Note: Descriptions are shown in the official language in which they were submitted.
<br/> CA 02447468 2003-10-29<br/>SIMPLIFIED ANTENNA STRUCTURES FOR LOGGING TOOLS<br/>Cross-reference to related applications<br/>This application is a continuation-in-part of U.S. Patent Application Serial <br/>No.<br/>10/113,686, filed March 29, 2002.<br/>Background of Invention<br/>Field of the Invention<br/>The invention relates generally to electromagnetic well logging apparatus. <br/>More<br/>specifically, antenna structures for such well logging apparatus.<br/> Background Art<br/> Electromagnetic (EM) based instruments for measuring properties of matter or<br/>identifying its composition are well known. The nuclear magnetic resonance <br/>(NMR)<br/>technique has been used to form images of biological tissues or to determine <br/>the composition<br/>of, for example, earth formations. The values of electrical conductivity for <br/>biological samples<br/>or for earth formations have been obtained through the use of electromagnetic <br/>induction<br/>tools. EM propagation well logging devices are also well known, and are used <br/>for measuring<br/>basic parameters such as amplitude and phase shift of EM waves being <br/>propagated through a<br/>medium in order to determine specific properties of the medium.<br/>Electrical conductivity (or its inverse, resistivity) is an important property <br/>of<br/>subsurface formations in geological surveys and prospecting for oil, gas, and <br/>water because<br/>many minerals, and more particularly hydrocarbons, are less conductive than <br/>common<br/>sedimentary rocks. Thus a measure of the conductivity is often a guide to the <br/>presence and<br/>amount of oil, gas, or water. Induction logging methods are based on the <br/>principle that<br/>varying electric currents, due to their associated changing magnetic flux, <br/>induce electric<br/>currents.<br/> Propagation logging instruments generally use multiple longitudinally-spaced<br/>transmitter antennas operating at one or more frequencies and a plurality of <br/>longitudinally<br/>spaced receiver pairs. An EM wave is propagated from the transmitter antenna <br/>into the<br/>formation in the vicinity of the borehole and is detected at the receiver <br/>antenna(s). A plurality<br/>of parameters of interest can be determined by combining the basic <br/>measurements of phase<br/>-1-<br/><br/> CA 02447468 2003-10-29<br/>and amplitude. Such parameters include the resistivity, dielectric constant <br/>and porosity of the<br/>formation as well as, for example, the degree to which the fluid within the <br/>borehole migrates into<br/>the earth formation.<br/>The transmitter antennas on induction logging instruments generate a time-<br/>varying<br/>magnetic field when a time-varying electric current is applied to them. The <br/>time-varying<br/>magnetic field induces eddy currents in the surrounding earth formations. The <br/>eddy currents<br/>induce voltage signals in the receiver antennas, which are then measured. The <br/>magnitude of the<br/>induced voltage signals varies in accordance with the formation properties. In <br/>this manner, the<br/>formation properties can be determined.<br/>Conventional antennas consist of coils mounted on the instruments with their <br/>axes<br/>parallel to the instrument's central or longitudinal axis. Therefore, the <br/>induced magnetic field is<br/>also parallel to the central axis of the well and the corresponding induced <br/>eddy currents make up<br/>loops lying in planes perpendicular to the well axis.<br/>The response of the described induction logging instruments, when analyzing <br/>stratified<br/>earth formations, strongly depends on the conductive layers parallel to the <br/>eddy currents.<br/>Nonconductive layers located within the conductive layers will not contribute <br/>substantially to the<br/>response signal and therefore their contributions will be masked by the <br/>conductive layers'<br/>response. Accordingly, the nonconductive layers are not detected by typical <br/>logging instruments.<br/> Many earth formations consist of conductive layers with non-conductive layers<br/>interleaved between them. The non-conductive layers are produced, for example, <br/>by<br/>hydrocarbons disposed in the particular layer. Thus conventional logging <br/>instruments are of<br/>limited use for the analysis of stratified formations.<br/>Solutions have been proposed to detect nonconductive layers located within <br/>conductive<br/>layers. U.S Pat. No. 5,781,436 describes a method that consists of selectively <br/>passing an<br/>alternating current through transmitter coils inserted into the well with at <br/>least one coil having its<br/>axis oriented differently from the axis orientation of the other transmitter <br/>coils.<br/>The coil arrangement shown in U.S. Pat. No. 5,781,436 consists of several <br/>transmitter<br/>coils with their centers distributed at different locations along the <br/>instrument and with their axes<br/>in different orientations. Several coils have the usual orientation, i.e., <br/>with their axes parallel to<br/>-2-<br/><br/> CA 02447468 2003-10-29<br/>the instrument axis, and therefore to the well axis. Others have their axes <br/>perpendicular to the<br/>instrument axis. This latter arrangement is usually referred to as a <br/>transverse coil configuration.<br/>Thus transverse EM logging techniques use antennas whose magnetic moment is<br/>transverse to the well's longitudinal axis. The magnetic moment m of a coil or <br/>solenoid-type<br/>antenna is represented as a vector quantity oriented parallel to the induced <br/>magnetic field, with<br/>its magnitude proportional to the corresponding magnetic flux. In a first <br/>approximation, a coil<br/>with a magnetic moment m can be seen as a dipole antenna due to the induced <br/>magnetic poles.<br/>In some applications it is desirable for a plurality of magnetic moments to <br/>have a<br/>common intersection but with different orientations. For example, dipole <br/>antennas could be<br/>arranged such that their magnetic moments point along mutually orthogonal <br/>directions. An<br/>arrangement of a plurality of dipole antennas wherein the induced magnetic <br/>moments are<br/>oriented orthogonally in three different directions is referred to as a <br/>triaxial orthogonal set of<br/>magnetic dipole antennas.<br/>A logging instrument equipped with an orthogonal set of magnetic dipole <br/>antennas offers<br/>advantages over an arrangement that uses standard solenoid coils distributed <br/>at different axial<br/>positions along the instrument with their axes in different orientations, such <br/>as proposed in U.S.<br/>Pat. No. 5,781,436.<br/>However, it is not convenient to build orthogonal magnetic dipole antennas <br/>with<br/>conventional solenoid coils due to the relatively small diameters required for <br/>logging<br/>instruments. Arrangements consisting of solenoid coils with their axes <br/>perpendicular to the<br/>well's central axis occupy a considerable amount of space within the logging <br/>instrument.<br/>In addition to the transmitter coils and the receiver coils, it is also <br/>generally necessary to<br/>equip the logging instrument with "bucking" coils in which the magnetic field <br/>induces an electric<br/>current in the receiver coils opposite and equal in magnitude to the current <br/>that is induced in the<br/>receiver coil when the instrument is disposed within a non-conducting medium <br/>such as, for<br/>example, air. Bucking coils can be connected in series either to the <br/>transmitter or the receiver<br/>coil. The receiver's output is set to zero by varying the axial distance <br/>between the transmitter or<br/>receiver coils and the bucking coils. This calibration method is usually known <br/>as mutual<br/>balancing.<br/> -3-<br/><br/> CA 02447468 2008-03-11<br/>79350-92<br/> Transverse magnetic fields are also useful for the<br/>implementation of NMR based methods. U.S. Pat.<br/> No. 5,602,557, for example, describes an arrangement that<br/>has a pair of conductor loops, each of which is formed by<br/>two saddle-shaped loops lying opposite one another and<br/>rotationally offset 90 relative to one another.<br/> A need remains for improved antenna structures and<br/>methods for producing same, particularly for antennas having<br/>oriented magnetic dipole moments.<br/> Summary of Invention<br/> One aspect of the invention provides an antenna<br/>adapted for a logging tool. The antenna comprises a core,<br/>the core including an electrical conductor disposed thereon<br/>such that the antenna has a first magnetic dipole moment<br/>substantially perpendicular to a longitudinal axis of the<br/>core.<br/> Another aspect of the invention provides a well<br/>logging tool. The tool comprises a support having at least<br/>one antenna mounted thereon and electrical circuitry coupled<br/>to the at least one antenna; wherein the at least one<br/>antenna comprises a dielectric core, the core having an<br/>electrical conductor disposed thereon to form a conductive<br/>path, the conductive path arranged to have a first magnetic<br/>dipole moment substantially perpendicular to a longitudinal<br/>axis of the core.<br/> Another aspect of the invention provides an<br/>antenna for use in a logging tool, comprising: a non-<br/>conductive core having an outer surface; and an electrical<br/>conductor printed on the non-conductive core; wherein the<br/>electrical conductor produces, when energized, a first<br/>- 4 -<br/><br/> CA 02447468 2008-03-11<br/>79350-92<br/>magnetic dipole moment substantially perpendicular to a<br/>longitudinal axis of the non-conductive core.<br/> Still another aspect of the invention provides a<br/>well logging tool comprising: a support having one or more<br/>antennas mounted thereon; and electrical circuitry coupled<br/>to the one or more antennas; wherein at least one of the one<br/>or more antennas comprises a non-conductive core having an<br/>electrical conductor printed thereon to form a conductive<br/>path, the conductive path being arranged to produce a first<br/>magnetic dipole moment substantially perpendicular to a<br/>longitudinal axis of the non-conductive core.<br/> Brief Description of Drawings<br/> FIG. 1 shows a logging instrument disposed in a<br/>well bore penetrating an earth formation.<br/> FIG. 2A is a schematic diagram of a transverse<br/>electromagnetic apparatus in accord with the invention.<br/>FIG. 2B is a schematic diagram of a transverse<br/> electromagnetic apparatus in accord with the invention.<br/>FIG. 3 is a schematic diagram of an antenna loop<br/> in accord with an embodiment of the invention.<br/> FIG. 4 is a schematic diagram of a transverse<br/>electromagnetic apparatus in accord with the invention.<br/>FIG. 5A is a diagram of a core structure of a<br/> transverse electromagnetic apparatus in accord with the<br/>invention.<br/>- 4a -<br/><br/> CA 02447468 2003-10-29<br/> FIG. 5B is a cross section of the core structure of FIG. 5A.<br/>FIG. 6 is a schematic diagram of a coil assembly in accord with the invention.<br/>FIG. 7A is a schematic diagram of a mutual balancing coil configuration in <br/>accord with<br/>the invention.<br/>FIG. 7B is a schematic diagram of another mutual balancing coil configuration <br/>in accord<br/>with the invention.<br/>FIG. 7C is a schematic diagram of another mutual balancing coil configuration <br/>in accord<br/>with the invention.<br/>FIG. 8 is a schematic diagram of a logging tool implementation in accord with <br/>the<br/>invention.<br/>FIG. 9 is a schematic diagram of another logging tool implementation in accord <br/>with the<br/>invention.<br/>FIG. 10 is a schematic diagram of an antenna configuration in accord with the <br/>invention.<br/>FIG. 11 illustrates a top view of the transverse electromagnetic apparatus as <br/>shown in<br/>FIG. 4.<br/>FIG. 12A shows an antenna configured with a printed conductive element in <br/>accord with<br/>the invention.<br/> FIG. 12B shows an exploded view of the indicated antenna section of FIG. 12A.<br/>FIG. 12C shows a cross-sectional view taken along a section line of FIG. 12B.<br/>FIG. 13 shows an antenna embodiment in accord with the invention.<br/>FIG. 14 is a schematic view of an antenna disposed within a downhole tool in <br/>accord<br/>with the invention.<br/>FIG. 15 is a flow chart of a process for producing an antenna in accord with <br/>the<br/>invention.<br/> -5-<br/><br/> CA 02447468 2003-10-29<br/> Detailed Description<br/>FIG. 1 shows a well (9) extending into an earth formation that includes layers <br/>of<br/>conductive (3) and non-conductive (5) material. A logging tool (7) is disposed <br/>within the well (9)<br/>on a wireline (11). The tool (7) includes transmitter coils (13), receiver <br/>coils (15) and bucking<br/>coils (17) with their axes parallel to the tool axis and thus the well axis. <br/>The magnetic field<br/>produced by the transmitter coils (13) induce eddy currents (19), which are <br/>detected by the<br/>receiver coils (15).<br/>FIG. 2A shows an arrangement for a transverse EM apparatus (21) in accordance <br/>with<br/>one embodiment of the invention. The transverse EM apparatus (21) includes a <br/>plurality of coils<br/>(23) disposed around a central axis (25) such that the coils' normal vectors <br/>(27) are<br/>perpendicular to the central axis (25).<br/>FIG. 2B shows another arrangement for the transverse EM apparatus (21) in <br/>accordance<br/>with an embodiment of the invention. In this case an additional coil (24) has <br/>been added to the<br/>arrangement of FIG. 2A such that its normal vector is parallel to central axis <br/>(25).<br/>FIGS. 2a and 2b show an orthogonal set of magnetic dipole antennas whose <br/>magnetic<br/>moments all have a common origin. This will provide, on a plane (26,28), i.e. <br/>at the same well<br/>depth, magnetic fields pointed in directions x,y for the arrangement of FIG. <br/>2A and x,y,z for the<br/>arrangement of FIG. 2B. A triaxial orthogonal set of magnetic dipole antennas, <br/>located at a<br/>selected distance from the transmitter, will correspondingly be able to <br/>receive and detect the<br/>eddy currents that travel in loops parallel and perpendicular to the tool <br/>axis.<br/>FIG. 3 shows one of the plurality of coils (23) of the invention in more <br/>detail. A coil (23)<br/>consists of two arcs (29) with their ends united by two lines (31). A current <br/>i traveling around the<br/>coil (23) induces a magnetic field B that surrounds each element of the coil. <br/>The y and z<br/>components of the magnetic field sum to zero due to the symmetry of the coil. <br/>Therefore, the coil<br/>has a magnetic moment m only parallel to the x coordinate.<br/>FIG. 4 shows an embodiment of a coil (23) of the invention. The coil (23) is <br/>composed of<br/>several loops (34) placed one within another. According to an embodiment of <br/>the invention, the<br/>coil (23) can be obtained by winding a single wire (55) around a central point <br/>(37).<br/>-6-<br/><br/> CA 02447468 2003-10-29<br/>The magnetic moments of the transverse dipole antenna embodiments of the <br/>invention<br/>can be determined as explained below.<br/>The modulus (M,,) of the magnetic moment m for a pair of coils (23) is equal <br/>to:<br/> M. = ZIxNXAxe~, (1)<br/>where IX is the current and NX is the number of turns and A~ is the <br/>approximate effective area<br/>defined by<br/>z /~<br/>efT rmandre! /;i<br/> AX = 2(r~a,! - )l hi sin , (2)<br/>rcoil i 2<br/>where h; is the saddle coil height, is the arc radius, rmandre! is the inner <br/>core radius, and l13; is<br/>the angle subtended by the arc formed by the coil as can be seen in FIG. 11. <br/>This result is a first<br/>approximation because the transverse magnetic moment is summed over all the <br/>turns forming the<br/>coil, since the angle ,l3! changes at each turn. It can be seen from Equation <br/>2 that the magnetic<br/>moment can be increased by increasing the height of the coil, where the arc <br/>radius is assumed<br/>constant.<br/>The modulus of the magnetic moment M,, of a saddle coil can be greater than <br/>the modulus<br/>of magnetic moment along the longitudinal axis of a solenoid coil for <br/>identical currents IX and IZ,<br/>where IZ is current of the solenoid coil typically used in well logging <br/>instruments. It can be<br/>shown that MZ of an axial solenoid wrapped on an insulator about a metal <br/>mandrel is<br/> MZ = I,N,Ar~, (3)<br/>where IZ is the axial current and N. is the axial number of turns and AZ~ is <br/>the effective area<br/>defined by<br/> Az~j = ~l~~i! - rmandre! 1- ~`reor! - rmandre! Xrini! + rmandre! ) , (4)<br/>where r.,;, is the coil radius.<br/>Next, the transmitter saddle-coil can be examined as a circuit constrained by <br/>its<br/>electrostatic characteristics. It can be shown that the resistance R, the <br/>inductance L, and the<br/>-7-<br/><br/> CA 02447468 2003-10-29<br/>capacitance C are all controlled by the geometry of the wire and/or trace. It <br/>is desirable to have a<br/>high quality factor Q, for example, for the transmitter, Q is defined as<br/> QWRL ~ (5)<br/>where w. is the resonant angular frequency of the circuit, R is the <br/>resistance, and L is the self-<br/>inductance of the saddle coil. The resistance of the coil is defined as<br/> R = e [1 + a(T - T(,)], (6)<br/>A<br/>where p is the resistivity, e is the total length of the wire, T is the <br/>temperature, To is the<br/>reference temperature, and A is the cross sectional area of the conductors <br/>that form the<br/>corresponding coil, ignoring skin depth effect. The approximate self-<br/>inductance of a saddle coil<br/>is given by the expression:<br/> aLnl 2a )+( bLn2 I+2 az +b2 - 5<br/> L= 0.004 p ' JN 3, (7)<br/>asinh~~-bsinh(a) -2(a+b)+ 4 (a + b)<br/>where a is the average width of the coil, b is the average height of the coil, <br/>p is the radius of the<br/>wire, u is the permeability constant, and N is the number of turns.<br/>It is desirable to obtain a quality factor (shown in Eq. 5) of around 10 to 20 <br/>for, for<br/>example, a saddle-coil transmitter. This can be achieved by increasing the <br/>resonance frequency<br/>of the corresponding circuit, increasing L, or decreasing R. A large quality <br/>factor Q may be<br/>achieved by using higher operating frequencies, with the caveat that the <br/>operating frequency<br/>affects the depth of investigation. For example, typical induction-type <br/>measurements would<br/>require frequencies around 15 kHz to 50 kHz, L can be increased by increasing <br/>b andJor N, but<br/>this would place demands on the magnitude of the capacitor (Wo =1I LC ) needed <br/>to series or<br/>parallel tune, for example, the transmitter circuit. It is also possible to <br/>decrease R by increasing<br/>the cross sectional area of the conductor.<br/> The self-resonance of the saddle coils is given by<br/>-8-<br/><br/> CA 02447468 2003-10-29<br/>(8)<br/>LCdA r<br/>where Cd;s, is the distributed capacitance per unit length of parallel wires. <br/>The approximate<br/>formula for the capacitance of two parallel wires is<br/> ~n<br/>Cdist _ (9)<br/>- ,<br/> cosh-' -<br/>a (C)<br/>where c is the distance between the conductors and a is the radius of the <br/>conductors. It is<br/>preferable that the resonance frequency wo be less than ws/3.<br/>Examination of the derived equations shows that the values of R, L, and C for <br/>the coils<br/>(23) can be controlled by varying, for example, the coil height h; and the <br/>number of turns N that<br/>form the coil. Equation 6 shows that the resistance R can be varied by <br/>altering these parameters.<br/>Similarly, the capacitance C can be controlled by either increasing or <br/>decreasing the distance<br/>between the conductors that fonm each turn, as derived from Equation 9.<br/>A transverse EM apparatus (32) according to one of the embodiments of the <br/>invention is<br/>shown in FIG. 4. The apparatus consists of a core (39) made out of dielectric <br/>material on which a<br/>plurality of coils (23) are mounted. The dielectric material can be ceramic, <br/>fiberglass, or other<br/>suitable materials and composites known in the art. According to one <br/>embodiment of the<br/>invention, the core (39) consists of an annular cylinder in which a metal rod <br/>(41) is inserted.<br/>The invention includes several configurations for disposing the coils (23) on <br/>the core<br/>(39). FIG. 5A and 5b show a core (39) in which specific cuts have been made to <br/>guide and retain<br/>the loops. The core (39) is composed of pin sections (41,41') and a channel <br/>section (43). The pin<br/>sections (41,41') are located at the core's ends and include a plurality of <br/>pins (45) in a matrix<br/>type arrangement. The channel section (43) is located between the pin sections <br/>(41,41) and is<br/>formed by a plurality of channels (47) that are parallel to the core's <br/>longitudinal axis<br/>(represented by a dashed line in FIG. 5B) and aligned with the channels (49) <br/>formed between the<br/>columns of the pin's matrix arrangement. The channels (49) provide guiding <br/>paths for inserting<br/>the conductors or wires (55) that form the coil(s).<br/>-9-<br/><br/> CA 02447468 2003-10-29<br/>A loop (51) is formed by inserting the wire in the channels (47) and wrapping <br/>a desired<br/>area (53) that includes both pin sections (41,41') and the channel section <br/>(43). For example, in<br/>order to form a loop, the wire (55) is inserted at one pin section (41') in a <br/>channel (49), the wire<br/>is then turned at a selected pin (45) and brought to the opposite pin section <br/>(41) by introducing it<br/>in the corresponding channels (47) of the channel section (43). Similarly, at <br/>the opposite pin<br/>section (41) the wire, exiting the channel (47) from the channel section (43), <br/>enters a<br/>corresponding channel (49). The wire (55) follows the channel (49) till the <br/>desired pins (45) are<br/>reached where the wire (55) is turned around and returned to the other pin <br/>section (41') through a<br/>corresponding channel (47). An additional loop (59) can be placed within a <br/>previously made<br/>loop (51) by repeating the procedure to cover a smaller area (61). The <br/>transverse EM apparatus<br/>(32) of FIG. 4 is an embodiment made by repeating this procedure to form a <br/>structure with as<br/>many coils as desired.<br/>In one embodiment of the invention the pins (45) are slanted with respect to <br/>the core's<br/>(39) outer surface (63). The slanting is directed toward the core (39) ends. <br/>The pins' orientation<br/>enables the wire (55) to be maintained in contact with the core's outer <br/>surface (63). Thus the<br/>wire (55) is also maintained within the corresponding channels (49). The <br/>slanted pins (45) also<br/>permit the wires to be held tighter to the core's outer surface, eliminating <br/>slack in the wire. The<br/>corners (65) of the slanted pins may be rounded to avoid damage to the wire <br/>(55).<br/>FIG. 6 shows another embodiment of the invention. In this embodiment, the <br/>coils (33) are<br/>affixed to an insulating sheet (67) according to the desired pattern. The <br/>coils (33) may be formed<br/>from any suitable electrical conductor, including wire or metallic foil. <br/>Alternatively, the coils<br/>may be formed by the deposition of conductive films on the insulating sheet as <br/>known in the art.<br/>Adhesives (e.g. polyimides, epoxies, and acrylics) may be used to bond the <br/>conductor to the<br/>insulating sheet.<br/>In the embodiment of FIG. 6, a plurality of coils (33) are disposed side by <br/>side and placed<br/>on an insulating sheet (67) to form a flexible circuit (69). Conductors (71) <br/>provide the<br/>corresponding electrical connection for energizing the coils (33). The <br/>flexible circuit (69) can be<br/>conformed about the core's exterior and attached to it via adhesives or <br/>mechanical fasteners. The<br/>insulating sheet can be any electrically nonconductive or dielectric film <br/>substrate, such as<br/>polyimide film or a polyester film having a thickness selected to enable <br/>bending or flexing.<br/>io-<br/><br/> CA 02447468 2003-10-29<br/>Methods used to produce the insulating sheet are described in U.S. Pat. No. <br/>6,208,031,<br/>incorporated by reference. The conductors (71) that are used to interconnect <br/>the coils (33) are<br/>preferably placed on the layers closest to the outside diameter of the <br/>invention. This aids in<br/>minimizing conductor (71) compression and forces the conductors (71) into <br/>tension, which<br/>greatly improves the reliability of the invention.<br/>The invention also includes techniques for mutually balancing a dipole <br/>antenna. FIGS. 7a<br/>and 7b show independently mutually balanced dipole antenna (73,74) embodiments <br/>of the<br/>invention. One technique entails selecting one or more loops within a main <br/>coil (75, 76). The<br/>selected loops constitute a separate coil (77, 78), referred to as a mutual <br/>balancing coil.<br/>A mutual balancing process of the invention entails cutting or leaving out <br/>several loops<br/>between the mutual balancing coil (77, 78) and the main coil (75, 76), thereby <br/>leaving a gap (79,<br/>80) between the coils, as shown in FIGS. 7a and 7b. In FIG. 7B, the mutual <br/>balancing<br/>arrangement is adapted to the core (74) as describe above, having channels to <br/>host the<br/>corresponding mutual balancing coil (78) and main coil (76), separated by a <br/>gap (80).<br/>FIG. 7C shows another antenna (74) embodiment of the invention adapted for <br/>mutual<br/>balancing. According to this embodiment, individual conductive elements or <br/>disks (72) are<br/>placed on the antenna within the main coil (76). This embodiment allows one to <br/>balance the<br/>antenna by placing appropriately sized disks (72) on the antenna until the <br/>desired balancing is<br/>achieved. The disks (72) may be formed of any conductive element, e.g. copper. <br/>The disks (72)<br/>may be bonded or affixed to the substrate using any suitable adhesive. The <br/>disk(s) (72) may also<br/>be placed within a recess formed in the substrate itself (not shown). <br/>Alternatively, the disk(s)<br/>may also be affixed to the sealer or potting compound (not shown) conunonly <br/>used to mount<br/>antennas on logging instruments as known in the art.<br/>The interleaved conductive loops forming the balancing coils (77, 78) and the <br/>conductive<br/>disks (72) excite opposing currents (by Lenz's law) that oppose the generated <br/>magnetic field to<br/>effectively reduce the magnetic moment of the main coil (75,76). These <br/>mutually balancing<br/>antennas of the invention provide greater flexibility for the placement of <br/>receiver arrays at<br/>different points along the tool axis. The mutual balancing antenna <br/>configurations of the<br/>invention may be used as receiver or bucking antennas.<br/>-ll-<br/><br/> CA 02447468 2003-10-29<br/>FIG. 8 shows a logging tool (80), according to one embodiment of the <br/>invention,<br/>disposed within a well on a wireline (11). The tool (80) has a transmitter <br/>antenna (81), a bucking<br/>antenna (83), and a receiver antenna (87). The bucking antenna (83) can be <br/>connected in inverse<br/>polarity to either the transmitter antenna (81) or to the receiver antenna <br/>(87). Transmitter<br/>electronic circuitry (89) is connected to the transmitter antenna (81) to <br/>provide time-varying<br/>electric currents to induce time-varying magnetic fields. Power supply (91) <br/>feeds the circuitry<br/>(89). Receiver circuitry (85) is connected to the receiver antenna (83) to <br/>detect and measure<br/>resulting EM signals.<br/>According to one embodiment of the invention, the bucking antenna (83) can be <br/>omitted<br/>by using a transmitter antenna (81) or a receiver antenna (87) adapted for <br/>independent mutual<br/>balancing as shown in FIGS. 7a, 7b, and 7c.<br/>FIG. 9 shows a drilling tool (92) disposed in a well (9) according to one <br/>embodiment of<br/>the invention. The drilling tool (92) has a transmitter antenna (93), a <br/>bucking antenna (95), and a<br/>receiver antenna (97). The bucking antenna (95) can be connected with an <br/>inverse polarity to<br/>either the transmitter antenna (93) or to the receiver antenna (97). The <br/>transmitter electronic<br/>circuitry (99) is connected to the transmitter antenna (93) to provide time-<br/>varying electric<br/>currents to induce time-varying magnetic fields. Power supply (103) feeds the <br/>circuitry (99).<br/>Receiver circuitry (101) is connected to the receiver antenna (97) to detect <br/>and measure resulting<br/>EM signals. The bucking antenna (95) may also be omitted in another embodiment <br/>by using<br/>antennas adapted for independent mutual balancing as shown in FIGS. 7a, 7b, <br/>and 7c. However,<br/>this may reduce effectiveness where one desires Mx, M, MZ to have a common <br/>origin.<br/>Those skilled in the art will appreciate that the antenna apparatus of the <br/>invention are not<br/>limited to use in any one particular type of measurement or exploration <br/>operation and that they<br/>may be disposed within a well bore on any type of support member, e.g., on <br/>coiled tubing, drill<br/>collars, or wireline tools.<br/>Parameters for the independently mutually balanced antennas (77, 78) of the <br/>invention<br/>are now presented. Cancellation of the undesired mutual coupling results in <br/>the following<br/>relationship:<br/> NeAe _ NRAR<br/>(10)<br/>LB LR<br/> -12-<br/><br/> CA 02447468 2003-10-29<br/>where the subscripts B and R represent the mutual balancing coil and the <br/>receiver coil,<br/>respectively, and N is the number of turns, A is the effective area of the <br/>coil, and L is the<br/>distance from the transmitter coil.<br/> Solving Equation 10 for AB gives the expression:<br/>3<br/> AB = NR LB AR . (11)<br/>NB LR<br/>Translation of the transverse coil for a small OLh is problematic, therefore a <br/>comparable AAB is<br/>added. To this end, the following relationship of a physical derivative is <br/>considered:<br/> AAB = dLB OLB (12)<br/>e<br/>For this statement to be true, the loop of area AAB should have an inductance <br/>much<br/>greater than its DC resistance. This is generally true because the resistance <br/>of a loop is typically<br/>in the sub-milli-ohm range. The inductance of a small circular loop of wire <br/>is:<br/> L0 =,u(2r-a 1- ~ K(k)-E(k) , (13)<br/>where a is the conductor radius, r is the loop radius, K(k)and E(k)are <br/>elliptic<br/>integrals, and<br/>kz = 4r(r - a) (14)<br/>(2r - a)z<br/>Put another way, this loop should generate a small opposing complex voltage in <br/>the<br/>receiver/bucking coil circuit. Equation 12 can be rewritten as<br/>z<br/>AAB = 3AROL NR LB<br/> ~ 3 . (15)<br/>NB LR<br/> The bucking loop radius can thus be shown to be<br/>y<br/>~B or r = 3ABOLB . (16)<br/>~c KLB<br/>-13-<br/><br/> CA 02447468 2003-10-29<br/>FIG. 10 shows an arrangement for a transmitter or receiver antenna according <br/>to an<br/>embodiment of the invention. This arrangement consists of a transverse EM <br/>antenna pair (105)<br/>(similar to FIG. 4) combined with a solenoid coil (107) oriented so that its <br/>dipole moment is<br/>parallel to the longitudinal axis of the instrument (represented by the z-<br/>axis). The solenoid coil<br/>(107) is surrounded by coils (109) that have their magnetic moments <br/>perpendicular to the<br/>solenoid's magnetic moment.<br/>Other embodiments of the invention may be implemented by "printing" the <br/>conductive<br/>coil(s) or elements directly onto the non-conductive core material through <br/>plating or other<br/>conventional deposition processes. One such embodiment comprises plating the <br/>entire outer<br/>diameter of the core with a conductive material and etching away the excess to <br/>form the coil.<br/>Another embodiment entails selectively plating only the shape of the coil onto <br/>the core through<br/>the use of masking techniques known in the art. Additional embodiments may <br/>also be<br/>implemented using other thin film growth techniques known in the art, such as <br/>spray coating and<br/>liquid phase epitaxy.<br/>Several processes are known to entirely or selectively coat a dielectric <br/>material with a<br/>conductive material such as copper. These include, but are not limited to, <br/>electroless plating and<br/>the various vapor deposition processes. These techniques allow one to produce <br/>a copper (or<br/>other conductive material) overlay in the shape of a saddle coil onto a <br/>ceramic or other dielectric<br/>material core.<br/>Electroless plating is one technique that may be used to implement the <br/>invention. This<br/>plating process enables the metal coating of non-conductive materials, such as <br/>plastics, glasses<br/>and ceramics. Compared to electroplating, the coatings derived from <br/>electroless plating are<br/>usually more uniform. The deposition is carried out in liquids (solutions), <br/>and is based on<br/>chemical reactions (mainly reductions), without an external source of electric <br/>current.<br/>Electroless plating is further described in GLENN O. MALLORY & JUAN B. HAJDU, <br/>ELECTROLESS<br/>PLATING (William Andrew Publishing, ISBN 0-8155-1277-7) (1990).<br/> Other embodiments of the invention may be implemented using known thin film<br/>deposition techniques. Deposition is the transformation of vapors into solids, <br/>frequently used to<br/>grow solid thin film and powder materials. Deposition techniques are further <br/>described in<br/>-14-<br/><br/> CA 02447468 2003-10-29<br/>KRISHNA SESHAN, HANDBOOK OF THIN FILM DEPOSITION PROCESSES AND TECHNIQUES, <br/>(William<br/>Andrew Publishing, ISBN 0-8155-1442-5) (2001).<br/>FIG. 12A shows an embodiment of the invention derived using a thin film <br/>technique as<br/>described above. As described above, the core (39) may be formed of any <br/>suitable dielectric<br/>material. It will be appreciated that practically any desired coil patterns <br/>may be derived using<br/>these techniques, including the mutual balancing configurations disclosed <br/>herein. Conductive<br/>disks (see item 72 in FIG. 7C) may also be added to the core (39) using these <br/>techniques.<br/>Connection points are shown at (40) for coupling the conductors to independent <br/>circuitry. FIG.<br/>12B shows an exploded view of the indicated antenna section of FIG. 12A, <br/>illustrating the<br/>conductor disposed on the core (39) surface. In this embodiment the non-coated <br/>core (39') has<br/>been masked during plating. Alternatively, the plating may also be removed <br/>from this area to<br/>form the desired pattern. FIG. 12C shows a cross-sectional view of the antenna <br/>(74) taken along<br/>a section of FIG. 12B. The conductive material is disposed on the outer <br/>surface of the core (39)<br/>to form the coil (23).<br/>Advantages of these printed coil embodiments include a more robust joint <br/>between the<br/>conductor and the dielectric core, which may be stronger than either material <br/>alone. Thus<br/>providing an antenna that can withstand the stresses and strains encountered <br/>in the downhole<br/>environment, particularly in while-drilling applications. The core is also <br/>easier to produce since<br/>it is basically featureless.<br/>While the antennas disclosed herein are generally shown as a one-piece annular <br/>surface<br/>of revolution, other embodiments of the invention may be implemented with the <br/>core formed in<br/>individual segments having individual conductive elements disposed thereon by <br/>any of the<br/>disclosed techniques. FIG. 13 shows such an embodiment. The core (39) provides <br/>a base<br/>forming a surface covering a ninety-degree sector. An independent saddle coil <br/>(23) is disposed<br/>thereon. Although the antenna (74) of FIG. 13 has an arcuate shaped core (39), <br/>it may be formed<br/>in practically any desired shape.<br/>Another embodiment of the invention may include a semi-curved or flat core <br/>(39), which<br/>can be disposed within a pocket or recess (120) formed in the logging/drilling <br/>tool (80, 90) as<br/>shown in FIG. 14. Feed thru wires (130/132) are run along the recess to <br/>connect to the coil (23)<br/>on the core (39) surface. The wires (130/132) couple the coil (23) to <br/>conventional electronics<br/>-I5-<br/><br/> CA 02447468 2003-10-29<br/>(not shown) adapted to energize the antenna with alternating current to <br/>transmit electromagnetic<br/>energy or to receive signals responsive to the receipt of electromagnetic <br/>energy as known in the<br/>art. A rubber overmold may also be disposed over the core (39) segment to <br/>completely<br/>encompasses the antenna (74) (not shown). A shield (not shown) may also be <br/>placed over the<br/>antenna (74) to protect the coil or provide electromagnetic energy focusing as <br/>known in the art.<br/>One or more of these independent antennas 74 could be placed on a downhole <br/>tool to provide a<br/>transverse magnetic dipole where desired with relative ease and repairs or <br/>replacement could be<br/>done in the field, reducing cost and delay.<br/>FIG. 15 illustrates a process for producing an antenna of the invention. An <br/>electrical<br/>conductor is disposed on a dielectric core at step (200). The conductor forms <br/>a conductive path<br/>arranged to have a first magnetic dipole moment substantially perpendicular to <br/>a longitudinal<br/>axis of the core. At step (205), the electrical conductor is adapted to be <br/>coupled with<br/>independent circuitry as known in the art.<br/>While the invention has been described with respect to a limited number of <br/>embodiments,<br/>those skilled in the art will appreciate that other embodiments can be devised <br/>which do not<br/>depart from the scope of the invention as disclosed herein. For example, the <br/>antennas of the<br/>invention may be configured using a combination of printed and wired coils. <br/>Multiple overlaid<br/>substrates may also be used to achieve modified couplings or to alter the <br/>magnetic moment(s) as<br/>desired. Using multiple-layered substrates would allow for antennas to be <br/>collocated on the<br/>support, e.g., a bucking and a receiver antenna. It will also be appreciated <br/>that the embodiments<br/>of the invention are not limited to any particular material for their <br/>construction. Any suitable<br/>material or compounds (presently known or developed in the future) may be used <br/>to form the<br/>embodiments of the invention provided they allow for operation as described <br/>herein.<br/>-16-<br/>
