CA1211841A - Borehole survey instrument - Google Patents

Abstract

Abstract A borehole survey instrument has a probe with a polarized light system for transmitting a signal representing the angular orientation of the probe to the surface. Light from a source in the probe is directed through a polarizing filter with an axis of polarization orthogonal to the longitudinal axis of the probe. The polarized light beam is transmitted to the surface through an optical fiber light conduit. The angle of polarization is detected with a rotating polarizing filter and provides a measure of the probe orientation. In surveying a borehole, azimuth is determined from inclinometer measurements. The probe orientation in vertical sections of the borehole is measured by the polarized light system. Two measures of borehole azimuth are combined, providing an improved measure of azimuth an boreholes near vertical and near horizontal.

Description

121184~

1 This invention relates to a borehole survey instrument having an improved means for determining the rotational orientation of the borehole probe as it moves -through a vertical borehole and to an improved method of determining borehole azimuth.
This application is related to Canadian patent Nos. 1,166,843 and 1,166,844 both issued May 8, 1984 to the present applicant.
A typical borehole survey instrument has a probe housing which is suspended from a cable and moved through the borehole. An inclinometer, for example an orthogonal triad of accelerometers, measures the angle of the local vertical with respect to the probe.
The probe is free to rotate about its longitudinal axis as it moves through the borehole. lt is necessary to measure the probe orientation to provide a reference for the inclinometer measuremenmts, in order to determine the borehold azimuth. It is known to measure orientation with a gyroscope or a magnetometer. Both have operating limitations which impair their reliability, degrade accuracy and contribute to high cost.
The above mentioned Canadian patents disclose borehole survey instruments which use multi section probes together with means for determining the incremental azimuth changes as the probe moves through the borehole. These instruments eliminate the gyroscope or magnetometer but have other disadvantages, including a long probe dimension and an ,.,~
; ~ .

121~41 accumu~ion of measurement error which reduoes accuracy of measurement.
The present instrument measures probe orientation direc~y using pa~arized electromagnetic radiat~n and transmits an orientation signal tc~ the surface through a conduit which maintains the axis of pr~A~ation of the signal. The instrument is particularly useful in providing a measure of probe orientatiDn as the probe traverses a ver~cal borehole section where orient~;~ n cannot be measured wi~ch an inc~i nometer .
One feature of the invention is that the boreho~e survey instrument has an improved means for deter~r~ng probe orientat~n including a pc~ar~7ed ~ight source in the probe whirh generates a beam of p~ized ~ight that has an a~;is of pclar~zation transverse to the ~ngitudinal ax~s of the probe and in fixed re~ion to the probe, together with an optical f~ber ~ight conduit for conduc~ng the p~a~zed ~ight to the surface and a means for detec~ing the plane o~ p~la~zat~n of the light received from the conduit. More par~c~ rly, the light source and a ~ing f er are fixed tc~ the pr~be, and the optical light fi3~er conduit is a part of the probe suspending cable.

2 5 Another feature of the inventic n is that the means for detec~ing tne p~ane of pn~ 7ation of the received light includes a ~ight sensor, a p~larizing fil~Pr interposed between the op~al fiber ~ight condui$ and the light sensor, means for rotating the fil~r to

3 0 modulate the ~ight ~mpir.ging on the sensor and means for determining the probe orientation from the phase 2ngle of the modu~ted light wi~h respect tc) the rc~ta~on of the second pt~ g f~ r.

lZ~841 A further feature of 'che invention is an improved and simpl;f~Pd method for determining borehole azimuth from successive inclinometer measurements.
Yet an~her feature of the inventicn is that S the borehcile survey system comprises a probe movable through the borehcile, an inc~ination sensor in the probe, means for deriving a firg measure of borehole azir.uth t~h a high degree of accuracy in a verticAl borehole and a reduced accuracy in a horizont probe borehc~e, means for deriving a second measure of borehole azimuth with a high degree of accuracy in a honzontal borehole and a reduced accuracy in a verti~al borehale and means for combining the first and second borehcae azimut~ measures in accordance with the borehoJe inc3ination.
And an~er feature of the invent~Dn is t~lat the accelerometer and other signals are transmitted from the probe to the surface by amplitude modulation of the pa~arized 'ight beam.
2 0 Further features and advantages of the invention w;ll readily be apparent from the ft~ wing specifica~ion and from the drawings, in which:
Eigure 1 is a ~iagram of an apparatus embodyir g the invention, including a sec~Dn through the borehc~le showing the probe;
Eigure 2 is a fragmentary diagrammatic illu~ a~on R the probe showing the palarized ~ight source and in~linometer;
Figures 3 and 4 are diagrammatic i~lustratiDns of the detecion of the angu~r orienta~Dn of the probe;
Figures 5-7 are geomctric di~grams used in descnbing ~e determ~na~on of boreh~le az~nu~;

~Z11841 Figure B is a diagram of a boreh~le survey appAratus with means for deriving and combining two measures of borehc~e azimuth: and Eigure 9 is a b~Dc~ diagram of a system for S transmi~ing sensor information bo the surface through amplitude modu~a~n of the polarized light beam.
In the survey system of Figure 1, a probe 20 is suspended from a cable 21 for movement through a borehole 22. The probe 20 is centered wi~ the 10 borehQle by suit~hk spacers 23 so t:hat the long~tudinal axis o$ the probe is oentered in the borehcle and may be cons;~?ered caincident w~th the borehale ax~s. Probe 20 is ~ee to rc~tate as it moves through the borehci~e.
Cable 21 passes over a rotating wheel which provides a 15 measure ~ of the distance of the probe downhcile. A
cable haisting mechanism for ~Dwering and raising probe 20 is nat shown tc) avoid complicating the drawing.
Br~e~ly, probe 20 includes means ~or measuring the inc~ination of the borehc~Le ~h respect 20 the vertical or gravi~ vector at sucoes~,e pc~nts a~Dng the b~reh~e. As will appear, this measurement in a slant borehc~e provi~es su~ nt informa~ion for a determina~n of the change ~ boxeho~e azimuth from paint ~ p~int. Many borehbles have a vertic~l section, 2 5 par icu~arly the ~ al section below the surface .
OrientatiDn of the probe in a vertical borehc~le is measured using the po~asized light system described bclow. S~gn 15 representing inclinat~n and or~Pntation of the probe are transmitted to the surface through 30 cab~e 21 and coupled t~ a detector 25. The output of the detector c in turn coup~ed wi~h a data processor 26 connected wi~h a keyhoard and disp~y 27 used to input data to and derive information from the system.

~21~41 The elements of the probe rel~vant to the invention are illustrated æhemadca~ly in F~gure 2. The probe has a housing 30 in which is located an inc~lometer 31 wh~ h is preferably made up of an S orthogonal triad of accelerl~meters 32, 33, 34 with thP~r sens ve axes designed X, Y, Z, respec~ively. The Z
a~s is shown as cc~ncident with the )Dng~tudinal ax~s of the probe. The X and Y axes define a p~ane at right ang~s to the Z axis. The accelerometers which 10 measure the local gravi~ vector are preferably servoed devices. Signals from the accelerometers define the inclination angle of the borehole from the vertical and in a slant ha~ the rotation angl~ of the probe with respect to a ver~c~l plane through the probe axis.
15 ~wo ac PlPrometers or ~ther angul~r re tionshi~s could be used, but the i~lustrated inc]inometer is prefe,~d.
A ~ight source 35, as a ~ight em;~ng diDde, is ~cated at the upper end of the probe 20. A
p~Lr~ins fi~er 36 is fixed in hous~g 30 tc~ p~ 7e the 20 light fro.n source 35 a~Dng an axis transverse to the ~ongitudinal axis of the probe. An opti~ al fiber light conduit 37 rec~ves ~lA~ed ~ight from filter 36 and conducts i~ ~ the sur~aoe. Optical fiber conduit 37 may be incorporated in h~ng cable 21. The end 37a 25 of the optical fiber ~ight oonduit is preferably fixed to the end of probe hou~ing 30. If, however, r~on of probe 20 as it moves through the boreh~le causes exoessive twisting of cable 21, the cable and op~-al fiber l;ght conduit 37 can be connected with the probe 30 hou~lng 37 through a swivel pint ~not shown) .
As probe 20 rotates about its axis, the planc of pol~iza~iDn of the light from LED 35 rotatcs the probe. The pl~ne of p~ za~on - ~211841 established by filter 36 is not substanti~lly modifi~d ci~her by refl~ction of the ~ight as it passes ~rough the ~iber optic light cond-~t 37 or by twis~ng of cable 5 21 and the light conduit. Accordingly, the ax~ of pa~ariza~on of the light detected at the surface represents the orientatiDn of the probe about its ~on~tudinal axis .
The pclarized light received at the surface 10 tlL-ou~h optiral fiber light conduit 37 ic directed to a light s~nsor 40 whi~h is oonnected with detector 25.
Intcrposed between the op'dcal fiber ~ht condu~t and sensor 40 is a second pc~larizing fi~er 41 which is rv~ted by a m~tor 42.
1~ The determinatiDn of the pro3 e orientat;~7r.
the po~arized light is i~lustrated in Figures 3 ar.d

4. As fi"er 41 r~ates, the light received by sensor 40 is a maximum when the pb~rization axes o~ t}.e fi~ers are c~n~dent and a m~nLnum when the axes are 90 2 0 displaced . The curves in Figures 3 and 4 p~t the re~e~ved light or sensor signal ampli~ude as a functiDn of the a; gu~ar po~i~n of fil~r 41. In Figure 3 the pola~ization axis of hl~or 36 is aligned with that of S~er 41 at tlle 0" po~i^n. Signal maximums occur at O- and 180. Signal m~mums occur wi~ ~i~or 41 at 90- and 270. In Figure 4, probe 20 is disp3aoed 90 f~m its rota~onal poC~ n in Figure 3. With ffltor d~cc 41 at ~e o poc~tian ~ the c~gnal amp3itude c m~nimum and t~lis a7ndiiiDn is repeated with the f~ r disc at 180. Signal maximums occur at 90 and 270.
In surveying a borehale, the probe 20 is G~nt~d to a known azimuth reference at the top of bo.ehaL 22. The angu~r re~ation between the output of sensor 40 and ratating ~31ter 41 is noted. As the pro~e moves l:hrough the boreh~ rotational orientation ~1184~ `

of the probe is correlated with the distance ~ of the probe along the borehole. It is necessary only that the angular velocity of the rotating filter 41 be much greater than the angular velocity of the probe 20.
05 Signals representing the output of sensor 40 and the angular position of filter disc 41 are coupled with detector 25 which measures the phase angle of the signal with respect to the angular position of filter 41 and determines the rotational position of probe 20. The relative difference between signal peaks and nulls may vary with system conditions, but the phase angle does not. The ~ignal is basically a half-wave rectified sine curve with a DC bias. Detector 25 may, for example, incorporate a data processor which applies a Fourier curve fit to the detector signal. The fundamental frequency component of the signal is twice that of the wheel rotation. Variable terms in the Fourier representation of the signal can be ignored. The phase angle of the signal uniquely identifies probe orientation.
The determination of the downhole position of the probe from the signals representing orientation of the probe about its longitudinal axis, distance downhole and the orientation of the probe with respect to gravity from inclinometer 31 will be described in connection with Figures 5, 6 and 7. In Figure 5, borehole 22 is depicted extending downwardly from the surface of the earth. A three-dimensional coordinate system, N
(north), E (east), G (gravity) has its origin at the intersection of the borehole with the surface The local surface area may be considered planar. With the probe 20 located at a point i inclinometer 31 measures the angle ~ between the transverse reference axis of the probe, i.e., the axis of polarization of filter 36, and ~21184~

the vertical plane 45 which oont~ns the longitudinal a.~is of the probe. The angle ~ is indicated between the po~ tion axis of fil~er 36 (sometimes referred to as the transverse referenoe axis of the probe) and a

5 line 46 normal ~ the ~Dngitudin~l ax~s of the probe and lying in vertical plane 45. The in~ination angle I of the probe and boreh~e at point i is shown as the ang~
between an extensiDn of the longitud~nal axis 47 of the probe and the ver~al ~ine 48 in p~ne 45.
~he intersection of ver~al plane 45 with the surface of the earth def~nes a ~ine 49 the onent~on of which is the a7~muth of the borehc~e at pc~nt i. ~he a7 muth angle A is measured ~ckwise from north, ~oking down on the surface of the earth.
The inc natiDn angle I is ca~u~ated from the accelerometer signals of inclinometer 31. 5imilarly, at any p~:rlt where the boreh~le axis is not ver~al, the angle ~ is c;~lrulatPd from the azcelerometer signals.
Where, however, the borehcile axis is ver~cal, there is no unique ver~cal plane and the ang~P ~ not defis~ed.
A typical borehc~le has an ini~l vert~al section and it is in traversing such a vertical se~n that the pci~srized ~ht rotational or~en~don detector is used.
As described above, the start of a survcy operation, 2 5 the probe onent2tiDn and the phase angle of the pnlAri7ed light sLgnal are noted. Changes ~n the rotational or~ntation of the probe as i~ is ~owered through a ver~cal borehcle se~n are recorded. When the probe leaves the vertical borehc~e sec~on, i~s orie:ltatiDn is known and provides the basis for furtl er det~r~ina~Dn of borehole azimuth.
h It was pcfinted out above that the angle ~ is determined from the acce~eroMeter sign ~ls . In the ~iu ~2~841 g patent 1,166,843 i~entified above, there is a disclosure that the angle ~ and boreh~le azimu'ch are related. The relati~nshi~ ic imp3i~y inv~Lved in a series o~ matrix oper~tiDns by which ~iu derives a representation of the boreh~le trajec~ry. An expli~ expression of the relationship is the basis of the method of determ~ning borehale azimut~ in accordance with the present inven~on.
In ~igure 7, two succes~ive borehcile pc~ints i and i+l are shown and it is assumed that the borehc~Le sec~on between the two pC~Lnts i a plane curve. ~his is n~ always true in a borehcJe but is a reasonab3e approxima~Dn and may be made as accurate as de~red by selectislg very sma~l distances between pcnnts. The p~ane Pi is a vert~cal plane contair~ing the ~Dngitudinal axis of the probe at the pc~nt i. Plane Pi has an azLmuth an ~le Ai and the inc~lination of the probe at p~t i is Ii. P~ane Pi+l is a ver~al p~ane containing t~e ~ngi'cudinal probe axis ~t p~nt i+l. The pl~ne R
2~ contains the p~ane borehale curve from pc~nt i to pc~nt i+l.
The vec~r j is a unit vector in the probe ~ordinate system, ini~dally a~igned ~h the E a~s of the global coordinate system. It can be shown using the method of Euler angles that the oomponents of the un~t vector ~ in the global coordinate system are:
N = -cosAcosIsin~-sinAcos~
E = -sinAcosIsin~+cosAcos~
G = sinIsin~;
In ~igure 7, the unit vec~or ~ is shown at bc~ ints i and i+l, rc*ated through an anqle O from the respective verti~al planes Pi and Pi+l so that the vectc)rs are perpendicular to the pl~ne of the- boreh~e 121~841 curve at b~th pa~nts. The two unil;s vec~rs wiU then have the same di~n and thus the same components in the NEG coordinate system. Therefore, N = -cosAicosIisin~i-sinAiCos~
-cosAi+lcosIi+lsin6i+l-sin~i+lcos9i+
E = -sinAicosIisin~i+cosAiCOs~i =
i+ l~:osI i+ lsin ~i+ l+~ osAi+ lcos ~i G = sinIisin9i z sinIi+lsin~i+l The change o~ azimuth angle fr~m p~int i to i+l may be expressed as i i+l i By ~inear com~natiDn of the N and E equatiDns, two new equat~ns may be derived in terms of ~i, cos~icosIlsin~i-sin~icosei =
cosIi+~,sin~i+l sin~icosIisin~i+cosQiCOS~i - cos~i+
An ang~e Yiis de~ined as Yi ~i+l ~i i+l i The equatiDns above may be combined with the equatiDn or the G oomponents of the unit vector, [cosIisinIi+l Yi]
lsinIi sinIi+lCsril s sinIicosIi+lsinyi 1 s~nIi-sinIi+lcosyil Cs~i [CosIisinIi+l i Yil i sinIicosyi-sinIi+l Sclving these equat~ns for sin Qi and cos Qi and d~ g one by the ~er t~ get tan Qi~ th-~f~7lr.wing rela~nship is de~ived:
Ai+ l~Ai = tan ~ cos I i+cos I i+ 1 ) s inyi s inIi s inI i+ 1- ~ l+cos I icos I~;+ 1 J cos Y i ¦

~21184~

.
Thus, the Ghange in azimu'ch angle between suc~essive pc~nts of the borehc le may be determmed from the inclinometer measurements at the two p~ints.
As pc~inted out above, t:he orientation of S probe 20 i5 measured directly at the easth's surfaoe~
The probe is then ~owered through the borehol~. So 3Dng as ~he borehole a~s is ver~ al, the probe or~Pntatio:l about its longitudinal axis is measured u~lizin g the p~larized light system . At the point the 10 ~orehcle-deviates from the vertical, changes in azimuth are determined by successive com.putatiDns of ~i and the azimuth anglP at any po~nt determined by adding the azimuth angle increments. Shoul~l the probe encounter another ver~cal borehc~e sectiDn, the or~entation of the lS probe about its IDngitudinal axis as it passes through the ver~cal section is moni~ored by the p~larized ~ight system .
If an addi~Dnal assumption is made that the borehole curve between p~ints is smooth and if the 2 0 paints are se~ected t~ be very ~lDse together so that ~ ~i+l is a sma~l angle, and Ii ~ Ii+l, then a ~ +l cosIi 2~ This relatiDnship is use ~ in visua~2ing behaviDr of the survey system and is suffi ~Pn~y accurate for,actual surveying in some app~catons.
The apparatus wh~h has been desc~bed is s~mpler than that of Canadian patent 1,166,843 in that it ut~es only one ~ ~nometer rather than two and the probe Ls a sing~ compact housing ra~ler than two hou~ngs ~ned by a connecion which is flexible bo ~ ~ .

- lZ1184~

1 bend along the axis of the borehole but which resists rotation between the housings about the borehole axis. These are significant differences from a mechanical standpoint. However, a more important difference in the present borehole survey instrument and method is in the nature of the derivation of the borehole azimuth. In the previously mentioned Canadian patents systems measurement errors are cumulative so that the accuracy of the measurements diminishes as more measurements are made. In the present system, there is a cancellation of errors so that the error in any azimuth measurement is a function of the difference between the initial azimuth and the final measure.
Tens of thousands of measurements may be made in surveying a borehole so that the difference in accuracy of the two systems is significant.
Another important difference is that the accuracy of the previously mentioned Canadian patents systems diminish in vertical or near-vertical boreholes. In the present system, the initial azimuth measurement may be made quite accurate and the polarized light system for measuring probe rotation minimizes errors introduced while the probe traverses a vertical borehole.
The accuracy of the present system diminishes, however, as the borehole approaches the horizontal where cosI goes to zero. If a borehole with a horizontal section is to be surveyed, the instrument of Figuire 1 is combined with the instrumenmt of Canadian patent 1,166,844 as shown in Figure 8. Here, probe 55 is suspended from cable 56 in borehole 57. Probe 55 has two sections 58, 59 connected by a flexible joint 60 of the character described in Canadian patent 1,166,844. Upper probe section 58 houses an inclinometer and a polarized light source as in Figure 2. Flexible joint 60 is provided ~! ~

wi~h means for generating signals representing the angle between the two probe sec~ons. The various signals are transmi~ed to the surface through cable 56 and are coupled wi~h receiver, detec~r and processor, 5 b~Dc)c 62. The pci~ed ~ight system and in~linometer c gnals are processed to develop a first measure of a7 muth A which has a high degree of accuracy in a ver~al boreh~. Signals from the inclinometer and from pint 60 are processed ~ develDp a seoond measure 10 of aamuth A' which has a high degree of accuracy in a horizont 1 borehciLe and a lesser degree of accuracy in a verti~al boreh~e. ~he azimuth sign ts A and A' are comhined in averaging circuit 63 in accordanoe with the probe in~linatiDn i to produce a compoCi~ azimuth signal Aave where AaVe - AcosI+A' (l-cosI) Signals from the various sensors in the probe are preferably transm;~ed to the surface in ~igi~al form by ampli~ude modu~don of the pa~ed light beam.
20 The system for accomplishing this is i~lustrated in b~Dck form in Figure 9. The v~ri^us sensors, e. g., accelerometers 32, 33 and 34 and the angle sensors of pint 60, ~igure 8, are represented at ~ck 65. The outputs of the sensors are selec~d individua~ly t~y a 25 mul~lexer and convertE~d from anal~g to di ~1 ~rm at blDck 66. The seri~l di i~ gnals are coupled t~ lamp 35 and modulate the inten~ty of the l;ght beam., The ~gnal from light sensor 40 has the waveform i~lustrated at 67 al~hough in practi~e the repetiiion rate of the 30 di ~1 c~gnals may be many times that i~lustrated. The signal from ~ight sensor 40 is connected with both t~e probe angl~ detector 68 and a di~l signal detector 69.
~he outputs of the dctectors are connected ~ h ~211841 pr~oessor 70. ~ the rela~ve ampJi~udes of the rec~Sed ~ne wave and the digi~l pulses are such that the di~al signa~s are ~Dst at the null of the analog signal, ~e di~ital data may be read only at the peaks of the 5 sine waves. ~n this ~tua~Dn the di ~l ~gn ls representing each sensor ou'q?ut may be repeated aYcPd ~DSS of sens~r informatiDn.

This application is a Divisional of co-pendin~.
Canadian patent application Serial No. 433,734 filed August 3, 1983.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A borehole survey system, comprising:
a borehole probe movable through the borehole;
an inclination sensor in said probe;
means connected with said sensor for deriving a first measure of borehole azimuth with a high degree of accuracy in a vertical borehole and a low degree of accuracy in a horizontal borehole;
means connected with said sensor for deriving a second measure of borehole azimuth with a high degree of accuracy in a horizontal borehole and a low degree of accuracy in a vertical borehole; and means for combining said first and second borehole azimuth measures in accordance with the borehole inclination.

2. The borehole survey system of claim 1 in which the means for deriving the first measure of borehole azimuth includes means for measuring the rotational orientation of the probe in the borehole.

3. The borehole survey system of claim 1 in which the means for deriving the second measure of borehole azimuth includes first and second flexibly connected probe sections and means for measuring the angle between the probe sections.

4. The borehole survey system of claim 1 in which the means for combining the first and second borehole azimuth measures establishes an average of the two azimuth measures weighted in accordance with borehole inclination.

5. The method of determining the difference in borehole azimuth at successive points along the borehole which comprises:
passing a probe with an inclinometer therein through the borehole, said probe defining a longitudinal axis;
establishing a source of polarized light within said probe so as to define a probe reference axis which is generally transverse to said longitudinal axis of said probe;
measuring the inclination angle between gravity and the borehole axis at said successive points;
measuring the angle between gravity and said probe reference axis at said successive points; and calculating the diference in borehole azimuth between said successive points from said four angular measurements.

6. The method of claim 5 for determining the difference .DELTA.i in borehole azimuth between successive points i and i+1 along the borehole, is given by the expression .DELTA.i = tan-1 where Ii is the inclination angle of the borehole axis at point i;
Ii+1 is the inclination angle of the borehole axis at point i+1;

Claim 6 continued...

.gamma.i = ?i+1??i;
?i is the angle between gravity and the probe reference axis at point i; and ?i+1 is the angle between gravity and the probe reference axis at point i+1.