The vast majority of gyro surveying performed for the industry, even today, still utilises rotor systems. A spinning gyro rotor tends to keep its axis pointing in the same direction. This is called Gyro Rigidity. If a force is applied which tends to change the direction of the spin axis, the axis will move at right angles to the direction of the applied force. If the spin axis is horizontal and you try to tilt it, the axis will turn. If the axis is horizontal and you try to turn it, the spin axis will tilt. This second characteristic of a gyro is called Precession.
In normal operational use, conditions such as bearing wear, Temperature Coefficients of Expansion (Inertial Mass Distribution – C of G) and System Attitude Change (related to a given well profile) all interact to generate a net force which acting on the Gyro Spin Axis cause the gyro to precess (drift from its initial orientation reference).
The gyro precession experienced during a survey has historically been corrected with a Drift Curve constructed with drift data samples recorded during the in-run and outrun survey. The frequency and duration of drift checks has tended to change over time but the basic premise has always been to take samples related to time, attitude change and temperature (particularly for deeper, hotter surveys). Drift checks were normally taken at least every 15 minutes, for a sample duration of 5 minutes with film systems. However, with the introduction of later Vernier scale readings, SRG and digital data, sampling criteria tended to change to 10 minute intervals with 3 minute sample duration.
Drift Checks during the survey were no longer relevant or required with the advent of earlier discreet sampling North Seeking Systems (more later). However, they are beneficial and recommended when operating the current North Seeking Systems in continuous Dynamic Mode where the individual or calculated survey sample stations are not determined by discreet north seek sensing.
The drift correction data is then either applied on its own or as a super set to the Gyro Calibration Model (current systems). The calculated real-time drift correction curve is tied and closed to the start and end reference data, be it a sight observation or north seek reference as in current technology systems
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