CA1189317A - Instruments for monitoring the direction of a borehole - Google Patents
Instruments for monitoring the direction of a boreholeInfo
- Publication number
- CA1189317A CA1189317A CA000409758A CA409758A CA1189317A CA 1189317 A CA1189317 A CA 1189317A CA 000409758 A CA000409758 A CA 000409758A CA 409758 A CA409758 A CA 409758A CA 1189317 A CA1189317 A CA 1189317A
- Authority
- CA
- Canada
- Prior art keywords
- axis
- outer gimbal
- borehole
- instrument
- rate
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000012544 monitoring process Methods 0.000 title claims abstract description 6
- 230000005484 gravity Effects 0.000 claims abstract description 26
- 238000000034 method Methods 0.000 claims description 9
- 238000005259 measurement Methods 0.000 abstract description 16
- 239000013598 vector Substances 0.000 description 9
- 230000005291 magnetic effect Effects 0.000 description 6
- 230000000694 effects Effects 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 238000010586 diagram Methods 0.000 description 2
- 229910000831 Steel Inorganic materials 0.000 description 1
- 230000000295 complement effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 238000005553 drilling Methods 0.000 description 1
- 239000010959 steel Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/02—Rotary gyroscopes
- G01C19/04—Details
- G01C19/16—Suspensions; Bearings
- G01C19/22—Suspensions; Bearings torsional
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B47/00—Survey of boreholes or wells
- E21B47/02—Determining slope or direction
- E21B47/022—Determining slope or direction of the borehole, e.g. using geomagnetism
Landscapes
- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Geology (AREA)
- Life Sciences & Earth Sciences (AREA)
- Mining & Mineral Resources (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Fluid Mechanics (AREA)
- Environmental & Geological Engineering (AREA)
- Geophysics (AREA)
- Geochemistry & Mineralogy (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Gyroscopes (AREA)
- Geophysics And Detection Of Objects (AREA)
- Perforating, Stamping-Out Or Severing By Means Other Than Cutting (AREA)
- Earth Drilling (AREA)
- Navigation (AREA)
Abstract
S P E C I F I C A T I O N
"Improvements in or Relating to Instruments for Monitoring the Direction of a Borehole"
ABSTRACT OF THE DISCLOSURE
An instrument for monitoring the direction of a borehole comprises an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis of the casing, a gyroscope unit mounted in the outer gimbal, a torque motor and a gravity sensor unit. At the beginning of a run the instrument is positioned at the mouth of the borehole. The rate of rotation about an axis transverse to the outer gimbal axis is then sensed by the gyroscope unit, and the outer gimbal is slewed by the torque motor in dependence on the sensed rate so as to align the transverse axis along an East/West direction. The instrument is then moved along the borehole and the rate of rotation about the outer gimbal axis is continually sensed by the gyroscope unit, the outer gimbal being torqued by the motor in dependence on the sensed rate to stabilize the outer gimbal about its axis. Simultaneously the components of gravity are sensed by the gravity sensor unit. The resulting measurements are used to determine the inclination and azimuth at a plurality of points along the length of the borehole.
"Improvements in or Relating to Instruments for Monitoring the Direction of a Borehole"
ABSTRACT OF THE DISCLOSURE
An instrument for monitoring the direction of a borehole comprises an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis of the casing, a gyroscope unit mounted in the outer gimbal, a torque motor and a gravity sensor unit. At the beginning of a run the instrument is positioned at the mouth of the borehole. The rate of rotation about an axis transverse to the outer gimbal axis is then sensed by the gyroscope unit, and the outer gimbal is slewed by the torque motor in dependence on the sensed rate so as to align the transverse axis along an East/West direction. The instrument is then moved along the borehole and the rate of rotation about the outer gimbal axis is continually sensed by the gyroscope unit, the outer gimbal being torqued by the motor in dependence on the sensed rate to stabilize the outer gimbal about its axis. Simultaneously the components of gravity are sensed by the gravity sensor unit. The resulting measurements are used to determine the inclination and azimuth at a plurality of points along the length of the borehole.
Description
,' ~~10\
This invention relates to instruments for monitoring the direction of a borehole either continuously or at a series of locations spaced along the length of the borehole, and to methods of surveying a borehole.
A spatial survey of the path of a borehole is usually derived from a series of values of the azimuth angle and the inclination angle taken along the length of the borehole. Measurements from which ~he values of these two angles can be derived are made at successive locations along the path of the boreholeg the distances between adjacent locations being accurately known.
In a borehole in which the earth's magnetic field is unchanged by the presence of the borehole itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the casing-fixed axes can be used to obtain values for 3:1
This invention relates to instruments for monitoring the direction of a borehole either continuously or at a series of locations spaced along the length of the borehole, and to methods of surveying a borehole.
A spatial survey of the path of a borehole is usually derived from a series of values of the azimuth angle and the inclination angle taken along the length of the borehole. Measurements from which ~he values of these two angles can be derived are made at successive locations along the path of the boreholeg the distances between adjacent locations being accurately known.
In a borehole in which the earth's magnetic field is unchanged by the presence of the borehole itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the casing-fixed axes can be used to obtain values for 3:1
2 _ the azimuth angle and the inclination angle7 the azimuth angle being measured with respect to an earth-fixed magnetic reference, for example magnetic North.
However, in situations in which the earth~ magnetic 5 field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magne-tic measurements can no longer be used to determine the azimuth angle relative to an earth-~ixed reference. In these circumst~nces, i.t is necessary to use a gyroscope instrument.
The Applicants' U.K. Patent Specification No.
1,509,293 dated the 26th March, 1975 describes such an instrument comprising a casing having its longitudinal axis coincident, in use, with the axis of the borehole, a .single-degree-of-freedom gyro comprising an outer gimbal mounted in the casing with its axis coincident with the longitudinal axis thereof~ an inner gimbal moun~ed in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the inner gimbal, means for sensing angular movement of the inner gimbal relative to the outer gimbal and means for applying a torque to the outer gimbal for rotating it in use about its axis so that the inner gimbal precesses back to its initial position, means for measuring the ~ngle o~ rotation of the casing about its longitudinal axis relative to the outer gimbal and a gravity sensor unit for measuring three components of gravity in three non-coplanar directions.
This instrument has proved to be extremely .~,....
reliable in practice and has been shown to be capable of an accura~y of up to approximately ~0.1 in inclination and ~1.0 in azimuth. The stated maximum inclination for such an instrument is generally considered to be about 7~ to the vertical since surveying at angles in excess of 60 leads to increasingly less accurate 6urveys as the inclination increases.
However~ with the trend towards high inclination drilling, there exîsts an increasing demand for an instrument with an accuracy in azimuth of the same order of magnitude as that attainable in inclination.
It is an ~bject of the invention to provide such an instrument.
According to the invention there is provided an instrument for monitoring the direction of a borehole 9 comprising an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole9 an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longi-tudinal a~is thereof9 a rate gyroscope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis, -torquing means for applyi~g a torque to the outer gimbal,first actuating means for actuating the torquing means when the instrument is disposed at the mouth of the borehole so as to pivot the outer gimbal about its axis,whereby,in use, the align~
ment of said transverse axis relative to an East/West `: :
However, in situations in which the earth~ magnetic 5 field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magne-tic measurements can no longer be used to determine the azimuth angle relative to an earth-~ixed reference. In these circumst~nces, i.t is necessary to use a gyroscope instrument.
The Applicants' U.K. Patent Specification No.
1,509,293 dated the 26th March, 1975 describes such an instrument comprising a casing having its longitudinal axis coincident, in use, with the axis of the borehole, a .single-degree-of-freedom gyro comprising an outer gimbal mounted in the casing with its axis coincident with the longitudinal axis thereof~ an inner gimbal moun~ed in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the inner gimbal, means for sensing angular movement of the inner gimbal relative to the outer gimbal and means for applying a torque to the outer gimbal for rotating it in use about its axis so that the inner gimbal precesses back to its initial position, means for measuring the ~ngle o~ rotation of the casing about its longitudinal axis relative to the outer gimbal and a gravity sensor unit for measuring three components of gravity in three non-coplanar directions.
This instrument has proved to be extremely .~,....
reliable in practice and has been shown to be capable of an accura~y of up to approximately ~0.1 in inclination and ~1.0 in azimuth. The stated maximum inclination for such an instrument is generally considered to be about 7~ to the vertical since surveying at angles in excess of 60 leads to increasingly less accurate 6urveys as the inclination increases.
However~ with the trend towards high inclination drilling, there exîsts an increasing demand for an instrument with an accuracy in azimuth of the same order of magnitude as that attainable in inclination.
It is an ~bject of the invention to provide such an instrument.
According to the invention there is provided an instrument for monitoring the direction of a borehole 9 comprising an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole9 an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longi-tudinal a~is thereof9 a rate gyroscope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis, -torquing means for applyi~g a torque to the outer gimbal,first actuating means for actuating the torquing means when the instrument is disposed at the mouth of the borehole so as to pivot the outer gimbal about its axis,whereby,in use, the align~
ment of said transverse axis relative to an East/West `: :
3~ 7 -direction is determined from the rate OI rotation about said tr~nsverse axis sensed by the gyroscope unitg second actuating means for ~ctuating the tor~uing means in response to the rate of rotation about the outer gimbal axis sensed by the gyroscope unit as the instrument is moved along the borehole so as to stabilize the outer gimbal about its axis 7 ~nd a gravity sensor unit for sensing two components of gravity in ~wo transverse ~irections. .
~O ~he use o~ a two-axis rate gyroscope enables an accuracy o~ better than +0.1 in inclination and ~0.2 in azimu-th to be achieved. A gyro-compassing technique may be used to align the outer gimbal with true North, and this eliminates ~the need for the casing ~5 reference alignment procedure presently employed wi~h con~entional gyroscopic instruments which can be a maj~r source of azimuth error. For inclinations in excess o~ 45 the outer gimbal may be torqued to maintain the high-side angle zero, and the rate measuremen~ o~ the ~yroscope about the transverse axis may be used ~o compute the azimuth as the instrument is traversed along the path of the borehole.
The invention also provides a method of surveying a borehole comprising positioning at the mouth of the borehole a sur~ey instrument comprising an elongate casing having its longitudinal axis coincident with the axis of the borehole 9 an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis thereof~ and a rat~ gyro-scope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis; sensing the rate of rotation about said tr~lsverse axis by means of the gyroscope unit and ap-plying a torque to the outer gimbal so as to pivot the outer gimbal about its axis9whereby the alignment of said transverse axis relative to an EasttWest direction is determined from the sensed rate; moving the survey instru-ment along the borehole; continually sensing the rate of rotatîon about the outer gimbal ~xis by means of the gyro-scope unit as the instrument moves along the borehole and applying a torque to the outer gimbal in dependence on the sensea rate so as to stabilise the outer gimbal about its axis; continually sensing two components of gravity in two transverse directions relative to the outer gimbal or the casing; and determining at least the inclination and azimuth of the borehole at a plurality of points along its length ~rom the sensed components of gravity.
In order that the invention may be more fully understuod, two preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic perspective view of a first instrument with its casing shown in section;
~igure 2 is a partially cut~away view of a 3 ~ ~7 dynamically tuned gyro forming part of the first instrument;
Figure 3 is a schematic representa-tion illustrating a transformation between two sets of reference axes;
~igures 4 to 6 are diagrams illus-trating various stages of the transformation shown in Figure 3; and ~igure 7 is a diagram illus-trating the relationship between -two sets of reference axes.
~
Referring to Figure 1I the instrument comprises, within a casing 10 whose longitudinal axis is coincident with the bore axis, a two axis dynamically tuned gyroscope . 12 mounted within an outer gimbal 13 on an outer gimbal shaft 14 provided with upper and lower outer gimbal bearings 16 and 18 supported by upper and lower outer gimbal bearing mountings 17 and 19. The tuned gyroscope 12 incorporates a gyro-wheel 46 having a spin axis perpendicular to the outer gimbal axis, a first pivot axis transverse to the spin axis and a second pivot axis perpendicular to the first pivot axis and transverse to the spin axis. The outer gimbal shaft 14 is also provided with a torque motor 22 and a resolver 26 supported by a resolver mounting 28. The resolver 26 incorporates a stator comprising a pair of coils with their axes or-thogonal to one another and a rotor comprising a corresponding pair of mutually orthogonal coils. If a reference signal is applied -to one of the coils on the rotor and the other coil on the rotor is grounded, and the outpu-ts of the two coils on the stator are a and b, then a/b is equal to the tangent of -the angle 01 between a reference direction with respect to the.casing 10 and a reference direction with respect to the outer gimbal shaft 14. The instrument also incorporates a gravity sensor unit 30 comprising three accelerometers mounted on the outer gimbal shaf-t 14.
Referring to Figure 2, the dynamically tuned gyroscope 12 incorporates a housing 32 which is fixed to the outer gimbal 13, a shaft 34 rotatable with respect to the housing 32 and the outer gimbal 13 about the spin axis 35 and provided with spin axis bearings 36, and a drive motor 40 comprising a rotor 42 attached to the shaft 34 and a stator 44 at-tached to the housing 32. The gyro-wheel 46 is coupled to the shaft 34 by a Hookes joint comprising an inner gimbal 48 pivo-tal about the first pivot axis by torsion springs 50 extending between the shaft 34 and the inner gimbal 48, the gyro-wheel 46 being pivo-tal about the second pivot axis perpendicular to the first pivot axis by torsion springs 52 extending between the inner gimbal 48 and the gyro-wheel 46.
The gyro-wheel 46 incorporates a permanent 3~
magnet ring 54 and an annular recess 56 immediately adjacent the permanent magnet ring 54 within which four torquing coils 57 to 60 fixed to the housing 32 extend, the coil 57 being disposed diame-trically opposite the coil 59 and the coil 58 being disposed diametrically opposite the coil 60. A series of pickoffs 62 ~ixed to the housing 32 serve to sense angular displacement of the gyro-wheel 46 about the two mutually perpendicular axes. In operation of the gyro the torque applied to the gyro-wheel 46 by the torsion springs 50 and 52 are offset by the negative torque generated due to the dynamic effect of the i~ler gimbal 48 which varies as the square root of the speed of the gyro-wheel 46. There is therefore only one speed, that is the -tuned speed, at which the positive spring torques are cancelled by the dynamic effect. At the tuned speed the gyro wheel 46 is decoupled from the shaft ~4 and so acts as a free gyro.
Figure 3 schematically illustrates a borehole 80 and various reference axes relative to which the orientation of the borehole 80 may be defined, these axes comprising a set of earth~fixed axes ON, OE and OV where OV is vertically down, ON is due North and OE is due East, and a set of casing-fixed axes OX, OY
and OZ where OZ lies along the local direction of the borehole at a measuring s-tation and OX and OY are in a plane perpendicular to this direction. The set of earth-fixed axes can be rotated into the set of casing-~ - 9 -fixed axes by -the following three cloc~wise ro-tations:
1) rotation about the axis OV through the azimuth angle ~ as shown in Figure 4, 2) rotation about the axis OE1 through the inclination angle ~ as shown in Figure 5 9 and 3) ro-ta-tion about the axis OZ -through the high-side angle 0 as shown in Figure 6.
Figure 7 schematically illustrates the relationship between the casing-fixed axes OX, OY and OZ and a set of outer-gimbal-fixed axes OX~, OY' and OZ' where the axes OZ and OZ' are coincident and are denoted by OZZ' in the figure. This figure also shows the relationship between the high-side angle ~ and the angle 01 measured by the resolver 26, 02 being the high-side angle which would be obtained if the instrument were to travel to a measurement location wi-thout rotation about the casing-fixed axis OZ. Clearly 02 is dependent on the shape of the path to be followed by the instrument. It will be apprecia-ted that 0 = 01 +
02 if the casing-fixed, earth-fixed and outer-gimbal-fixed axes are coincident at the mouth of the borehole.
The three accelerometers of the sensor unit are arranged to sense components of gravity gX ~ gY' and gz, along the three mutually orthogonal outer-gimbal-fixed axes OX', OY' and OZ 7, the axis OZ' being coinciden~with the bore axis. Alternatively the three accelerometers may be mounted on the casing 10 and arranged to sense components of gravity gX' gy and gz ~ ~93~
- 10 ~
along three mutually orthogonal casing-fixed axes OX, OY and OZ.
If -the accelerometers are mounted on the casingy -then the gravity vector g = gx.~x~gy.uy+gz~z where ~X~ Uy and ~z are the unit vectors in the casing-fixed axes directions OX, OY and OZ respectively. If the accelerometers are mounted on the outer gimbal, then the gravity vector g = gX ? . Ux ~ +gyi-uy~+gzl-uz~ where Ux,, Uyl and Uzl are the unit vectors in -the outer-gimbal-fixed axes directions OXI, OY' and OZ' repsectively.
Thus, gX~ ~ gXcs 01 ~ gysi~ 01 ----------(A) gy~ = gxsin 01 + gycos 01 ----------(B) ~;z~ = gz ....................... (C) If UN, UE and Uv are unit vectors in the earth-fixed axes directions ON, OE and OV respectively, then according to the definition of the angles 0, ~ and the ~ec-tor operation equation UNEv = ~ Uxyz represents the transformation relationship between -the sets of unit vectors in the two frames where, cos ~ -sin ~ O
sin ~ cos ~ O {~
- cOs e O sin 4 o 1 o {9}
=sin ~ O cos 4 3:~7 cos 0 -sin 0 0 sin0 cos 0 0 = ~0 O 0 1 ~
The vector operation ~XY~ = ~0~ UNEV
represents the transformation relationship in -the opposite direction.
Thç ins-trument may be operated in three distinct measurement phases to obtain three separate measurements.
Firstly, with the instrument disposed vertically at the mouth of the borehole, that is with the OZ'axis aligned with the OV axis, a gyro-compassing technique can be used to align a reference angular position of the outer gimbal 13 with true North. The rate of rotation of the gyro-wheel 46 about the OX'axis due to the rotation of the earth as measured by the appropriate pickoffs 62 of the tuned gyro 12 is fed back to the -torque motor 22 by way of suitable control circuitry and used to slew the outer gimbal 13 until the earth~s rate of rotation measured about -the OX'axis by the tuned gyro 12 is zero when the spin axis 35 (OY'axis) must lie North/South and the OX axis must lie East/West. This North-seeking gyro-compassing phase eliminates the need for the casing-reference alignment procedure presently employed with conventional gyroscopic instruments which can be a major source of azimuth error.
In a second measurement phase, applicable to borehole inclinations of 0 to 45 to the vertical, the 33:~7 _ ~2 --inclination of the borehole is measuxed either continu~
ously or a~ a series of locations along its length by the surveying method described in U.K. Specification No. ~,50~,293 except that the initial alignment reference is obtained as described above with reference to the first measurement phase. The rate of rotation o~ the gyro-wheel 46 about the OZ'axis as measured ~y the appropriate pickoffs 62 Df the tuned gyro ~2 is fed back by appropriate control circuitry to the torque motor 22 and used to stabilize the outer gimbal 13 about the OZ'axis so as to maintain the alignment of the OY'~xis in the vertical pl~ne lying North/South.
The outer gimbal 13 therefore behaves as a single-axis stabilized platform about its axis O~which coincides with the casing axis OZ. r~hus, the net rotation o~ the casing 10 about the OZ axis measured with respect to an outer gimbal reference is equal to the sum of all rotations of the casing 10 about the instant3neous directions of OZ as the instrument is moved along the borehole path and is clearly independent of the path fvllowed.
With the instrument described in U.K.Speci~ication No. 1,509,293 outer-gimbal drift rates about the OZ
axis are of the order of 1 to 10 per hour and drif~
rate checks are made during the survey. In this manner a rate measurement accuracy of the order of 0.5 per hour can be achieved. By contrast, using the instrument described above, a rate measuremerlt accuracy of the ~~( -- ~3 --order of 0.1 per hour can be achieved and there is no necessity to stop for drif-t rate checks during -the survey. With suitable programming o~ the system to correct the accelerometer outputs for the effects of the instrument transversing a non-rectilinear path during surveying, -this measurement phase can be carried out in one continuous operation.
In a third measurement phase, applicable to borehole inclinations in excess of 45 to the vertical, the outer gimbal 13 is torqued by the torque motor 22 -to maintain the high-side angle 0 as measured by the gravity sensor unit 30 zero. If the three accelerometers yield gravity components along the casing-fixed axes OX, OY and Z of gX~ gy and gz respectivelyl the inclination angle ~ will be given by:
= atan (gx2 ~ gy2) / (gx2 -~ gy2 ~ gz2)~
and the high-side angle 0 will be given by:
0 = atan (gy / ~gX) gX' gy and gz must be corrected for the effects of the instrument traversing a non-rectilinear path.
The rate measurement of the gyro wheel 46 about the OX' axis as measured by the appropriate pickoffs 62 o~ the tuned gyro 12 can then be used to compute the azimuth angle ~ as the instrument is traversed along the path of the borehole. The ra-te measured about the OX' axis, rX = WX ~12X where~x is the rate of rotation of the instrument about the OX~ axis and S~x is the rate of rotation of the earth about the OX~ axis. Since 0 = O, the azimuth angle ~may be computed from the time integral ~ , where ~ = - ~X / sin ~ = -(rx-Q X)sin 4 where Q X = RT.cos ~ .cos ~ -~ RR.sin ~, RT = RE.cos~
and RR = RE.sin ~ where RE is the earth's rate of ro-tation about its axis and 1 is the geographical latitude.
The second and third measurement phases are mutually complementary since, for incli.na-tions in excess of 45 to the vertical, the second measurement phase would -tend to give increasingly inaccurate results with increasing inclination, whereas, for inclinations of 0 to 45 to the vertical, the third measurement phase would give increasingly inaccurate results with decreasing inclination.
~3~D~
If the instrument is moved in such a manner that the longitudinal instrument axis oZ t remains parallel to the borehole axis during the traverse, then the rates of rotation of the instrument about the casing-fixed axes are defined as wx, wy and ~z respectively.
If the instantaneous.rates of rotation are defined in terms of the rates 4 and ~, where these rates are defined by the changes in borehole parameters ~ and ~ as the instrument moves along the boreho1e path, then the rate of rotation o~ the instrument can be defined in the earth-fixed frame by:
~p = _ ~.sin ~.~N + ~.cos ~ .UE + ~ V
Operating on the vector ~p to transform it in-to 3~7 casing-fixed components yields:
~0~{~{~ p = (-sin~.cos0. ~ + ~.sin0 ) Ux ~ (sin~.
sin0. ~ + ~.cos0 )~y + (cos~. ~ ) Rz Thus, WX = -sin~.cos0. ~ .sin0 .~.............. ,............ ~1) w y = sin~.sin0. ~ + ~.cos0 o~ (2) = cos~. ~ ............................ ...... (3) Solving Equations 1 and 2 above for ~ and 4 ~O yields:
wx.cos~ - wy.sin0 } / sin~ .................. (4) = wX.sin0 + wy.cos0 ........................... (5) If the magnitude of the earth~s rate of rotation about its axis is RE, then the earth's rate of rotation can be defined in the earth-fixed frame by:
E E Cos ~ N RE.s_nl ~V or ~E = RT-~N ~ RR'~
Operating on the vector RE to transform it into casing-fixed components yields:
SlX = RT.cos ~.cos~.cos0 + RR.sin9.cos0 - RT.sin ~.sin0 (6) ~ y = -RT.cos~ .cos~.sin0 - RR.sin~.sin0 - RT.sin~ .cos0 (7) n z = RT.cos~ .sin~ - RR.cos~ (8) where ~x, J~y and Q z are the rates of rotation of the earth about the casing-fixed axes.
~O ~he use o~ a two-axis rate gyroscope enables an accuracy o~ better than +0.1 in inclination and ~0.2 in azimu-th to be achieved. A gyro-compassing technique may be used to align the outer gimbal with true North, and this eliminates ~the need for the casing ~5 reference alignment procedure presently employed wi~h con~entional gyroscopic instruments which can be a maj~r source of azimuth error. For inclinations in excess o~ 45 the outer gimbal may be torqued to maintain the high-side angle zero, and the rate measuremen~ o~ the ~yroscope about the transverse axis may be used ~o compute the azimuth as the instrument is traversed along the path of the borehole.
The invention also provides a method of surveying a borehole comprising positioning at the mouth of the borehole a sur~ey instrument comprising an elongate casing having its longitudinal axis coincident with the axis of the borehole 9 an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis thereof~ and a rat~ gyro-scope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis; sensing the rate of rotation about said tr~lsverse axis by means of the gyroscope unit and ap-plying a torque to the outer gimbal so as to pivot the outer gimbal about its axis9whereby the alignment of said transverse axis relative to an EasttWest direction is determined from the sensed rate; moving the survey instru-ment along the borehole; continually sensing the rate of rotatîon about the outer gimbal ~xis by means of the gyro-scope unit as the instrument moves along the borehole and applying a torque to the outer gimbal in dependence on the sensea rate so as to stabilise the outer gimbal about its axis; continually sensing two components of gravity in two transverse directions relative to the outer gimbal or the casing; and determining at least the inclination and azimuth of the borehole at a plurality of points along its length ~rom the sensed components of gravity.
In order that the invention may be more fully understuod, two preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic perspective view of a first instrument with its casing shown in section;
~igure 2 is a partially cut~away view of a 3 ~ ~7 dynamically tuned gyro forming part of the first instrument;
Figure 3 is a schematic representa-tion illustrating a transformation between two sets of reference axes;
~igures 4 to 6 are diagrams illus-trating various stages of the transformation shown in Figure 3; and ~igure 7 is a diagram illus-trating the relationship between -two sets of reference axes.
~
Referring to Figure 1I the instrument comprises, within a casing 10 whose longitudinal axis is coincident with the bore axis, a two axis dynamically tuned gyroscope . 12 mounted within an outer gimbal 13 on an outer gimbal shaft 14 provided with upper and lower outer gimbal bearings 16 and 18 supported by upper and lower outer gimbal bearing mountings 17 and 19. The tuned gyroscope 12 incorporates a gyro-wheel 46 having a spin axis perpendicular to the outer gimbal axis, a first pivot axis transverse to the spin axis and a second pivot axis perpendicular to the first pivot axis and transverse to the spin axis. The outer gimbal shaft 14 is also provided with a torque motor 22 and a resolver 26 supported by a resolver mounting 28. The resolver 26 incorporates a stator comprising a pair of coils with their axes or-thogonal to one another and a rotor comprising a corresponding pair of mutually orthogonal coils. If a reference signal is applied -to one of the coils on the rotor and the other coil on the rotor is grounded, and the outpu-ts of the two coils on the stator are a and b, then a/b is equal to the tangent of -the angle 01 between a reference direction with respect to the.casing 10 and a reference direction with respect to the outer gimbal shaft 14. The instrument also incorporates a gravity sensor unit 30 comprising three accelerometers mounted on the outer gimbal shaf-t 14.
Referring to Figure 2, the dynamically tuned gyroscope 12 incorporates a housing 32 which is fixed to the outer gimbal 13, a shaft 34 rotatable with respect to the housing 32 and the outer gimbal 13 about the spin axis 35 and provided with spin axis bearings 36, and a drive motor 40 comprising a rotor 42 attached to the shaft 34 and a stator 44 at-tached to the housing 32. The gyro-wheel 46 is coupled to the shaft 34 by a Hookes joint comprising an inner gimbal 48 pivo-tal about the first pivot axis by torsion springs 50 extending between the shaft 34 and the inner gimbal 48, the gyro-wheel 46 being pivo-tal about the second pivot axis perpendicular to the first pivot axis by torsion springs 52 extending between the inner gimbal 48 and the gyro-wheel 46.
The gyro-wheel 46 incorporates a permanent 3~
magnet ring 54 and an annular recess 56 immediately adjacent the permanent magnet ring 54 within which four torquing coils 57 to 60 fixed to the housing 32 extend, the coil 57 being disposed diame-trically opposite the coil 59 and the coil 58 being disposed diametrically opposite the coil 60. A series of pickoffs 62 ~ixed to the housing 32 serve to sense angular displacement of the gyro-wheel 46 about the two mutually perpendicular axes. In operation of the gyro the torque applied to the gyro-wheel 46 by the torsion springs 50 and 52 are offset by the negative torque generated due to the dynamic effect of the i~ler gimbal 48 which varies as the square root of the speed of the gyro-wheel 46. There is therefore only one speed, that is the -tuned speed, at which the positive spring torques are cancelled by the dynamic effect. At the tuned speed the gyro wheel 46 is decoupled from the shaft ~4 and so acts as a free gyro.
Figure 3 schematically illustrates a borehole 80 and various reference axes relative to which the orientation of the borehole 80 may be defined, these axes comprising a set of earth~fixed axes ON, OE and OV where OV is vertically down, ON is due North and OE is due East, and a set of casing-fixed axes OX, OY
and OZ where OZ lies along the local direction of the borehole at a measuring s-tation and OX and OY are in a plane perpendicular to this direction. The set of earth-fixed axes can be rotated into the set of casing-~ - 9 -fixed axes by -the following three cloc~wise ro-tations:
1) rotation about the axis OV through the azimuth angle ~ as shown in Figure 4, 2) rotation about the axis OE1 through the inclination angle ~ as shown in Figure 5 9 and 3) ro-ta-tion about the axis OZ -through the high-side angle 0 as shown in Figure 6.
Figure 7 schematically illustrates the relationship between the casing-fixed axes OX, OY and OZ and a set of outer-gimbal-fixed axes OX~, OY' and OZ' where the axes OZ and OZ' are coincident and are denoted by OZZ' in the figure. This figure also shows the relationship between the high-side angle ~ and the angle 01 measured by the resolver 26, 02 being the high-side angle which would be obtained if the instrument were to travel to a measurement location wi-thout rotation about the casing-fixed axis OZ. Clearly 02 is dependent on the shape of the path to be followed by the instrument. It will be apprecia-ted that 0 = 01 +
02 if the casing-fixed, earth-fixed and outer-gimbal-fixed axes are coincident at the mouth of the borehole.
The three accelerometers of the sensor unit are arranged to sense components of gravity gX ~ gY' and gz, along the three mutually orthogonal outer-gimbal-fixed axes OX', OY' and OZ 7, the axis OZ' being coinciden~with the bore axis. Alternatively the three accelerometers may be mounted on the casing 10 and arranged to sense components of gravity gX' gy and gz ~ ~93~
- 10 ~
along three mutually orthogonal casing-fixed axes OX, OY and OZ.
If -the accelerometers are mounted on the casingy -then the gravity vector g = gx.~x~gy.uy+gz~z where ~X~ Uy and ~z are the unit vectors in the casing-fixed axes directions OX, OY and OZ respectively. If the accelerometers are mounted on the outer gimbal, then the gravity vector g = gX ? . Ux ~ +gyi-uy~+gzl-uz~ where Ux,, Uyl and Uzl are the unit vectors in -the outer-gimbal-fixed axes directions OXI, OY' and OZ' repsectively.
Thus, gX~ ~ gXcs 01 ~ gysi~ 01 ----------(A) gy~ = gxsin 01 + gycos 01 ----------(B) ~;z~ = gz ....................... (C) If UN, UE and Uv are unit vectors in the earth-fixed axes directions ON, OE and OV respectively, then according to the definition of the angles 0, ~ and the ~ec-tor operation equation UNEv = ~ Uxyz represents the transformation relationship between -the sets of unit vectors in the two frames where, cos ~ -sin ~ O
sin ~ cos ~ O {~
- cOs e O sin 4 o 1 o {9}
=sin ~ O cos 4 3:~7 cos 0 -sin 0 0 sin0 cos 0 0 = ~0 O 0 1 ~
The vector operation ~XY~ = ~0~ UNEV
represents the transformation relationship in -the opposite direction.
Thç ins-trument may be operated in three distinct measurement phases to obtain three separate measurements.
Firstly, with the instrument disposed vertically at the mouth of the borehole, that is with the OZ'axis aligned with the OV axis, a gyro-compassing technique can be used to align a reference angular position of the outer gimbal 13 with true North. The rate of rotation of the gyro-wheel 46 about the OX'axis due to the rotation of the earth as measured by the appropriate pickoffs 62 of the tuned gyro 12 is fed back to the -torque motor 22 by way of suitable control circuitry and used to slew the outer gimbal 13 until the earth~s rate of rotation measured about -the OX'axis by the tuned gyro 12 is zero when the spin axis 35 (OY'axis) must lie North/South and the OX axis must lie East/West. This North-seeking gyro-compassing phase eliminates the need for the casing-reference alignment procedure presently employed with conventional gyroscopic instruments which can be a major source of azimuth error.
In a second measurement phase, applicable to borehole inclinations of 0 to 45 to the vertical, the 33:~7 _ ~2 --inclination of the borehole is measuxed either continu~
ously or a~ a series of locations along its length by the surveying method described in U.K. Specification No. ~,50~,293 except that the initial alignment reference is obtained as described above with reference to the first measurement phase. The rate of rotation o~ the gyro-wheel 46 about the OZ'axis as measured ~y the appropriate pickoffs 62 Df the tuned gyro ~2 is fed back by appropriate control circuitry to the torque motor 22 and used to stabilize the outer gimbal 13 about the OZ'axis so as to maintain the alignment of the OY'~xis in the vertical pl~ne lying North/South.
The outer gimbal 13 therefore behaves as a single-axis stabilized platform about its axis O~which coincides with the casing axis OZ. r~hus, the net rotation o~ the casing 10 about the OZ axis measured with respect to an outer gimbal reference is equal to the sum of all rotations of the casing 10 about the instant3neous directions of OZ as the instrument is moved along the borehole path and is clearly independent of the path fvllowed.
With the instrument described in U.K.Speci~ication No. 1,509,293 outer-gimbal drift rates about the OZ
axis are of the order of 1 to 10 per hour and drif~
rate checks are made during the survey. In this manner a rate measurement accuracy of the order of 0.5 per hour can be achieved. By contrast, using the instrument described above, a rate measuremerlt accuracy of the ~~( -- ~3 --order of 0.1 per hour can be achieved and there is no necessity to stop for drif-t rate checks during -the survey. With suitable programming o~ the system to correct the accelerometer outputs for the effects of the instrument transversing a non-rectilinear path during surveying, -this measurement phase can be carried out in one continuous operation.
In a third measurement phase, applicable to borehole inclinations in excess of 45 to the vertical, the outer gimbal 13 is torqued by the torque motor 22 -to maintain the high-side angle 0 as measured by the gravity sensor unit 30 zero. If the three accelerometers yield gravity components along the casing-fixed axes OX, OY and Z of gX~ gy and gz respectivelyl the inclination angle ~ will be given by:
= atan (gx2 ~ gy2) / (gx2 -~ gy2 ~ gz2)~
and the high-side angle 0 will be given by:
0 = atan (gy / ~gX) gX' gy and gz must be corrected for the effects of the instrument traversing a non-rectilinear path.
The rate measurement of the gyro wheel 46 about the OX' axis as measured by the appropriate pickoffs 62 o~ the tuned gyro 12 can then be used to compute the azimuth angle ~ as the instrument is traversed along the path of the borehole. The ra-te measured about the OX' axis, rX = WX ~12X where~x is the rate of rotation of the instrument about the OX~ axis and S~x is the rate of rotation of the earth about the OX~ axis. Since 0 = O, the azimuth angle ~may be computed from the time integral ~ , where ~ = - ~X / sin ~ = -(rx-Q X)sin 4 where Q X = RT.cos ~ .cos ~ -~ RR.sin ~, RT = RE.cos~
and RR = RE.sin ~ where RE is the earth's rate of ro-tation about its axis and 1 is the geographical latitude.
The second and third measurement phases are mutually complementary since, for incli.na-tions in excess of 45 to the vertical, the second measurement phase would -tend to give increasingly inaccurate results with increasing inclination, whereas, for inclinations of 0 to 45 to the vertical, the third measurement phase would give increasingly inaccurate results with decreasing inclination.
~3~D~
If the instrument is moved in such a manner that the longitudinal instrument axis oZ t remains parallel to the borehole axis during the traverse, then the rates of rotation of the instrument about the casing-fixed axes are defined as wx, wy and ~z respectively.
If the instantaneous.rates of rotation are defined in terms of the rates 4 and ~, where these rates are defined by the changes in borehole parameters ~ and ~ as the instrument moves along the boreho1e path, then the rate of rotation o~ the instrument can be defined in the earth-fixed frame by:
~p = _ ~.sin ~.~N + ~.cos ~ .UE + ~ V
Operating on the vector ~p to transform it in-to 3~7 casing-fixed components yields:
~0~{~{~ p = (-sin~.cos0. ~ + ~.sin0 ) Ux ~ (sin~.
sin0. ~ + ~.cos0 )~y + (cos~. ~ ) Rz Thus, WX = -sin~.cos0. ~ .sin0 .~.............. ,............ ~1) w y = sin~.sin0. ~ + ~.cos0 o~ (2) = cos~. ~ ............................ ...... (3) Solving Equations 1 and 2 above for ~ and 4 ~O yields:
wx.cos~ - wy.sin0 } / sin~ .................. (4) = wX.sin0 + wy.cos0 ........................... (5) If the magnitude of the earth~s rate of rotation about its axis is RE, then the earth's rate of rotation can be defined in the earth-fixed frame by:
E E Cos ~ N RE.s_nl ~V or ~E = RT-~N ~ RR'~
Operating on the vector RE to transform it into casing-fixed components yields:
SlX = RT.cos ~.cos~.cos0 + RR.sin9.cos0 - RT.sin ~.sin0 (6) ~ y = -RT.cos~ .cos~.sin0 - RR.sin~.sin0 - RT.sin~ .cos0 (7) n z = RT.cos~ .sin~ - RR.cos~ (8) where ~x, J~y and Q z are the rates of rotation of the earth about the casing-fixed axes.
Claims (9)
1. An instrument for monitoring the direction of a borehole, comprising an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis thereof, a rate gyroscope unit mounted in the outer gimbal and adapted to provide outputs in-dicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis;
torquing means for applying a torque to the outer gimbal, first actuating means for actuating the torquing means when the instrument is disposed at the mouth of the bore-hole so as to pivot the outer gimbal about its axis,where-by, in use, the alignment of said transverse axis relative to an East/West direction is determined from the rate of rotation about said transverse axis sensed by the gyro-scope unit, second actuating means for actuating the torquing means in response to the rate of rotation about the outer gimbal axis sensed by the groscope unit as the instrument is moved along the borehole so as to stabilise the outer gimbal about its axis, and a gravity sensor unit for sensing two components of gravity in two trans-verse directions.
torquing means for applying a torque to the outer gimbal, first actuating means for actuating the torquing means when the instrument is disposed at the mouth of the bore-hole so as to pivot the outer gimbal about its axis,where-by, in use, the alignment of said transverse axis relative to an East/West direction is determined from the rate of rotation about said transverse axis sensed by the gyro-scope unit, second actuating means for actuating the torquing means in response to the rate of rotation about the outer gimbal axis sensed by the groscope unit as the instrument is moved along the borehole so as to stabilise the outer gimbal about its axis, and a gravity sensor unit for sensing two components of gravity in two trans-verse directions.
2. An instrument according to claim 1, further comprising resolver means for sensing the angle of rotation of the outer gimbal about its axis relative to the casing.
3. An instrument according to claim 1, further comprising means for determining the high-side angle of the instrument as it is moved along the borehole from the sensed components of gravity, and third actuating means for actuating the torquing means in response to the rate of rotation about said transverse axis sensed by the gyroscope unit as the instrument is moved along the borehole at high inclination angles so as to maintain the high-side angle zero.
4. An instrument according to claim 1, wherein the gravity sensor unit is mounted on the outer gimbal.
5. An instrument according to claim 1, wherein the gravity sensor unit is mounted on the casing.
6. An instrument according to claim 1, wherein the gravity sensor unit is adapted to sense three components of gravity in three non-coplanar directions.
7. An instrument according to claim 1, wherein the gyroscope unit is a two-axis dynamically tuned gyroscope.
8. A method of surveying a borehole comprising positioning at the mouth of the borehole a survey instrument comprising an elongate casing having its longitudinal axis coincident with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis thereof, and a rate gyroscope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the other gimbal axis and an axis transverse to the outer gimbal axis;
sensing the rate of rotation about said transverse axis by means of the gyroscope unit and applying a torque to the outer gimbal so as to pivot the outer gimbal about its axis, whereby the alignment of said transverse axis relative to an East/West direction is determined from the sensed rate; moving the survey instrument along the borehole; continually sensing the rate of rotation about the outer gimbal axis by means of the gyroscope unit as the instrument moves along the borehole and applying a torque to the outer gimbal in dependence on the sensed rate so as to stabilise the outer gimbal about its axis;
continually sensing two components of gravity in two transverse directions relative to the outer gimbal or the casing; and determining at least the inclination and azimuth of the borehole at a plurality of points along its length from the sensed components of gravity.
sensing the rate of rotation about said transverse axis by means of the gyroscope unit and applying a torque to the outer gimbal so as to pivot the outer gimbal about its axis, whereby the alignment of said transverse axis relative to an East/West direction is determined from the sensed rate; moving the survey instrument along the borehole; continually sensing the rate of rotation about the outer gimbal axis by means of the gyroscope unit as the instrument moves along the borehole and applying a torque to the outer gimbal in dependence on the sensed rate so as to stabilise the outer gimbal about its axis;
continually sensing two components of gravity in two transverse directions relative to the outer gimbal or the casing; and determining at least the inclination and azimuth of the borehole at a plurality of points along its length from the sensed components of gravity.
9. A method according to claim 8, wherein, at high inclination angles of the borehole, the rate of rotation about said transverse axis is continually sensed by means of the gyroscope unit as the instrument moves along the borehole and a torque is applied to the outer gimbal in dependence on the sensed rate so as to maintain the high-side angle as determined from the sensed components of gravity zero.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8125409 | 1981-08-20 | ||
| GB8125409 | 1981-08-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1189317A true CA1189317A (en) | 1985-06-25 |
Family
ID=10524052
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000409758A Expired CA1189317A (en) | 1981-08-20 | 1982-08-19 | Instruments for monitoring the direction of a borehole |
Country Status (10)
| Country | Link |
|---|---|
| JP (1) | JPS5873807A (en) |
| BR (1) | BR8204849A (en) |
| CA (1) | CA1189317A (en) |
| CH (1) | CH651890A5 (en) |
| DE (1) | DE3230889A1 (en) |
| FR (1) | FR2511727B1 (en) |
| IT (1) | IT1153556B (en) |
| MX (1) | MX152695A (en) |
| NL (1) | NL8203007A (en) |
| NO (1) | NO164193C (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4434654A (en) * | 1982-08-09 | 1984-03-06 | Sundstrand Data Control, Inc. | Borehole orientation detection system employing polarized radiation |
| DE19625720C1 (en) * | 1996-06-27 | 1997-09-04 | Brueckner Grundbau Gmbh | Determining position accuracy of bore holes and slots in building base |
| US7997157B2 (en) * | 2008-02-11 | 2011-08-16 | Honeywell International Inc. | Control moment gyroscope |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| ZA73638B (en) * | 1972-02-03 | 1973-10-31 | Selco Mining Corp Ltd | Bore hole logging device |
| GB1509293A (en) * | 1976-03-16 | 1978-05-04 | Russell A | Instruments for measuring the orientation of a borehole |
| FR2410725A1 (en) * | 1977-12-02 | 1979-06-29 | Sagem | IMPROVEMENTS MADE TO DEVICES FOR MEASURING THE AZIMUT AND THE INCLINATION OF A DRILL LINE |
| FR2410724A1 (en) * | 1977-12-02 | 1979-06-29 | Sagem | IMPROVEMENTS MADE TO DEVICES FOR EXPLORATION, IN AZIMUT AND INCLINATION, OF A DRILLING LINE |
-
1982
- 1982-07-27 NL NL8203007A patent/NL8203007A/en not_active Application Discontinuation
- 1982-08-19 CH CH4955/82A patent/CH651890A5/en not_active IP Right Cessation
- 1982-08-19 DE DE3230889A patent/DE3230889A1/en not_active Withdrawn
- 1982-08-19 MX MX194071A patent/MX152695A/en unknown
- 1982-08-19 CA CA000409758A patent/CA1189317A/en not_active Expired
- 1982-08-19 IT IT22898/82A patent/IT1153556B/en active
- 1982-08-19 BR BR8204849A patent/BR8204849A/en unknown
- 1982-08-19 NO NO822825A patent/NO164193C/en unknown
- 1982-08-20 FR FR828214389A patent/FR2511727B1/en not_active Expired
- 1982-08-20 JP JP57144567A patent/JPS5873807A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| NO164193B (en) | 1990-05-28 |
| IT8222898A0 (en) | 1982-08-19 |
| DE3230889A1 (en) | 1983-03-03 |
| IT1153556B (en) | 1987-01-14 |
| MX152695A (en) | 1985-10-11 |
| BR8204849A (en) | 1983-08-02 |
| CH651890A5 (en) | 1985-10-15 |
| FR2511727B1 (en) | 1989-02-03 |
| NO822825L (en) | 1983-02-21 |
| JPS5873807A (en) | 1983-05-04 |
| NL8203007A (en) | 1983-03-16 |
| FR2511727A1 (en) | 1983-02-25 |
| NO164193C (en) | 1990-09-05 |
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