Note: Descriptions are shown in the official language in which they were submitted.
~37~
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THIS INVENTION relates to a drill hole survey instrument.
For accurate location of diamond drill hole intersections
there is a need to survey the hole Eor both dip and
azimuth. The measurement of dip seldom presents great
problems. ~lowever, the measurement of azimuth has generally
been by use oE instruments based on a magnetic compass. In
the last Eew years various instruments Eor small diameter
holes have been developed that use a gyroscope as the
azimuth reference (ie. independent of any magnetic influ-
ence). These instruments have several disadvantages.
(a) Due to a very small available space and the need for
the gyrorotor to be installed within at least 3 gimbal
axis the rotor is always very small and is required to
spin at very high speeds (40,000 - 60,000 rpm) to
achieve even a low measure of gyroscopic stability. All
gimbal bearings have to be very light to achieve the
desired Ereedom from Eriction. Thus these instruments
are not particularly rugged.
(b) Accuracy of these types of instruments is limited due
to the fact that gyro drift is not constant while the
instrument is lowered down the hole.
(c) All gyro instruments require very high precision in
their manufacture and are very costly to develop. Thus
the final cost of a complete instrument is very high.
Typically $50,000 for a complete instrument.
Gyroscopic instruments have one great advantage over
magnetic instruments in that they retain their accuracy in
the presence of magnetic rocks. Most sulphide nickel ore
bodies in Australia are located in highly magnetic rocks as
are some iron ore bodies such as the magnetite iron ore
deposits of Savage River in Tasmania.
This invention provides an instrument that does not rely on
magnetic north as an azimuth reference system yet has the
potential to be more rugged, cost less to develop and
manufacture and be more accurate than a gyro instrument.
1~37~9
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In one form the inventlon resides in a drill hole survey
instrument comprising a first pair oE inclinometers with
sensitive axes that are co-planar and at 90 to each other,
mo~mted at one end of a torsionally rigid member of known
length and a second pair of inclinometers similarly mounted
at the other end of the rigid member so ~hat when the
instrument is vertical the sensitive axes lie in two vert-
ical planes at 90 to each other.
The invention will be better understood by reference to the
following description of one specific embodiment thereof
when read in conjunction with the accompanying drawings
wherein:
Figure 1 is a diagrammatic representation of the inclinom-
eters with the longitudinal axis oE the instrument
vertical;
Figure 2 is a generalised diagrammatic representation of
the instrument;
Figure 3 is a typical correction chart for an
inclinometer;
Figure 4 is a typical correction chart for analogue
multiplexer and A/D converter; and
Figures 5, 6 and 7 are circuit diagr~ms showing the
electronic circuitry of the instrument.
Like all other down hole survey instruments this instrument
is designed to give the dip and azimuth of the drill hole
at selected points down the hole.
The dip of a drill hole is the angle in a vertical plane
between the axis of the drill hole and a horizontal line.
Thus a vertical hole has a dip of 90 a horizontal hole a
dip of 0. The azimuth of a drill hole is the angle
measured in a horizontal plane between some datum (gener-
ally grid north) and the vertical plane that contains the
drill hole axis.
As a drill hole is a continuous curve the term drill hole
axis is generally accepted as meaning the tangent to the
drill hole at any specified point.
~L37
- 4
This instrument does not measure the absolute azimuth at
any one point in a clrill hole but measures the change in
azimuth over a given length (say ~Om) in a manner analogous
to a surface survey using a chain and theoclolite. Thus the
azimuth oE each leg of the survey is only relative to the
adjacent legs until one leg ls related to a traverse leg of
defined azimuth. In this case the top of the hole.
The change in azimuth is determined by mounting 2 inclinom-
eters with sensitive axes that are co-planar and at 90 to
each other, to two similarly mounted inclinometers by a
torsionally rigid tube of known length. The two sets of
inclinometers are aligned so that when the instrument is
vertical the sensitive axis lie in 2 vertical planes 90 to
each other (See Figure 1).
Th~ts externally the instrument appears as a long slender
tube that can be lowered down the inside of the drill rods
by a cable. The pendulum units are contained in pressure
casings and their inclinations are sensed electronically
and transmitted to the surface by the cable and displayed
as digital numbers.
If the electronic pendulum units are tilted in a plane
parallel to their sensitive axis their output voltage is 5
times the sine of the angle of tilt. If the pendulum units
are tilted in a plane at an angle Y to the sensitive axis
then the output voltage is 5 times the sine of the angle of
tilt multiplied by the cosine of angle Y. Thus if ~c is
the output voltage of pendulum unit A, and ~ is the output
voltage of pendulum unit B then:
cx~ = 5 . SIN D . COS Y
As the sensitive axis of B is at 90 to the sensitive axis
of A:
= 5. SIN D . COS (90-Y)
where D is the angle of inclination of the pair from
vertical and Y is the angle between the vertical plane
~37~!39
- 5 -
containing the sensitive axis of pendulum unit A and the
vertical plane through the directlon of tilt.
As it is conventional to measure the dip of the hole as the
angle from the horizontal we can let D = the dip of the
hole.
thus the equations become:
cx~ = 5 . COS D . COS Y
~ = 5 . COS D . SIN Y
Thus these are two simultaneous equations from which values
Eor DIP and angle Y can be obtained.
i.e. Y = TAN 1 (~ /~c) (1)
and D = COS 1 (~/5 COS Y) (2)
or D = COS 1 (~ /5 SIN Y) (3)
equation (2) is used when SIN Y approaches 0
and equation 3 is used when COS Y approaches 0
Using the lower pair of pendulums it is possible to cal-
culate the dip at the lower part of the instrument D2 and
another Y value i.e. Y2.
If we let R = Y2 - Yl then:
tan Az = tan _ cos ~ (Dl - D2)
2 2 sin ~ (Dl D2) (4)
where ~ Az is the change in azimuth over the length of
the instrument.
For proof of equation (4) see "A Method of Surveying Drill
Holes by Oriented Drill Rods". L.A. Dahners and C.J. Cohen.
(U.S. Bureau of Mines R.I. 3773 August, 1944).
372~9
I~CLINOMETER UNITS AND DISPLAY OF VOLTAGES oc AI~D B
_
Inclinomete~ units suitable Eor use in this instrument are
those manufactured by Schaevitz Engineering Model LSRP.
These are small cylindrical devices speciEically designed
to be used in pairs twlth their sensitive axis at 90).
They have a diameter of 36.3mm and a height of 40.6mm.
Their small size allows them to be fitte~ inside an instrum-
ent pressure casing that can pass down the inside of BQ
drill rods.
They are completely sealed solid state ~mits in which a
pendulus mass is held in a null position by a torque motor
which provides a restoring force. The magnitude of this
restoring force is proportional to the sine of the angle of
inclination of the unit from vertical. This restoring force
is output from the unit as a voltage ranging from -5
through O to -~5 volts for angles of -90 through 0 to +90.
The pendulus mass is suspended on a horizontal arm from a
vertical axis however the OtltpUt Erom the device has been
considered conceptually in Figure 1 as that due to a
pendulum pivoted on a horizontal axis only for ease of
diagrammatic explanation.
The output voltages from the 4 inclinometer units Al, Bl,
A2 and B2 are sequentially switched by use of an analogue
multiplexer to a 13 bi~ analogue ~o digital (AtD) converter
which converts the voltage to a burst of digital pulses
where the number of digital pulses is proportional to the
voltage. These bursts of digital pulses are -transmitted to
the surEace module through the single coaxial cable that is
used to lower the instrument up and down the hole. The
pulses are counted by the surface module which has switch
positions such that the output of each inclinometer can be
displayed at will. Power for the inclinometers, analogue
multiplexer and A/D converter are derived by power supply
circuitry within the probe which in turn receives its power
from the surface down the same cable used ~or pulse
transmission. Complete details of the electronic circuitry
of the instrument are shown in Figures 5, 6 and 7.
~L9 37~
-- 7 --
INSTRUMENT PROBE DESIGN
The instru~ent probe consists of an upper secti,on, a lower
section and extension pieces.
The upper section contains all the circuitry necessary for
the operation of the probe and the inclinometers Al and Bl.
It has a connector at the top end to connect to the
supporting cable, the lower end also has fittings such that
the extension pieces can connect the upper section to the
lower section both mechanically and electrically.
The lower section only contains the lower pair of inclinom-
eters A2 and B2. It also has a connector for joining it
with the extension pieces to the upper section.
The extension pieces are 3m long connection rods that carry
conductors between the lower section and the upper
section. These extension pieces also serve to maintain
alignment of the Al and A2 inclinometer and the Bl and B2
inclinometers while maintaining them a set distance apart.
Thus with three 3 metre extension pieces the actual separa-
tion of the upper and lower pair of inclinometers would be
10 metres (the extra 1 metre being made up in the upper and
lower sec-tions of the probe). In prac~ice any number of
extension pieces could be used depending on requirements.
All external parts of the probe are made of stainless steel
to prevent corrosion and the entire probe and all seals and
connectors are designed to operate under hydrostatic
pressures of 140KPa (2,000 lbs/sq inch) such that the
instrument can operate to depths of in excess of 1000m. (It
should be noted that the drilling fluid inside the drill
rods can have a specific gravity greater than 1).
The outside diameter of the end sections of the upper and
lower parts of the instrument have a diameter such that
they permit the instrument to just slide down the inside of
~Q drill rods (I.D. 46mm) yet still maintain alignment of
the londgitudinal axis of the instrument with that of the
drill rods. Provision is also made so that collar rings can
~L372
- 8
be placed over the end sections so that the instrument can
be used in holes larger than BQ (i.e. NQ or PQ).
~ABLE AND WINCH SYSTEM
The cable and winch system Eor lowering and raislng the
probe in the hole and electrically connecting the probe to
the surface module is a commercially available system.
Various lengths oE cable can be fitted to the winch (typic-
ally 600M). The cable has a diameter of 2.54mm and a
breaking strength of 455 kgm. The winch is a simple hand
wound system with cable depth readout.
SURFACE MODULE
This is a simple instrument case which is connected to the
instrument probe by the winch cable and is also connected
to a battery power supply (18 - 24V DC).
The surface module displays numbers proportional to the
output voltages oE the individual inclinometers.
The relationship is as follows:-
( Displayed Number _ 1 10.625 = Output Voltage
4096
It would be entirely feasible to incorporate a micro-
processor in the surface module such that dip and azimuth
of the hole could be calculated from the digital numbers
available.
The dip and azimuth could then be displayed~ printed or
recorded on magnetic tape.
A complete description of the electronics oE the surface
module are given in Figures 5, 6 and 7 of the drawings.
METHOD OF SURVEYING WITH THE INSTRUMENT
.
The instrument is assembled with a suitable number of
1~3~299
_ 9 _
extension pieces to give the desired instrument length (say
10m) and lowered into the hole until it is just proud of
the clrill collar. A sighting device is attached to the top
of the instrument by way oE suitable mo~mting hole such
that the instrument can be rotated to bring the sensitive
axis of inclinometer unit Al in line with a suitable datum
point, (normally a peg si~uated grid north of the drill
collar).
The displayed values for each inclinometer unit are read
and these values are converted to the voltages ~Cl, ~ 1'
~2' ~ 2. These values are then inserted into equations 1,
2, 3 and 4 to give the dip of the hole at the top and
bottom oE the instrument and the change in azimuth over the
length of the instrument. The initial azimuth of the hole
can be calculated from Dipl and Yl. The initial azimuth and
change in azimuth then permits the azimuth at the lower end
of the instrument to be calculated. The instrument is then
lowered by an amount exactly equivalent to its effective
length (in this case lOm) and the procedure repeated.
This entire process can then be repeated until the bottom
of the hole is reached. The process can be repeated on the
way back up the hole and on reaching the surface the
instrument can then be aligned with the sensitive axis of
the inclinometer unit Al pointing to the same datum. Thus
permitting a closure adjustment to be carried out.
LIMITATIONS AND ACCURACY OF INSTRUMENT
The obvious limitations of the instrument are that it will
not go down a hole smaller than BQ. BQ is the smallest
sized hole that is commonly drilled although on rare
occasions AQ holes are drilled. These are generally short
holes and it is rare for them to be surveyed. The instrum-
ent cannot be used conveniently in any hole where the dip
is such that the instrument will not slide down the hole,
(i.e. holes dipping less than about 30). As the azimuth of
a hole that is vertical is undefinable the instrument
cannot be used in a hole that starts off exactly vertical
~13~
- 10 -
nor can it be used in a hole that at some depth becomes
exactly vertlcal. In point of Eact a truly, vertical hole
is very rare as a hole with a vertica] trend, in detail
actually spiraLs around a vertical line.
Apart from errors due to misalignment oE the instrument
which can be eliminated by using the correct closure adjust-
ments the errors of the instrument are due to:
(a) Non linearity oE the output voltage of the
inclinometer.
(b) Non linearity of the analogue multiplexer and A/D
converter system.
As the inclinometers are supplied wlth calibration data it
is possible to make up correction charts for each individ-
ual inclinometer. A typical correction chart for an inclino-
meter is shown in Figure 3.
It is also possible to make a correction chart for the
analogue multiplexer and A/D converter system by setting up
precisely known voltages at the analogue multiplexer and
noting the readings at the surface module. A typical correc-
tion chart for the analogue multiplexer and A/D converter
is shown in Figure 4.
Thus the error of the instrumen~ approaches the error
produced by the resolution of the A/D converter.
Repeated testing has shown that the repeatibility of the
A/D conversion has a standard deviation of typically 1.2
over the range of +4.5 volts to -4.5 volts. Thus over this
range we can be 95% sure that the correct value displayed
is within 2 standard deviations (i.e. ~2.4 counts) over the
range of -4.5 V to +4.5 volts. It should be noted that if
the instrument is used in a hole always dipping more than
30 the output voltages lie in the range of -4.33 volts to
+4.33 volts. Figure 6 also shows the experimentally
obtained standard deviation over the above voltage range.
