AU716441B2 - Method for electromagnetically exploring an earth formation - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT r.

C C

C.

C C C. 9 Applicant(s): SCHLUMBERGER TECHNOLOGY B.V.

Invention Title: METHOD FOR ELECTROMAGNETICALLY EXPLORING AN EARTH

FORMATION

Application No: Filing Date:

C

The following statement is a full description of this invention, including the best method of performing it known to me/us: 2

TITLE

Method For Electromagnetically Exploring An Earth Formation o* BACKGROUND OF THE INVENTION The subject matter of the present invention 15 relates to method for electronically exploring an earth :.'formation. This application is a divisional of co-pending application 74072/96 that relates to an induction logging sonde adapted to be disposed in a wellbore including a S°."folded" sensor array, the sensor array including a transmitter coil and a plurality of bucking coils and receiver coils, the plurality of bucking coils and receiver coils further including a plurality of cowound bucking coils and receiver coils, each of the plurality of cowound bucking coils and receiver coils being cowound together on 25 the same bobbin, the transmitter coil being located at an end of the array, the plurality of bucking coils and receiver coils including the plurality of cowound bucking coils and receiver coils being located only on one side of the transmitter coil thereby shortening the sensor array of the present invention relative to the prior are sensor array of other prior art induction logging tools.

Over the past two decades, the electronics industry has been packing more and more circuitry into smaller and smaller packages. However, in the past decade and half, since digital well logging tools were introduced, the tendency has been to pack more circuitry into the H:\ann\Keep\Perm\74072 96 DIV.doc 15/04/98 3 logging tools. Examples of such circuitry packed into the logging tools include self-test, automatic calibration, more measurement channels, more sensors, etc. As a result, a conventional well logging toolstring combination of today is longer, in length, than its counterpart toolstring of fifteen years ago. The measurements, made by the conventional well logging toolstring, are more accurate and plentiful, and the environmental corrections are more automatic, but the increased toolstring length, of the conventional well logging tool, means that more rathole must be drilled and the rig-up time (the time it takes to assemble the toolstring) takes much longer.

One example of a conventional well logging 15 induction tool is disclosed in U.S. Patent 5,157,605 to Chandler et al, entitled "Induction Logging Method and Apparatus Including Means for Combining In-Phase and Quadrature Components of Signal Received at Varying Frequencies and Including use of Multiple Receiver Means Associated With a Single Transmitter", the disclosure of which is incorporated by reference into this specification.

In the Chandler patent, the induction tool includes an induction array, and the induction array 25 includes a transmitter coil, a receiver coil disposed on both sides of the transmitter coil, and a secondary or bucking coil disposed on both sides of the transmitter coil and interposed between the transmitter coil and each receiver coil. While the measurements made by the induction logging tool of the Chandler patent are accurate and plentiful, the length of the induction logging tool string which included the induction array of the Chandler patent is approximately forty (40) feet. As a result, it may be difficult to place the induction logging toolstring of the Chandler patent in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.

H:\ann\Keep\Perm\74072 96 DIV.doc 15/04/98 4 SUMMARY OF THE INVENTION According to the present invention there is provided a method for electromagnetically exploring an earth formation traversed by a deviated borehole, comprising the steps of: a) lowering an induction logging tool having a length less than twenty feet into the borehole, the tool comprising a transmitter coil and a plurality of cowound receiver coils and bucking coils disposed adjacent one end of the transmitter coil, wherein the bucking coil of each cowound receiver coil and bucking coil is associated with the receiver coil of a different cowound receiver coil and bucking coil; b) applying a source of electrical energy to the transmitter coils; 20 c) producing a plurality of receiver signals thereby producing an output signal representative of a characteristic of the formation.

Further scope of applicability of the present invention 25 will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, -while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description.

\\melb-files\home$\Monique\Keep\speci\6195098.doc 20/12/99 4a BRIEF DESCRIPTION OF THE DRAWINGS A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: figure 1 illustrates the prior art induction logging tool disclosed in the Chandler patent which further includes an array section; figure 2 illustrates the induction logging tool of the present invention which is shorter in length than the prior art induction logging tool of figure 1; figure 3 illustrates in greater detail the induction logging tool of the present invention shown in figure 2 which further includes an array section called the new "folded array" in accordance with the present invention; figure 4 illustrates a detailed construction-of the array section of the prior art induction Slogging tool of figure 1; 0 figure 5 illustrates a detailed construction of the new "folded array" section of the induction logging tool of figure 3 of the present invention; figure 6 illustrates a further more detailed construction of the new folded array section of 25 the induction logging tool of the present invention illustrated in figure 5, the new folded array section including a "plurality of cowound receiver coils and bucking coils"; and -figures 7 through 12 illustrate further, more detailed, construction views of each of the plurality of cowound receiver coils and bucking coils of figure 6.

9 i DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Refering to figure 1, a prior art induction logging tool 10 includes a sensor array 12, an electronics section 14, a transmitter electronics section 16, and a spontaneous potential (SP) sub 18.

While the measurements made by the prior art induction logging tool 10 of figure 1 are accurate and plentiful, the length of the tool 10, including the sensor array, is approximately fourty (40) feet. As a result of this 40 foot length, it may be difficult to place the prior art induction logging tool 10 of figure 1 in wellbores having a minimum Srathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.

15 The sensor array 12 of the prior art induction logging tool 10 of figure 1 is discussed in U.S. Patent 5,157,605 to Chandler et al, the disclosure of which has already been incorporated by reference into this specification. Other patents also discuss various other features of the prior art induction logging tool 10 of figure 1, and these other patents include: U.S. Patent 5,041,975 to Minerbo et al directed to borehole correction, (2) 20 U.S. Patent 5,355,088 to Howard Jr directed to a method and apparatus for determining invasion parameters, U.S. Patent 5,184,079 to Barber directed to a method and apparatus for eliminating the effects of a dip angle on collected data, and U.S. Patent 5,379,216 to Head directed to a volume transform system. The Minerbo et al patent, the Howard Jr patent, the Barber patent, and the Head patent are each incorporated by reference into this specification.

Referring to figure 2, an induction logging tool 22 in accordance with the present invention is illustrated. This induction logging tool 22 includes a sensor array 22c, an electronics section 22a, and a spontaneous potential (SP) electrode 22e. However, the length of the logging tool 22 of the present invention in figure 2 is approximately 16 feet. Compare the length of the induction logging tool 22 of the present invention shown in figure 2 (16 feet) with the length of the prior art induction logging tool 10 shown in figure 1 (40 feet). This reduction in the length of the induction logging tool 22, from 40 feet to 16 feet, will allow the induction logging tool 22 of figure 2 to be placed in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.

6 Referring to figure 3, the induction logging tool 22 of the present invention shown in figure 2 is illustrated again in greater detail in figure 3.

In figure 3, the induction logging tool 22 of the present invention shown in figure 3 includes an electronics section 22a including upper standoffs 22al, a compensator section 22b, a sensor array.22c (also illustrated as the sensor array 22c in figure a lower standoff 22d, and a nose section 22e. The nose section 22e is disclosed in prior pending application serial number 08/330,397 filed 10/27/94 corresponding to attorney docket number 20.2635, entitled "Method and Apparatus for measuring mud resistivity in a •wellbore including a probe having a bottom electrode located at the bottom of a tool string and propagating a current from and to the bottom electrode in a direction parallel to a longitudinal axis of the tool string", the disclosure of which is incorporated by reference S: into this specification.

Referring to figure 4, the sensor array 12 of the prior art induction logging tool 10 of figure 1 is illustrated. This sensor array 12 is adequately described in the U.S. Patent 5,157,605 to Chandler already incorporated herein by reference.

20 The sensor array 12 of figure 4 includes a transmitter coil 12a disposed in the center, a plurality of bucking coils and receiver coils disposed on one side of the transmitter coil 12a, and a further plurality of bucking coils and receiver coils disposed on the other side of the transmitter coil 12a.

For example, the following bucking coils and receiver coils exist on a first side of the transmitter coil I2a: the 6 inch bucking coil 12b, the 6 inch receiver coil 12c, the 12 inch bucking coil 12d, the 12 inch receiver coil 12e, the 21 inch bucking coil 12f, the 21 inch receiver coil 12g, the 39 inch bucking coil 12h, the 39 inch receiver coil 12i, the 72 inch bucking coil 12j, and the 72 inch receiver coil 12k.

In addition, the following bucking coils and receiver coils exist on a second side (opposite to the first side) of the transmitter coil 12a: the 9 inch bucking coil 12L, the 9 inch receiver coil 12m, the 15 inch bucking coil 12n, the 15 inch receiver coil 12p, the 27 inch bucking coil 12q, and the 27 inch. receiver coil 12r.

7 In figure 4, note the dotted line 24 passing through the transmitter coil 12a. This dotted line represents a "fold line". Assume that it were possible to fold (or rotate) the second side of the sensor array 12 of figure 4 about the dotted line 24 and overlay the second side of sensor array 12 (which includes the 9 inch bucking coil 12L) over the first side of the sensor array 12 (which includes the 6 inch bucking coil 12b). If this "folding operation" were possible, the result of the "folding operation" would be the sensor array 22c (hereinafter called "the folded array 22c") shown in figure 5 in accordance with the present invention.

Referring to figure 5, the folded array 22c of the induction logging tool 22 of figures 2 and 3 of the present invention is illustrated.

In figure 5, the folded array 22c includes a transmitter coil 22c 1 disposed on one side of the array, and a 6 inch bucking coil 22c5, a plurality of cowound receiver coils and 15 bucking coils 22c2, a further plurality of individual receiver coils 22c3, and at least one further bucking coil 22c4, all of which are disposed only on one side of the transmitter coil 22c 1. The transmitter coil 22c I is located at an end 26 of the new folded array 22c, S and the plurality of cowound receiver coils and bucking coils 22c2, the 6 inch bucking coil 22c5, the individual receiver coils 22c3, and the one further bucking coil 22c4 are located 20 adjacent to and on only one side of the transmitter coil 22c 1 of the new folded array 22c.

Each of the plurality of cowound receiver coils and bucking coils 22c2 in figure 5 include a receiver coil and a bucking coil which is really associated with another receiver coil Furthermore, each of the plurality of cowound receiver coils and bucking coils 22c2 in figure 5 are formed by winding both the receiver coil and its bucking coil (N) in a bifilar way on the same ceramic bobbin starting from the center of the bobbin and winding the receiver coil and the bucking coil in opposite outward directions.

In addition, although it is important that each cowound receiver coil and bucking coil 22c2 be formed by winding the receiver coil and the bucking coil on the same ceramic bobbin, recalling that each bucking coil is associated with its own corresponding receiver coil it is also important that the number of coil-turns for each bucking coil (N) and each receiver coil be carefully selected. That is, for each cowound receiver coil and bucking coil 22c2, two criteria must exist and be satisfied at the same time: 8 The turns ratio between the turns of a particular receiver coil and its corresponding bucking coil must be carefully selected in order to balance or null the direct mutual coupling between the transmitter and receiver array, and When the aforementioned turns ratio is carefully selected, the receiver coil and the bucking coil associated with another receiver coil must nevertheless be cowound on the same ceramic bobbin. When the receiver coil and the bucking coil are cowound on the same ceramic bobbin, the bucking coil and the receiver coil are both located at approximately the same distance from the transmitter coil.

As a result, the functional operation of the new folded array 22c of the present invention of figure 5 is the same manner as functional operation of the prior art sensor array 12 of figure 4, the functional operation being discussed in U.S. Patent 5,157,605 to Chandler et al; "however, the new folded array 22c of the present invention of figure 5 (part of the induction logging tool 22 of figures 2 and 3) is shorter in length than the prior art sensor array 12 of figures 1 and 4 thereby allowing the new induction logging tool 22 of figures 2 and 3 to be placed in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.

Referring to figure 6, a further more detailed construction of the new folded array 22c of figures 2, 3, and 5 of the new induction logging tool 22 of figures 2 and 3 is illustrated.

If it were possible to fold (or rotate) the second side of the induction section 12 of figure 4 about the dotted line 24 and overlay the second side of induction section 12 of figure 4 (which includes the 9 inch bucking coil 12L) over the first side of the induction section 12 of figure 4 (which includes the 6 inch bucking coil 12b), previously called "a folding operation", a plurality of the receiver coils and bucking coils disposed on one side of the array 12 of figure 4 would overlay a corresponding plurality of the receiver coils and bucking coils disposed on the other side of the array 12 of figure 4, and the new folded array section 22c of figures 5 and 6 would be the result.

9 The induction logging tool of the prior art shown in figure 4 includes a plurality of receiver coils and bucking coils. Each'of the receiver coils and bucking coils are defined as follows: 1. A 6 inch receiver coil and corresponding 6 inch bucking coil 2. A 9 inch receiver coil and corresponding 9 inch bucking coil 3. A 12 inch receiver coil and corresponding 12 inch bucking coil 4. A 15 inch receiver coil and corresponding 15 inch bucking coil A 21 inch receiver coil and corresponding 21 inch bucking coil 6. A 27 inch receiver coil and corresponding 27 inch bucking coil 7. A 39 inch receiver coil and corresponding 39 inch bucking coil 8. A 57 inch receiver coil and corresponding 57 inch bucking coil S•9. A 72 inch receiver coil and corresponding 72 inch bucking coil Referring to the above list of receiver coils and bucking coils, it is necessary to define certain nomenclature used in this specification: If the bucking coil consists of the 9 inch bucking coil, the receiver coil consists of the 9 inch receiver coil, but the receiver coil consists of the 6 inch receiver coil, the previously mentioned coil in the above list.

0 Similarly, if the bucking coil consists of the 39 inch bucking coil, the receiver coil consists of the 39 inch receiver coil, but the receiver coil consists of the 27 inch receiver coil, the previously mentioned coil in the above list.

In figure 6, the new folded array 22c includes the transmitter coil.22c 1, the 6 inch bucking coil 22c5, the "plurality of cowound receiver coils and bucking coils" 22c2, a temperature sensor 22c6, the individual receiver coils 22c3, and the one further bucking coil 22c4, all of which are disposed on one side only of the transmitter coil 22c L. The advantage of the new folded array 22c relative to the prior art sensor array 12 is as follows: the new folded array 22c of figures 5 and 6 is shorter in length than the sensor array 12 of figure 4. This shortened length of the folded array 22c allows the new induction logging tool 22 of figures 2 and 3 to be placed in wellbores which have a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.

10 In figures 5 and 6, the plurality of cowound receiver coils and bucKing coils 22c2 of the new folded array 22c include the following: a 6 inch receiver/9 inch bucking coil 2a in figures 5 and 6, the 6 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 9 inch bucking coil from the sensor array 12 of figure 4, a 9 inch receiver/12 inch bucking coil 2b in figures 5 and 6, the 9 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 12 inch bucking coil from the sensor array 12 of figure 4, a 12 inch receiver/15 inch bucking coil 2c in figures 5 and 6, the 12 inch receiver coil Sfrom the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 15 inch S* bucking coil from the sensor array 12 of figure 4, a 15 inch receiver/21 inch bucking coil 2d in figures 5 and 6, the 15 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 21 inch bucking coil from the sensor array 12 of figure 4, 20 a 21 inch receiver/27 inch bucking coil 2e in figures 5 and 6, the 21 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 27 inch bucking coil from the sensor array 12 of figure 4, and a 27 inch receiver/39 inch bucking coil 2f in figures 5 and 6, the 27 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 39 inch bucking coil from the sensor array 12 of figure 4.

From the aforementioned description of the plurality of cowound receiver coils and bucking coils 22c2, it is evident that each of the plurality of cowound receiver coils and bucking coils 22c2 in figures 5 and 6 include a receiver coil and a bucking coil However, the bucking coil is associated with another receiver coil it is not associated with the receiver coil In addition, the plurality of cowound receiver coils and bucking coils 22c2 are all located on one side of the transmitter 22c 1, the transmitter 22c I itself being located at the end 26 of the array. The bucking coil and the receiver coil of each cowound receiver coil and bucking coil 22c2 is wound on the same ceramic bobbin. As a result, the bucking coil and the receiver coil are positioned 11 at approximately the same distance from the transmitter coil 22cl. Furthermore, the turns ratio for a particular receiver coil and bucking coil [that is, the number of turns of a particular receiver coil divided by the number of turns of its corresponding bucking coil must be carefully selected in order to balance or null the direct mutual coupling between the transmitter and receiver array.

Referring to figures 7 through 12, a detailed construction of each of the plurality of cowound receiver coils and bucking coils 22c2 is illustrated, these illustrations depicting the manner by which each cowound receiver coil and bucking coil 22c2 are cowound together about the same bobbin and about the same bobbin pin.

In figure 7, the bucking coil 32, 34 and the receiver coil 38, 40 associated with one of the plurality of cowound receiver coils and bucking coils 22c2 are cowound about a nominal center 36 of a bobbin 30. As a result, the bucking coil 32, 34 and the receiver coil 38, are located at approximately the same distance from the transmitter coil 22c 1.

Recall that each of the plurality of cowound receiver coils and bucking coils 22c2 are cowound together about the same ceramic bobbin. Each of the plurality of cowound receiver coils and bucking coils consist of a bucking coil, and a receiver coil. For example, the 6 inch receiver/9 inch bucking coil 2a consists of the 6 inch receiver coil (the receiver coil) and the 9 inch bucking coil (the bucking coil). In figure 7, the bucking coil of a particular cowound receiver coil and bucking coil (one of 2a-2f of 22c2) consists of a first bucking coil 32 (hereinafter called "Buck C1 32") and a second bucking coil 34 (hereinafter called "Buck C2 and the receiver coil of the same particular cowound receiver coil and bucking coil (one of 2a-2f of 22c2) consists of a first receiver coil 38 (hereinafter called "Rec C1 which is interleaved in part with the first bucking coil 32, and a second receiver coil 40 (hereinafter called "Rec C2 which is interleaved in part with the second bucking coil 34. Note from the above description that the first receiver coil 38 is "interleaved in part" with the first bucking coil 32 and that the second receiver coil 40 is "interleaved in part" with the second bucking coil 34. The above reference to "interleaving" will be discussed in greater detail below with reference to figure 10 of the drawings.

Therefore, in our example, the cowound receiver coil and bucking coil "the 6 inch receiver/9 inch bucking coil 2a" includes the 6 inch receiver coil and the 9 inch bucking coil, the 6 inch receiver coil being represented by the first receiver coil 38 of figure 7 plus the second receiver coil 40 of figure 7 (Rec C1 38 Rec C2 40), the 9 inch bucking coil 12 being represented by the first bucking coil 32 of figure 7 plus to the second bucking coil 34 of figure 7 (Buck Cl 32 Buck C2 34).

Similarly, the cowound receiver coil and bucking coil "the 27 inch receiver/39 inch bucking coil 2f" includes the 27 inch receiver coil and the 39 inch bucking coil, the 27 inch receiver coil being represented by the first receiver coil 38.of figure 7 plus the second receiver coil of figure 7 (Rec Cl 38 Rec C2 40), the 39 inch bucking coil being represented by the first bucking coil 32 of figure 7 plus the second bucking coil 34 of figure 7 (Buck C1 32 Buck C2 34).

Therefore, each cowound receiver coil and bucking coil (2a-2f) 22c2 of figure 6 consists of a particular receiver coil and a particular bucking coil the particular receiver coil consisting of (Rec Cl 38 Rec C2 40) of figure 7, the particular bucking coil (N) consisting of (Buck Cl 32 Buck C2 34) of figure 7. In addition, in figure 7, the 15 particular receiver coil and the particular bucking coil are both positioned about the nominal center 36, the nominal center 36 being further positioned at a distance from the transmitter coil 22cl. Therefore, the particular receiver coil and the particular bucking coil are both located at approximately the same distance from the transmitter coil 22cl. As a result, the receiver coil and the bucking coil of each 20 of the plurality of cowound receiver coils and bucking coils (2a-2f of 22c2) are each positioned at approximately the same distance from the transmitter coil 22c 1. This concept will be better understood by reading the following detailed description.

Figure 7 illustrates the first half of the bucking coil 32 (Buck Cl 32) interleaved with the first half of the receiver coil 38 (Rec C1 38), and the second half of the bucking coil 34 (Buck C2 34) interleaved with the second half of the receiver coil 40 (Rec C2 40), each being wound about the nominal center 36 of the bobbin 30. However, figure 7 does not illustrate exactly how such coils 32, 34, 38, 40 are cowound together about the bobbin The following discussion with reference to figures 8 through 12 will provide a description of exactly how such coils 32, 34, 38, and 40 are cowound together about the same ceramic bobbin In figure 8, recalling (from figure 7) that Buck Cl 32 interleaved with Rec Cl 38 is disposed on one side of the nominal center 36 and is wound about the bobbin 30, and Buck C2 34 interleaved with Rec C2 40 is disposed on the other side of nominal center 36 and is wound about the bobbin 30, figure 8 illustrates the nominal center 36 on the bobbin 13 However, in figure 8, a plurality of bobbin pins 42 are transversely disposed relative to the nominal center 36, the plurality of bobbin pins 42 being held together by an epoxy 44. The receiver coil and bucking coil 32, 34, 38,40 of figure 7 associated with each of the plurality of cowound receiver coils and bucking coils 2a through 2f of figure 6 are initially connected to one of the bobbin pins 42 of bobbin 30 before the coils are cowound about the same bobbin 30. Figure 9 better illustrates how each cowound receiver coil and bucking coil (32, 34, 38,40) of figure 7 of the plurality of cowound receiver coils and bucking coils (2a-2f) 22c2 of figure 6 are connected to bobbin pins 42 and cowound about the same bobbin In figure 9, the "Buck C1 32" portion of the bucking coil of figure 7 interleaved with the "Rec C1 38" portion of the receiver coil of figure 7 (associated with one of the cowound receiver coils and bucking coils 22c2) is shown connected to a bobbin pin 42 of bobbin In figure 9, note how the Buck C1 32 bucking coil is interleaved with the Rec C1 38 15 receiver coil on the one side of the nominal center 36 of the bobbin 30. In figure 9, the Buck C1 32 bucking coil includes an end which is shown to be stripped of its outer (0 insulating layer thereby exposing a wire pin 48. The wire pin 48 has an end portion which is physically wrapped around a bobbin pin 42, one of the bobbin pins 42 shown in 20 figure 8. Further reference to figure 10 will better illustrate the co-winding of each of the receiver coils and bucking coils 32, 38 and 34, 40 about the bobbin 30 when the end portion 50 of each wire pin 48 of either the bucking coil 32, 34 or the receiver coil 38, 40 is wrapped around the bobbin pin 42 in the manner illustrated in figure 9.

t. 0 0 In figure 10, the first half of the bucking coil 32 (Buck C1 32) is shown interleaved with the first half of the receiver coil 38 (Rec C1 38) on the left side of nominal center 36 and is wound around the bobbin 30 in the winding direction indicated by arrow 52 (upwardly in figure 10). However, note that the bucking coil turns 32 are initially interleaved with the receiver coil turns 38; however, when the bucking coil turns 32 wrap around bobbin 30, in the direction indicated by arrow 52, to the point in figure 10 indicated by numeral 56, the bucking coil turns 32 begin to cut across several successive receiver coil turns 38. This happens because the bucking coil 32 ends, at 56, but the receiver coil 38 continues to wrap around the bobbin 30. As a result, the bucking coil 32 cuts across several successive receiver coil turns 38.

However, the second half of the bucking coil 34 (Buck C2 34) is shown interleaved with the second half of the receiver coil 40 (Rec C2 40) on the right side of nominal center 36 14 and is wound around the bobbin 30 in the winding direction indicated by the arrow 54 (downwardly in figure 10). However, note that the bucking coil turns 34 are initially interleaved with the receiver coil turns 40; however, when the bucking coil turns 34 wrap around bobbin 30, in the direction indicated by arrow 54, to the point in figure 10 indicated by numeral 58, the bucking coil turns 34 begin to cut across several successive receiver coil turns 40. This happens because the bucking coil 34 ends, at 58, but the receiver coil continues to wrap around the bobbin 30. As a result, the bucking coil 34 cuts across several successive receiver coil turns The end portions 50 of each bucking coil 32, 34 and each receiver coil 38, 40 of figure 7 of each of the plurality of cowound bucking coils and receiver coils 2a-2f of 22c2 of figure 6 are wrapped around the following bobbin pins 42: the end portion 50 of the first half of the bucking coil 32 (Buck C1 32) of figure 7 is initially wrapped around the bobbin pin No. 2 of figure 10 and the bucking coil 32 is wrapped around the bobbin 30 in the direction of arrow 52 (upwardly in figure (2).the end portion 50 of the first half of the receiver coil 38 (Rec C1 38) of figure 7 is initially wrapped around bobbin pin number 4 of figure 10 and the receiver coil 38 is wrapped around bobbin 30 in the direction of arrow 52 (upwardly in figure the end portion 50 of the second half of the bucking coil 34 (Buck C2 34) of figure 7 is initially wrapped around bobbin pin no. 2 of figure 10 and the bucking coil 34 is wrapped around bobbin 30 in the direction of arrow 54 (downwardly in figure 10), and the end portion 50 of the second half of the receiver coil 40 (Rec C2 40) of figure 7 is initially wrapped around bobbin pin number 7 of figure 10 and the receiver coil 40 is wrapped around bobbin 30 in the direction of arrow 54 (downwardly in figure In figure 1 I, the bucking coil portion 32 and 34 of figure 10, which cuts across several successive receiver coils 38 and 40 of figure 10, is. illustrated in greater detail. Since the bucking coil portion 32 and 34 ends abruptly, at 56 and 58, but the receiver coils 38, 15 continue to wrap around the bobbin 30, the bucking coil portion 32 and 34 must cut across the successively wrapped receiver coils 38 and 40, the end of the bucking coil portion 32, 34 being soldered to a bobbin pin 42, at In figure 12, recall that each of the plurality of cowound bucking coils and receiver coils 2a-2f 22c2 of figure 6 includes a bucking coil and a receiver coil where the bucking coil further includes a first bucking coil 32 (Buck Cl 32) and a second bucking coil 34 (Buck C2 34), and the receiver coil further includes a first receiver coil 38 (Rec C1 38) and a second receiver coil 40 (Rec Cl 40). However, the first bucking coil 32 is interleaved in part with the first receiver coil 38 when the coils 32, 38 are wrapped around a bobbin 30 in one direction (arrow 52), and the second bucking coil 34 is interleaved in part with the second receiver coil 40 when the coils 34, 40 are wrapped around the same bobbin 30 in a direction opposite to the one direction (arrow 54).

Therefore, when the first bucking coil 32.("Buck Cl 32") and the first receiver coil 38 ("Rec Cl 38") are wound in an interleaved fashion on one side of the nominal center 36 in the manner shown in figures 7 and 10, and when the second bucking coil 34 ("Buck C2 34") and the second receiver coil 40 ("Rec C2 40") are wound in an interleaved fashion on the other side of the nominal center 36 in the manner shown in figures 7 and the result is shown in figure 12.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

For the purposes of this specification it will be clearly understood that the word "comprising" means "including but not limited to", and that the word "comprises" has a corresponding meaning.

Claims (7)

1. A method for electromagnetically exploring an earth formation traversed by a deviated borehole, comprising the steps of: a) lowering an induction logging tool having a length less than twenty feet into the borehole, the tool comprising a transmitter coil and a plurality of cowound receiver coils and bucking coils disposed adjacent one end of the transmitter coil, wherein the bucking coil of each cowound receiver coil and bucking coil is associated with the receiver coil of a different cowound receiver coil and bucking coil; b) applying a source of electrical energy to the transmitter coils; c) producing a plurality of receiver signals thereby 20 producing an output signal representative of a characteristic of the formation. *o S**

2. The method of claim 1, wherein the source of electrical energy includes at least two oscillating 9** i" 25 signals, the frequencies fl and f2 of each signal being substantially constant and being different from each other.

3. The method of claim 2, wherein step further S. comprises producing a first receiver signal at frequency fl 30 and a second receiver signal at frequency f2; separating the first receiver signal of frequency fl into its in-phase and quadrature component relative to the first oscillating signal; and, separating the second receiver signal of frequency f2 into its in-phase and quadrature components relative to the second oscillating signal. \\melb~iles\home\Monique\Keep\peci\6195-9.doc 20/12/99 17

4. The method of claim 3, wherein step further comprises combining the in-phase and quadrature components of the first receiver signal with the in-phase and quadrature components of the second receiver signal thereby producing the output signal representative of a characteristic of the formation.

The method of claim 1, wherein the source of electrical energy includes an oscillating signal having a single frequency.

6. The method of claim 5, wherein the plurality of receiver signals have the same frequency as the oscillating signal.

7. A method for electromagnetically exploring an earth formation traversed by a deviated borehole substantially as herein described with reference to and as 20 illustrated in accompanying figures 2, 3, and 5 to 12. Dated this 20th day of December 1999 •SCHLUMBERGER TECHNOLOGY B.V. By their Patent Attorneys 25 GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia •o* o \\melbfiles\home\Moniue\Keep\speci\61950-98.doc 20/12/99