BT2 user manual - H-5911-9004-02-B Page | 10
3 Boretrak2 deployments – gyro introduction
The Boretrak2 is fitted with an Inertial Measurement Unit (IMU) which incorporates a 3-axis MEMS-based gyro and
a 3-axis accelerometer.
The IMU integrates information from both the gyro and the accelerometer, and the output orientation is determined
by fusing the outputs of both these sensors through a software-based Kalman filter model.
The advanced IMU solution ensures that the attitude and orientation of the Boretrak2 probe can always be tracked
as it is deployed along a borehole, whether it is pointing up, down or horizontal.
3.1 Gyro drift
The results from a gyro may drift with time. If you lay a gyro flat and absolutely still, it will output a heading. If you
leave the gyro in exactly the same position, the raw heading that it outputs will gradually change, even though the
gyro is not changing position or rotating. This is ‘drift’ and it is effectively an error that will affect raw data output
from a gyro.
The results from a gyro may also drift with temperature, with severe temperature changes causing greater drift in
the gyro output.
Processing in the Boretrak2 firmware and Carlson Boretrak software helps to mitigate these effects on the gyro in
the installed IMU. The raw gyro drift over a given time is kept to a minimum and constant checks on the drift
throughout the deployment ensure that temperature changes are mostly accounted for. If the accelerometers detect
that the Boretrak2 is held static at any point, then the gyro is effectively locked so that any drift during this period is
ignored.
However, from the moment the Boretrak2 is first aligned, the accuracy of the gyro is slowly deteriorating due to an
accumulating angular error. This means that the deployment operation should be as prompt and efficient as
possible.
In ideal circumstances, during operations, the drift of the gyro will be <1.0° per 30 minutes.
Each time the Boretrak2 is aligned in a known orientation, any error is reset to zero. Therefore, it is worthwhile
realigning the probe as often as is practically possible during field operations.
3.2 Gyro alignment
The gyro in the Boretrak2 does not output a heading with respect to true north. The gyro can measure the angle
that it is rotated through as it is deployed or as it is moved around on the surface, but it cannot determine which
direction it is pointing in when it is switched on.
For this reason, we must manually define the starting, ‘reference’ heading of the probe at the beginning of an
operation. Once we have determined this starting orientation, the gyro can then keep track of the changing heading
as it is moved around and deployed.
After the Boretrak2 is powered ON, and before it is deployed, the unit must undergo an alignment which is when
this starting heading is established.
During the alignment the probe should be located within the operations area, ready for the deployment to
commence. The probe should be laid out horizontally (roughly +/- 20° from horizontal) on a solid, stable surface. An
alignment jig is supplied to provide an easy mounting point for the probe.
The alignment lasts approximately 3 seconds. During this period, the Boretrak2 probe must be motionless. Site the
probe away from areas where knocks or excessive vibration may occur. If movement is detected, the alignment
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