AU2019201449B2 - Borehole survey instrument and method - Google Patents

Abstract

A method for determining an azimuth of a borehole using a borehole survey instrument, the borehole survey instrument comprising a rate sensor having a sensing axis, wherein the sensing axis is perpendicular to a longitudinal axis of the borehole survey instrument, the method comprising determining a first horizontal earth rate component and a fixed bias drift term of the rate sensor by rotating the rate sensor about the longitudinal axis of the borehole survey instrument and collecting data along the sensing axis at at least two different orientations which lie in a plane perpendicular to the longitudinal axis, rotating the rate sensor about a second axis and collecting data along the sensing axis at a further orientation lying outside of the plane perpendicular to the longitudinal axis, determining a second horizontal earth rate component using the previously determined fixed bias drift term and the data collected at the further orientation, calculating the azimuth using the first horizontal earth rate component and the second horizontal earth rate component. 11130850 _1 (GHMatters) P110785.AU N Er1 I ErxcosP Er4 W 270 go E Er x sinW 180 Er3 S Er2 Fig. 1 rotation axis rotating reference frame F&h Earth Equator ------ ---- ------ inertial reference frame Fig. 2

Description

Borehole Survey Instrument and Method

Technical field

This invention relates to borehole survey instruments and methods of using borehole survey instruments.

Background

It is common practice to require that the geometric position of a borehole is determined by means of a survey instrument. Typically such survey instruments consist of earth’s magnetic field sensors (magnetometers) combined with earth’s gravitational field sensors (accelerometers) to measure angles from vertical and magnetic north. When combined with the depth of the borehole, the coordinates of the borehole can be calculated.

However, in a number of applications it is not possible to use magnetic measurements due to local interference effects caused by adjacent metallic elements or magnetised geology. In these situations, gyroscopic measurements are often employed to measure orientation relative to true north as such measurements are not susceptible to magnetic interference. This requires accurate measurements the earth’s rate of rotation to be taken.

Gyro sensors that are capable of measuring the earth’s rate of rotation with adequate accuracy and stability from a single stationary measurement are typically quite large and very expensive. Such sensors are not suitable for borehole survey and mapping applications, 25 which generally require instruments of a diameter of less than 45mm.

Although lower performance gyro sensors can be utilised to measure the earth’s rate of rotation, such sensors have a variability of output between power cycles, as well as with time and temperature. This variability means that the sensor input axis, or sensing axis, has to be 30 rotated in order to remove any fixed bias drift errors present. It is common practice to rotate the sensing axis about the longitudinal axis of a borehole survey instrument (the axis of the instrument coincident with the borehole axis) and take multiple measurements at a number of orientations. For example, four successive measurements may be taken which are separated from each other by 90°. This allows a fixed bias drift term to be calculated using data from at

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2019201449 01 Mar 2019 least one diametrically opposed pair of measurements in order to remove bias errors of the gyro sensor. Typically, the gyro sensor sensing axis is perpendicular to the longitudinal axis of the borehole survey instrument. Therefore, rotation of the gyro sensor sensing axis around the longitudinal axis allows the gyro sensor to measure the variation in the earth’s horizontal rotation, from which the orientation with respect to true north can be calculated. Such techniques are typically referred to as gyrocompassing.

The above gyrocompassing technique works well when the borehole is vertical or close to vertical, but provides poor results if the borehole is horizontal or close to horizontal. This is because as the gyro sensor rotation plane moves away from the horizontal, the measurable horizontal component of the earth’s rotation diminishes.

Gyrocompassing also requires the borehole survey instrument to be stationary within the wellbore when measurements are taken. This is time consuming and elongates the overall drilling procedure.

WO2011146986 describes a survey tool incorporating a two axis mechanical gyro mounted on a rotary platform which may be indexed by 180 degrees to remove the bias and measure the earth’s rate along multiple directions, as previously described. Also described is a rotary mechanism for rotating the device around a second pitch axis. The pitch rotation and the roll rotation are adjusted at each survey point to align the sensitive axes of the gyro device in the horizontal plane. Two axis mechanical gyros of this type are known to be susceptible to gravity induced errors and are typically limited to operation in this single horizontal plane for optimum performance to be achieved. Also, the process of indexing the rotary platform 25 requires the device to be rotated to a fixed pitch angle to engage the indexing drive and subsequently moved back to the horizontal position to take survey data.

Because the two axis gyro is placed into a horizontal plane at each point in the survey the survey instrument is able to operate in both vertical and horizontal boreholes using a single 30 operating mode. The mechanisms required to provide this functionality are however relatively complex and time consuming to implement.

It is an object of the invention to provide a borehole survey instrument and method which overcomes disadvantages associated with the prior art.

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Summary

According to the invention in one aspect, there is provided a method for determining an azimuth of a borehole using a borehole survey instrument, the borehole survey instrument comprising a rate sensor having a sensing axis, wherein the sensing axis is perpendicular to a longitudinal axis of the borehole survey instrument, the method comprising determining a first horizontal earth rate component and a fixed bias drift term of the rate sensor by rotating the rate sensor about the longitudinal axis of the borehole survey instrument and collecting data along the sensing axis at at least two different orientations which lie in a plane perpendicular to the longitudinal axis, rotating the rate sensor about a second axis and collecting data along the sensing axis at a further orientation lying outside of the plane perpendicular to the longitudinal axis, determining a second horizontal earth rate component using the previously determined fixed bias drift term and the data collected at the further orientation, calculating the azimuth using the first horizontal earth rate component and the second horizontal earth rate component.

Optionally, the second earth rate component is determined without collecting data along the sensing axis at an additional further orientation.

Optionally, the second earth rate component is determined by only collecting data along the sensing axis at the further orientation.

Optionally, when the rate sensor is in the further orientation the sensing axis is parallel to the longitudinal axis.

Optionally, when the rate sensor is in the further orientation the sensing axis is not parallel to the longitudinal axis, and wherein determining the second horizontal earth rate component further comprises removing a component perpendicular to the longitudinal axis by using the data collected when the sensing axis was in the plane perpendicular to the longitudinal axis.

Optionally, the first horizontal rate component and the fixed bias drift term of the rate sensor may be determined from more than two sensing axis orientations in the plane perpendicular to the longitudinal axis, provided that the relative angle of each orientation is known. 35 Preferably, these orientations are at equally-spaced angles.

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Optionally, the first horizontal rate component and the fixed bias drift term of the rate sensor may be determined by continually measuring the rate sensor output while rotating its sensing axis about the longitudinal axis.

According to the invention in a second aspect, there is provided a method for determining an azimuth of a borehole using a borehole survey instrument, the borehole survey instrument comprising a rate sensor having a sensing axis, the method comprising determining a first horizontal earth rate component by rotating the rate sensor about a longitudinal axis of the borehole survey instrument and collecting data along the sensing axis for two or more orientations of the sensing axis which lie in a plane perpendicular to the longitudinal axis, determining a second horizontal earth rate component by rotating the rate sensor about a second axis which is perpendicular to the longitudinal axis, and collecting data along the sensing axis at a pair of orientations comprising a further orientation and an additional further orientation, wherein the further orientation is at an angle of +Θ degrees from the longitudinal axis and wherein the additional further orientation is at an angle of -Θ from the longitudinal axis, wherein -0 is equal and opposite to +0 with respect to the longitudinal axis, and wherein 0 is less than 90 degrees, and calculating the azimuth using the first horizontal earth rate component and the second horizontal earth rate component.

Determining the second horizontal earth rate component may comprise subtracting the data collected at one of the further orientation and the additional further orientation from the other of the further orientation and the additional further orientation.

The two or more orientations perpendicular to the longitudinal axis may comprise a first orientation and a second orientation which are separated by 180 degrees.

The two or more orientations perpendicular to the longitudinal axis may comprise a first orientation at an angle of +0 degrees from an initial position and a second orientation at an 30 angle of -0 degrees from an initial position, wherein 0 is less than 90 degrees. Determining the first horizontal earth rate component may comprise subtracting the data collected at one of the first orientation and the second orientation from the other of the first orientation and the second orientation.

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The first horizontal earth rate component may be determined before the second horizontal earth rate component (or vice versa).

Optionally, the borehole survey instrument further comprises an inclination sensor, and the method further comprises collecting, by the inclination sensor, inclination data indicative of the inclination of the borehole, and determining an inclination angle of the borehole using the inclination data.

Optionally, the rate sensor is rotated about the second axis to collect data in the further orientation if the determined inclination angle falls within a horizontal mode range.

Optionally, for subsequent azimuth measurements, if the inclination angle falls outside of the horizontal mode range, the borehole survey instrument operates in a vertical mode, in which the second horizontal earth rate component is determined by rotating the rate sensor about the longitudinal axis of the borehole survey instrument, and collecting data at further orientations within the plane perpendicular to the longitudinal axis.

Optionally, if the determined inclination angle is outside of the horizontal mode range, the rate sensor is not rotated about the second axis to determine the second horizontal earth rate component.

Optionally, the method further comprises switching to and from the vertical mode based on the inclination angle.

Optionally, the horizontal mode range is between -45 degrees and +45 degrees from a horizontal plane.

Optionally, the horizontal mode range is between -70 degrees and +70 degrees from a horizontal plane.

Optionally, the method further comprises determining, using the inclination data, the orientation of a horizontal plane.

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Optionally, the method further comprises setting the locations of first and second orientations in the plane perpendicular to the longitudinal axis such that the data is collected in the horizontal plane.

Optionally, the rate sensor is rotated about the second axis to align the sensing axis such that the further orientation is in the horizontal plane.

Optionally, the horizontal plane is perpendicular to a line which extends from the rate sensor to the centre of the earth.

Optionally, the duration of time for which data is collected at the further orientation is greater than the duration of time for which the data is collected at first and second orientations in the plane perpendicular to the longitudinal axis.

Optionally, collecting data at the further orientation comprises taking a plurality of measurements.

Optionally, the borehole survey instrument comprises a plurality of rate sensors, each rotatable about the longitudinal axis, and the method further comprises collecting, by each of the plurality of rate sensors, data along respective sensing axes.

The sensing axes of the plurality of rate sensors may have orientations perpendicular to the longitudinal axis. The sensing axes of the plurality of rate sensors may have different sensing axes. The rate sensors can be at any angle with respect to one another because 25 each can be used to derive an independent azimuth measurement which can be subsequently combined to improve the overall azimuth measurement accuracy.

Optionally, the plurality of rate sensors are further rotatable about an axis parallel to the second axis, and the method further comprises collecting, by each of the plurality of rate 30 sensors, data along respective sensing axes at orientations parallel to the further orientation.

Optionally, the borehole survey instrument further comprises an inclination sensor coupled to the rate sensor, the method further comprises rotating the inclination sensor about the second axis and collecting inclination data along the inclination sensor sensing axis to

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2019201449 01 Mar 2019 determine the angle through which the rate sensor should be rotated to collect data at the further orientation.

Optionally, the rate sensor is a MEMS gyro sensor.

Optionally, the second axis is perpendicular to the longitudinal axis.

Optionally, the data collected by the rate sensor is the earth rate rotation data indicative of the earth’s horizontal rotation.

Optionally, the inclination data comprises data indicative of the earth’s gravitational acceleration.

According to the invention in a further aspect, there is provided a borehole survey instrument for use in determining an azimuth of a borehole, the borehole survey instrument comprising: a rate sensor enclosed in a housing and having a sensing axis along which data is collected, wherein the sensing axis is perpendicular to a longitudinal axis of the housing, a rotation drive means coupled to the rate sensor and configured on actuation to rotate the rate sensor about the longitudinal axis to allow data to be collected along the sensing axis at at least two different orientations which lie in a plane perpendicular to the longitudinal axis, and a rotation mechanism coupled to the rate sensor and comprising a controller configured to actuate the rotation mechanism to rotate the rate sensor about a second axis by less than 180 degrees, such that data can be collected along the sensing axis at a further orientation, the further orientation lying outside of the plane perpendicular to the longitudinal axis.

Optionally, the third orientation is located at an angle of + Θ degrees from the longitudinal axis, and wherein Θ is less than 90 degrees.

Optionally, the rotation mechanism is further configured, under actuation by the controller, to 30 rotate the rate sensor about the second axis to an additional further orientation located at - 0 degrees from the longitudinal axis, wherein - 0 is equal and opposite to +0.

Optionally, the two or more orientations perpendicular to the longitudinal axis comprise a first orientation and a second orientation which are separated by 180 degrees.

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Optionally, the two or more orientations perpendicular to the longitudinal axis comprise a first orientation located at an angle of +0 degrees from an initial position and a second orientation located at an angle of -Θ degrees from an initial position, and wherein Θ is less than 90 degrees.

The borehole survey instrument may further comprise a three-axis inclination sensor having an inclination sensing axis perpendicular to the longitudinal axis of the borehole instrument and configured to collect inclination data indicative of the inclination angle of the borehole.

The controller may be configured to actuate the rotation mechanism to cause rotation of the rate sensor about the second axis if the inclination data falls within a horizontal mode range, and wherein if the inclination data falls outside of the horizontal mode range, the rotation drive means is actuated to cause rotation of the rate sensor about the longitudinal axis of the borehole survey instrument to collect data at further orientations within the plane perpendicular to the longitudinal axis.

Optionally, the horizontal mode range is between -45 degrees and +45 degrees from a horizontal plane tangential to an earth surface.

Optionally, the borehole survey instrument further comprises a plurality of rate sensors, each rotatable about the longitudinal axis and configured to collect data along respective sensing axes.

The three-axis inclination sensor may be a three-axis accelerometer and wherein a first axis 25 of the three-axis accelerometer is aligned to the longitudinal axis and a second axis and a third axis of the three-axis accelerometer are aligned along axes orthogonal to the longitudinal axis.

The borehole survey instrument may further comprise an inclination sensor coupled to the 30 rate sensor such that the inclination sensor is configured to rotate with the rate sensor on actuation of the rotation mechanism.

The inclination sensor may comprise an inclination sensor sensing axis coincident or parallel with the sensing axis of the rate sensor.

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Optionally, the rate sensor is a MEMS gyro sensor.

Optionally, the second axis is perpendicular to the longitudinal axis.

The second axis may be perpendicular to the longitudinal axis.

Optionally, the rate sensor is coupled to the rotation mechanism via a geared arrangement. It will be appreciated that a geared arrangement does not necessarily provide ratio-ed movement.

The rotation mechanism may comprise a lead screw or worm drive arrangement.

According to the invention in a further aspect, there is provided a method for configuring a borehole survey instrument for continuous measurement of an azimuth of a borehole such that the borehole survey instrument is operable in a roll stabilised or a gravity stabilised mode depending on an inclination angle of the borehole, the method comprising collecting, by an inclination sensor, inclination data indicative of the inclination angle of the borehole, and using the inclination data to change the orientation of a rate sensor sensing axis depending on the inclination angle, and, if the inclination data indicates that the inclination angle is sufficiently close to horizontal to be operated in a gravity stabilised mode, the rate sensor sensing axis is oriented and held in a plane perpendicular to a longitudinal axis of the borehole survey instrument, and if the inclination data indicates that the inclination angle is sufficiently close to the vertical to be operated in a roll stabilised mode, the rate sensor sensing axis is aligned to the longitudinal axis and then rotated as the azimuth of the 25 borehole rotates to cancel outputs of the rate sensor.

Optionally, the inclination sensor comprises an inclination sensing axis coincident or parallel with the longitudinal axis, and two additional inclination sensing axes aligned along two orthogonal axes, and if the borehole survey instrument is operating in the roll stabilised 30 mode, the method further comprises collecting, by the inclination sensor, data indicative of the azimuth of the borehole.

Optionally, the inclination sensor comprises an inclination sensing axis coincident or parallel with the longitudinal axis, and two additional inclination sensing axes aligned along two 35 orthogonal axes, and if the borehole survey instrument is operating in the gravity stabilised

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2019201449 01 Mar 2019 mode, the method further comprises collecting, by the rate sensor, rotation rate data indicative of a rotation of the borehole survey instrument in a horizontal plane, and calculating, by integration of the rotation rate data, data indicative of the change in azimuth angle of the borehole.

Optionally, the inclination angle sufficiently close to horizontal to be operated in the gravity stabilised mode is between -70° and +70° from a horizontal plane.

Optionally the inclination angle sufficiently close to vertical to be operated in the roll stabilised mode is between -90° and -20° for a downwardly inclined borehole or between +90° and +20° for an upwardly inclined borehole.

Optionally, a hysteresis loop is used when switching between the gravity stabilised mode range and the roll stabilised mode.

Brief description of the drawings

Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a representation of the output of a gyro sensor as the gyro sensor is rotated about the longitudinal axis of a borehole survey instrument;

Figure 2 is representation of the measurement of the earth’s horizontal rate at a latitude λ;

Figure 3 is a schematic view of a borehole survey instrument according to an embodiment of the invention;

Figure 4 is a schematic view of a sensor module system of the borehole survey instrument of 30 Figure 3, with a gyro sensor sensing axis perpendicular to the longitudinal;

Figure 5 shows a schematic view of a controller for use with a borehole survey instrument;

Figure 6 is a schematic view of the sensor module system of Figure 4, with the gyro sensor 35 sensing axis parallel to the longitudinal;

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Figure 7 is a schematic view of a sensor module system according to a further embodiment, with a gyro sensor sensing axis perpendicular to the longitudinal;

Figure 8 is a schematic view of the sensor module system of Figure 7, with the gyro sensor sensing axis parallel to the longitudinal;

Figure 9a is a schematic view of a sensor module system for a borehole survey instrument according to a further embodiment, with a gyro sensor sensing axis perpendicular to the longitudinal;

Figure 9b is a schematic view of the sensor module system of Figure 9a, with the gyro sensing axis parallel to the longitudinal;

Figure 10 is a schematic view of the borehole survey instrument according of Figure 9a; and

Figure 11 is a schematic view of a sensor module system according to a further embodiment.

Detailed description

Generally disclosed herein are borehole survey instruments and methods of using borehole survey instruments to determine the azimuth of a borehole with an inclination angle approaching the horizontal. Borehole survey instrument according to an embodiment of the invention comprises a rate sensor having a sensing axis, along which data can be collected 25 and which is perpendicular to a longitudinal axis of the borehole survey instrument. A rate sensor encompasses any sensor capable of measuring the earth’s rate of rotation. The rate sensor may be a gyro sensor. The borehole survey instrument may comprise a rotation drive means operable to rotate the rate sensor about a longitudinal axis of the borehole survey instrument. The borehole survey instrument further comprises a rotation mechanism 30 configured to rotate the gyro sensor about a second axis, which may be perpendicular to the longitudinal axis of the borehole survey instrument.

When using the borehole survey instrument in a borehole with an inclination substantially vertical or close to vertical, rotation of the rate sensor about the longitudinal axis provides an 35 accurate indication of the horizontal component of the earth rotation at any given angular

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2019201449 01 Mar 2019 position. As the borehole approaches the horizontal, rotation about the longitudinal axis will sense primarily the variation in the vertical earth component, since the sensing axis is likely to be aligned in a substantially upward or downward direction, with minimal sensitivity to the variation in the horizontal earth rotation rate. As the borehole trajectory approaches the horizontal, the rate sensor is rotated about the longitudinal axis between at least two orientations, in a common plane. The two orientations may be 180 degrees apart. The rate sensor is then additionally rotated about the second axis such that the sensing axis moves back into a position in which it is able to measure the earth’s horizontal rate component in a third orientation, which is substantially in the horizontal plane. The rate sensor may be rotated such that the rate sensor’s sensing axis is coincident or parallel with the longitudinal axis of the borehole survey instrument. In exemplary borehole survey instruments, the rate sensor may be rotated about the second axis by 90 degrees with respect to the plane in which the first two measurements were taken.

A rotation of the rate sensor about the second axis by 90 degrees with respect to the plane in which the first two measurements were taken maximises the sensitivity to the earth’s horizontal rate measurement if the borehole is exactly horizontal. In situations where the borehole is not exactly horizontal the rate sensor may be rotated about the second axis through an angle that ensures that the rate sensor’s sensing axis is substantially in the horizontal plane. This maximises sensitivity to the earth’s horizontal rate measurement.

Rotating a rate sensor’s sensing axis about the longitudinal axis of a borehole survey instrument removes fixed bias errors of the rate sensor. For the purpose of the below explanation, the rate sensor is a gyro sensor.

Figure 1 shows the output of a gyro sensor when rotated about the longitudinal axis of a borehole survey instrument. In Figure 1 it is assumed that the sensing axis of the gyro sensor is horizontal (in other words, parallel to the earth’s surface) and that the gyro sensor is rotated about the vertical (i.e. the longitudinal axis of the instrument, with the instrument in 30 the vertical).

As used herein, the term “vertical” encompasses the local vertical; that is, the direction towards the centre of the earth (see the vector Erv in Figure 2). The term “horizontal” encompasses the local horizontal; that is, a direction perpendicular to the vertical as defined 35 above (see the vector Erh in Figure 2), or in other words, parallel to the earth’s surface. More

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2019201449 01 Mar 2019 generally, relative terms such as perpendicular, longitudinal, upper, lower etc. are used herein to aid description and need not be limiting on the scope of the invention.

As described above, the gyro sensor measures the earth’s rotation. In particular, when the gyro sensor axis is aligned horizontally, the gyro sensor measures the horizontal component of the earth’s rotation vector (referred to throughout the specification as the earth’s horizontal rate). Note that although the magnitude of the earth’s rotation rate is constant, the horizontal component of the earth’s rotation vector depends on latitude. The horizontal rate for a particular latitude, λ, (see Figure 2), is given by the formula:

Erh = 15.041 * cos(A).

Figure 1 shows that as the gyro sensor is rotated about the longitudinal axis of the borehole survey instrument, the output of the gyro sensor describes a sinusoidal waveform. This output is made up of components of earth’s horizontal rate, Er, as well as a fixed bias drift term, D of the gyro sensor.

The orientation or azimuth angle with respect to true north is defined as Ψ, with the maximum measured rate being at Ψ = 0°, corresponding to when the gyro sensor’s sensing axis is aligned to true north.

It is known that measurements of the earth’s rotation may be used to determine the alignment of true north using either continuous measurement during rotation of the gyro 25 sensor sensing axis about the longitudinal axis, or by taking measurements at multiple discrete angular orientations. This is because as the gyro sensor is rotated about the vertical, the horizontal orientation of the gyro sensor’s sensing axis with respect to true north changes, making it possible to determine the azimuth. This is often referred to as gyrocompassing or northseeking.

An exemplary gyrocompassing technique is described below to illustrate how the azimuth may be determined. In exemplary gyrocompassing techniques, four measurements of the earth’s horizontal rate components may be taken when the gyro sensor is rotated about the longitudinal axis of the borehole survey instrument. The four measurements may be taken at 35 orientations separated by 90 degrees. Alternatively, measurements may be taken

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2019201449 01 Mar 2019 continuously as the gyro sensor is rotated about the longitudinal axis of the borehole survey instrument. The earth’s rotation at measurement orientations at 180 degrees apart have equal but opposite values (i.e. Er3 = -Er1 and Er4 = -Er2), see for example Figure 1. It will be appreciated that due to the sinusoidal variation in the horizontal earth rate component, measurements taken around the east or west directions (at Ψ = 90°and Ψ = 270°) provide greater sensitivity because the earth rate component variation with angle change is a maximum whereas it is a minimum around the north and south directions (see Figure 1).

At each orientation ‘n’ at which a measurement is taken (i.e. at Ψ, Ψ + 90°, Ψ + 180°, and Ψ + 270°), the output of the gyro sensor includes a fixed bias drift term, D (which may be referred to as a bias), and an earth’s horizontal rate component, Ern. The gyro output, Ω_η, at each of the orientations is therefore as follows:

At n=1,

At n=2,

At n=3,

At n=4,

Ω₄— Er1 + D

Ω₂= Er2 + D

Ω₃ = -Er1 + D

Ω₄ = -Er2 + D

Although the orientations that are 180 degrees apart (η =Ψ + 0°/180° and 0 = ^ + 90°/270°) result in a reversal of the measured earth’s horizontal rate component, Ern, the fixed bias drift term, D, remains substantially constant over the time duration of the earth rate measurements. Consequently, by subtracting gyro sensor outputs that are 180 degrees apart, the fixed bias drift term, D, is eliminated, leaving twice the earth’s rate, 2.Ern:

Ω₄ — Ω₃ = (Erl + D)—(—Erl + D) = 2. Erl

Ω₂ — Ω₄ = (Er2 + D)— (—Er2 + D) = 2. Er2

The earth’s horizontal rate comprises two components: a first horizontal earth rate component and a second horizontal earth rate component. The two resultant earth’s rate 30 components (derived from the mean value of the two sets of 180 degree points) represent the sine component of the earth’s horizontal rate (points 2 and 4 in Figure 1) and the cosine component of the earth’s horizontal rate (points 1 and 3 in Figure 1).

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These components can be used to calculate the azimuth,Ψ, which is the angle of the sensing axis of the gyro sensor with respect to true north, as follows:

2019201449 01 Mar 2019 /Erl — Er3

Ψ = atari --------\Er2 — Er4.

— atari

Erl\

Er2/

Similarly, it can be seen that by adding the measurements that are 180 degrees apart, the earth’s horizontal rate component, Ern, is eliminated and the fixed bias drift term, D, can be established:

Ω₁ + Ω₃ = (Erl + D) + (-Erl + D) = 2.D Ω₂ T 124 ⁼ (Er2 + D) + (—Er2 + D) = 2. D

When the angle of a borehole is within up to 70 degrees of the vertical, the above method can be used to establish the azimuth, to determine the direction of the borehole survey instrument with respect to true north within the borehole.

However, the method will not work well when the instrument approaches the horizontal (e.g. for angles greater than 70 degrees from the vertical). This is because the sensing axis of the gyro sensor may be close to vertical during some measurements and in these positions the magnitude of the horizontal earth rate component to be measured becomes significantly smaller. These measurements will therefore not provide an accurate value for the earth’s horizontal rate component and the accuracy of the azimuth angle measurement Ψ will be similarly degraded.

As such, for boreholes in which the borehole angle increases from the vertical towards the horizontal, rotation of the gyro around the instrument longitudinal axis is only able to provide measurements enabling one of the horizontal earth rate components to be derived.

It therefore becomes necessary to measure the earth’s horizontal rate along a second axis of 30 the borehole survey instrument in order to determine the second horizontal earth rate component, and allow Ψ to be calculated as described above. As such, when the borehole survey instrument approaches horizontal, a further measurement, Er5, is required in a different plane to the plane in which the measurements Er1 - Er4 were taken.

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In addition to azimuth data, it is often desired to collect inclination data during a borehole survey. The inclination angle of a borehole may be defined as the angle of the borehole from horizontal (where the horizontal is represented as Erh in Figure 2).

The inclination of the borehole may be determined using well-known techniques utilising inclination sensors. An inclination sensor encompasses any sensor capable of determining the inclination angle. In exemplary borehole survey instruments, the inclination sensor is an accelerometer. The accelerometer may be mounted within the borehole survey instrument to measure the gravitational acceleration. In alternative borehole survey instruments, the inclination sensor may be a tilt sensor or an inclinometer.

Exemplary borehole survey instruments comprise three accelerometers (or a single threeaxis accelerometer device). The accelerometers are typically mounted with one accelerometer sensing axis aligned to the longitudinal axis of the borehole survey instrument and the other two accelerometer sensing axes aligned along each of the other orthogonal axes. One of the orthogonal accelerometer axes is aligned with the orthogonal sensing axis of the gyro sensor. When the survey instrument is in a perfectly vertical borehole the accelerometer (or the accelerometer axis of the three axis device) aligned along the borehole survey instrument longitudinal axis will sense the earth’s gravitational field (i.e. +/-g), with the other two accelerometer sensing axes, sensing zero gravity. As the borehole deviates from vertical one or both of the orthogonal accelerometer axis sensors will start to sense a component of earth’s gravity depending on the direction of the change in inclination. The inclination angle and roll angle of the survey instrument may be derived in a conventional manner by combining the outputs of the three accelerometer axes. The roll angle defines the 25 rotation of the borehole survey instrument itself about the longitudinal axis.

The inclination angle (determined using an inclination sensor) and the azimuth angle (determined using the rate sensor) of the survey instrument enables the orientation of the survey instrument to be fully determined.

Bias errors in the accelerometer axes may be cancelled in a similar manner to that applied to the rate sensor. The accelerometer bias is typically calculated by subtracting the measured accelerometer output at orientations separated by 180 degrees. This enables accelerometers of lower performance to be utilised, such as MEMS devices. Such devices are available in 35 both single and multiple axis packages, which are small and relatively low cost.

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In addition to providing data to allow the orientation of the survey instrument to be determined, the data outputted by the inclination sensor may be used to determine when the borehole survey instrument should operate in a vertical survey mode and when the borehole survey instrument should operate in a horizontal survey mode. Exemplary borehole survey instruments may be operable in the vertical survey mode if the inclination angle of the borehole measured by the accelerometer is within a vertical mode range. Exemplary borehole survey instruments may be operable in the horizontal survey mode if the inclination of the borehole measured by the accelerometer is within a horizontal mode range.

In exemplary borehole survey instruments, the vertical mode range may be used if the inclination angle of the borehole is between -90° and -45° (or between 45° and 90° for an upwardly inclined borehole). In exemplary borehole survey instruments, the horizontal mode range may be used if the inclination angle of the borehole is between -45° and +45° (where vertically downwards is defined as -90° and horizontal as 0°).

While exemplary survey instruments will operate optimally when configured in vertical mode for inclination angles between -90° and -45° (or between 45° and 90° for an upwardly inclined borehole) and in horizontal mode when the inclination angle is between -45° and +45°, the modes are not restricted to these ranges. The vertical or horizontal mode ranges may be extended to cover a wider range of inclination angles, with some reduction in sensitivity, as described below.

Alternative borehole survey instruments may be configured to operate in the vertical mode 25 when the determined borehole inclination angle is between -90° and -20° (or between +90° and +20° for an upwardly inclined borehole). Alternative borehole survey instruments may be configured to operate in the horizontal mode when the determined inclination angle is between -70° and +70°. That is, the horizontal mode range may be between -70°and +70°. Extending the horizontal or vertical mode range will minimise the requirement to switch 30 operating modes as the inclination changes, and for boreholes where the inclination angle variation is limited no mode changes may be required.

In exemplary borehole survey instruments the vertical mode range may be between

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-90° and any angle between -45° and -20° (or between +90° and any angle between +45° and +20° for an upwardly inclined borehole). In exemplary borehole survey instruments the horizontal mode range may be defined as being between any angle between -45° and +45° but may be extended up to any angle between -70° and +70°.

Further, the inclination data may be used to set the angular locations around the borehole survey instrument’s longitudinal axis, at which the Er1 and Er3 measurements are taken, such that the measurements are performed in the horizontal plane to maximise their sensitivity to earth’s horizontal rotation rate.

Figure 3 shows a schematic representation of a borehole survey instrument 200 according to an embodiment of the invention. The borehole survey instrument 200 is generally cylindrical in cross section. The diameter of the borehole survey instrument 200 may be less than 45mm and preferably less than 42mm to allow insertion within a typical borehole drill rod diameter, such as that found in the mining and minerals industry.

The borehole survey instrument 200 comprises a housing 202 and a sensor module system 203. The exemplary sensor module system 203 of Figure 3 comprises a sensor module support 206 which may be a housing and a rotation mechanism (not shown in Figure 3). The borehole survey instrument 200 further comprises a rotation drive means 208, and a bearing 210. The sensor module system 203 is coupled to the rotation drive means 208 via a drive shaft 212 such that when the rotation drive means 208 is actuated, the sensor module system 203 is freely rotatable around the longitudinal axis 224 of the survey instrument through 360°. Alternative arrangements may be utilised to provide the capability of rotating 25 the borehole survey instruments through 360° about the longitudinal axis.

In the borehole survey instrument 200, the sensor module system 203 is coupled to a support shaft 214, fixed within the rotational bearing 210. The bearing 210 and the rotational drive means 208 are rigidly fixed to the housing 202 via attachments 216 and 218 30 respectively.

The sensor module 204 may comprise one or more rate sensors, for example gyro sensors 220, for sensing rotation and one or more inclination sensors 223 for sensing inclination. The inclination sensors may comprise accelerometers, inclinometers or tilt sensors.

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Figure 4 shows a schematic view of the sensor module system 203. The sensor module system 203 comprises the sensor module 204, the sensor module housing 206 and a rotation mechanism 226.

The sensor module housing 206 is generally cylindrical in cross section. The diameter of the sensor module housing 206 may be less than 33mm to allow insertion of the sensor module housing 206 within a borehole survey instrument 200. However, the sensor module housing 206 may be of any cross section (size or shape) provided that insertion into the borehole survey instrument 200 is possible. The sensor module housing 206 comprises a longitudinal axis 224, which may coincide with an axis of a borehole into which the borehole survey instrument 200 is inserted.

In the sensor module system 203 of Figure 4 the sensor module 204 is enclosed within the sensor module housing 206. The sensor module 204 comprises a single rate sensor. In the sensor module 204 of Figure 4, the rate sensor is a gyro sensor 220. However it will be appreciated that in alternative borehole survey instruments, the sensor module 204 may comprise multiple rate or gyro sensors, and/or other sensors, such as accelerometers, enclosed within the sensor module housing 206 (as shown in Figure 3).

In the borehole survey instrument 200, the gyro sensor 220 is a micromachined silicon (MEMS) gyro sensor. MEMS gyro sensors are less sensitive to orientation dependant (gsensitive) drift errors that are typical of spinning mass type gyro sensors. MEMS sensors also have a form factor advantage of being small. However, any gyro or rate sensor of 25 suitable size and performance could be used within the borehole survey instrument 200.

The gyro sensor 220 comprises a sensing axis 230 along which measurements may be taken. In the borehole survey instrument 200 shown in Figure 4, the sensing axis 230 is perpendicular to the longitudinal axis 224.

The rotation mechanism 226 is also enclosed in the sensor module housing 206. The rotation mechanism 226 is provided with a controller (not shown) configured to actuate the rotation mechanism 226 when certain conditions are fulfilled. For example, the controller may actuate the rotation mechanism 226 to rotate the rate sensor about the second axis if 35 the inclination data indicates that the borehole survey instrument should be operating in a

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2019201449 01 Mar 2019 horizontal mode (as described above). Similarly, the controller may be configured to actuate the rotation mechanism 226 to enable the borehole survey instrument to switch between the horizontal mode and the vertical mode dependent on the inclination data.

Figure 5 schematically illustrates components of the controller 2 of the rotation mechanism 226 and rotation drive means 208 in accordance with an embodiment. It can be seen that the controller 2 comprises a CPU 2a which is configured to read and execute instructions stored in a RAM memory 2b which may be a volatile memory. The RAM 2b stores instructions for execution by the CPU 2a and data used by those instructions. For example, instructions may be provided to control the rotation of the rotation mechanism 226 dependent on the inclination data determined by the inclination sensor.

The controller 2 further comprises non-volatile storage 2c, such as, for example, flash memory, although it will be appreciated that any other form of non-volatile storage may be used. Computer readable instructions for controlling the rotation of rotation mechanism 226 and rotation drive means 208 may be stored in the non-volatile storage 2c. The controller 2 further comprises an I/O interface 2d to which peripheral devices used in connection with the rotation mechanism 226 and rotation drive means 208 may be connected. For example, an input 2e (shown in the form of a keypad) may be provided to allow user interaction with the rotation mechanism 226 and rotation drive means 208. While not shown, it will be appreciated that other input or output devices may be connected to the I/O interface, such as a display. In other embodiments, however, the rotation mechanism 226 and rotation drive means 208 may not be provided with means for a user to interact directly with the rotation mechanism 226. Interaction with the rotation mechanism 226 and rotation drive means 208 25 may be entirely through a connected device.

The I/O interface 2d may further comprise a port 2f to allow the connection of I/O devices, such as data storage devices. For example, the port 2f may be a USB port to allow connection of USB flash drives. A communications interface 2i may also be provided. The 30 communications interface 2i may provide for short range connections to other devices (e.g.

via Bluetooth, near-field communication (NFC), etc.), and/or for connection to networks such as the Internet, for longer range communication. The CPU 2a, RAM 2b, non-volatile storage 2c, I/O interface 2d and communications interface 2i are connected together by a bus 2j.

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It will be appreciated that the arrangement of components illustrated in Figure 5 is merely exemplary, and that the controller 2 may comprise different, additional or fewer components than those illustrated in Figure 5.

The gyro sensor 220 is coupled to the rotation mechanism 226. In the sensor module housing 206, the gyro sensor 220 is mounted on a gyro sensor platform 228. The gyro sensor platform 228 may be coupled to the rotation mechanism 226, such that rotation of the gyro sensor platform 228 rotates the gyro sensor 220.

The rotation mechanism 226 is configured, under the control of the controller, to rotate the gyro sensor platform 228, on which the gyro sensor 220 is mounted, about a second axis 232. As discussed above, the controller may actuate the rotation mechanism 226 to rotate the gyro sensor platform 228 if the inclination angle determined by the inclination sensor indicates that the borehole survey instrument should be switched between the vertical mode and horizontal mode.

The second axis 232 is orientated at an angle to the longitudinal axis 224 of the borehole survey instrument 200. In the borehole survey instrument 200 shown in Figure 4, the second axis 232 is perpendicular to the longitudinal axis 224 of the borehole survey instrument. In the sensor module system 203 shown in Figure 4, the second axis 232 is perpendicular to the sensing axis 230 of the gyro sensor 220.

The rotation mechanism 226 may be configured such that the gyro sensor 220 is rotatable about the second axis 232 by less than 180 degrees, or preferably by less than 120 degrees.

In the borehole survey instrument 200, the rotation mechanism 226 is configured to rotate the gyro sensor 220 about the second axis 232 by up to around 90 degrees.

The rotation mechanism 226 may comprise a rack and pinion or geared arrangement, as shown in Figure 4. The drive force is provided by means of a drive motor 234, which is fixed 30 to the sensor module housing 206. The drive motor may provide rotation of the gyro sensor

220 via a geared arrangement. The drive motor 234 rotates a motor gear wheel 236 which is coupled to the motor 234. The motor gear wheel 236 is engaged with a geared arrangement 238 such that rotation of motor gear wheel 236 induces a counter rotation in the geared arrangement 238. The geared arrangement 238 is coupled to a geared portion 240 on the 35 gyro sensor platform 228 (see Figure 6). The geared arrangement 238 is attached to the

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2019201449 01 Mar 2019 sensor module housing 206 (not shown) to allow free rotation of the geared arrangement 238 about an axis parallel to the second axis 232. The gyro sensor platform 228 is attached to the sensor module housing 206 by means of pins 242 which allow rotation of the gyro sensor platform 228 about the second axis 232. The geared portion 240 is configured such that the gyro sensor 220 may be rotated about the second axis 232 by less than 180 degrees. In the gyro sensor platform 228 of Figure 4, the geared portion 240 is provided by a semi-circular projection. Alternative borehole survey instruments may utilise any other rotation mechanisms capable of allowing a rotation of the gyro sensor about the second axis by less than 180 degrees.

Figure 6 shows the same sensor module system 203 as shown in Figure 4. In Figure 6, the gyro platform 228 has been rotated about the second axis 232 to align the gyro sensing axis 230 with the longitudinal axis 224 of the borehole survey instrument (and sensor module housing 206). In this example the rotation about the second axis was 90°.

Gyro devices that meet the performance requirements for borehole surveying applications typically consist of a gyro sensor system comprising a gyro sensor and associated control electronics to operate the gyro sensor. For MEMS gyro sensor systems, the gyro sensor may comprise a MEMS gyro sensor. The MEMS gyro sensor may comprise a packaged micromachined resonant sensor structure. Control electronics to drive and detect the resonator motion and to provide the gyro output are also provided. For lower performance gyros the control electronics is typically implemented as an ASIC (Application Specific Integrated Circuit) which can be very small and is low cost when produced in high volumes. The ASIC and MEMS sensor (not shown) may be contained within the same package (for 25 example, in the embodiment shown in Figures 4 and 6). Due to the low signal levels and precision required, higher performing sensors are typically implemented using discrete electronics mounted on printed circuit boards, with the MEMS gyro sensor contained in a separate package. These control electronics are necessarily significantly larger and may be produced as rectangular boards with a width which fits within the diameter of the survey tool. 30 Rotation of such devices around the longitudinal axis of the survey instrument is relatively straightforward however these gyro sensor systems are not suitable for rotation around the orthogonal axes of the borehole survey instrument as the length of the boards exceeds the diameter of the survey instrument. This limitation may be overcome by separating the control electronics and gyro sensor with interconnections provided by means of a flexible cable.

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Figures 7 and 8 schematically depict a sensor module system 303 which forms part of a survey instrument (the rest of the survey instrument corresponds with the survey instrument of Figure 3 and is not depicted). Many of the features of the sensor module system 303 are similar to those described above in respect of the sensor module system 203 of Figures 4 and 6. As such, a description of these features is not given again here and corresponding reference numerals are used to identify them in Figures 7 and 8. Thus, 306 is the sensor module housing, 324 is the longitudinal axis of the survey instrument, 328 is the gyro sensor platform, and 332 is the second axis.

In this embodiment, the sensor module system 303 has a gyro sensor system 347 which comprises control electronics 344 and a gyro sensor 348 which are separated and interconnected by a flexible cable 346. The gyro sensor 348 is a MEMS gyro sensor 348. The control electronics 344 are mounted on a separate printed circuit board. The control electronics 344 may include the controller 2 depicted in Figure 5. As described above, in implementations where the rate sensor (in this case, a MEMS gyro sensor) is rotatable about the second axis 332 by less than 180 degrees, a relatively short flexible cable 346 may be utilised to connect the MEMS gyro sensor 348 and the control electronics 344. As such, more complex rotational arrangements that prevent the flexible cable becoming tangled during a rotation of significantly more than 180 degrees (e.g. 360 degrees) about a second axis do not need to be utilised.

In Figure 7, the sensing axis 330 of the MEMS gyro sensor 348 is perpendicular to the longitudinal axis 324 of the borehole survey instrument. In Figure 8, the sensing axis 330 is parallel to the longitudinal axis 324 of the borehole survey instrument. The MEMS gyro 25 sensor 348 is fixed into the gyro platform 328 which is coupled to the rotation mechanism 326. The printed circuit board comprising the control electronics 344 is fixed to the sensor module housing 306 and remains stationary as the rotation mechanism 326 rotates the MEMS gyro sensor 348. Electrical connection between the MEMS gyro sensor 348 and the control electronics 344 is maintained via the flexible cable 346.

Figure 8 shows the same sensor module system 303 as shown in Figure 7. In Figure 8, the gyro platform 328 has been rotated about the second axis 332 to align the gyro sensing axis 330 with the longitudinal axis 324 of the borehole survey instrument (and sensor module housing 306). This is a rotation of 90 degrees. The length of the flexible cable 346 is such 35 that when the gyro platform 328 is rotated about the second axis 332, the flexible cable 346

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2019201449 01 Mar 2019 is pulled taut (see Figure 8). In other embodiments, the flexible cable 346 may be longer such that it is not pulled taut. In other embodiments, the rotation of the gyro platform may be less than 90 degrees. Where this is the case, the flexible cable 346 may be shorter, i.e. not long enough to accommodate a rotation of 90 degrees. Rotation of the gyro platform by less than 90 degrees will cause less flexing of the flexible cable 346.

In another embodiment, the rotation mechanism may comprise a lead screw or worm drive arrangement as shown in Figures 9a and 9b. Figures 9a and 9b depict a sensor module system 903 which forms part of a survey instrument (the rest of the survey instrument corresponds with the survey instrument of Figure 3 and is not depicted). Many of the features of the sensor module system 903 depicted in Figures 9a and 9b are similar to those described above in respect of the sensor module system 203 of Figure 4 and the sensor module system 303 of Figures 7 and 8. As such, a description of these features is not given again here and corresponding reference numerals are used to identify them. Thus, 924 is the longitudinal axis of the survey instrument, 928 is the gyro sensor platform, 932 is the second axis, 944 are control electronics, and 948 is the gyro sensor.

In the embodiment of Figures 9a and 9b, the drive force is provided by means of a drive motor 950. The drive motor 950 may be mounted to a sensor module housing (not depicted). The drive motor 950 may have an axis of rotation generally parallel to the longitudinal axis 924 of the survey instrument. On actuation (e.g. when instructed by the controller 2), the drive motor 950 rotates a lead screw 952 which is coupled to the motor. The lead-screw 952 is engaged with a crank lever 954 such that rotation of the lead screw 954 moves the crank lever in an axial direction (which may be generally parallel with the 25 longitudinal axis 924 of the survey instrument). The crank lever 954 is coupled to the gyro sensor platform 928 (see Figures 9a and 9b) such that the gyro sensor platform 928 rotates about the second axis 932 on axial movement of the crank lever 954. Specifically, the coupling of the crank lever 954 to the gyro sensor platform 928 is offset from the second axis 932 about which the gyro sensor platform rotates.

Figure 9b shows the same arrangement as Figure 9a. However, in Figure 9b the gyro sensor platform 928 has been rotated about the second axis 932 by 90 degrees. The motor 950 has driven the lead-screw 952 to rotate, and this in turn has caused the crank lever 954 to move axially. The axial movement of the crank lever 954 has rotated the gyro sensor

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2019201449 01 Mar 2019 platform 928. A flexible cable 946, which carries signals to and from the gyro sensor platform 928 bends in order to accommodate the rotation of the gyro sensor platform.

Although in Figure 9b the gyro sensor platform 928 has been rotated about the second axis 932 by 90 degrees, the gyro sensor platform 928 may be rotated about the second axis 932 by any angle (provided that the length and flexibility of the flexible cable 946 can accommodate the rotation). The sensing platform 928 may be rotated about the second axis 932 by less than 90 degrees. This advantageously reduces the amount of flexing of the flexible cable 946, as is described further below. It may also allow a shorter flexible cable to be used.

As mentioned above, gyrocompassing is one method of conducting a borehole survey. In a gyrocompassing method, the borehole survey instrument is inserted into a borehole and measurements are taken at predetermined points along the borehole (for example, measurements may be taken every 10m), while the instrument is stationary.

The borehole survey instruments described above can be used to determine the azimuth of the borehole at discrete points in the borehole using the method described below. The survey instrument may be switched off between measurements to reduce power consumption. However this may introduce additional bias drift errors as the devices warms up and cools down during power cycles.

The below method is described with reference to Figures 7 and 8. However, it should be understood that the below method could similarly be applied to a borehole survey instrument 25 comprising the sensor module system 203 of Figures 4 and 6, or the sensor module system

903 of Figures 9a and 9b.

The MEMS gyro sensor 348 is rotated about the longitudinal axis 324 of the borehole survey instrument to determine a first horizontal earth rate component and the fixed bias drift term, 30 D, as outlined above. That is, data (Er1 and Er3) is collected along the MEMS gyro sensor’s sensing axis 330 at at least a first orientation and a second orientation. The first orientation and the second orientation may be separated by 180 degrees, with each of the first orientation and second orientation lying in a plane perpendicular to the longitudinal axis 324.

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In this instance the first horizontal earth rate component is most advantageously measured with the MEMS gyro sensor sensing axis 330 oriented substantially in the horizontal plane.

The horizontal plane is the plane parallel to the earth’s surface, or equivalently the plane perpendicular to a line which extends from the gyro sensor to the centre of the earth.

The output of inclination sensors, e.g. accelerometers or inclinometers, (not shown in Figures 7 and 8) may be used to determine the orientation of the sensor platform 328 with respect to the horizontal plane as previously described. The platform may then be rotated around the longitudinal axis 324 of the instrument to align the Er1 and Er3 measurement directions in the horizontal plane to optimise the measurement accuracy of the first horizontal earth rate component.

Alternatively a conventional four point gyrocompass can be performed which will provide first and second horizontal earth rate components. These may then be processed to determine the azimuth,¹/⁷ (the angle of the sensing axis of the gyro sensor with respect to true north) without the requirement to align the platform to optimise the horizontal measurement accuracy. This four point compass will however take longer to perform.

As described above, at angles of inclination of the borehole approaching horizontal, it is not possible to determine the second horizontal earth rate component solely by rotation of the MEMS gyro sensor 348 about the longitudinal axis 324.

For the measurement of the second horizontal earth rate component, the rotation mechanism 326 is actuated, by the controller, to rotate the MEMS gyro sensor 348 about the second axis 25 332.

The MEMS gyro sensor 348 may be rotated by 90 degrees about the second axis 332, such that the sensing axis 330 of the MEMS gyro sensor 348 becomes coincident with the longitudinal axis 324 (see Figure 8). In alternative embodiments, the MEMS gyro sensor 348 30 may be rotated by any angle less than 180 degrees about the second axis 332. The sensing axis 330 may be rotated into a position in which the second horizontal earth rate component may be accurately determined. For example, where the survey instrument is not precisely horizontal the gyro sensor sensing axis may be rotated such that it is in a substantially horizontal plane to optimise sensitivity to the horizontal earth rate. The rotation of the gyro 35 sensor 348 may be less than 90 degrees.

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Once the MEMS gyro sensor 348 has been rotated into a position in which the second horizontal earth rate component may be accurately determined, data is collected along the sensing axis 330 at a third orientation. The third orientation lies outside of the plane perpendicular to the longitudinal axis in which the first and second orientations are measured. In exemplary methods, data is collected along the sensing axis 330 in a third orientation parallel to the longitudinal axis 324. In exemplary methods, data is collected along the sensing axis in a plane parallel to the horizontal plane. The data may be collected along the sensing axis in some other plane which is not perpendicular to the longitudinal axis 324.

The fixed bias drift term, D, is determined from the first rotation of the MEMS gyro sensor 348 about the longitudinal axis 324 using method described above (that is, using the equation Ω₁ + Ω₃ = (Erl + D) + (-Erl + D) = 2. D). Therefore, there is no requirement to rotate the sensing axis 330 through 180 degrees about the second axis 332. This is because the measurement data t2₅ collected at the third orientation, can be corrected using the previously determined fixed bias drift term, D to give the value of Er5 (i.e. Er5 = Ω₅ - D). The second horizontal earth rate component can therefore be determined by obtaining only one measurement Er5 following rotation of the MEMS gyro sensor 348 about the second axis 332.

In the case where the third orientation is parallel to the longitudinal axis 324, Ψ can be determined by taking measurements at three orientations and using the equation:

/Erl — Er3

Ψ = atan ------— \ 2. Er5

In another embodiment, the third orientation is not parallel to the longitudinal axis 324. This may be the case for example if the gyro sensor 348 cannot be rotated through a full 90 degrees about the second axis (this may be difficult to achieve in some situations). Where 30 this is the case the azimuth Ψ can still be determined. The measurement t2₅ comprises three components. The first component is the previously determined fixed bias term, D. The second component is the Earth rate component parallel to the longitudinal axis, Er5. The third component is the Earth rate component in the direction parallel to the intersection between the plane orthogonal to the longitudinal axis 324 and the plane containing the

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2019201449 01 Mar 2019 longitudinal axis 324 and the sensing axis of the third orientation. This third component is referred to herein as ErO.

Typically, the direction of the third component ErO, is coincident with the direction of one of the previously determined components Er1, Er2, Er3 or Er4, so ErO is equal to the respective previously determined component. If the direction of the third component is not coincident with that any of the previously determined components, ErO can be determined from the previously determined components and the angle between the direction of the third component and the direction of any of the previously determined components.

Defining 0 as the angle of rotation about the second axis 332, the gyro sensor output at the third orientation, Ω5, is:

Ω₅ = D + Er5. sin(0) + ErO. cos(0)

Ω₅ — D — ErO. cos(0)

The azimuth Ψ can then be determined using the same equation as for the case where the third orientation is parallel to the longitudinal axis.

The accuracy of the earth rate measurements, Er1, Er3 and Er5, can be improved by increasing the measurement duration (which mitigates the effect of gyro noise) or by increasing the number of measurements taken at each orientation.

During the first rotation of the MEMS gyro sensor 348 (about the longitudinal axis 324) two 25 measurements are taken in order to determine Er1 and Er3. In contrast, a single measurement, Er5, is taken at the third orientation when the MEMS gyro sensor 348 is rotated about the second axis 332. As such, the time taken to collect data during the rotation about the longitudinal axis is greater than the time taken to collect data when the MEMS gyro sensor 348 is rotated about the second axis 332. The process by which Er5 is obtained 30 would therefore be less accurate if the same time was used for each earth rate measurement. This may be mitigated by increasing the duration of the Er5 measurement to provide comparable noise levels for the two earth rate component values. Alternatively, two or more measurements at the third orientation may be taken in order to increase the accuracy of the data collected at the third orientation. One or more measurements at a

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2019201449 01 Mar 2019 fourth orientation may be taken. The fourth orientation may also not be parallel to the longitudinal axis and not perpendicular to the longitudinal axis. Although measurements at a fourth orientation (or more orientations) may be used, in practice no particular benefit is provided from doing so. The third orientation is preferably the largest achievable rotation of the gyro sensor 348 about the second axis towards the longitudinal direction. This is because the largest achievable rotation will give the greatest signal to noise ratio for the measurement at the third orientation. Given this, moving to a fourth orientation, which would correspond to a smaller achievable rotation towards the longitudinal direction and thus a lower signal to noise ratio, is not desirable.

In an alternative embodiment the second horizontal earth rate component may be determined without using a previously obtained bias measurement. This may be done by using third and fourth orientations whose rotation angles are equal and opposite to each other. This is set out below with reference to Figure 10, which shows the sensor module system 903 of Figures 9a and 9b. However, the below method could similarly be applied to a borehole survey instrument comprising the sensor module system 203 of Figures 4 and 6 or the sensor module system 303 of Figures 7 and 8.

In one embodiment, the gyro sensor 948 may be rotated about the second axis 932 to a third orientation located at any angle +0 from an original position. The original position may be a position in which the sensing axis of the gyro sensor 948 is perpendicular to the longitudinal axis 924, as shown in Figure 10. That is, the position prior to rotation about the second axis 932. The third orientation may lie outside of the plane perpendicular to the longitudinal axis in which the measurements at the first and second orientations were collected.

In this case, as noted above the gyro sensor output at the third orientation, Ω₅, is:

Ω₅ = D + Er5. sin(0) + ErO. cos(0)

The gyro sensor 948 may be rotated about the second axis 932 to a fourth orientation 30 located at an angle of -0 with respect to the original position (see Figure 10). The gyro sensor may be rotated in the opposite direction to the direction of rotation utilised to reach the third orientation. In this case, the gyro sensor is rotated in the opposite direction by an angle of 20. The third and fourth orientations are therefore separated by an angle of 20. The angle of the fourth orientation is equal and opposite to the angle of the third orientation (with 35 respect to the longitudinal axis 924).

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In this case, the gyro output at the fourth orientation Ωθ is:

Ω₆ = D + Er5.sin(—θ') + Er0.cos(—Θ)

Subtracting the gyro sensor output at the fourth orientation from the gyro sensor output at the third orientation gives:

Ω₅ — Ω₆ =Er5.sin(0)+ ErO. cos(0) + D — Er5. sin(—Θ) — ErO. cos(—Θ) — D

From which, Er5 can be determined:

„ _c (^5 ^— kl₆) ^ErS= 2. si„(e)

As such, the second horizontal earth rate component can be determined if Θ is known and without needing to determine the bias and the third rate component ErO.

Thus, the second horizontal earth rate component can be determined from gyro sensor measurements in third and fourth orientations that are not 180° apart. For example, the second horizontal earth rate may be determined by rotating the gyro sensor 948 through an angle of ±30 degrees. In this particular case, where θ = 30°:

ErE> = Ω₅ — Ω₆.

Rotating to orientations separated by less than 180 degrees is advantageous as it reduces the amount of flexure of the flexible cable 946. Flexure of the cable may introduce an inaccuracy into the output of the system. As such, Θ may be substantially any value less than 90 degrees. Additionally, the same quality of measurement of Er5 can be obtained as would be obtained by rotating through 90° from the original position.

The angle of rotation does however have an impact on the resolution of the measurement obtained and the resolution decreases as Θ decreases, therefore increasing uncertainty. The resolution may scale as a function of sin(0). Therefore, it may be advantageous for θ to be at least 30 degrees, in which case resolution is reduced by no more than 50% (although θ of less than 30 degrees may be used). Since resolution scales as a function of sin(0), having θ

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2019201449 01 Mar 2019 greater than 60 degrees will provide only a small potential increase of measurement resolution, whilst at the same time requiring a large range of flexure of the flexible cable 946 (which may itself reduce measurement resolution). For this reason, it may be advantageous for θ to be no more than 60 degrees (although angles greater than 60 degrees may be used).

Optionally, the same technique may be used to determine the first horizontal earth rate component. That is, the gyro sensor 948 may be rotated about the longitudinal axis 924 to the first orientation located at any angle +0 from an initial position. The initial position may be an intersection between a vertical plane and the plane which is perpendicular to the longitudinal axis. The gyro sensor 948 may then be rotated about the longitudinal axis 924 to the second orientation located at an angle of -0 with respect to the initial position. Again, by rotating the gyro sensor about the longitudinal axis 924 by equal and opposite angles less than 90 degrees, the angular difference between the first and second orientations can be less than 180 degrees. Rotation of the gyro sensor about the longitudinal axis 924 by equal and opposite angles of less than 90 degrees allows determination of the first horizontal earth rate component.

If determination of the gyro sensor bias is required, enough measurements should be taken which allow separation of the bias from the earth rate components. This may be two measurements in the plane perpendicular to the longitudinal axis which are separated by 180 degrees (as explained above). Alternatively, it may be at least three measurements separated by known angles in the plane perpendicular to the longitudinal axis. The known angles of separation should equal 360 degrees when combined together. The known angles of separation may be equal to each other. Although this is not essential it provides the 25 lowest uncertainty for the bias and the earth rate components. It also simplifies calculation of the bias (the trigonomics used is simpler). In a further alternative approach the bias may be determined by continually measuring the rate sensor output while rotating its sensing axis about the longitudinal axis (as mentioned further above).

In the above described embodiment first and second measurements with the sensing axis separated by 180 degrees in the plane perpendicular to the longitudinal axis are used. The sensing axis is then rotated about an axis perpendicular to the longitudinal axis and a further measurement is performed. This further measurement is referred to above as the third measurement. However, as mentioned above, more than two measurements in the plane 35 perpendicular to the longitudinal axis may be performed. Where this is the case the further

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2019201449 01 Mar 2019 measurement could be considered not to be a third measurement. For this reason, in this document when the term “third measurement” is used this may be interpreted as meaning “further measurement”. Similarly, the term “third orientation” may be interpreted as meaning “further orientation”. In a similar manner the term “fourth measurement” may be interpreted as meaning “additional further measurement”, and the term “fourth orientation” may be interpreted as meaning “additional further orientation”.

As explained above, measurement resolution may be reduced as the size of the angle θ reduces. Because rotation about the longitudinal axis 924 is relatively easily achieved, it may be advantageous to use θ of 180 degrees.

Alternative borehole survey instruments may comprise a further gyro sensor, which may be a MEMS gyro sensor, which is only rotatable about the longitudinal axis 324. This gives the advantage of reducing noise on measurements taken during the initial rotation about longitudinal axis by 1A/n, where n is the number of sensors rotatable about the longitudinal axis. Similarly, exemplary borehole survey instruments may comprise further gyro sensors capable of being rotated about the second axis 332. This will similarly have the effect of further reducing the noise and providing multiple measurements of earth’s rate.

Figure 11 shows a sensor module system 1103 according to another embodiment of the invention. Many features of the sensor module system 1103 are similar to the sensor module system 303 of Figures 8 and 9, and as such a description of these features is not given again here and corresponding reference numerals are used to identify them. Thus 1128 is the gyro sensor platform, 1124 is the longitudinal axis, 1126 is the rotation 25 mechanism and 1132 is the second axis.

Borehole survey instruments may further comprise an inclination sensor 1150 coupled to the rotation mechanism 1126. The inclination sensor may be an accelerometer, which may be a single or multi-axis device with an accelerometer sensing axis coincident with the sensing 30 axis 1130 of the MEMS gyro sensor (not visible).

The inclination sensor 1150 may be coupled to the rotation mechanism 1126 such that rotation of the inclination sensor 1150 about the second axis 1132 is possible on actuation of the rotation mechanism 1126. In the embodiment of Figure 11, the inclination sensor 1150 is 35 mounted on the gyro sensor platform 1128. For example, the inclination sensor 1150 may

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2019201449 01 Mar 2019 be mounted on a printed circuit board 1152, which is in turn mounted on the gyro sensor platform 1128. In this implementation the MEMS gyro sensor (not visible), is electrically connected to the circuit board 1152 on a first side, and the inclination sensor 1150 is electrically connected to the circuit board 1152 on a second side. This circuit board 1152 is electrically connected to gyro control electronics 1144 via flexible cable 1146. Some of the components of the control electronics 1144 may be relocated to the circuit board 1152 allowing the size of printed circuit board of the control electronics 1144 to be reduced. The control electronics 1144 may include the controller 2 depicted in Figure 5.

The inclination sensor 1150 may be rotated about the second axis 1132 to determine the orientation to which the MEMS gyro sensor (not visible) would need to be rotated to align the gyro sensor sensing axis 1130 with the horizontal plane. This has the advantage of ensuring that the maximum earth’s horizontal rate would be measured by the gyro sensor, maximising the signal to noise level. In alternative borehole survey instruments, alternative tilt sensors, inclination sensors or encoders may be utilised to determine the angle that the gyro sensor should be rotated.

In the above described embodiments the calculation of the borehole survey instrument direction at inclination angles around horizontal may be based upon multiple measurements of the earth’s rate of rotation, using both axial and cross axial components.

An advantage of the borehole survey instrument described above is that with the single gyro sensor (comprising a single sensing axis) which is rotatable about a second axis, a compact and low cost instrument maybe realised that not only has all angle gyrocompass capability, 25 but also has the ability to use the axial (longitudinal) rate measurement, as a means to inertially roll stabilise the sensor module platform 228, 328, 428. This enables the instrument to have different survey modes which the instrument may switch between. The survey modes may be optimised for the geometry of the borehole at any particular time. This may include the ability to take continuous measurements from vertical to horizontal, eliminating 30 the need to stop at discrete points in the borehole to establish a gyrocompass reference.

The gyrocompass mode has the advantage that it provides an absolute and accurate measurement of the azimuth angle at each survey point. It does however require the survey instrument to be stationary during the gyrocompass process, and this results in a relatively 35 elongated overall survey duration. Alternative continuous survey modes are known which

11130850_1 (GHMatters) P110785.AU

2019201449 01 Mar 2019 allow survey data to be taken while the survey instrument is in motion along the length of the borehole. These techniques are generally quicker and provide particular advantages for specific geometries or inclination angles within the borehole.

Continuous mode techniques determine relative changes in azimuth as the survey instrument is conveyed along the borehole. As such, they require an absolute azimuth reference at some point during the survey. This may be from a gyrocompass survey. The continuous mode techniques also require the gyro bias to be measured and calibrated to remove an offset, as the rate output is either integrated directly to provide azimuth angle or used to inertially stabilise the sensing module rotation. In this case, the total bias is the sum of bias error of the gyro sensor and the Earth rate component parallel to the sensing axis of the gyro sensor. The total bias may be measured during an initial gyrocompass survey. Alternatively, the total bias may be measured by holding the instrument stationary and determining the average output from the gyro sensor over a fixed period (e.g. around 15 seconds). Since the gyro sensor is also stationary, its output is the sum of its fixed bias error and the Earth rate component parallel to its sensing axis. This process may be repeated occasionally during the survey process to obtain updates to the total bias, which may change as the temperature of the gyro sensor changes or because of changes in the Earth rate component due to changes in the direction of the tool.

For borehole inclination angles that are relatively close to horizontal the above described borehole survey instrument may be operated in a so called gravity stabilised mode in which the rate sensor is aligned with its sensing axis 230, 330 perpendicular to the instrument longitudinal axis 224, 324, 924 as shown in Figures 4,7 and 9a. This mode may also be 25 referred to as a ‘high side’ mode. The ‘high side’ is the angular direction of the upward vertical in the XY plane of the survey instrument (where the Z direction is the longitudinal axis of the survey instrument). In exemplary methods, the borehole survey instrument may operate in the ‘high side’ gravity stabilised mode when the inclination of the borehole is within a gravity stabilised mode range. A Gravity roll angle from vertical is often referred to as High 30 Side angle.

In exemplary methods, the gravity stabilised mode range may be used If the inclination of the borehole is between -45° and +45° (where vertically downwards is defined as -90° and horizontal as 0°). Alternative borehole survey instruments may be configured to operate in

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2019201449 01 Mar 2019 the gravity stabilised mode when the determined inclination angle of the borehole is between

-70° and +70°.

The gravity stabilised mode will now be described with reference to Figure 3. In the gravity stabilised mode, the output from the inclination sensor 223 in the sensor module housing 206 is used to maintain the sensor module 204 orientation in the XY plane at a constant angle (where Z is along the longitudinal axis 224 of the borehole instrument). . That is, the orientation of the sensor module housing 206 within the borehole survey instrument needs to be such that at all points during the survey, the gyro sensor sensing axis 230 is maintained in a direction as close to vertical as possible within the XY plane (if the borehole survey instrument is in a horizontal borehole, the sensing axis 112 would be maintained in a vertical direction in the XY plane). During the survey, as the borehole survey instrument moves through the borehole, the borehole survey instrument itself may rotate, altering the direction of the rate sensor sensing axis within the XY plane away from vertical. The data collected by the inclination sensor 223 can be used to feedback information on the rotation of the borehole survey instrument, and the rotation drive means 208 is actuated by a controller to rotate the sensor module housing 206 about the longitudinal axis 224 of the borehole survey instrument to maintain the rate sensor sensing axis in the direction as close to vertical as possible within the XY plane. As a result, the inclination sensor with its sensing axis aligned orthogonally to both the rate sensor sensing axis and longitudinal axis is zero i.e. it is aligned horizontally.

For a perfectly horizontal borehole the rate sensor sensing axis would therefore be aligned vertically and would directly measure any rotation, Ω_Η, in the borehole angular direction in the 25 horizontal earth plane. The rate output from the rate sensor may then be integrated to provide a measurement of the incremental angle change of the borehole in the horizontal plane.

Any deviation in the inclination angle, Θ, of the wellbore from horizontal will decrease the 30 measurement sensitivity of the rate sensor, as the rate sensor will only sense a component, Or, of the rotation rate Ωη, given by:

Ω_β = Ω_Η cos Θ

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This mode therefore works optimally for angles close to horizontal and is not applicable at angles close to vertical. For example, the gravity stabilised mode may be used at angles within ±70° of horizontal, or even at angles within ±80° of horizontal in some instances. The inclination angle, Θ, may be derived using the accelerometer outputs Gx, Gy and Gz as follows:

2019201449 01 Mar 2019

Where, ^gr ⁼ J^Gx + ^GY

This enables the value of Ωη, and hence the angle increment, to be derived.

The requirements for the gyro sensor when used in a gyrocompass mode are only for relatively low input rates (typically <±0.1deg/sec) as the survey measurements are performed with the gyro sensor and the borehole survey instrument stationary. This is in contrast with the continuous survey modes, where the gyro may be subjected to transient high rotation rates which could exceed ±100deg/s in borehole paths which are highly deviated and the survey instrument is moving at high speed. This may be the case in surveys where the tool is dropped and allowed to freefall down the hole. When used in a gravity stabilised mode, the above described borehole survey instruments provide the advantage of enabling the angular orientation of the gyro sensor sensing axis to be adjusted with respect to the longitudinal axis, by rotating the sensing axis about the second axis 232. The gyro sensor will then only sense a component, ω_κ, of the rotation rate around the high side axis, ω, given by:

a>_R = ω. sin(y)

Where γ is the angle between the gyro sensing axis and the instrument axis 224. This effectively extends the range of operation in this mode to higher input rates.

Continuous survey techniques may also be applied for surveying boreholes at inclination angles around vertical using a ‘roll stabilised’ operating mode. In exemplary methods, the borehole survey instrument may operate in the ‘roll stabilised’ operating mode when the inclination of the borehole is within a roll stabilised range. In exemplary methods, the roll

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2019201449 01 Mar 2019 stabilised mode range may be used if the inclination angle of the borehole is between -90° and -45° (or between +90° and +45° for an upwardly inclined borehole). Alternative borehole survey instruments may be configured to operate in the roll stabilised mode when the determined inclination angle is between -90° and -20° (or between +90° and +20° for an upwardly inclined borehole).

In the ‘roll stabilised’ operating mode, the survey instrument may be initially configured as shown in Figure 6 or 9b, with the rate sensor sensing axis initially aligned to the longitudinal axis 224, 324. In this mode the rate sensor is used to measure the rotation rate of the sensor module housing. The output of the rate sensor is fed back to the controller, which controls the rotation mechanism to rotate the sensor module such that the output of the rate sensor is driven to a fixed target value. Typically, this target value is the previously measured gyro sensor bias which means that the sensor module does not rotate when the borehole survey instrument is stationary. Alternatively, this target value may be zero, which means that the sensor module will rotate at a rate equal and opposite to the measured gyro sensor bias when the borehole survey instrument is stationary. The subsequent data processing would need to apply a correction for this fixed rate of rotation.

If the borehole survey instrument is rotated about its longitudinal axis 224, through some angle, the rotation feedback mechanism causes the sensor module to rotate by an equal and opposite angle with respect to the instrument housing and thus remain static. If however, the borehole survey instrument is inclined from vertical and its azimuth is changed, that is it is rotated about the vertical, the gyro sensor will only measure the component of the azimuth change that is in its sensing axis. This is equal to the azimuth change multiplied by the 25 cosine of the angle between the longitudinal axis and the vertical. Thus, the rotation feedback mechanism will cause the sensor module to rotate with respect to the instrument housing by an opposite angle that is less than the azimuth angle change.

For a constant inclination angle from vertical Θ, the X and Y axis accelerometer outputs, Gx 30 and Gy, would vary sinusoidally as the azimuth angle rotates. These outputs may be used to derive the high side angle, a, using the equation:

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The incremental changes in high side angle, Δα, may be used to derive changes in the

2019201449 01 Mar 2019 azimuth angle, ΔΆ where:

ΔΨ =

Δα sin0

The initial azimuth angle, Ψο, may be measured using a gyro compass survey and therefore the azimuth angle Ψν is given by:

Ar/

Ψ_Ν=Ψ_Ν.₁+11Ψ= Ψν-₁+—^.

The survey instrument described is capable of being configured to operate in modes optimised for both vertical and horizontal surveying. It is also operable in both gyrocompass and continuous modes. The survey instrument may be switched, under the control of the controller, between modes during the course of a borehole survey to optimise the accuracy for the geometry of the borehole at any given point, based on measurements provided by the accelerometers. When switching between modes using the controller it is advantageous to extend the angular operating range of the respective roll and gravity stabilised modes such that the switching does not occur at the ±45° inclination points. For example, if the borehole is initially near vertical (i.e. close to +90°) and operating in a roll stabilised mode and changes inclination towards horizontal, the switching from the roll stabilised to gravity stabilised mode can be set at +40°. If the inclination angle subsequently changes back towards vertical the switching back to roll stabilised mode can be set to occur at +50°. This ‘hysteresis loop’ implementation avoids the possibility of rapid multiple changes occurring around +45° for repeated small positive and negative inclination angle changes which might otherwise induce short interruptions to the survey and consume power unnecessarily. Other 25 values for the hysteresis loop may be used. The values may be centred around 45°.

In embodiments of the invention the rate sensor (e.g. MEMS gyro sensor) is rotatable about a second axis by an angle of less than 180 degrees. This is advantageous because rotation of 180 degrees or more about the second axis is difficult to achieve in practice. Rotation of 30 180 degrees or more may require brushes or slip-rings to maintain electrical contacts during the rotation. Rotations of less than 180 degrees may be achieved without requiring brushes or slip-rings. In embodiments of the invention the rate sensor (e.g. MEMS gyro sensor) may for example be rotatable about a second axis by an angle of up to 170 degrees, or for

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2019201449 01 Mar 2019 example by an angle of up to 140 degrees. In embodiments of the invention the rate sensor (e.g. MEMS gyro sensor) may for example be rotatable about a second axis by an angle of degrees or more.

While references have been made herein to a controller or controllers it will be appreciated that control functionality described herein can be provided by one or more controllers. Such controllers can take any suitable form. For example control may be provided by one or more appropriately programmed microprocessors (having associated storage for program code, such storage including volatile and/or non-volatile storage). Alternatively or additionally control may be provided by other control hardware such as, but not limited to, application specific integrated circuits (ASICs) and/or one or more appropriately configured field programmable gate arrays (FPGAs).

The skilled person will be able to envisage other borehole survey instruments and methods and features thereof without departing from the scope of the appended claims.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense,

i.e. to specify the presence of the stated features but not to preclude the presence or addition 25 of further features in various embodiments of the invention.

Claims (20)

1. CLAIMS:

   1. A method for determining an azimuth of a borehole using a borehole survey instrument, the borehole survey instrument comprising a rate sensor having a sensing axis, wherein the sensing axis is perpendicular to a longitudinal axis of the borehole survey instrument, the method comprising:

   determining a first horizontal earth rate component and a fixed bias drift term of the rate sensor by rotating the rate sensor about the longitudinal axis of the borehole survey instrument and collecting data along the sensing axis at at least two different orientations which lie in a plane perpendicular to the longitudinal axis, rotating the rate sensor about a second axis and collecting data along the sensing axis at a further orientation lying outside of the plane perpendicular to the longitudinal axis, determining a second horizontal earth rate component using the previously determined fixed bias drift term and the data collected at the further orientation, calculating the azimuth using the first horizontal earth rate component and the second horizontal earth rate component.
2. 2. A method according to claim 1, wherein the second earth rate component is determined without collecting data along the sensing axis at an additional further orientation.
3. 3. A method according to claim 1 or 2, wherein the second earth rate component is determined by only collecting data along the sensing axis at the further orientation.
4. 4. A method according to any one of the preceding claims, wherein when the rate sensor is in the further orientation the sensing axis is parallel to the longitudinal axis.
5. 5. A method according to any one of claims 1 to 3, wherein when the rate sensor is in the further orientation the sensing axis is not parallel to the longitudinal axis, and wherein determining the second horizontal earth rate component further comprises removing a component perpendicular to the longitudinal axis by using the data collected when the sensing axis was in the plane perpendicular to the longitudinal axis.
6. 6. A method for determining an azimuth of a borehole using a borehole survey instrument, the borehole survey instrument comprising a rate sensor having a sensing axis, the method comprising:

   determining a first horizontal earth rate component by rotating the rate sensor about a longitudinal axis of the borehole survey instrument and collecting data along the sensing axis for two or more orientations of the sensing axis which lie in a plane perpendicular to the longitudinal axis, determining a second horizontal earth rate component by rotating the rate sensor about a second axis which is perpendicular to the longitudinal axis, and collecting data along the sensing axis at a pair of orientations comprising a further orientation and an additional further orientation, wherein the further orientation is at an angle of +Θ degrees from the longitudinal axis and wherein the additional further orientation is at an angle of -Θ from the longitudinal axis, wherein -0 is equal and opposite to + 0 with respect to the longitudinal axis, and wherein 0 is less than 90 degrees, and calculating the azimuth using the first horizontal earth rate component and the second horizontal earth rate component.
7. 7. A method according to claim 6, wherein determining the second horizontal earth rate component comprises subtracting the data collected at one of the further orientation and the additional further orientation from the other of the further orientation and the additional further orientation.
8. 8. A method according to any one of the preceding claims, wherein the two or more orientations perpendicular to the longitudinal axis comprise a first orientation and a second orientation which are separated by 180 degrees.
9. 9. A method according to any one of claims 6 to 8, wherein the first horizontal earth rate component is determined before the second horizontal earth rate component.
10. 10. A method according to any one of the preceding claims, wherein the borehole survey instrument further comprises an inclination sensor, and the method further comprising:

    collecting, by the inclination sensor, inclination data indicative of the inclination of the borehole, and determining an inclination angle of the borehole using the inclination data.
11. 11. A method according to claim 10, wherein the rate sensor is rotated about the second axis to collect data in the further orientation if the determined inclination angle falls within a horizontal mode range, and wherein, for subsequent azimuth measurements, if the inclination angle falls outside of the horizontal mode range, the borehole survey instrument operates in a vertical mode, in which the second horizontal earth rate component is determined by rotating the rate sensor about the longitudinal axis of the borehole survey instrument, and collecting data at further orientations within the plane perpendicular to the longitudinal axis.
12. 12. A method according to claim 10 or 11, wherein the method further comprises determining, using the inclination data, the orientation of a horizontal plane, and wherein the method further comprises setting the locations of first and second orientations in the plane perpendicular to the longitudinal axis such that the data is collected in the horizontal plane.
13. 13. A method according to claim 11, wherein the rate sensor is rotated about the second axis to align the sensing axis such that the further orientation is in the horizontal plane.
14. 14. A borehole survey instrument for use in determining an azimuth of a borehole, the borehole survey instrument comprising:

    a rate sensor enclosed in a housing and having a sensing axis along which data is collected, wherein the sensing axis is perpendicular to a longitudinal axis of the housing, a rotation drive means coupled to the rate sensor and configured on actuation to rotate the rate sensor about the longitudinal axis to allow data to be collected along the sensing axis at at least two different orientations which lie in a plane perpendicular to the longitudinal axis, and a rotation mechanism coupled to the rate sensor and comprising a controller configured to actuate the rotation mechanism to rotate the rate sensor about a second axis by less than 180 degrees, such that data can be collected along the sensing axis at a further orientation, the further orientation lying outside of the plane perpendicular to the longitudinal axis.
15. 15. A borehole survey instrument according to claim 14, wherein the further orientation is located at an angle of + 0 degrees from the longitudinal axis, and wherein 0 is less than 90 degrees.
16. 16. A borehole survey instrument according to claim 15, wherein the rotation mechanism is further configured, under actuation by the controller, to rotate the rate sensor about the second axis to an additional further orientation located at -0 degrees from the longitudinal axis, wherein - 0 is equal and opposite to +0.
17. 17. A borehole survey instrument according to any one of claims 14 to 16, wherein the two or more orientations perpendicular to the longitudinal axis comprise a first orientation and a second orientation which are separated by 180 degrees.
18. 18. A borehole survey instrument according to any one of claims 14 to 17, wherein the two or more orientations perpendicular to the longitudinal axis comprise a first orientation located at an angle of +0 degrees from an initial position and a second orientation located at an angle of -0 degrees from an initial position, and wherein 0 is less than 90 degrees.
19. 19. A borehole survey instrument according to any one of claims 14 to 18, further comprising a three-axis inclination sensor having an inclination sensing axis perpendicular to the longitudinal axis of the borehole instrument and configured to collect inclination data indicative of the inclination angle of the borehole.
20. 20. A method according to any one of claims 10 to 13, wherein the borehole survey instrument is operable to continuously measure the azimuth of the borehole in a roll stabilised or a gravity stabilised mode depending on an inclination angle of the borehole, the method further comprising:

    collecting, by the inclination sensor, inclination data indicative of the inclination angle of the borehole, and using the inclination data to change the orientation of the rate sensor sensing axis depending on the inclination angle, wherein, if the inclination data indicates that the inclination angle is sufficiently close to horizontal to be operated in a gravity stabilised mode, the rate sensor sensing axis is oriented and held in a plane perpendicular to a longitudinal axis of the borehole survey instrument, and wherein if the inclination data indicates that the inclination angle

    2019201449 19 Apr 2020 is sufficiently close to the vertical to be operated in a roll stabilised mode, the rate sensor sensing axis is aligned to the longitudinal axis and then rotated as the azimuth of the borehole rotates to cancel outputs of the rate sensor.