GB2052757A - Drill hole survey instrument - Google Patents

Abstract

A drill hole survey instrument comprising a first pair of inclinometers with sensitive axes a1, b1 that are co- planar and at 90 DEG to each other, mounted at one end of a torsionally rigid member of known length and a second pair of inclinometers with sensitive axes a2 b2 similarly mounted at the other end of the rigid member so that when the instrument is vertical the sensitive axes lie in two vertical planes at 90 DEG to each other. <IMAGE>

Description

SPECIFICATION Drill hole survey instrument 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 for both dip and azimuth. The measurement of dip seldom presents great problems. However, the measurement of azimuth has generally been by use of instruments based on a magnetic compass.

In the last few years various instruments for small diameter holes have been developed that use a gyroscope as the azimuth reference (i.e.

independent of any magnetic influence). These instruments have several disadvantages.

(a) Due to the very small available space and the need for the gyrorotor to be installed within at least 3 yimbal 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 freedom from friction. 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 vary 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 3n 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.

In one form the invention resides in a drill hole survey instrument comprising a first pair of inclinometers with sensitive axes that are coplanar and at 900 to each other, mounted 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 that when the instrument is vertical the sensitive axes lie in two vertical planes at 900 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: Fig. 1 is a diagrammatic representation of inclinometers with the longitudinal axis of the instrument vertical; Fig: 2 is a generalised diagrammatic representation of the instrument; Fig. 3 is a typical correction chart for an inclinometer; Fig. 4 is a typical correction chart for analogue multiplexer and AID converter; and Figs. 5, 6 and 7 are circuit diagrams 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 hoie.

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 900 a horizontal hole a dip of 00. The azimuth of a drill hole is the angle measured in a horizontal plane between some datum (generally 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.

This instrument does not measure the absolute azimuth at any one point in a drill hole but measures the change in azimuth over a given length (say 10 m) in a manner anaiogous to a surface survey using a chain and theodolite. Thus the azimuth of each leg of the survey is only relative to the adjacent legs until one leg is 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 inclinometers with sensitive axes that are co-planar and at 900 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 900 to each other (See Fig. 1).

Thus 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 electronicaily 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 a is the output voltage or pendulum unit A, and oB is the output voltage of pendulum unit B then: a= 5. SIN D . COS Y As the sensitive axis of B is at 900 to the sensitive axis of A 5 = 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 containing the sensitive axis of pendulum unit A and the vertical plane through the direction 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: a=5.COSD.COSY p . COS D . SINY Thus these are two simultaneous equations from which values for DIP and angle Y can be obtained.

i.e. Y = TANKS (a) (1) and D = COS-1 (a/5 COS Y) (2) or D = COS~1 (p/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 calculate the dip at the lower part of the instrument D2 and another Y value i.e. Y2.

If we let R = Y2Y1 then

where hAz 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.l. 3773 August 1944).

INCLINOMETER UNITS AND DISPLAY VOLTAGES cc AND B Inclinometer units suitable for use in this instrument are those manufactured by Schaevitz Engineering Model LSRP. These are small cylindrical devices specifically designed to be used in pairs (with their sensitive axis at 900). They have a diameter of 36.3 mm and a height of 40.6 mm. Their small size allows them to be fitted inside an instrument pressure casing that can pass down the inside of BQ drill rods.

They are completely sealed solid state units 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 0 to + volts for angles of -90 through 0 to +900.

The pendulus mass is suspended on a horizontal arm from a vertical axis however the output from 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 A1, B1, A2 and B2 are sequentially switched by use of an analogue multiplexer to a 1 3 bit analogue to digital (A/D) 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 surface 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 for pulse transmission.Complete details of the electronic circuitry of the instrument are shown in Figs. 5, 6 and 7.

INSTRUMENT PROBE DESIGN The instrument probe consists of an upper section, a lower section and extension pieces.

The upper section contains all the circuitry necessary for the operation of the probe and the inclinometers A1 and B1. 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 inclinometers A2 and B2. It also has a connector for joining it with the extension pieces to the upper section.

The extension pieces are 3 m long connecting rods that carry 5 conductors between the lower section and the upper section. These extension pieces also serve to maintain alignment of the A and A2 inclinometer and the B1 and B2 inclinometers while maintaining them a set distance apart. Thus with three 3 metre extension pieces the actual separation of the upper and lower pair of inclinometers would be 10 metres (the extra 1 metre being made up in the upper and lower sections of the probe). In practice 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 140 KPa (2,000 Ibsjsq inch) such that the instrument can operate to depths of in excess of 1000 m. (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 BQ drill rods (I.D 46 mm) yet still maintain alignment of the longitudinal axis of the instrument with that of the drill rods. Provision is also made so that collar rings can be placed over the end sections so that the instrument can be used in holes larger than BQ (i.e. NQ or PO).

CABLE AND WINCH SYSTEM The cable and winch system for lowering and raising the probe in the hole and electrically connecting the probe to the surface module is a commercially available system. Various lengths of cable can be fitted to the winch (typically 600 M).

The cable has a diameter of 2.54 mm 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 (18-24 V DC).

The surface module displays numbers proportional to the output voltages of the individual inclinometers.

The relationship is as follows: Displayed Number - 1 } 10.r,25 4096 = Output Voltage It would be entirely feasible to incorporate a microprocessor 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 of the surface module are given in Figs. 5, 6 and 7 of the drawings.

METHOD OF SURVEYING WITH THE INSTRUMENT The instrument is assembled with a suitable number of extension pieces to give the desired instrument length (say 10 m) and lowered into the hole until it is just proud of the drill collar. A sighting device is attached to the top of the instrument by way of suitable mounting hole such that the instrument can be rotated to bring the sensitive axis of inclinometer unit A, in line with a suitable datum point (normally a peg situated grid north of the drill collar).

The displayed values for each inclinometer unit are read and these values are converted to the voltages al, sst, a2, p2s These values are then inserted into equations 1, 2, 3 and 4 to give the dip of the hole at the top and bottom of the instrument and the change in azimuth over the length of the instrument. The initial azimuth of the hole can be calculated from Dip, and Y,. 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 10 m) 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 As 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 instrument 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 300). As the azimuth of a hole that is vertical is undefinable the instrument cannot be used in a hole that starts off exactly vertical nor can it be used in a hole that at some depth becomes exactly vertical. In point of fact a truly, vertical hole is very rare as a hole with a vertical trend, in detail actually spirals around a vertical line.

Apart from errors due to misalignment of the instrument which can be eliminated by using the correct closure adjustments the errors of the instrument are due to: (a) Non linearity of the output voltage of the inclinometer (b) Non linearity of the analogue multiplexer and AID converter system.

As the inclinometers are supplied with calibration data it is possible to make up correction charts for each individual inclinometer.

A typical correction chart for an inclinometer is shown in Fig. 3.

It is also possible to make a correction chart for the analogue multiplier 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 correction chart for the analogue multiplexer and A/D converter is shown in Fig. 4.

Thus the error of the instrument approaches the error produced by the resolution of the A/D converter.

Repeated testing has shown that the repeatability 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 300 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.

Claims (6)

1. A drill hole survey instrument comprising a first pair of inclinometers with sensitive axes that are co-planar and at 90 to each other, mounted 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 that when the instrument is vertical the sensitive axes lie in two vertical planes at 900 to each other.

2. A drill hole survey instrument as claimed in claim 1 wherein the inclinometers are solid state units in which a pendulus mass is held in a null position by a torque motor which provides a restoring force.

3. A drill hole survey instrument as claimed in claim 2 wherein the inclinometers are fitted inside an instrument pressure casing mounted at each end of a length of tube which forms the rigid member and which is suspended from a length of coaxial cable through which signals generated by the inclinometers are transmitted to the surface.

4. A drill hole survey instrument as claimed in claim 3 wherein output voltages from the inclinometers are sequentially switched by use of an analogue multiplexer to an analogue to digital converter which converts the voltage to a burst of digital pulses.

5. A drill hole survey instrument as claimed in any one of the preceding claims wherein a sighting device is attached to the top so that the instrument can be aligned with a suitable datum.

6. A drill hole survey instrument substantially as herein described with reference to the accompanying drawings.