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For instance, the measurement of angular rate provided by a gyroscope will include: • a bias component which is predictable and is present each time the sensor is switched on and can therefore be corrected following calibration • a temperature dependent bias component which can be corrected with suitable calibration • a random bias which varies from gyroscope switch-on to switch-on but is constant for any one run • an in-run random bias which varies throughout a run; the precise form of this error varies from one type of sensor to another. The fixed components of error, and to a large extent the temperature induced variations, can be corrected to leave residual errors attributable to switch-on to switch-on variation and in-run effects, i.e. the random effects caused by instabilities within the gyroscope. It is mainly the switch-on to switch-on and in-run variations which influence the performance of the survey system in which the sensors are installed. Gyro mass unbalance The performance of a mechanical gyroscope is extremely sensitive to mass unbalance in the rotor suspension, i.e. non- coincidence of the rotor centre of gravity and the centre of the suspension mechanism. Minute mechanical changes sufficient to affect gyro performance can arise as a result of shock and vibration to which the survey tool may be subjected; either down-hole or at surface as a result of knocks sustained during transport and surface handling. Movements of the rotor centre of gravity with respect to the suspension mechanism of a few nano-metres will produce changes is mass unbalance that are sufficient to give rise to significant changes in measurement accuracy.
Summary : Variations in the residual systematic bias components and the g-dependent bias caused by changes in mass- unbalance present the major concern in survey systems incorporating mechanical gyroscopes. Survey correction techniques are implemented, either during or after a survey operation, in order to compensate for survey inaccuracies resulting from the effects of these particular gyro errors.
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