GB2314163A

GB2314163A - Orientation measurement instruments

Info

Publication number: GB2314163A
Application number: GB9709488A
Authority: GB
Inventors: Symong Shih; Jr Albert A Alexy
Original assignee: Western Atlas International Inc
Current assignee: Western Atlas International Inc
Priority date: 1996-06-14
Filing date: 1997-05-09
Publication date: 1997-12-17
Also published as: NO972699L; NO972699D0; GB9709488D0

Abstract

The orientation of an instrument, e.g. a well logging instrument 10 in a well bore 2, is determined with respect to the earth's gravity and magnetic field by means of apparatus which includes a plurality of magnetometers 8,12 each including sensing coils. Each of the coils has a sensitive axis separated from the sensitive axis of the other coils by a known angle so that a direction of an apparent magnetic field can be determined. The apparent magnetic field includes both the earth's magnetic field and interference from magnetic anomalies. The apparatus also includes accelerometers 18 each having a sensitive axis separated from the sensitive axis of the other accelerometers by a known angle so that a direction of the earth's gravity may be determined. The apparatus includes means for calculating the magnitudes of the directional components of the apparent magnetic field caused by the magnetic anomalies from the measurements made by the sensing coils and the accelerometers, so that the directional components of the earth's magnetic field can be determined. In a preferred embodiment, the sensing coils of each magnetometer, and the accelerometers, are mutually orthogonal. One coil from each magnetometer is set parallel to the instrument axis 14 and in opposite polarity to the other axially parallel coil. The other coils therefore lie in a plane perpendicular to the axis, and are separated from each other by a known angle.

Description

ORIENTATION MEASUREMENT INSTRUMENTS The present invention relates to orientation measurement instruments, and in particular, but not exclusively, to methods and apparatus for reducing errors in geomagnetically-based orientation measurements caused by magnetic anomalies in the orientation instrument housing.

Orientation measurement instruments are used for, among other uses, determining the geographic orientation of oil wells drilled through earth formations, particularly when such wells are not drilled vertically. A common type of orientation measuring instrument includes three mutually orthogonal flux-gate magnetometers, each of which measures the magnitude of a component of the earth's magnetic field coaxial with its sensitive axis. The magnetic field component measurements are used to determine the direction of the earth's magnetic field relative to the orientation measuring instrument. The instrument also typically includes acceleration sensors to determine the direction of the earth's gravity with respect to the instrument. The measurements of the earth's gravitational and magnetic fields can be combined to determine the geographic orientation of the instrument. The instrument is inserted into the wellbore by various means of conveyance, including specially adapted drill collars in a measurement-whiledrilling (MWD) apparatus, or by armored electrical cable as part of a "wireline" instrument string. Because the instrument, for all practical purposes, is coaxial with the wellbore the measurements of the earth's magnetic field direction and gravity correspond to the direction of the wellbore with respect to the earth's magnetic field and gravity.

The magnetometers and associated electronic circuits are typically encased in a non-magnetic metal housing to exclude wellbore fluids from the interior of the instrument while enabling measurement of the earth's magnetic field. Nonmagnetic metal housings, however, sometimes have magnetic anomalies. The anomalies can include so called "hot-spots" in the housing itself, and sources of magnetic interference which originate axially above or below the non-magnetic housing. The magnitude and position of the magnetic anomalies within the instrument is generally not entirely predictable. The effect of the magnetic anomalies on the magnetometer measurements, therefore, is not entirely predictable.

There are various methods to estimate the magnitude of the effect of magnetic anomalies on the magnetometer measurements. A method described in U. S. patent no. 4,510,696 issued to Roesler provides a calculation for the minimum amount of non-magnetic housing which must be included above and below the magnetometers to measure accurately the component of the earth's magnetic field which is coaxial with the instrument axis. The method in the Roesler '696 patent, however, does not disclose or suggest any method of quantifying the effect of "hot spots" in the housing or other magnetic anomalies which are not coaxial with the instrument axis (referred to as "cross-axial" anomalies which generate magnetic field components in a plane perpendicular to the instrument axis).

A method is described in U. S. patent no. 4,682,421 issued to van Dongen et al for quantifying cross-axial magnetic anomalies. The method in the van Dongen et al '421 patent includes measuring cross-axial components of the earth's magnetic field while rotating the instrument about its axis through a plurality of rotary orientations. While the method in the van Dongen '421 patent is generally suitable for use in a measurement-while-drilling (MWD) or similar drillstring conveyed apparatus, instruments conveyed by wireline (electrical cable) typically do not include the capability of being rotated about the instrument axis while disposed in the wellbore. Building rotational capability into a typical wireline conveyed apparatus can be difficult and expensive. Making magnetic measurements in a plurality of rotary orientations also consumes considerable extra operating time.

Various aspects of the present invention are exemplified by the attached claims.

Another aspect of the present invention provides a method of determining the orientation of a wellbore with respect to the earth's magnetic field without the need to rotate the instrument about its axis to determine the effect of cross-axial magnetic anomalies.

Another aspect of the present invention is an apparatus for determining the orientation of an instrument with respect to the earth's gravity and magnetic fields In a preferred embodiment, the apparatus includes first and second magnetometers.

Each magnetometer includes three sensing coils. Each one of the sensing coils has a sensitive axis separated from the sensitive axis of the other coils by a known angle, so that a direction of an apparent magnetic field can be determined. The apparent magnetic field includes both the earth's magnetic field and interference from magnetic anomalies. The preferred embodiment of the apparatus also includes accelerometers each having a sensitive axis separated from the sensitive axis of the other accelerometers by a known angle so that a direction of the earth's gravity may be determined. The apparatus includes means for calculating the magnitudes of the directional components of the apparent magnetic field caused by the magnetic anomalies from the measurements made by the sensing coils and the accelerometers, so that the directional components of the earth's magnetic field can be determined.

In one preferred embodiment, the sensing coils of each magnetometer are mutually orthogonal. One coil from each magnetometer is aligned parallel to the instrument axis and in opposite polarity to the corresponding coil from the other magnetometer. The other coils therefore lie in a plane perpendicular to the instrument axis. These other coils are separated from each other by a known angle.

The sensitive axes of the accelerometers are also mutually orthogonal, and are parallel to the sensitive axes of the sensing coils in one of the magnetometers.

Another embodiment uses three magnetometers each having mutually orthogonal sensing coils. Each magnetometer has one coil aligned parallel to the instrument axis, and the other coils therefore line in the plane perpendicular to the instrument axis. The other coils are separated from each other by known angles.

A method embodying another aspect of the present invention includes the steps of measuring components of an apparent magnetic field substantially parallel to the instrument axis, and measuring components of the apparent magnetic field in radially spaced apart directions in a plane substantially perpendicular to the instrument axis of said instrument. The radially spaced apart directions are separated from each other by known angles. A direction of the earth's gravity with respect to orientation of the instrument is determined by measuring acceleration in mutually orthogonal directions. The total magnitude and dip direction of the earth's magnetic field is determined, and magnitudes of directional components of the magnetic field induced by the magnetic anomalies are determined by combining the measurements of the components of the apparent magnetic field, the direction of gravity and the total magnitude and dip angle of the earth's magnetic field, thereby enabling calculation of magnitudes of directional components of the earth's magnetic field with respect to the orientation of the instrument.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a well logging instrument, including magnetometers and accelerometers, inserted into a wellbore; Figure 2 shows first and second magnetometers, and the accelerometers of the instrument in greater detail; Figure 3 shows a diagram of the relative orientations of the first and second magnetometers; Figure 4 shows an example of a magnetic anomaly in the housing and definitions of its axial distances to each of the magnetometers; Figure 5 shows an embodiment of the invention using three magnetometers; and Figure 6 shows a diagram of the relative orientations of the first' second and third magnetometers of the alternative embodiment.

Figure 1 shows a well logging instrument 10 disposed in a wellbore 2. The instrument 10 can be extended into the wellbore 2 at one end of an armored electrical cable 4. The instrument 10 could also be part of a measurement-whiledrilling (MWD) instrument system and so the invention is not limited to instruments which are extended into the wellbore 2 by means of the cable 4. The cable 4 can provide electrical power to the instrument 10 and can transmit signals to the earth's surface for recording, data processing and interpretation. The signals typically correspond to measurements made by various sensors in the instrument 10, which include triaxial magnetometers such as shown at 8 and 12 and which will be further explained, and include mutually orthogonal accelerometers, shown in package 18. The instrument 10 can include a telemetry/signal processing unit (SPU) 6. The SPU 6 can be programmed to generate the signals for transmission to the earth's surface, and to control the operation of the various sensors (including the magnetometers 8 and 12) in the instrument 10. The invention does not require transmission of these signals to the earth's surface. The signals may also be recorded in the instrument 10 itself by a suitable data recording device (not shown).

The instrument 10 typically includes other sensors (not shown) for measuring various properties of the earth formations penetrated by the wellbore 4.

The magnetometers 8, 12 can be triaxial, flux-gate types well known in the art. Each magnetometer 8, 12 typically includes three, mutually orthogonal sensing coils, which will be luther explained. Each of the sensing coils measures the magnitude of a component of the apparent magnetic field which is parallel with the sensitive axis of that particular sensing coil. Preferably one of the sensing coils in each magnetometer 8, 12 is oriented substantially parallel to the axis 14 of the instrument 10. As will be further explained, the apparent magnetic field can include the earth's magnetic field and the effect of any magnetic anomalies. The portion of the instrument housing 16 in which the magnetometers 8, 12 are located is typically formed from a substantially non-magnetic material such as monel.

Monel and other substantially non-magnetic materials can have some residual magnetism, which is one form of the aforementioned magnetic anomalies. The residual magnetism may affect the portion of the measurements made by each one of the sensing coils which results from the earth's magnetic field. Another form of magnetic anomaly can be electromagnetic interference from other sensors (not shown) forming part of the instrument.

The arrangement of the magnetometers 8, 12 and the method of calculating the earth's magnetic field in the invention enables determination of the magnitude of the effect of the magnetic anomalies, thereby enabling determination of magnitudes of the components of the earth's magnetic field, as will be further explained.

The accelerometer package 18 can include three mutually orthogonal accelerometers of any type known in the art useful for measuring accelerations of about 1 g. The accelerometers in the package 18 are preferably oriented to that the sensitive axis of each accelerometer is substantially parallel with the sensitive axis of one of the sensing coils (not shown in Figure 1) in either one of the magnetometers 8, 12, as will be further explained The arrangement ofthe magnetometers 8, 12 and the accelerometer package 18 can be better understood by referring to Figure 2. One magnetometer 8, referred to for convenience as the first magnetometer preferably has one of its sensing coils, shown at 8C, substantially parallel to the instrument axis 14. Sensing coil 8C is referred to as the Z-axis coil for purposes of the description of the invention. The other two sensing coils, 8A and 8B are preferably substantially orthogonal to the Z-axis coil 8C, and to each other. Sensing coil 8A is referred to for this description as the X-axis coil, and sensing coil 8B is referred to as the Yaxis coil. The signal magnitudes from each of the sensing coils 8A, 8B and 8C in the first magnetometer 8 can be represented by vector notations B,, Bn and B,, respectively.

The other magnetometer 12, referred to as the second magnetometer, can be located in the housing 16 near the first magnetometer 8. The second magnetometer 12 also can include three substantially mutually orthogonal X-, Y-, and Z-axis sensing coils 12A, 12B, 12C, respectively. The signal magnitude of the sensing coils 12A, 12B, 12C in the second magnetometer 12 for convenience can be represented by vector notations Baas B"2, Bz2, respectively, similar to the representations for the signals from the coils in the first magnetometer 8. Z-axis coil 12C can also be substantially parallel to the instrument axis 14. Z-axis coil 1 2C is also preferably opposite in polarity to the Z-axis coil 8C in the first magnetometer 8. As will be further explained, a substantially uniform magnetic field inducing a signal in sensing coil 8C will generate substantially the same magnitude but opposite polarity signal in sensing coil 12C. The reason for inverting the polarity of Z-axis coil 12C with respect to its counterpart 8C in the first magnetometer 8 is so that the effect of a magnetic anomaly with respect to the Z-axis will be indicated by a difference between the signals generated by coil 12C and coil 8C, since the effect of the earth's magnetic field will substantially cancel when the signals from coil 8C and coil 12C are added.

The X-axis and Y-axis coils 12A, 12B of the second magnetometer 12 are preferably oriented at a fixed, known angle with respect to the orientation of the X-axis and Y-axis coils 8A, 8B of the first magnetometer 8. This angle is shown in Figure 3 as +. The magnitude of angle 9 is not critical to embodiments of the invention, it is only necessary that it be known. As a practical matter, + should be sufficiently large to provide substantial differences in signal magnitude between corresponding coils 8A, 12A and 8B, 12B, respectively, of each magnetometer 8, 12.

It is also to be explicitly understood that the sensing coils 8A, 8B, 8C, 12A, 12B, 12C in each magnetometer 8, 12, need not be mutually orthogonal for proper operation of embodiments of the invention. It is only necessary that the angle subtended between each of the sensing coils be known, and that the angle subtended be known between any sensing coil and an orientation reference with respect to the housing 16, in order to be able to calculate the orientation of the housing 16 with respect to the earth's magnetic field. References with respect to the housing 16 typically include the instrument axis 14, and a reference indicator (not shown) on the exterior of the housing 16 located on a line parallel to the Xaxis of coil 8A in Figure 2. The trigonometric relations to calculate orientation from signals generated by non-orthogonal sensing coils are well known in the art.

Selecting the X-axis as the positional reference for the reference indicator (not shown) is strictly a matter of convenience for the present description of the invention and is not to be construed as a limitation on the invention.

It is also to be understood that the arrangement of magnetometers 8, 12 described herein is a matter of convenience for the system designer in order to use commercially available magnetometers. Commercially available magnetometers typically include three mutually orthogonal sensing coils. The system designer may just as easily elect to build the apparatus of the invention using individual sensing coils, arranged as described herein, without the necessity of packaging such sensing coils in the form of two separate commercially available magnetometers. It is contemplated that the sensing coils can be arranged so that the X- and Y-axis sensing coils, 8A, 12A and 8B, 12B, respectively, are substantially coplanar and radially spaced apart by angle + as shown in Figure 3. The Z-axis sensing coils, 8C, 12C, irrespective of the chosen arrangement of the other sensing coils, are preferably arranged as shown in Figure 3 for signal processing reasons which will be further explained.

The accelerometers of package 18 are shown individually in Figure 2 at 18A, 18B, 18C. The accelerometers 18A, 18B, 18C are preferably oriented so that the sensitive axis of each accelerometer is mutually orthogonal to the sensitive axes of the other accelerometers. The accelerometers 18A, 18B, 1 8C are also preferably arranged so that the sensitive axis of each one is parallel with the sensitive axis of one of the sensing coils in either one of the magnetometers 8, 12. As shown in Figure 2 the package 18 includes an X-axis accelerometer 1 8A which is substantially parallel with X-axis sensing coil 8A, a Y-axis accelerometer 1 8B which is substantially parallel with Y-axis sensing coil 8B, and a Z-axis accelerometer 18C which is substantially parallel with the instrument axis 14 and Z-axis coil 8C. The magnitude of the component of the earth's gravity which is parallel with the sensitive axis of each accelerometer 18A, 18B, 1 8C can be represented by vector notation as Gx, G^ > Gz, respectively. The accelerometers 18A, 18B, 18C are substantially unaffected by any magnetic anomalies in the housing 16 or the instrument 10 and therefore their measurements can reliably be used to determine the direction of gravity with respect to the instrument 10.

Methods for calculating the gravitational direction with respect to the instrument 10 from the component measurements made by each accelerometer 18A, 18B, 18C, are known in the art.

While the orientation of the sensing coils in each magnetometer 8, 12 and the accelerometers 18A, 18B, 18C with respect to each other has been generally described as mutually orthogonal, it is to be understood that other non-orthogonal orientations could also provide the gravity and magnetic field component measurements necessary for the invention to perform as described herein.

Orthogonal orientation of the sensing coils in each magnetometer 8, 12 and the accelerometers 18A, 18B, 18C, however, has the particular advantage of providing the highest resolution in determining the direction of the earth's gravity and magnetic fields. The reason, as can be explained, for example, in the case of mutually orthogonal accelerometers, is that if one of the accelerometers is parallel to the earth's gravity, then the other two orthogonal accelerometers will indicate substantially zero gravitational acceleration. Cross-axial components of the measured quantity have the smallest effect, and therefore the least deleterious effect on system resolution, when the accelerometers (or the magnetometer sensing coils) are mutually orthogonal.

Having described the preferred arrangement of accelerometers and magnetometer sensing coils, the manner in which the measurements provided by the magnetometers (8, 12 in Figure 2) and the accelerometers (18A, 18B, 18C in Figure 2) can be combined to resolve the effect of magnetic anomalies can now be explained. The gravitational component measurements Gx, G^7 Gz made by accelerometers 18A, 18B, 18C, respectively, combine as shown in equation (1) to yield the earth's total gravitational field Gr:

If the accelerometers are mutually orthogonal as shown in Figure 2, the inclination, I, of the tool axis 14 with respect to the earth's gravity (vertical) can be determined from the gravitational component measurements by the expression:

If the accelerometers are oriented other thin mutually orthogonal, the trigonometric relations to calculate the inclination are well known in the art and can be easily used provided that the accelerometer orientations are known. An angle, F, subtended between the reference marker and vertical (sometimes called the "gravity toolface") can be calculated from the gravitational component measurements by the expression:

F = tan-dI (3) where the reference marker, as previously explained, is typically defined as being on a line perpendicular the tool axis 14, and parallel to the X-axis. Therefore the reference marker would generally be parallel to the sensitive axis of X-axis sensing coil 8A and X-axis accelerometer 18A. The use of these gravitationally derived orientation values will be further explained.

A total magnetic field, B" determined by combining measurements of all three sensing coils in each magnetometer, should be substantially the same at each magnetometer 8, 12 since both magnetometers 8, 12 are affected by the same total magnetic field. This total magnetic field includes the earth's magnetic field and the effect of any magnetic anomalies. The total field expressed in terms of the magnetic component measurements can be represented by the following equation:

It is the earth's magnetic field to which the orientation of the instrument 10 is desired to be determined. The contribution of the earth's magnetic field to the total magnetic field can be represented in terms of the axial components of the earth's magnetic field B, B", B, (along the X-, Y- and Z-axes, respectively).

Measurements actually made by each sensing coil, therefore, consist of an axial component of the earth's magnetic field and an axial component of the magnetic field emitted by any magnetic anomalies (the anomalous magnetic field), these axial components being parallel to the sensitive axis of the particular sensing coil. The anomalous magnetic field's axial components can be represented by bBxs bB", & Bz.

The measurements made by each of the sensing coils 8A, 8B, 8C of the first magnetometer 8, for example, can therefore be defined in terms of the axial components of the earth's magnetic field plus the anomalous field axial components, by the expressions: B10 = B12 - 5B1 Byo = ByZ - 6By (5) Bzo = Bzl - 6Bz Similar expressions can be readily derived for the signals generated by sensing coils 12A, 12B, 12C of the second magnetometer 12.

The magnitude of the earth's magnetic field, B" and an inclination angle of the earth's magnetic field (called the magnetic dip angle, D) with respect to gravitational vertical are known at any position on the surface of the earth. These two values, total magnitude and dip angle can be obtained from various geomagnetic surveys well known in the art. The magnitude of the apparent magnetic field, Bp derived from the sensing coil measurements has previously been defined in terms of its X-, Y- and Z-axis components in equation (4), and this apparent magnetic field can be further defined in terms of the intrinsic (the earth's magnetic field) axial components and the anomalous field axial components by substitution of equation (5). The magnetic dip angle, D, of the earth's magnetic field B, can be derived from the intrinsic field components (or conversely can be used to generate relative magnitudes of intrinsic field axial components) by the expression:

The magnitude of a component of the earth's magnetic field which is substantially perpendicular to the Z-axis (and which therefore lies in planes including the sensitive axes of the X- and Y-axis sensing coils lying therein, referred to as the X-Y plane) will be substantially the same at both magnetometers 8, 12. This component, R, can be defined by the expression:

When there is a magnetic anomaly, in the housing 16 or elsewhere along the instrument 10, the X- and Y-components of the earth's magnetic field as detected by the X- and Y-axis sensing coils in each magnetometer 8, 12 will, as previously explained, include an "offset" from the intrinsic (earth's) field axial component magnitudes, the intrinsic component magnitudes represented by Bxe and B, The offset has an indeterminate magnitude and direction since its source may not be known. The offset, as previously explained can be represented by its X- and Y-axis components oBx and ABr. The X-, and Y-axis sensing coil measurements actually made by each magnetometer 8, 12 can, as previously explained, be represented by the following expressions: BX1 = Bxo + #BX (8) BY1 = BY0 + 6By By substitution of the relationships of equation (5) the following expressions can be derived: (BX1 - #BX)2 + (BY1 - # Y)2 = R (9) (BX2 - #BX)2 + (BY2 - # Y)2 = R2 BX2 = B,cosO + BnsinO (10) BY2 = Brlco sS - BX1sin# (11) two + bBy] = (BY0 +6By)cOs - (BX0 + #BX)sin# (12) Equation (7) can be rearranged to generate a relationship between the X- and Y-axis axial components of the anomalous magnetic field as shown herein:

From equations (7) and (13) it is possible to solve by iteration for Bxo, B", oBx and oBr. The iteration technique preferably used to solve for the intrinsic (earth's) magnetic field axial components and the anomalous field (offset) axial components is known as "perturbation theory", and is described for example in, A Treatise on the Mathematical Theorv of Elasticitv. A. E. H. Love p. 189-190, or in Numerical Recipes in FORTRAN - The Art of Scientific Computing W. H. Press et al, Cambridge University Press, p. 65-67. As is understood by those skilled in the art' solution of Boa, B bBx and oBy will directly enable determination of the relative magnetic orientation of the instrument axis 14 and consequently the wellbore (2 in Figure 1).

Having solved for BX0, B oBx and & By it is then possible to solve for Ba and & Bz. Referring now to Figure 4, a magnetic anomaly or "hot spot" is shown at H on the housing 16. In Figure 4 only the Z-axis sensing coils 8C and 12C are shown for clarity of the illustration. The magnetic anomaly H is located at an axial distance represented by r2 from the approximate measurement point 12CA of sensing coil 12C, and is located an axial distance represented by r1 from the measurement point 8CA of sensing coil 8C. Measurement points 12CA and 8CA are themselves separated by an axial distance represented by r. Similarly as for the X- and Y-axis components, the Z-axis component of the total magnetic field detected by each sensing coil 8C, 12C can be expressed as: BZ0 + Bz = Bz. An expression defining Bz can be developed as follows:

since the Z-axis coils are preferably opposite in polarity, the difference in the their signal magnitudes, represented by ABz, can be expressed by the following equation:

Equation (15) can be rewritten as:

and equation (16) can be further simplified to the following expression: 2r(Bm+5B AB 2 if r < r1 (17) r1 Finally, an expression for the Z-axis component of the anomalous magnetic field, oBz, caused by the hot spot H can be shown by the following equation:

As previously explained, the total signal from all three coils 8A, 8B, 8C at the first magnetometer 8 can be expressed as:

which leads to an expression for the Z-component of the earth's magnetic field BZ0:

can be approximated by the following expression which includes the previously described measurements of the axial com BZ0 = 1/GZ (GtB0cosD - GXBX0 - GYBY0 (21) so that ultimately B, and oBz can be solved through an iteration process similar to the one used to derive the X- and Y-axis axial components.

Having determined all three axial components of the earth's magnetic field with respect to the known orientation along the housing 16, it is then possible to determine the geomagnetic direction of the instrument axis 14, and therefore, if desired, the wellbore (2 in Figure 1).

It is also possible to include additional magnetometer sensing coils to the first embodiment of the invention at different angular spacings therebetween to explicitly determine the effect of magnetic anomalies on the magnetometer measurements. Figure 5 shows an embodiment of the instrument 10 having three commercially available magnetometers 8, 12, 20. Just as in the first embodiment, each of the magnetometers 8, 12, 20 in the present embodiment can include three mutually orthogonal sensing coils: X-axis coil 8A, Y-axis coil 8B, and Z-axis coil 8C for the first magnetometer 8. Respective X-axis, Y-axis and Z-axis coils are provided for the second 12 and third 20 magnetometers as shown at 12A, 12B, 12C and 20A, 20B and 20C. Preferably all the Z-axis coils 8C, 12C, 20C are substantially parallel with the instrument axis 14, but as in the first embodiment this is a matter of convenience for the system designer and is not to be construed as a limitation on the invention. Preferably all the X- and Y-axis coils 8A, 8B, 12A, 12B, 20A, 20C lie in planes perpendicular to the instrument axis 14. The X- and Y-axis coils for each individual magnetometer 8, 12, 20 are typically angularly spaced at 90 degrees from each other. Preferably the X- and Y-axis coils of the different magnetometers 8, 12,20 are radially spaced apart spaced at some nominal, known angle from each other. The angular separation of the X- and Y-axis coils of the different magnetometers 8, 12, 20 can be better understood by referring to Figure 6. X-axis coil 8A from the first magnetometer 8 is shown as separated by angle +, from X-axis coil 12A from the second magnetometer 12 and is angularly separated from the X-axis coil 20A of the third magnetometer 20 by angle +2. It should be noted that just as in the first embodiment of the invention, the X- and Yaxis sensing coils can be arranged to be substantially co-planar if the system designer elects to use individual sensing coils. It is also acceptable to omit the third Z-axis sensing coil 20C since this would generate a redundant measurement of the Z-axis component of the total (apparent) magnetic field.

Considering the X- and Y-axis component measurements generated by each magnetometer, represented for the first 8, second 12 and third 20 magnetometers as boo BY1, B B, Bx3, BY3, respectively, the following solution can be derived for the earth's intrinsic magnetic field components Bxa, B, and the X- and Y-axis offset component magnitudes ABX and Obey. Just as for the first embodiment of the invention, the actual sensing coil measurements for the first magnetometer 8 can be defined in terms of the intrinsic magnetic field plus the offset as: Bxo = BXl-6Bx (22) Bro = Bn~6BY with similar expressions that can be derived for the second 12 and third 20 magnetometer sensing coils. The sum of the squares of the axial component measurements of the earth's magnetic field is substantially constant for all three magnetometers as shown by the following equation:

and therefore by substitution of equation (22) and its counterparts for the second 12 and third 20 magnetometers: R2 = BX12-2BX1#BX+#BX2+BY12-2BY1#BY+#BY2 - BX22-2BX2#BX+#BX2+BY22-2BY2#BY+#BY2 (24) = BX32-2BX3#BX+#BX2+BY32-2BY3#BY+#BY2 By defining variables C, and C2 as follows: C1 = (B2 B2)+(B2 B2) (25) C2 = (BX32-BX12)+(BY22-BY12) and substituting C, and C2 into equation 24, solutions for the X- and Y-axis offset (anomalous field) component magnitudes can be derived as follows: #BX = C1(BY1-BY3) - C2(BY1-BY2) (26) 2[(Bxl-BY2)(BX1-BX3) - (BY1-BY3)(BX1-BX2)] and: ôB = C1(BX1-BX3)-C2(BY1-BY2) (27) 2[(BY1-BY2)(BX1-BX3)-(BY1-BY3)(BX1-BX2)] The offset component magnitudes can be subtracted from the sensing coil measurements to provide the intrinsic magnetic field components. After solving the X- and Y-axis components of the intrinsic magnetic field, the Z-axis component is easily derived from the equation for the total magnetic field, which as stated previously, is known for any location on the surface of the earth and can be determined from a geomagnetic survey known in the art.

If desired by the system designer, four or more magnetometers could be provided having each one's X- and Y-axis sensing coils separated by a known angle. Using four magnetometers would provide three additional solutions to the intrinsic magnetic field, which could materially improve the quality of the solution.

The number of magnetometers is practically limited by considerations such as the space available in the housing 16 and the available locations on the telemetry format and memory of the SPU (6 in Figure 1).

The embodiments of the invention disclosed herein are meant to serve only as an example of the invention. Those skilled in the art will readily devise other embodiments which do not depart from the spirit of the invention.

Claims

1. An apparatus for determining orientation of an instrument with respect to the earth's magnetic field, comprising: magnetometers each including sensing coils, each of said sensing coils having a sensitive axis separated by a known angle from the sensitive axis of each of the other sensing coils, so that a direction of an apparent magnetic field can be determined with respect to said instrument, said apparent magnetic field including the earth's magnetic field and interference from magnetic anomalies; a plurality of accelerometers each having a sensitive axis separated by a known angle from the sensitive axis of each of the other accelerometers so that a direction of the earth's gravity can be determined; and means for calculating magnitude of components of said apparent magnetic field caused by said magnetic anomalies from measurements made by said sensing coils and said accelerometers, so that direction of the earth's magnetic field can be determined by subtracting said components of said apparent magnetic field from directionally corresponding components of the earth's magnetic field.

2. An apparatus as claimed in claim 1, comprising means for calculating orientation of said instrument with respect to earth's gravity and magnetic field from measurements of said accelerometers and said components of the earth's magnetic field determined by said means for calculating.

3. An apparatus as claimed in claim 1 or 2, wherein said sensitive axis of a selected one of said sensing coils from each of said magnetometers is positioned substantially parallel to an axis of said instrument, and said sensitive axis of a first one of said selected sensing coils is positioned in opposite polarity to a second one of said selected sensing coils.

4. An apparatus as claimed in claim 3, wherein said sensitive axes of said sensing coils in each one of said magnetometers are substantially mutually orthogonal.

5. An apparatus as claimed in claim 4, wherein corresponding ones of said sensing coils in each of said magnetometers lie in planes substantially perpendicular to said axis of said instrument, and said sensitive axes of said corresponding sensing coils are separated from each other by said known angle.

6. An apparatus as claimed in claim 1, wherein said sensitive axes of said accelerometers are substantially mutually orthogonal.

7. An apparatus for determining the orientation of an instrument with respect to the earth's magnetic field, comprising: at least two magnetometers each including three mutually orthogonal sensing coils for determining component magnitudes of an apparent magnetic field, said apparent magnetic field including the earth's magnetic field and an anomalous magnetic field, each of said sensing coils in each magnetometer having a sensitive axis separated by a known angle from the sensitive axes of said sensing coils in the other ones of said magnetometers; and means for calculating the magnitude and direction of said anomalous magnetic field from measurements of said sensing coils so as to calculate magnitude and direction of the earth's magnetic field.

8. An apparatus as claimed in claim 7, wherein the sensitive axis of a selected one of said sensing coils from each one of said magnetometers is substantially parallel to an axis of said instrument.

9. An apparatus as claimed in claim 8, wherein the sensitive axes of corresponding ones of said sensing coils in each of said magnetometers lie in planes substantially perpendicular to an axis of said instrument, and said sensitive axes of said corresponding sensing coils are separated from each other by said known angle.

10. A method of determining orientation of an instrument with respect to the earth's magnetic field comprising: measuring components of an apparent magnetic field substantially parallel to an axis of said instrument, said apparent magnetic field including the earth's magnetic field and a magnetic field induced by magnetic anomalies; measuring components of said apparent magnetic field in a plurality of radially spaced apart directions in planes substantially perpendicular to said axis of said instrument, said radially spaced apart directions separated by known angles; determining a direction of the earth's gravity with respect to said instrument; determining total magnitude and dip direction ofthe earth's magnetic field; and calculating magnitudes of directional components of said magnetic field induced by said magnetic anomalies by combining said measurements of said components of said apparent magnetic field, said direction of gravity and said total magnitude and dip angle of the earth's magnetic field, thereby enabling calculation of magnitudes of directional components of the earth's magnetic field with respect to said orientation of said instrument.

11. A method as claimed in claim 10, wherein said step of determining said direction of gravity includes measuring acceleration in mutually orthogonal directions and solving for said direction of gravity.

12. A method as claimed in claim 10 or 11, wherein said total magnitude and said dip direction of the earth's magnetic field is provided by a geomagnetic survey.

13. A method as claimed in claim 10, 11 or 12, wherein said steps of measuring said components of said apparent magnetic field substantially parallel to said axis and in said plane includes: measuring strength of said apparent magnetic field in mutually orthogonal directions at a first location, wherein one of said directions is substantially parallel to said axis of said instrument; measuring strength of said apparent magnetic field in mutually orthogonal directions at a second location, wherein one of said directions is substantially parallel to said axis of said instrument and is opposite in polarity to said one of said directions at said first location; and said mutually orthogonal directions which lie in said plane at said first location being separated from said mutually orthogonal directions which lie in said plane at said second location by a known angle.

14. A method of determining orientation of an instrument with respect to the earth's magnetic field comprising: measuring components of an apparent magnetic field substantially parallel to an axis of said instrument, said apparent magnetic field including the earth's magnetic field and a magnetic field induced by magnetic anomalies; measuring components of said apparent magnetic field in radially spaced apart directions in planes substantially perpendicular to said axis of said instrument, said spaced apart directions separated by known angles, said step of measuring said components in said plane including at least three pairs of axes, said at least three pairs of axes separated from each other by known angles; and calculating magnitudes of directional components of said magnetic field induced by said magnetic anomalies by combining said measurements of said components of said apparent magnetic field substantially parallel to said axis of said instrument and in said plane, thereby enabling calculation of directional components of the earth's magnetic field with respect to said instrument.

15. A method as claimed in claim 14, wherein said step of measuring said components lying in said plane includes the axes within each pair of said axes being orthogonal to each other, said pairs separated from each other by known angles.

16. An apparatus for determining orientation of an instrument with respect to the earth's magnetic field substantially as hereinbefore described with reference to the accompanying drawings.

17. A method for determining orientation of an instrument with respect to the earth's magnetic field substantially as hereinbefore described with reference to the accompanying drawings.