Well logging apparatus for obtaining azimuthally sensitive formation resistivity measurements Number:7,436,184 from the United States Patent and Trademark Office (PTO) owispatent

Title: Well logging apparatus for obtaining azimuthally sensitive formation resistivity measurements

Abstract: An apparatus for making azimuthally sensitive resistivity measurements of a subterranean formation is disclosed. The apparatus includes a magnetically permeably ring deployed about an electrically conductive tool body. An AC voltage supply is coupled to the tool body on opposing sides of the magnetically permeable ring, with at least one connecting conductor crossing outside the ring. Exemplary embodiments of this invention may further include one or more current sensing electrodes deployed in and electrically isolated from an outer surface of the tool body and may be utilized to make azimuthally resolved formation resistivity measurements.

Patent Number: 7,436,184 Issued on 10/14/2008 to Moore

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Inventors:	Moore; Robert A (Katy, TX)
Assignee:	PathFinder Energy Services, Inc. (Houston, TX)
Appl. No.:	11/080,777
Filed:	March 15, 2005

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Current U.S. Class:	324/347 ; 166/66.5; 324/338; 324/355
Current International Class:	G01V 3/08 (20060101); E21B 31/06 (20060101)
Field of Search:	324/338-358,369,366,333,368 166/66.5 702/6,7,9 367/25,82 175/50,406

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References Cited [Referenced By]

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Primary Examiner: Aurora; Reena

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Claims

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I claim:

1. A logging while drilling resistivity tool comprising: a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends, the tool body disposed to be operatively coupled with a drill string and deployed in a subterranean borehole such that the logging while drilling tool may acquire resistivity measurements without removing the drill string from the borehole; at least one magnetically permeable ring deployed about the tool body between the first and second longitudinally opposed ends thereof; an AC voltage supply electrically connected to the first and second ends of the tool body with at least one connecting conductor deployed exterior to an outer surface of the magnetically permeable ring; and a receiver.

2. The logging while drilling tool of claim 1, wherein the magnetically permeable ring has a relative permeability of greater than about 10,000.

3. The logging while drilling tool of claim 1, wherein the receiver comprises at least one electrode deployed in an outer surface of the tool body, an outer surface of the electrode electrically isolated from the outer surface of the tool body.

4. The logging while drilling tool of claim 3, wherein the receiver comprises a plurality of electrodes longitudinally spaced along the tool body.

5. The logging while drilling tool of claim 3, wherein the receiver comprises a plurality of electrodes circumferentially spaced about the tool body.

6. The logging while drilling tool of claim 1, wherein the at least one connecting conductor comprises a rigid member deployed about the magnetically permeable ring, the voltage supply being connected to the second end of the tool body via the member, the member providing an electrically conductive path exterior to an outer surface of the magnetically permeable ring.

7. The logging while drilling tool of claim 1, wherein the magnetically permeable ring is deployed in a recess in an outer surface of the tool body.

8. The logging while drilling tool of claim 1, wherein the receiver comprises a toroidal receiver deployed about the tool body, the toroidal receiver including an electrical conductor wound about a magnetically permeable toroidal core.

9. A logging while drilling tool comprising: a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends; at least one magnetically permeable ring deployed about the tool body between the first and second longitudinally opposed ends thereof; an AC voltage supply electrically connected to the first and second ends of the tool body with at least one connecting conductor deployed exterior to an outer surface of the magnetically permeable ring; at least one current sensing electrode deployed in an outer surface of the tool body.

10. The logging while drilling tool of claim 9, comprising a plurality of current sensing electrodes longitudinally spaced along the tool body.

11. The logging while drilling tool of claim 9, comprising a plurality of current sensing electrodes circumferentially spaced about the tool body.

12. The logging while drilling tool of claim 9, wherein: a neck portion of the current sensing electrode is mechanically and electrically connected to the tool body; and an outer surface of the current sensing electrode is electrically isolated from the outer surface of the tool body.

13. The logging while drilling tool of claim 12, further comprising a current measuring transformer deployed about the neck portion of the current sensing electrode.

14. A logging while drilling tool comprising: a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends; at least one magnetically permeable ring deployed about the tool body between the first and second longitudinally opposed ends thereof; an electrically conductive, rigid sleeve deployed about the magnetically permeable ring; an AC voltage supply electrically connected to the first end of the tool body and the sleeve; at least one electrode deployed in an outer surface of the tool body, an outer surface of the electrode electrically isolated from the outer surface of the tool body; and at least one current measurement module disposed to measure electrical current in the electrode.

15. The logging while drilling tool of claim 14, wherein the sleeve is threadably coupled with the tool body.

16. The logging while drilling tool of claim 14, further comprising an insulating layer deployed between the magnetically permeable ring and at least one of the tool body and the sleeve.

17. The logging while drilling tool of claim 14 wherein the magnetically permeable ring is deployed in the sleeve.

18. The logging while drilling tool of claim 14, wherein the magnetically permeable ring is deployed in a recess in the tool body.

19. The logging while drilling tool of claim 14, comprising a plurality of electrodes longitudinally spaced along the tool body.

20. The logging while drilling tool of claim 14, comprising a plurality of electrodes circumferentially spaced about the tool body.

21. A logging while drilling tool comprising: a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends; longitudinally spaced first and second magnetically permeable rings deployed about the tool body, a central region of the tool body located between the first and second magnetically permeable rings; at least one AC voltage supply disposed to provide an AC voltage difference between the central region of the tool body and the longitudinally opposed ends of the tool body with at least one connecting conductor deployed exterior to an outer surface of each of the rings; at least one electrode deployed in an outer surface of the central region of the tool body, an outer surface of the electrode electrically isolated from an outer surface of the tool body; and at least one current measurement module disposed to measure electrical current in the electrode.

22. The logging while drilling tool of claim 21, wherein a first terminal of the voltage supply is electrically connected to the central region of the tool body and a second terminal of the voltage supply is electrically connected to each of the first and second ends of the tool body.

23. The logging while drilling tool of claim 21, comprising first and second AC voltage supplies, the first AC voltage supply electrically connected to the first end and the central region of the tool body, at least one connecting conductor between the first end and the central region of the tool body deployed external the first magnetically permeable ring, the second AC voltage supply electrically connected to the second end and the central region of the tool body, at least one connecting conductor between the second end and the central region of the tool body deployed external to the second magnetically permeable ring.

24. The logging while drilling tool of claim 21, further comprising first and second rigid sleeves deployed about the corresponding first and second magnetically permeable rings.

25. An inductive choke for a logging while drilling tool, the inductive choke comprising: an electrically conductive drill collar; an electrically conductive, rigid sleeve mechanically and electrically coupled to the drill collar, the rigid sleeve deployed about an outer surface of the drill collar; and a magnetically permeable ring deployed about an outer surface of the drill collar, the magnetically permeable ring deployed radially between an inner surface of the sleeve and the outer surface of the drill collar.

26. The inductive choke of claim 25 further comprising an AC voltage supply electrically connected to the drill collar and the sleeve.

27. The inductive choke of claim 25, further comprising an electrical insulating layer deployed radially between the magnetically permeable ring and at least one of the drill collar and the sleeve.

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Description

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BACKGROUND OF THE INVENTION

The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. One such electrical measurement technique is utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Any references to the determination or use of resistivity in this application are intended to generically include conductivity as well. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.

Prior art logging while drilling tools utilized to measure formation resistivity, typically utilize one or more wound toroidal core antennas (also referred to as toroidal transmitters and toroidal receivers) deployed in an insulating media along the exterior of the drill collar. As generally described in the prior art, the wound toroidal core antenna induces an electrical current in the drill collar. The electrical current enters the formation on one side of the toroidal transmitter and returns to the drill collar on the other side of the toroidal transmitter. Measurement of the current enables a formation resistivity to be determined.

For example, Redwine et al., in U.S. Pat. No. 3,408,561, disclose an LWD apparatus in which a toroidal receiver is deployed about a drill collar near the drill bit and a toroidal transmitter is deployed about the drill collar uphole of the toroidal receiver. In use, the voltage induced in the toroidal receiver provides an indication of the resistivity of the formation. Aarps, in U.S. Pat. No. 3,305,771, discloses a similar apparatus, but including a pair of toroidal transmitters and a pair of toroidal receivers.

Clark et al., in U.S. Pat. No. 5,235,285, disclose a technique intended to provide vertically and azimuthally resolved resistivity at multiple depths of investigation. An LWD tool including a tubular drill collar having longitudinally spaced first and second wound toroidal core antennas is utilized. The upper antenna is configured as a transmitter while the lower antenna is configured as a receiver. The tool further includes three longitudinally spaced button electrodes deployed in the drill collar between the wound toroidal core antennas. The button electrodes are intended to provide a return path for electrical current flow from the formation to the drill collar, with the current in the button electrodes being measured to determine a lateral resistivity of the regions of the formation opposing the electrodes. The longitudinal spacing of the button electrodes is intended to provide vertically resolved resistivity at multiple depths of investigation. Clark et al. further disclose rotating the drill collar to obtain azimuthally resolved resistivity.

The above described prior art resistivity tools are similar in that each includes two or more wound toroidal core antennas (one configured as a transmitter and the other configured as a receiver) deployed about a drill collar. These antennas create inductive impedances along the otherwise highly conductive drill collar. It is also known in the art to use such inductance in impede the unwanted flow of electrical current into other sections of the drill string or bottom hole assembly. For example, in one such device, magnetically permeable rings are deployed about an electrically conductive drill collar. The rings are positioned below a resistivity tool having wound toroidal antennas, and thus increase the electrical impedance between the resistivity tool and the adjacent bottom hole assembly. A protective, fiberglass sleeve may be deployed around the magnetically permeable rings to reduce the risk of mechanical damage to the rings. This type of device is sometimes referred to as an inductive choke.

While prior art LWD resistivity tools have been used successfully in comrnercial drilling applications, utilization of a multiple turn toroidal transformer is often problematic. A typical wound toroidal core antenna has a primary winding including many turns of insulated wiring about a toroidal core. Construction and protection of the relatively large toroidal core (e.g., typically having a diameter in the range of 4 to 10 inches) and winding are problematic, especially for use in the demanding downhole environment associated with geophysical drilling. Wound toroidal core antennas utilized in drilling applications are subject to high temperatures (e.g., as high as 200 degrees C.) and pressures (e.g., as high as 15,000 psi) as well as various (often severe) mechanical forces, including, for example, shocks and vibrations up to about 650 G per millisecond. Mechanical abrasion from cuttings in the drilling fluid and direct hits on the antenna (e.g., from drill string collisions with the borehole wall) have been known to damage wound toroidal core antennas. Moreover, it is typically expensive to fabricate and maintain wound toroidal core antennas capable of withstanding the above described downhole environment.

Therefore, there exists a need for an improved apparatus for making azimuthally sensitive resistivity measurements of a subterranean formation. In particular, an apparatus not requiring a wound toroidal core antenna may be potentially advantageous for making such azimuthally sensitive resistivity measurements in LWD applications.

SUMMARY OF THE INVENTION

The present invention addresses one or more of the above-described drawbacks of prior art techniques for making azimuthally sensitive resistivity measurements of a subterranean formation. Embodiments of this invention include at least one magnetically permeable ring deployed about an electrically conductive tool body. The tool body is configured for coupling with a drill string. An AC voltage supply is coupled to the tool body on opposing sides of the magnetically permeable ring, with at least one connecting conductor crossing outside the ring. The magnetically permeable ring decreases the admittance of the tool body (i.e., increases the resistance to flow of alternating current) such that an AC voltage difference may be sustained between the opposing sides of the tool body. Exemplary embodiments of this invention may further include one or more current sensing electrodes deployed in and electrically isolated from an outer surface of the tool body. In such exemplary embodiments, azimuthally sensitive formation resistivity may be determined via measurement of the AC current in the electrode(s). Rotation of the tool in the borehole and measurement of the azimuth via a conventional azimuth sensor enables one to determine the azimuthal variation of formation resistivity.

Exemplary embodiments of the present invention may advantageously provide several technical advantages. For example, embodiments of this invention do not require the use of a toroidal transmitter or a toroidal receiver deployed about the tool body. Rather, the combination of the AC voltage supply coupled directly to the tool body and the magnetically permeable ring(s) function as a transmitter. As such, exemplary embodiments of this invention may provide for improved reliability at reduced costs as compared to prior art azimuthal resistivity tools.

In one aspect the present invention includes a logging while drilling resistivity tool. The tool includes a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends. The tool body is disposed to be operatively coupled with a drill string and deployed in a subterranean borehole such that the logging while drilling tool may acquire resistivity measurements without removing the drill string from the borehole. The tool also includes at least one magnetically permeable ring deployed about the tool body between the first and second longitudinally opposed ends and an AC voltage supply electrically connected to the first and second ends of the tool body. At least one connecting conductor is deployed exterior to an outer surface of the magnetically permeable ring. The tool also includes a receiver.

In another aspect, this invention includes a logging while drilling tool including a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends. The tool also includes at least one magnetically permeable ring deployed about the tool body between the first and second longitudinally opposed ends and an electrically conductive, rigid sleeve deployed about the magnetically permeable ring. The tool further includes an AC voltage supply electrically connected to the first end of the tool body and the sleeve. At least one electrode is deployed in an outer surface of the tool body and a current measurement module is disposed to measure electrical current in the electrode.

In still another aspect this invention includes a downhole tool. The downhole hole tool includes a substantially cylindrical, electrically conductive tool body including first and second longitudinally opposed ends. The tool body further includes a blade deployed thereon, the blade being configured to extend outward from the tool body. At least one magnetically permeable ring is deployed about the tool body between the first and second longitudinally opposed ends, and an AC voltage supply is electrically connected to the first and second ends of the tool body. The downhole tool further includes at least one current sensing electrode deployed in an outer surface of the blade.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic representation of a portion of a prior art downhole tool having a toroidal transformer deployed about a drill collar.

FIG. 1B depicts an electrical circuit representation of the prior art tool shown on FIG. 1A.

FIG. 2 is a schematic representation of an offshore oil and/or gas drilling platform utilizing an exemplary embodiment of the present invention.

FIG. 3A is a schematic representation of a portion of a downhole tool according to the present invention.

FIG. 3B depicts an electrical circuit representation of the tool shown on FIG. 3A.

FIG. 3C depicts an exemplary electrical circuit representation of the tool shown on FIG. 3A deployed in a subterranean borehole.

FIG. 4 depicts an exemplary embodiment of a downhole tool according to the present invention.

FIG. 5A depicts, in cross section, a portion of the embodiment of FIG. 4 showing an exemplary sleeve assembly deployed about the tool body.

FIG. 5B depicts, in cross section, a portion of the embodiment of FIG. 4 showing an exemplary electrode deployed in a tool body.

FIG. 6 depicts an alternative embodiment of a downhole tool according the present invention.

DETAILED DESCRIPTION

FIG. 1A schematically illustrates a wound toroidal core antenna 90 deployed about a drill collar 80 as utilized in various prior art downhole resistivity measurement tools. In a typical prior art apparatus, the wound toroidal core antenna 90 includes a toroidal core 94 having multiple windings (N-turns) of insulated wire 96 wrapped thereabout. An AC voltage supply 92 is coupled to the ends of the insulated wire 96. AC current passing through the windings induces a magnetic field in the toroidal core 94 circumferentially about the drill collar 80. The circumferential magnetic field further induces an AC potential difference in the drill collar 80 such that there is a potential difference between upper 82 and lower 84 sides thereof. It will be appreciated that terms used in this disclosure such as "upper" and "lower" are intended merely to show relative positional relationships in the described exemplary embodiments and are not limiting of the invention in any way. As described briefly above in the Background Section of this disclosure, the potential difference causes an electrical current to flow from one side of the drill collar (e.g., upper side 82) through the borehole and subterranean formation to the other side of the drill collar (e.g., lower side 84). Such current flow through the formation (induced by the wound toroidal core antenna 90) and measurement thereof is the basis for certain prior art resistivity logging techniques.

With further reference now to FIG. 1B, an electrical circuit representation of the prior art arrangement shown in FIG. 1A is illustrated. As described in the prior art, the combination of the wound toroidal core antenna 90 and drill collar 80, as shown in FIG. 1A, is essentially a N:1 stepdown transformer. Thus, when AC voltage source 92 (providing V volts) is coupled to the N-turn primary winding 96, a secondary voltage with a magnitude of V/N is induced between the upper 82 and lower 84 sides of the drill collar. The two sides 82 and 84 of the drill collar are approximately separate quasi-equipotential surfaces having a potential difference of V/N.

One possible alternative approach for providing a potential difference between upper and lower portions of a drill collar is to electrically isolate the two portions of the drill collar. For example, an electrical insulator may be deployed between the two portions of the drill collar and a voltage may be applied therebetween, for example via a conventional AC voltage supply. While such an approach is seemingly straightforward, it is not likely to provide a viable solution. Of particular significance, a drill collar having first and second portions separated by an electrical insulator is not rigid enough for downhole drilling applications owing to the relatively poor mechanical properties of conventional electrical insulators (as compared, for example, to stainless steel). Thus, another alternative approach is required in order to replace wound toroidal core antennas in certain downhole resistivity measurement tools.

Referring now to FIGS. 2 through 6, exemplary embodiments of the present invention are illustrated. FIG. 2 schematically illustrates one exemplary embodiment of a logging while drilling tool 100 in use in an offshore oil or gas drilling assembly, generally denoted 10. In FIG. 2, a semisubmersible drilling platform 12 is positioned over an oil or gas formation (not shown) disposed below the sea floor 16. A subsea conduit 18 extends from deck 20 of platform 12 to a wellhead installation 22. The platform may include a derrick 26 and a hoisting apparatus 28 for raising and lowering the drill string 30, which, as shown, extends into borehole 40 and includes a drill bit 32 and a measurement tool 100. Embodiments of LWD tool 100 include at least one magnetically permeable ring 120 deployed about the tool body 110 (FIG. 3A). Exemplary embodiments of LWD tool 100 may further optionally include (i) one or more electrodes 140 configured to locally measure the current flow between the tool body 110 and the formation and (ii) an azimuth sensor 180, which are advantageously longitudinally spaced from ring 120. Azimuth sensor 180 may include substantially any sensor that is sensitive to its azimuth on the tool 100 (e.g., relative to high side), such as one or more accelerometers, magnetometers, and/or gyroscopes. Drill string 30 may further include a downhole drill motor, a mud pulse telemetry system, and one or more other sensors, such as a nuclear logging instrument, for sensing downhole characteristics of the borehole and the surrounding formation.

It will be understood by those of ordinary skill in the art that the deployment illustrated on FIG. 2 is merely exemplary for purposes of describing the invention set forth herein. It will be further understood that the measurement tool 100 of the present invention is not limited to use with a semisubmersible platform 12 as illustrated on FIG. 2. Measurement tool 100 is equally well suited for use with any kind of subterranean drilling operation, either offshore or onshore.

In the embodiment shown on FIG. 2, azimuth sensor 180 is longitudinally spaced from and deployed at substantially the same azimuthal (circumferential) position as the electrode 140. It will be appreciated that this invention is not limited to any particular layout (positioning) of the electrode(s) 140 and the azimuth sensor(s) 180 on the tool 100. For example, in an alternative embodiment (not shown) electrode 140 and an azimuth sensor 180 may be deployed at substantially the same longitudinal position. Moreover, it will also be appreciated that this invention is not limited to any particular number of electrodes 140 and/or azimuth sensors 180. Furthermore, as described in more detail below, certain exemplary methods of this invention do not rely on azimuth measurements or electrode measurements and hence do not require a downhole tool having an azimuth sensor or an electrode.

Referring now to FIG. 3A, a portion of one exemplary embodiment of LWD tool 100 from FIG. 2 is schematically illustrated. LWD tool 100 is typically a substantially cylindrical tool, being largely symmetrical about longitudinal axis 70. In the exemplary embodiment shown, magnetically permeable ring 120 is deployed about (external to) a substantially cylindrical conductive tool body 110 (e.g., a drill collar). The tool body is configured for coupling to a drill string (e.g., drill string 30 on FIG. 2) and therefore typically, but not necessarily, includes conventional threaded pin and box ends (not shown). An AC voltage supply 130 is electrically connected to the tool body 110 on opposing sides 112 and 114 of the magnetically permeable ring 120, with at least one connecting conductor 132 crossing the exterior (outer surface 122) of the ring 120. Such opposing sides are also referred to herein as upper 112 and lower 114 sides for clarity of exposition. It will be understood that the application is not limited by such terminology. Application of an AC current between the upper 112 and lower 114 sides of the tool body 110 induces a circumferential magnetic flux in the ring 120. The magnetic flux in turn decreases the admittance (i.e., increases the impedance) between the upper 112 and lower 114 sides of the tool body 110, which enables an AC potential difference to be supported therebetween.

With further reference now to FIG. 3B, an electrical circuit representation of the exemplary embodiment shown in FIG. 3A is illustrated. AC voltage source 130, having an AC voltage of V volts, is electrically connected to upper 112 and lower 114 sides of the tool body 110 with one of the conductors routed exterior to the magnetically permeable ring 120 (as shown on FIG. 3A). As described above, the applied AC voltage induces a magnetic flux in the ring 120, which in turn reduces the admittance Y of the tool body between the upper 112 and lower sides 114. Such a reduction in admittance Y enables the tool body to support an AC voltage difference of V volts between the upper 112 and lower 114 sides. While the admittance of the tool body 110 may be significantly reduced, it is not reduced to zero. Therefore, a current of VY will flow in the tool body 110 between the upper 112 and lower 114 sides thereof.

It will be appreciated that in the configuration shown on FIG. 3A (and also on FIGS. 4 and 6 as described in more detail below) the magnetically permeable ring 120 increases the inductance of the portion of the tool body 110 located internal to the ring 120, thereby converting the otherwise conductive tool body 110 into an inductor. The impedance of such an inductor (the portion of the tool body located internal to the ring 120) is substantially proportional to the both the frequency of the AC voltage source 130 and the magnetic permeability of the ring 120. As described in more detail below (and as shown on FIG. 5A), multiple rings, each having a high magnetic permeability, may be used to increase the inductance (and impedance) of the tool body and therefore to reduce current flow in the tool body 110 between the upper 112 and lower 114 sides.

With reference again to FIG. 3A, it is generally advantageous to configure LWD tool 100 so that the admittance between the upper 112 and lower 114 sides of the tool body is reduced (i.e., the impedance is increased) as much as possible in order to decrease current flow in the tool body 110 between the upper 112 and lower 114 sides. This may be accomplished, for example, by utilizing a magnetically permeable ring 120 having a high magnetic permeability. While a magnetically permeable ring 120 having substantially any suitable magnetic permeability may be utilized, one having a relative magnetic permeability of greater than about 10,000 is preferred. In such preferred embodiments, magnetically permeable ring 120 may be fabricated from, for example, Supermalloy, Amorphous Alloy E, and Permalloy 80 (available from Magnetics, Inc.) and Metglas.RTM. 2714A and Metglas.RTM. 2605 (available from Allied-Signal). Increasing the number of magnetically permeable rings 120 deployed about tool body 110 and the physical dimensions thereof (i.e., the radial thickness and longitudinal width of the rings 120) also tends to decrease the admittance between the opposing sides 112 and 114 of the tool body 110 by increasing the magnetic flux in the ring 120. However, it will be appreciated that in many applications there may be a tradeoff between a desire to further lower the admittance