Wide angle inclinometer

Improved apparatus for measuring the angular deviation from the vertical of the end of the drill string which includes a housing adapted for insertion in the drill string, a pendulum pivotally mounted in the housing and capable of swinging through large angles, and a substantially perfectly balanced catch link pivoted at the same point as the pendulum and normally coaxially aligned with the end of the drill string. Means are provided for coupling the pendulum and catch link together at a preselected angle and for measuring the deflection of the coupled pendulum and catch link from the axis of the end of the drill string so that the angular deviation from the vertical of the end of the drill string may be determined. Means are provided for signalling the measure of the deflection to the surface.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
This invention relates to apparatus for measuring the angular deviation of 
the end of a drill string, and more particularly to apparatus of the mud 
pressure pulse signalling type. 
(b) Description of the Prior Art 
Patents and publications showing the general state of the prior art 
include: U.S. Pat. Nos. 2,329,732; 2,435,934; 2,762,132; 2,824,380; 
3,431,654 and Society of Petroleum Engineers of AIME Paper Number SPE 765. 
The most pertinent prior art patents are U.S. Pat. Nos. 3,176,407 and 
3,466,755, which teach apparatus which includes a pendulum within a 
housing placed in the drill string. The pendulum is suspended from a rod 
that is axially movable in the housing downwardly in response to mud 
pressure and upwardly in response to the cessation of mud flow. The upward 
movement of the rod is limited by a set of concentric stop rings of 
decreasing ascending diameter adapted to engage the pendulum, dependent 
upon the deviation of the pendulum from the axis of the string. A coding 
system is provided whereby relatively short movements of the pendulum and 
rod are translated into relatively long movements of a knob headed shaft 
extending radially outwardly from the housing. In addition to magnifying 
the length of movement of the pendulum and rod, the coding system inverts 
the movement whereby a minimum movement of the pendulum and rod gives rise 
to a maximum movement of the shaft and a maximum movement of the pendulum 
and rod gives rise to a minimum movement of the shaft. The inversion 
causes the shaft movement to be directly proportional to the deviation 
measured. The measure of the deviation is signaled to the surface by 
pressure pulses created by means of the interaction of the knob with a 
plurality of pulse rings in the string above the housing. The number of 
pulses detected at the surface is directly proportional to the angular 
deviation of the string. With pressure pulse signalling type deviation 
measuring devices, deviation may be measured each time a length of pipe is 
added to the string, and thereby allow substantially continuous monitoring 
of deviation. 
Because of length and diameter constraints, the devices of the prior art 
are limited to measuring angles in a range from 0 to approximately 10 
degrees. However, in directional drilling programs it is necessary to 
measure accurately angles much in excess of 10 degrees. Presently, such 
large angles are measured by various multi-shot survey instruments. 
However, the use of such instruments is costly in that considerable 
valuable rig time is expended in running such instruments into the hole. 
Additionally, multi-shot surveys are not well suited for substantially 
continuous monitoring of hole deviation, as would be desirable in 
directional drilling programs, and as is possible with deviation measuring 
apparatus of the pressure pulse signalling type. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a deviation measuring 
apparatus of the pressure pulse signalling type that is capable of 
measuring large angles. 
Briefly stated, in the apparatus of the present invention, the pendulum of 
the prior art is replaced by a pendulum capable of freely swinging through 
large angles, a balanced catch link normally coaxially aligned with the 
end of the drilling string when mud is being pumped through the string, 
and means for coupling the pendulum and catch link together at a 
preselected known angle and deflecting the pendulum some relatively small 
but unknown angle from the vertical. The unknown angle is equal to the 
difference between the preselected angle at which the pendulum and catch 
link are now coupled together and the deviation from the vertical of the 
drill string. When the flow of mud through the string is interrupted, the 
pendulum and catch link move upwardly along the drill string axis until 
the catch link is free from being restrained coaxially with the drill 
string. When the coaxial restraint is removed, the pendulum returns to the 
free-swing position thereby deflecting the catch link away from the axis 
of the string by the same angle as the pendulum was collected when the 
catch link was restrained. The deflection of the catch link is then 
measured in the same way that the prior art devices measured the 
deflection of the prior art pendulum and the measure of that deflection is 
signaled to the surface in the conventional manner thereby enabling the 
driller to determine the deviation from the vertical of the drill string.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the apparatus of the preferred embodiment is 
designated generally by the numeral 11. Apparatus 11 is contained within a 
tubular sub 12 adapted for coaxial insertion in a drill string above the 
bit 13. Apparatus 11 is comprised generally of means 150 for forming 
pressure pulses to signal to the surface the measure of the deviation of 
sub 12, means 151 for coding angular deviation into pressure pulses, and 
means 152 for measuring the angular deviation of sub 12. 
As with prior art devices, pulse forming means 150 includes a plurality of 
pulse rings 14, and a shaft 16 having a knob 15 on the end thereof. Shaft 
16 is biased upwardly by the force of a main spring 29, as illustrated in 
FIG. 2A, against the downward flow of mud 153 through sub 12. When the 
flow of mud 153 through sub 12 is interrupted, shaft 16 drives knob 15 
upwardly to a position adjacent one pulse ring 14, the position being 
determined by the angular deviation of sub 12, as will be described 
hereinafter. When the flow of mud 153 is resumed, knob 15 is forced 
downwardly and a series of pressure pulses are created as knob 15 passes 
each pulse ring 14, which pressure pulses may be read at the surface by 
the driller and converted into angular deviation, as is well known in the 
art. 
Coding means 151 is taught in the prior art and includes a plurality of 
thimbles 18, a coding tube 19, a plurality of coding balls 20, and a 
shoulder 30 at the lower end of shaft 16, illustrated in FIG. 2A. Coding 
tube 19 has therein a plurality of holes 31, illustrated in FIG. 2A, 
containing coding balls 20. Holes 31 are spaced apart distances which 
correspond to the spacing of pulse rings 14. Telescopically disposed 
within coding tube 19 is coding rod 17. Coding rod 17 is axially movable 
within coding tube 19 upwardly when the flow of mud 153 is interrupted by 
means of a pendulum lift spring 40, illustrated in FIG. 2A, and downwardly 
by the force of the mud 153 on knob 15. The force of the mud 153 on knob 
15 is transmitted to coding rod 17 by a seating spring 75 disposed in a 
chamber at the upper rod of shaft 16. Coding rod 17 has a plurality of 
radially enlarged thimbles 18 thereon, which thimbles are spaced apart to 
be adjacent to a particular set of coding balls 20 when the catch link 25 
is limited in upward travel by a particular stop ring 27, as will be 
described hereinafter. Thimbles 18 serve to thrust balls 20 radially 
outward through holes 31 into contact with the inside surface of shaft 16, 
as shown in phantom in FIG. 2A. Radially outwardly held balls 20 engage 
shoulder 30 and thereby limit the upward travel of shaft 16. Thimbles 18 
are positioned on coding rod 17 such that the upward travel of shaft 16 
and therefor of knob 15 corresponds inversely to the upward travel of 
catch link 25 so that the length of upward travel of shaft 16 is 
proportional to the angular deviation of catch link 25 from the axis of 
sub 12. 
Referring now to FIG. 2A, the upper portion of the apparatus of the 
preferred embodiment is illustrated. The apparatus is contained within a 
tubular housing 32 which is in several parts threaded together to aid in 
the assembly thereof. Main spring 29 is compressed between a shoulder 33 
at the lower end of shaft 16 and a shoulder 34 at the top of a tubular 
member 35 contained within housing 32. Coding rod 17 terminates at the 
lower end thereof in a radially narrowed portion 36 inserted and connected 
by soldering or the like to a washer 37. Washer 37 is connected to a guide 
bushing 21 by means of a pair of snap rings 38 and 39. Guide bushing 21 is 
axially movable within coding tube 19 by means of pendulum lift spring 40, 
which is compressed between a radially enlarged portion 41 of guide 
bushing 21 and a lower collar 42 which is held against the lower end of 
coding tube 19 by means of snap ring 43. Pendulum lift spring 40 functions 
to move coding rod 17 axially upwardly and to lift guide bushing 21, which 
in turn lifts pendulum 24 by means of an actuator rod 22. Actuator rod 22 
is coaxially mounted within guide bushing 21 and is restrained from moving 
axially therewithin by a shoulder 44 in guide bushing 21 that engages a 
radially enlarged portion 45 of actuator rod 22 and snap ring 38. The 
upward travel of guide bushing 21 is limited by a retaining ring 46 which 
engages coding tube 19. 
At the lower end of actuator rod 22 is connected an orientor 23 which 
serves to orient the pendulum 24 with the low side of the hole during 
rotation of sub 12. Orientor 23 is freely rotatable upon actuator rod 22 
by means of bearings 47 and 48. Bearings 47 and 48 are housed within a 
radially enlarged portion 49 of orientor 23 and are spaced apart by means 
of a spacer 50. Bearings 47 and 48 are retained on actuator rod 22 by 
means of a washer 51 and screw 52 threadedly engaged at the bottom of 
actuator rod 22 and a bearing clamp nut 53 threadedly engaged in the top 
of radially enlarged portion 49. 
Orientor 23 includes a counter weight 54 that is radially asymmetrical with 
respect to the axis of actuator rod 22 and which hangs toward the low side 
of the hole and thereby orients the elements of the apparatus therebelow 
with the low side of the hole. During rotation of sub 12, counter weight 
54 hangs toward the low side of the hole and thereby eliminates any 
wobble. 
Referring now to FIGS. 2B and 3, the lower portion 55 of orientor 23 is 
pivotally connected to pendulum 24 by means of journal 56 mounted in a 
pair of bearings 57 and 58. Journal 56 is inserted through a hole through 
lower portion 55 and is held in place by means of a set screw 59. The ends 
60 and 61 of journal 56 are radially narrowed to accommodate thereon 
bearings 57 and 58 respectively. Journal 56 is located in lower portion 55 
of orientor 23 at right angles to counter weight 54 so that pendulum 24 
hangs and seeks plumb toward the low side of the hole. 
Also pivoted about journal 56, by means of a pair of bearings 84 and 85, is 
catch line 25. Catch link 25 includes a tubular lower member 64 and a 
counter weight, designated generally by the numeral 65. Catch link 25 
extends through a void section 68 in the interior of pendulum 24. 
Lower portion 64 of catch link 25 has telescopically disposed therewithin a 
plunger 59 having a pawl 70 attached to the upper end thereof and a button 
71 at the lower end thereof. Plunger 69 is biased axially downwardly 
relative to outer portion 64 by means of a spring 72 compressed between 
button 71 and a shoulder 73 formed at the lower end of outer portion 64. 
Catch link 25 has, near the lower end of lower portion 64, a hook 76 for 
engaging stop rings 27 as will be described hereinafter. 
While catch link 25 has an appearance generally similar to a pendulum of 
the prior art devices, catch link 25 is substantially perfectly balanced 
about journal 56 by means of counter weight 65 and has no gravity 
responsive characteristics. Counterweight 65 is comprised of two halves 66 
and 67, which are connected together by a screw 80 and a bolt 81 and nut 
82 spaced apart by spacer 83. Halves 66 and 67 include auxiliary weights 
86 and 87 respectively which are attached by screws or the like. 
It is contemplated that catch link 25 will be substantially perfectly 
balanced about journal 60 during the manufacture thereof; however, to 
correct any imbalance, adjustment screws 88 and 89 are provided. 
Adjustment screws 88 are tapped in auxiliary weights 86 and 87 and lie on 
a plane containing the axes of catch link 25 and journal 56 and movement 
thereof axially will correct any longitudinal imbalance about journal 56. 
Adjustment screws 89 are tapped normal to the axis of catch link 25 and 
movement thereof in and out will correct any imbalance caused by the axial 
asymmetry of catch link 25. 
Since adjustment screws 88 and 89 are tapped normal to each other, 
adjustment screws 88 and 89 may be adjusted independently to substantially 
perfectly balance catch link 25 about journal 56. If upon manufacture and 
assembly of catch link 25 it is found that catch link 25 is not 
substantially balanced about the axis of journal 56, catch link 25 may be 
balanced by first orienting the longitudinal axis horizontally and moving 
adjustment screws 88 axially until catch link 25 has no tendency to rotate 
under the influence of gravity. When the longitudinal axis of catch link 
25 is oriented horizontally, the position of adjustment screws 89 does not 
in any way cause any moment of torque. After adjustment screws 88 have 
been properly positioned, the orientation of catch link 25 is changed such 
that the longitudinal axis thereof is vertical and adjustment screws 89 
are positioned such that catch link 25 again has no tendency to rotate 
under the influence of gravity. When the longitudinal axis of catch link 
25 is vertically aligned, adjustment screws 88 have no tendency to produce 
a torque about journal 56. When adjustment screws 88 and 89 have been 
positioned as aforesaid, catch link 25 is substantially perfectly balanced 
about the axis of journal 56. 
Pendulum 24 is of hemispherical construction and is adapted to swing 
through a large angle, which in the preferred embodiment is greater than 
56 degrees. Pendulum 24 has a void 68 in the center thereof to accommodate 
catch link 25 and a pair of notch plates 90 and 91. The top portion of 
pendulum 24 that lies at the low side of the hole, is milled down to 
compensate for the weight removed by virtue of void 68. A pair of 
adjustment screws 94 and 95 are provided to balance pendulum 24 about the 
axis thereof so that the axis thereof will normally align with the 
vertical. 
In other words, the pendulum axis may be considered to be an imaginary line 
which passes through both the pivot point and the center of gravity of the 
pendulum. When the pendulum is free-swinging and at rest in this 
illustrative embodiment, the axis will always be vertically oriented 
(within predetermined maximum limits) regardless of the angle of the axis 
of the drill string relative to vertical. 
Disposed within void 68 in pendulum 24 are a pair of arcuate serrated notch 
plates 90 and 91. Notch plates 90 and 91 have therein a plurality of 
generally upwardly facing V-shaped notches, each designated generally by 
the numeral 97. Each notch 97 includes a first side 98 which lies on a 
line intersecting the center of journal 56 and therefore the pivot point 
of pendulum 24, and a second side 99 which slants upwardly from the bottom 
100 to the top 101 of each notch 97, as best seen in FIG. 4. The first 
side 98 of notches 97 form a plurality of surfaces, each of which may 
cause the pendulum axis to be deflected by catch link 25. The angular 
spacing between each adjacent side 98 of each notch 97 is equal, and in 
the preferred embodiment it is contemplated that such separation is 7 
degrees. Each bottom 100 of each notch 97 lies along an arc having its 
center at the center of journal 56 and each top 101 of each notch 97 lies 
on a concentric inner arc. 
Notch plate 90 and pawl 70 function to couple pendulum 24 and catch link 25 
together so that angles much larger than heretofore measurable can be 
measured. Pawl 70 is attached to plunger 69 by means of a screw 74. Pawl 
70 has an alignment side 102 which lies on the axis of plunger 69 and a 
biasing side 103. When the mud pumps are shut off and actuator rod 22 
begins to move axially upward under the influence of pendulum lift spring 
40, button 71 tends to remain biased into scat 110 by the action of spring 
72. Plunger 69 and pawl 74 thus tend to remain stationary while pendulum 
24 moves upwardly. As a result of this relative movement, biasing surface 
103 will engage the top of one of the notches. The particular notch 
engaged will be selected as a result of (a) the angular orientation from 
vertical assumed by the axis of the plunger 69 which is coaxial with sub 
12, and (b) the relative orientation of the axis of pendulum 24, which 
will always be vertical when the pendulum is free-swinging. Biasing 
surface 103 engages the tops 101 of the selected notch 97 and deflects 
pendulum 24 such that its axis is no longer vertical until alignment side 
102 engages a first side 98 of the notch 97, at which point catch link 25 
and pendulum 24 are coupled together. 
Since catch link 25 is substantially perfectly balanced, the coupling 
thereof to pendulum 24 does not affect the plumb seeking properties of 
pendulum 24. Thus, when button 71 of catch link 25 is not restrained 
within seat 110, the axis of pendulum 24 returns to vertical and deflects 
catch link 25 away from the axis of sub 12. 
The slope of the second side 99 of notch 97 is greater than that of biasing 
side 103 of pawl 70 so that the knife edge 104 does not contact anything 
and thus cannot become blunted or bent. It can be visualized by reference 
to FIG. 4 that if knife edge 104 were to contact side 99, knife edge 104 
would tend to curl toward first side 98 and thereby introduce error into 
the system. 
Adjacent to void 68 in pendulum 24 is a lower body sight hole, which is 
normally plugged by a plug 112. Single hole 111 provides means by which 
pawl 70 and notch plates 90 may be inspected without dissembling housing 
32. 
Below lower body sight hole 111 are a plurality of stop rings, designated 
generally by the numeral 27. Stop rings 27 form a plurality of concentric 
shoulders for engaging hook 76 of catch link 25, and are spaced axially 
apart such that when hook 76 engages one particular shoulder 113, one 
particular thimble 18 will be positioned within coding tube 19 such that 
appropriate coding balls 20 will be thrust into the path of shoulder 30 in 
shaft 16. The diameter of each stop ring 27 is such that the upward travel 
of hook 76 will be limited according to the angular deviation of catch 
link 25 from the axis of sub 12 and in the preferred embodiment the 
diameters are such that increments of one degree are measured. 
In the preferred embodiment, since the angular separation between each 
notch 97 is 7 degrees, it is therefore necessary that stop rings 27 be 
configured to measure angles from 0 to 7 degrees. As is shown in FIG. 2B, 
six stop rings 27 are provided, whereby maximum deviation of catch link 25 
is measured by the engagement of hook 76 with the shoulder designated 114 
and minimum deviation is measured when hook 76 engages no stop rings 27; 
rather the axially upward travel of catch link 25 is limited by the 
engagement of the upper end of guide bushing 21 with retaining ring 46, 
illustrated in FIG. 2A. 
Below stop rings 27 is catch link seating ramp assembly 118, as best seen 
in FIG. 2B. Seating ramp assembly 118 includes an upper portion 119, which 
abutts stop rings 27 and a lower frustrum shaped lower portion 120. 
Seating ramp assembly 118 has therein a pair of holes 121 and 122 and 
ports 123 and 124. Holes 121 and 122 and ports 123 and 124 allow for the 
flow of fluid 154 with which apparatus 11 is filled. Fluid 154 
conveniently takes the form of commercially available 300 centistoke 
silicone fluid, and serves to lubricate the various moving parts of the 
apparatus, damp oscillation of pendulum 24 and catch link 25, and slow 
somewhat the movement of the parts upwardly and downwardly. It has been 
found that the reliability of the apparatus is greatly enhanced by the 
addition of approximately 0.25% by weight of dioleyl hydrogen phosphite to 
the fluid 154. 
Lower portion 120 of seating ramp 119 serves to guide button 71 of catch 
link 25 into seat 110 as pendulum 24 and catch link 25 move downwardly 
under the influence of mud pressure on knob 15. Seat 110 has an initial 
frustrum shaped portion 125, which has a slope substantially equal to that 
of lower portion 120 of seating ramp 119, and a lower cylindrical portion 
126. Portion 126 has an inside diameter substantially equal to the 
diameter of button 71 and functions to restrain catch link 25 in axial 
alignment with sub 12 as pawl 70 engages one of the notches 97. Seat 110 
also serves to restrain button 71 from further axially downward movement 
to thereby disengage pawl 70 from notch 97 when knob 15 is again pushed 
down by the drilling mud. This allows pendulum 24 to return to plumb. 
Below seating ramp 118 is a cylindrical restricter block 131 that 
accommodates various fluid control means, including a check valve 132, 
orifice discs 133 and a relief valve (not shown). Orifice discs 133 serve 
to inhibit the downward flow of fluid 154 through apparatus 11, thereby 
slowing the downward movement of knob 15 past pulse rings 114 to space 
apart the pressure pulses generated thereby. Check valve 132 closes 
against downward the flow of fluid 154 and opens to upward fluid flow to 
allow the parts of the apparatus to travel upwardly more easily under the 
influence of main spring 29 and pendulum lift spring 40. The relief valve 
(not shown) serves to protect the instrument against damage from excessive 
pressure within housing 32. Restricter block 131, seating ramp assembly 
118, and stop rings 27 are retained in the lower portion of housing 32 by 
means of a snap ring 135. 
Fluid 154 displaced within housing 32 by the movement of the various parts 
is accommodated by an axially moveable piston 137 located below snap ring 
135. Piston 137 sealingly engages the inside of housing 32 by means of a 
quad-ring 138. Piston 137 has a pair of radial ports 139 and 140 therein 
for the flow of fluid 154 into a cylindrical chamber 141 therein. The 
fluid within chamber 141 serves to lubricate bearings 127 and 128 which 
mount seat 110. The lower portion of chamber 141 is sealed by a plug 142, 
which is held in place by means of a snap ring 143. The lower portion of 
piston 137 is threaded to provided means by which a threaded tool can be 
engaged to remove piston 137 from housing 32 for inspection and servicing. 
Threadedly engaged in the center of restricter block 131 is a restricter 
block insert 145. Restricter block insert 145 has a cylindrical lower 
portion 146 that extends into chamber 141, and an upper threaded portion, 
which contains seat bearing assembly 147. Seat bearing assembly 147 
contains therein bearings 127 and 128 and spacer 129, which contain seat 
110. Bearing 128 is retained at the bottom of seat 110 by a screw and 
washer 151. Seat 110 is thus rotatable relative to sub 12 so that the 
rotation of sub 12 will not impart any rotation to catch link 25. 
The operation and geometry of the apparatus of the present invention may 
best be understood by reference to FIGS. 5-7 wherein is illustrated a 
situation where the angular drift from the vertical of the string is equal 
to an angle A plus B: where A is a known angle equal to the angular 
separation of notches 79 times the number of the notch engaged minus one, 
and B is an unknown angle to be measured. Since in the preferred 
embodiment of the invention the angular separation of notches 97 is 7 
degrees, and pawl 70 is shown in FIGS. 5-7 in a position to engage the 
fifth notch 97, angle A is equal to 28 degrees. 
Throughout FIGS. 5-7, the line designated by the numeral 105 represents the 
axis of sub 12, and therefore the axis of the end of the string, and the 
line designated by the numeral 106 designates the vertical. FIG. 5 
illustrates the situation where the mud pumps are in operation and catch 
link 25 is fully seated within seat 110 and substantially coaxial with 
axis 105 of sub 12. Spring 72 is fully compressed and pawl 70 is 
disengaged from notch plate 90. Pendulum 24 is thus free to swing and the 
axis thereof is aligned with the vertical 106. Therefore, the angular 
separation between the axes of pendulum 24 and catch link 25 is equal to 
the angular drift from the vertical of the end of the string, or A plus B. 
FIG. 6 illustrates the situation a short time after the mud pumps have been 
deactuated wherein pendulum 24 is shown lifted slightly under the 
influence of pendulum lift spring 40; however, button 71 of plunger 69 is 
still biased into lower cylindrical portion 126 of seat 110 and pawl 70 
has engaged the fifth notch 97. Catch link 25 is still coaxial with axis 
105 of sub 12; however, the axis of pendulum 24, has been deflected B 
degrees from the vertical 106 to a portion 107 by the interaction of pawl 
70 with notch 97. Pendulum 24 and catch link 25 are thus coupled together 
such that the axes thereof are A degrees apart. 
FIG. 7 illustrates a still further point in time when button 71 has become 
disengaged from seat 110. With catch link 25 no longer restrained to 
remain coaxial with axis 105 of sub 12, the axis of pendulum 24 returns to 
the vertical 106 and the axis of catch link 25, now designated by numeral 
108, is deflected B degrees from the axis 105 of sub 12. Since the catch 
link is fully balanced, it holds itself in a position fixed to the 
pendulum such that the pendulum axis is vertical. The upward travel of 
pendulum 24 is limited by the engagement of hook 76 with a stop ring 
designated by the numeral 130 and, through coding system 151 knob 15 moves 
axially upwardly to a position adjacent the pulse ring 14 designated by 
the numeral 134 in FIG. 1. Upon resumption of the flow of mud 153, the 
driller would detect three pressure pulses and would thus know that angle 
B is equal to 2 degrees. The driller would then determine that the angular 
drift of the end of the string is equal to 30 degrees, i.e., the known 
angle of 28.degree. plus 2.degree.. Also upon resumption of the flow of 
mud 53 pendulum 24 would be driven axially downwardly and the apparatus 
would return to the configuration illustrated in FIG. 5 whereupon the 
drift of the end of the string may again be measured upon subsequent 
interruption of the flow of mud 153. If, during subsequent drilling, the 
drift remains constant at the illustrative angle of 30 degrees, then at 
each interruption in drilling, the driller will detect three pulses. If, 
however, the drift were to increase to, for example, 31 degrees, the 
driller would detect four pulses. Similarly, if the drift were to decrease 
to, for example, 29 degrees, the driller would detect two pulses. 
It becomes apparent that in the preferred embodiment of the present 
invention the driller will always detect from one to seven pulses, which 
will indicate angles from 0 to 6 degrees within each 7 degree range formed 
by notches 97. For example, one pressure pulse detected by the driller 
could indicate that the angular drift of the end of the string is equal to 
0 degrees. However, one pressure pulse could also indicate that the 
angular drift of the end of the string is equal to 7 degrees or 14 degrees 
or some other multiple of 7 degrees. Similarly, four pressure pulses 
detected by the driller could indicate that the angular drift of the end 
of the drill string is equal to 3 degrees from the vertical; however, four 
pulses also could indicate a drift of 10 degrees or 17 degrees or some 
other angle equal to a multiple of 7 degrees plus 3 degrees. It is 
therefore necessary that the driller know initially the angular deviation 
of the end of the string and then keep a continuous record of subsequent 
measurements. 
For example, the apparatus of the present invention could be inserted in 
the string when drilling is first commenced and the drift from the 
vertical of the string is substantially 0. Upon each interruption of the 
flow of mud 153, a reading may be taken and recorded. As the angular drift 
of the end of the string increases, the number of pulses would increase 
until the drift angle became larger than 7 degrees, at which point the 
number of pulses would drop from 7 to 1. Because of the nature of the 
environment and the equipment used, there is virtually no possibility of 
an ambiguous reading being detected. By keeping continuous record of the 
readings taken, the deviation of the hole may be mapped. In an alternative 
method, the initial measure of drift of an already started hole may be 
obtained by measuring the drift angle of the bottom of the hole with 
conventional measuring means and then commencing measuring of the drift 
angle with the apparatus of the present invention. 
Further modifications and alternative embodiments of the apparatus of this 
invention will be apparent to those skilled in the art in view of this 
description. Accordingly, this description is to be construed as 
illustrative only and is for the purpose of teaching those skilled in the 
art the manner of carrying out the invention. It is to be understood that 
the form of the invention herewith shown and described is to be taken as 
the presently preferred embodiment. Various changes may be made in the 
shape, size and arrangement of parts. For example, equivalent elements or 
materials may be substituted for those illustrated and described herein, 
parts may be reversed, and certain features of the invention may be 
utilized independently of the use of other features, all as would be 
apparent to one skilled in the art after having the benefit of this 
description.