US 3704749 A
An orienting device for a gun or jet perforator or the like to permit setting at a selected angle to a ferrous element such as an adjacent casing string, comprising an exciter coil producing an alternating electromagnetic field and a pair of receiver coils longitudinally spaced from the exciter coils, the disposition of the receiver coils being such that the voltages induced therein vary differentially with the angle presented by the ferrous element by reason of the distortion of the otherwise axially symmetrical field. Electronic means are provided to convert the differential voltages to a pulsed signal which is received at the surface and caused to register the orientation angle. Motor means are provided to rotate the device. All operating power, control signals, and information signals are transmitted by a single conductor cable serving also to suspend the device.
Description (OCR text may contain errors)
United States Patent Estes et al.
 METHOD AND APPARATUS FOR TOOL ORIENTATION IN A BORE HOLE  Inventors: James D. Estes; Kirby De Gough,
both of Houston, Tex.
 Assignee: N L Industries, Inc., New York,
 Filed: May 6, 1971  Appl. No.: 140,813
3,032,107 5/1962 Rumble et al.... .....175/4.5l
3,175,608 3/1965 Wilson ..l75/4.5l 3,307,626 3/1967 Bielstein... 166/255 3,307,642 3/1967 Smith ..l75/4.5l 3,342,275 9/]967 Mellies ..175/4.5l
& am Hi i Dec. 5, 1972 Primary Examiner-David H. Brown Attorney-Delmar H. Larsen, Robert L. Lehman, Fred Floresheimer and Roy F. House  ABSTRACT An orienting device for a gun or jet perforator or the like to permit setting at a selected angle to a ferrous element such as an adjacent casing string, comprising an exciter coil producing an alternating electromagnetic field and a pair of receiver coils longitudinally spaced from the exciter coils, the disposition of the receiver coils being such that the voltages induced therein vary differentially with the angle presented by the ferrous element by reason of the distortion of the otherwise axially symmetrical field. Electronic means are provided to convert the differential voltages to a pulsed signal which is received at the surface and caused to register the orientation angle. Motor means are provided to rotate the device. All operating power, control signals, and information signals are transmitted by a single conductor cable serving also to suspend the device.
15 Claims, 11 Drawing Figures PATENTEDHEB 5 m2 SHEET 3 [IF 4 Era .9.
INVENTOR5. JAM65 0- 557 55 [/EBV P660U6H METHOD AND APPARATUS FOR TOOL ORIENTATION IN A BORE HOLE This invention relates to an apparatus and procedure for orienting a tool, such as a perforator, in a bore hole, such as an oil well, and more particularly in a well bore which contains two or more casing strings in a side-byside relationship.
For many operations carried out by means of tools lowered in bore holes, often to great depths, it is essential to be able to determine the orientation of the tool when emplaced at the selected depth. Many tools are lowered to depth on a cable, and it will be readily understood that during the course of lowering the tool, the latter may rotate considerably, so that its orientation can only be determined with certainty by some device operating at the depth of the tool itself.
This problem is particularly acute in the case wherein an oil or gas well is completed so as to permit production from more than one zone. Such multiple zone completions are frequently carried out by running two or more strings of casing in a side-by-side relationship into' a single well bore which penetrates all the zonesin question. In order to produce a particular zone, a gun perforator is run through one of the strings of casing and placed opposite the zone to be produced. The gun in such a case is of the type which fires all ofits bullets or jets in a single direction, and these bullets or jets must be directed so that the other casing strings or strings will not be perforated. In this manner, each zone to be produced is perforated from a different casing string, so that it is possible to produce each zone independently of the others. It will be appreciated that in order to do this with certainty and safety, the orientation of the perforating tool must be known just before firing, with respect to the other casing strings.
A useful review article on the subject appears in World Oil, Feb. 1, 1962, pages 52-59, by R. W. Scott.
Representative prior art solutions to the problem outlined above are US. Pat. No. 3,426,850, to McDuffie; US. Pat. No. 3,426,849, to Brumble; and US. Pat. No. 3,426,851, to Arendt.
The McDuffie patent describes a method of orienting a gun perforator under the general conditions already outlined, in which selected strings of casing contain radioactive material which is used as an orientation target by a radiation detector used in conjunction with a perforating gun. The practical disadvantage here is that the strings not being perforated must be prepared in advance with a radioactive substance.
The Brumble patent shows a multiple line system wherein one line operates a perforating gun designed to fire in only one direction and also contains a directed source of radiation, while radiation detectors are operated by other line or lines in the other strings of casing. The practical disadvantage of this system is that it requires running wire lines into all casing strings.
The Arendt patent shows a single line system wherein the line controls a tool containing a perforating gun designed to tire in only one direction, a source of radiation, and a collimated radiation detector. The practical disadvantage here is that the resulting density readings may be seriously affected by the presence or absence of cement around the casings as well as bythe distance between the casings.
An object of the present invention is to provide an apparatus and procedure for orienting a tool such as a gun perforator designed to shoot in only onedirection in a bore hole such as an oil well which contains two or more casing strings in a side-by-side relationship, so that the perforator may be fired in a desired direction so as to safely and reliably clear the other casing strings which are not desired to be perforated, and moreover to accomplish this by an essentially electromagnetic means, free of the hazards associated with radioactive materials and capable of a high degree of precision.
Another object of the invention is to provide such an apparatus and procedure that may be operated from the surface on a single conductor wire line.
Other objects of the invention will appear as the description thereof proceeds.
In the drawings,
FIG. 1 is a general view, combining a schematic representation of the above-ground installation as well as an enlarged cross-sectional view of the bore hole and casings at the depth of the perforator assembly. FIGS. 2 and 3 are cross sections taken at the approximate depth of the perforator assembly showing typical arrangements with respectively two casing strings and three casing strings in a single bore hole.
FIG. 4 is a block diagram showing the circuits which may be used in the orientation device.
FIGS. 5, 6, 7, and 8 show different possible arrangements of receiver coils used in the invention.
FIG. 9 shows an optional checking arrangement for use with the invention.
FIG. 10 shows a still further optional checking arrangement for use with the invention.
FIG. 11 supplements FIG. 4 in showing-wave forms at various points of the circuit.
Generally speaking, and in accordance with illustrative embodiments of the invention, we provide an orienting device which for exemplary and illustrative purposes is preferably and most conveniently unitized with the perforating device as a single tool for lowering into the bore hole, and more particularly into the casing to be perforated. The orienting device comprises an exciter coil which produces an electromagnetic field which in an isotropic embodiment is symmetrical about its axis. Spaced longitudinally from the exciter coil, either above or below but in our preferred embodiment below is a receiver means. In the preferred embodiment, two receiver coils are disposed therein, one of which, conveniently termed a reference coil, is preferably although no necessarily disposed coaxially with the tool and therefore with the bore hole, while the other coil, which may for convenience be termed a sensor coil, is disposed at an angle with respect to the axis of the tool. As will be described in detail below, means are provided to energize the exciter coil and to detect the voltages induced in the reference and sensor coils, and means are also provided to rotate the orienting device together with the perforating gun so that a favorable orientation may be selected and achieved prior to firing the gun.
Coming now to a detailed description of the invention as depicted in various embodiments in the drawings, FIG. 1 shows a vertical cross section of a bore hole 10 which contains two casing strings, 11 and 12. Both strings are cemented in the bore hole, sections of the cement being shown at 13, 14, and 1S. Suspended in the bore hole by a cable 16 is a unitized tool comprising a belly spring section 17 which maintains any selected position of the tool in'the hole, and also functions as a centralizer; a motor section 18 which serves to rotate the balance of the tool therebelow as desired and controlled from the surface; an exciter coil means 19; an electronic section 20; a receiver means 21, functioning as a non-symmetrical electromagnetic field detector; and finally the device 22, the orientation of which is to be adjustably controlled, and which in our preferred embodiment is a perforating gun, two bullet portions thereof being shown at 23 and 24. In the disposition shown in FIG. 1, the perforating gun has been oriented so that when the bullet sections 23 and 24 are fired, the perforations will be formed in the formation 25 to be produced without the bullets or jets having disturbed the integrity of the other casing string 12.
FIG. 1 also shows in schematic form hoist means 27 and a power and recording instrument means 28.
FIG. 2 is a section taken horizontally through FIG. 1 just above the tool itself, showing the top of the belly spring section 17 in place in casing string 11, in side-byside relationship with the second casing strip 12.
As mentioned, there may be several casing strings instead of just two, and FIG. 3 shows three such casing strings for an arrangement in all other respects essentially similar to that of FIGS. 1 and 2.
Reverting now to FIG. 1, cable 16 is of the conventional type used in wire line operations of the type concerned, and thus not requiring detailed description. It comprises a steel cable strong enough to support the assembly, in the interior of which is an insulated copper conductor serving to supply electrical power to the tool and also to convey the electrical signals to the surface.
The belly spring section 17 is again of conventional construction, containing belly or bow springs serving primarily to maintain the tool at whatever point in the hole at which it has been positioned by the operator. A secondary function is to centralize the tool in the hole.
The exciter coil means 19, which as already men tioned may be rotated at will by motor section 18, may be simply a coil which when energized causes an alternating electromagnetic field to be formed in the surrounding region which is symmetrical about the axis of the device and accordingly symmetric about the axis of the casing string 11, by virtue of the centralizing action of the belly spring section 17, for the condition of an isotropic environment. Thus, the configuration of the electromagnetic field surrounding exciter coil 19 is independent of the rotation thereof within the casing string. However, in actual use, the surrounding medium is not isotropic, but is strongly anisotropic by reason of the pressure of the additional casing string 12 as shown in FIGS. 1 and 2 or indeed several such strings as shown in FIG. 3. Thus, when exciter coil 19 is energized, the electromagnetic field in the surroundings, which ,of course includes the casing strings as well as the bore hole and the surrounding formation, is severely distorted from what would otherwise be axial symmetry by the presence of the additional casing string or strings. Moreover, as already mentioned, the particular configuration of the field in a given case, such as that illustrated in FIG. 1, remains essentially unchanged as motor section 18 rotates the balance of the tool including the exciter coil 19 and the receiver means 21. The departure from symmetry occasioned by the fact that the device 22 may have some unilateral bullet portions 23 and 24 is quite negligible.
Coming now .to the receiver means 21, in the preferred embodiment this comprises two receiver or pickup coils 30 and 31 as shown in FIG. 5, in which the exciter coil portion of means 19 is shown as 26. Alternative configurations for the pair of receiver coils are shown in FIGS. 6, 7, and 8, wherein the coil pairs are 32 and 33; 34 and 3S; and 36 and 37 respectively.
By way of general explanation, it may be noted that the receiver coils 30 and 31 and like pairs as in the other FIGS. 6, 7, and 8, are disposed in one of any number of configurations, all of which are characterized by the fact that, first, both coils act as pickup coils in that they respond to the alternating electromagnetic field produced by exciter coil 26; second, that under completely isotropic conditions, that is, in the absence of any ferromagnetic material such as a second casing string which would cause the electromagnetic field produced by exciter coil 26 to depart from axial symmetry, the voltage induced in each coil is independent of rotation of the pair of coils about the axisof the device; third, that in the presence of a distorting element such as an adjacent second casing string, the ratio of the voltages produced in the two coils of any given pair will change as the device is rotated about its axis.
Thus, for example, considering the configuration of FIG. 5, if this device is centrally disposed in a single casing string and thusin an isotropic environment, substantially equal voltages will be induced in both coils 30 and 31, assuming that they are of like construction, this assumption being made merely to simplify the-explanation. Moreover, as the device is rotated about the axis shown in FIG. 5, the voltages induced in both coils will remain equal, independent of the azimuth. On the other hand, suppose that a second casing string is alongside the device on the right hand side thereof as the figure is viewed. Under this condition, coil 31 will have a higher voltage induced in it than that of coil 30 because of the effect of the ferromagnetic casing string in concentrating the lines of force on that side. If under this same assumed condition, the device of FIG. 5 is rotated through then the voltages induced in coils 30 and 31 will be approximately equal; and if rotation is continued for an additional 90, thus bring coil 30 closest to the second casing string and coil 31 farthest away, then coil 30 will have a higher voltage induced therein than coil 31.
It is convenient to consider one of each pair of receiver coils as a reference coil, and the second of the pair as a sensor coil. Thus, in FIGS. 5, 6, 7, and 8, it is convenient to regard coils 30, 32, 34, and 36 respectively as the reference coil of each pair, and coils 31, 33, 35, and 37 respectively as the sensor coils.
It will be clear that even for a fixed separation of exciter coil and receiver coils, the voltage induced in the latter will be subject to considerable variation, both in amplitude and phase, as caused for example by varying casing diameter, casing wall thickness, proximity to casing collars, and the like. However, our arrangement avoids any difficulty on this score, since we use a pair of coils and utilize the ratio of the voltage induced in the .lvmm mun two coils and moreover register this ratio in the form of a selected function of angular rotation with respect to the axis of the device.
From the foregoing, it will be clear how the variation in relative voltage arises in the receiver coil pair for the other embodiments shown in FIGS. 6, 7, and 8. In FIGS. 6 and 7, reference coils 32 and 34 produce a voltage which is substantially independent of rotation, while sensor coils 33 and 35 are highly dependent upon their orientation with respect to the adjacent casing string or strings because the axes of these two coils 33 and 35 are set at a considerable angle from the vertical axis of the device itself. The variation in the relative voltages of the two receiver coils in the device of FIG. 8 as the device is rotated follows the same explanation as already given in the case of FIG. 5.
It will be apparent that a wide choice of dimensions, configurations, and the like is possible for the exciter and receiver coils, and in any specific design these will be dictated by sound engineering considerations. Thus, exciter coil 26 being essentially a power source may be wound with relatively heavy gauge wire over an ironwire core of substantial length, such as 12 -inches. While the exciter coil may be operated over 'a wide range of frequencies, we prefer to drive it at about 60 Hz., at a power consumption of to watts. The receiver coils function essentially to develop voltage, and may be conveniently and preferably wound with much finer wire and a correspondingly larger number of turns, and on a much shorter core, such as l to 2 inches. They do not necessarily have to be identical; we prefer to have the reference coil about twice as large as the sensor coil. A convenient and preferred spacing is to place the pair of receiver coils 24 inches from the center of the exciter coil 26.
Returning to the overall view as shown in FIG. 1, we find it convenient to energize the electronic section 20, the motor section 18, and, when required, the perforating device 22 by direct current. 40 volts of applied voltage operates only the electronic section; when the voltage is increased to 70 volts, the direct current electric in motor section 18 is activated to rotate that portion of the tool below it. The perforating device may be fired when desired by applying a voltage exceeding 100 volts, which may simply be a pulse as short as 100 milliseconds. The arrangement of the electrical circuits so as to differentially respond to these varying voltages is conventional and so well known in the art that it need not be described in detail here.
The signals from the two pickup coils and 31 and the like may be compared in any desired fashion to ascertain which orientation of the tool gives a peak response for the sensor coil. A system which we find convenient and which we prefer, although others will occur to those skilled in the art, is shown in block outline in FIG. 4, and in somewhat more detail in FIG. 11.
Referring to FIG. 7, this shows the relative positions of exciter coil 26, reference coil 34 and sensor coil 35, although it will be clear that the description applies to other selected dispositions of the pickup coils, as indicated for example in FIGS. 5, 6, and 8. As indicated in FIG. 11, the signal from 'coil 34 is split, a portion going to reference amplifier 40 and a part to phase shifter 41. The latter signal is mixed with the signal in mixer'42. The typical wave form for the signal from reference coil 34 is shown at 43, as a typical sine wave. As previously explained, this is independent of the orientation of the tool. The typical wave forms of the signal from sensor coil 35 are shown at 44 for three different conditions of operation. That designated 1 is for the condition of no external pipe being present. As previously explained, the electromagnetic field produced by exciter coil 26 under this condition is axially symmetrical, so that there is no induced voltage in sensor coil 35 but merely a low amount of noise. The second wave form shown at 44 is for the condition of the'tool aimed at the external pipe, and shows the induced sinisoidal voltage. The third wave form shown at 44 is for the condition of the tool aimed directly away from the pipe, which gives rise to a like voltage exactly 180 out of phase from the second condition. The effect of the phase shifter is shown at 45.-
The mixed signal leaving mixer 42 has one of three wave forms, for. the three conditions discussed, depending upon the amplitude and phase of the signal from coil 35, giving the results shown at 46. This is fed to amplifier 47, the output of which is a square wave, the phase of which is proportional to the ratio of the voltage in coil 35 to that of coil 34. The wave form for the three conditions discussed is shown at 48. This signal is fed to gate 49, which is opened by the negative part of the square wave 50 produced by amplifier 40. Only positive parts of the signal from amplifier 47 which occur at the same time as the negative parts of the reference signal from amplifier 40 are passed on to integrator 51. Thus, gate 49 functions as a phase detector producing a pulse, the width of which is proportional to the phase difference between the reference signal from amplifier 40 and the sensor signal from amplifier 47, as indicated by the three typical wave forms shown at 52. The pulses are integrated in integrator 51 to give a DC voltage which is proportional to the cosine of the angle of the tool with the external casing, as indicated at 53. This voltage output from 51 is applied to voltage-controlled pulse generator 54, the output of which is a 50 microsecond, 40 volt pulse, the rate of which is controlled by the input voltage, as indicated at 55. These pulses are fed to the line driver 56, and applied to the wire line, where they are detected at the surface in unit 28 as shown in FIG. 1. In this unit, the pulses are separated from the exciter voltages present in the line, and are amplified and counted and their integrated value is used to drive a recording pen on a strip chart recorder, all in accordance with means known to those skilled in the art. The maximum deflection registered at the surface then corresponds to the azimuthal location of the external string or strings of casing.
It is entirely possible and indeed we prefer to use a single conductor as already described for line 16, since the applied DC voltages, the alternating voltage for driving the exciter coil, and the pulse output from electronic section 20 do not mutually interfere with each other and may readily be sorted out as needed by means known in the art.
Occasionally, the down-the-hole conditions may be so complex and obscure that it may be desirable to run a check on the operation of the device. For example, there may be as many as five strings of casing in a sideby-side relationship, or the separation between the casings may be extreme. Two means of making such a check are shown in FIGS. 9 and 10.
"UN", Anna In FIG. 9, the device in casing string 11 is as previously described. However, for this check, the exciter coil 26 is shut off, and an auxiliary battery-operated exciter unit 60, comprising a battery section 61, an alternator section 62, and an exciter coil 63 is lowered into casing string 12 on a separate wire line 64. It will be clear that this auxiliary exciter device 60 will produce an alternating electromagnetic field strongly asymmetrical with respect to the pickup coils in casing string'll, so that the orientation of the latter may be doubly checked. At the same time, a check on the depth of the pickup coils is given by noting how far the device 60 must be lowered on its wire line 64 to achieve maximum response. The device and procedure of FIG. 9 has a further utility in that it may be used to orient the perforator in cases where the bore holes contain casing strings of non-ferrous metal, such as aluminum alloys, or of plastic.
A second type of check is shownin FIG. 10 in which the tool in casing string 11 is as previously described. In this procedure, the exciter coil is operated as usual, but when a check is desired, a length of transformer iron 70, which may for example be approximately 1 inch in diameter and 2 feet long, or like ferromagnetic'device, is lowered on its wire line 71 into casing string 12. When it is opposite the tool in casing string 11, and more particularly when it is opposite approximately the midpoint between the exciter and'pickup coils, it will greatly increase the asymmetry of the electromagnetic field already brought about by the presence of casing string 12.
Reverting now to our invention broadly, it will be seen that it accomplishes its objects. In addition to the advantages already discussed, it may be further mentioned that a serious limitation in prior art density system discussed earlier is its unreliability when used in small diameter tubing, disposed within substantially larger casing, such as often occurs. The device of the present invention can be produced in various diameters with excellent reliability, and is operative even under the condition just mentioned.
Our invention has been described in terms of our preferred embodiment, in which the electromagnetic field produced by the exciter coil, and more particularly any asymmetry in this field, is detected by receiver means 21 in the form of two pickup coils. It is entirely possible to use other types of receiver means 21 capable of detecting the electromagnetic field and responsive to any asymmetry thereof. For example, magnetometers of various types, such as those based on the Hall effect, flux gate magnetometers and rubidium vapor magnetometers may be used. Of course when the receiver means 21 embodies a magnetometer of the type which is primarily responsive to a steady state magnetic field, it should be used with appropriate circuitry so as to be responsive to the essentially alternating electromagnetic field involved in the present invention. Devices of this type are well known and have been highly developed both as to miniaturization and sensitivity. All such receiver means 21, including those of the coil type described hereinabove in detail, may be termed electromagnetic field sensors responsiveto the magnitude and the configuration of the electromagnetic flux.
We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a personskilled in the art.
Having described the invention, we claim:
1. A device for sub-surface emplacement in a bore hole adjacent to a ferrous element for determining the orientation of said element with respect to said device comprising:
an exciter coil producing an electromagnetic field in the region surrounding said coil, said field having axial symmetry in the absence of said element but having axial asymmetry in the presence of said element; reference coil longitudinally spaced from said exciter coil and adapted for the production of an induced voltage from said field; sensor coil adjacent said reference coil likewise adapted for the production of an induced voltage from said field, said sensor coil being positioned non-symmetrically with respect to the longitudinal axis of said exciter coil; means for rotating said device in said bore hole; means for determining the ratio of said induced voltage in said sensor coil to said induced voltage from said reference coil as a function of the angular orientation of said device with respect to said ferrous element. 2. A device in accordance with claim l wherein said reference coil is coaxial with said exciter coil and said sensor coil is positioned at an angle to the axis of said exciter coil and said reference coil.
3. A device in accordance with claim 1 wherein said reference coil and said sensor coil are spaced from the axis of said exciter coil and each has its axis parallel to the axis of said exciter coil.
4. A device in accordance with claim 1 wherein said reference coil and said sensor coil are each spaced from the axis of said exciter coil and wherein each has its axis positioned at a like angle from said exciter coil axis.
5. An orientatable perforating device for subsurface use and adapted to perforate in a preselected orientation comprising, in combination:
rotating motor means attached to said positioning means;
exciter coil means likewise attached to said positioning means, said exciter coil means producing an electromagnetic field which is axially symmetrical with respect to said device when in an isotropic environment;
receiver means longitudinally spaced from said exciter coil means and rotatable by said motor means, said receiver meanscomprising an electromagnetic field sensor responsive to the magnitude and the configuration of said electromagnetic field, said sensor providing an electrical signal indicative of said magnitude and said configuration;
means for transmitting said signal to the surface;
perforating means actuatable from said surface and carried by said device and rotated by said motor means.
1 (Ann mm" 6. A device in accordance with claim in which said receiver means comprises a reference coil and a sensor coil, each adapted for the production of an induced voltage from said field.
7. A device in accordance with claim 5 in which said sensor is a magnetometer.
8. An orientatable perforating device for subsurface use and adapted to'perforate in a preselected orientation comprising, in combination, positioning means;
rotating motor means attached to said positioning means; exciter coil means likewise attached to said position ing means, said exciter coil means producing an electromagnetic field which is axially symmetrical with respect to said device when in an isotropic environment; receiver coil means longitudinally spaced from said exciter coil means and rotatable vby said rotor means and including a reference coil capable of producing an induced voltage from said electromagnetic field and a sensor coil means capable of producing an induced voltage from said field, said sensor coil means having its axis oriented at an angle to the longitudinal axis of said device so that when said field is axially non-symmetrical, the voltage induced in said sensor coil varies with its relative orientation with respect to said non-symmetrical field;
means responsive to the ratio of the induced voltage in said sensor coil to the induced voltage in said reference coil and producing a signal indicative of said ratio which is transmitted to the surface;
perforating means actuatable from said surface and carried by said device and rotated by said motor means.
9. A method for orienting an actuatable subsurface device in a bore hole containing a first casing string and a second casing string, to a preselected orientation 10 comprising:
positioning in said first casing string a source of electromagnetic flux normally axially symmetric with said first casing string but asymmetrically distorted by the presence of said second casing string;
positioning a rotatable receiver means longitudinally spaced from said source within said first casing string, said receiver-means being responsive to the magnitude and the configuration of said elec tromagnetic flux and providing an electrical signal indicative of said configuration; transmitting said signal to said surface;
rotating said receiver means so as to cause a registration at said surface of its orientation with respect to said second casing string;
rotating said sub-surface actuatable device into said preselected position;
subsequently actuating said device.
10. The method in accordance with claim 9 wherein said device is a perforator.
11. The method in accordance with claim 9 wherein said source of electromagnetic flux is placed in said second casing string, and said rotatable receiver means is not necessarily longitudinally spaced from said source.
12. The method in accordance with claim 9 wherein said receiver means comprises a reference coil capable o producing an induced voltage from said electromagnetic flux and a sensor coil means capable of producing an induced voltage from said flux, said sensor coil means having its axis oriented at an angle to the longitudinal axis of said casing string.
13. The method in accordance with claim 12 wherein said device is a perforator.
14. The method in accordance with claim 9 wherein said receiver means is a magnetometer.
15(The method in accordance with claim 14 wherein said device is a perforator.