide and four to six inches in length. Each of the slots 81 are positioned radially outwardly in the drill collar from an antenna mounting recess 82 within which is positioned an oval loop transmitting antenna 83 having the long direction of the loop positioned in the same direction as the long direction of the slot 81.
Each loop 83 may comprise one or more turns of electrical conductor and the ends thereof are carried by tubular conduit 84 from the antenna mounting recess 82 through the steel body of the drill collar 81 to the instrument cavity 74 where they are connected to the output of a radio frequency transmitter.
Similarly, directly below the transmitter antennas 81 are identical first receiving slot or aperture antennas 91 radially inwardly of which are formed first receiving antenna mounting cavities 92. The first slot receiving antennas 91 are spaced a distance on the order of a few inches in the axial direction from the transmitting antennas 81 and are in axial alignment therewith. Mounted within each cavity 92 is a first oval loop-type receiving antenna 93 comprising one or more turns of conducting wire the ends of which are carried by means of the tubular guide 94 for connection to a receiver positioned within the instrument cavity 74.
The tool of the present invention also contemplates an array of circumferentially spaced second aperture antennas 101 directly below and in axial alignment with the first receiving aperture antennas 91. The second receiving apertures antenna 101 is also spaced a distance on the order of a few inches in the axial direction from the first antennas 91. Each of the second aperture antennas 101 are in communication with second receiving antenna mounting cavities 102. Mounted within each cavity 102 is a second oval looptype receiving antenna 103 also comprising one or more turns of conducting wire the ends of which are carried by a tubular guide 104 for connection for a receiver located in the instrument cavity 74.
The interior of both the antenna mounting cavities and the aperture or slot antennas are filled with a tough resilient insulating material to suspend both the transmitting and receiving antennas protecting them from the harsh environment of the borehole while drilling is in progress as well as closing the external periphery of the drill collar from entry of fluids and forming a continuous outer surface.
As discussed above, the transmitting antennas 83 are driven at an R.F. frequency on the order of 2 MHz while the dimensions of the slot or aperture antennas are considerably less than quarter or half wavelengths at that frequency. Thus, rather than acting as resonators to propagate electromagnetic energy into the surrounding formation, the slot antennas act as "columnators" to produce an induced electromagnetic field into the formation in both vertically and horizontally narrowly defined regions. The loop antennas 83 are operated in a mode whereby the induced electromagnetic field pattern contains a very strong H field component in the near field to maximize the penetration and definition of the field within the formation.Similarly, the slot or aperture receiving antennas 91 serve to define a narrow field pattern which is used to excite the oval loop receiving antenna 93 in response to the received signal produced by the conductivity of the formation behaving in response to the induced field.
Thus, if the three transmitting antennas 81 are disposed circumferentially around the drill collar 1 5 and are simultaneously excited with an R.F. signal having a strong H field component in the near field and the receiving slot antennas 91 are each simultaneously sampled, the received signals are indicative of the conductivity of the formation adjacent each of the respective receiving antennas slots 91.If, thereafter, the drill stem 1 8 is moved axially down the borehole and a subsequent transmission is had from each of the three circumferentially spaced slot transmitting antennas 81 and the received signal at the three circumferentially spaced receiving slot antennas 91 is compared with the earlier reading, there is given an indication of the manner in which the formation surrounding the borehole has changed with distance along the borehole. More particularly, this change in conductivity information as a function of both azimuth and longitudinal distance along the borehole can be processed by conventional software to produce an indication of the dip of the formations through which the borehole passes.
In another preferred mode of operation the signal induced into the formation through the transmitted aperture antennas 81 is received by both the array of first receiving aperture antennas 91 and the array of second received aperture antennas 1 01. The phase difference between the signals received by the first antennas 91 and the second antennas 101 is indicative of a charateristic of the formation adjacent the borehole. This phase difference is measured between the output of the two received signals by a phase comparator located in the instrument cavity 74 with the receivers. Movement of the tool down the borehole for several measured movements of phase difference enables the evaluation of formation dip and bed thickness by conventional signal analysis techniques.
It should be understood that an essential feature of the present invention is the use of a slot or aperture antenna in conjunction with the non-propagating field components of electromagnetic energy having a frequency on the order of 2 MHz, i.e. the induction field, to create in the subresonant region of the antenna a highly directional, large amplitude non-propagating H field which penetrates to a substantial depth within the formation surrounding the borehole wall. The induced field mode of the exciting signal is able to penetrate much deeper distances outside the borehole wall than could be accomplished with a resonant propagated microwave signal as is taught by the prior art.
As can be seen by the above disclosure, the system and method of the present invention enables an accurate determination of the values of formation parameters determined with vertical and azimuthal accuracy in borehole as part of a measuring while drilling system.
It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description.
While the method and apparatus shown and described has been characterized as shown as being preferred, it will be obvious that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the following claims.