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Figure 61: Straight plot of sensor readings.
In the field, the driller could observe raw sensor readings at several toolfaces (a rotational shot or cluster shot) and record the values on the x and y axes.
Figure 62: Adding a mathematical sine wave to the sensor readings.
If a mathematical sine wave was then made to fit this variation, the average value could then be derived as if the readings had been taken evenly around the clock.
In this example, the magnetometer is clearly carrying a positive magnetic field value that is not going away.
This rotational data can then be used to correct the x and y magnetometer values for this bias. This still leaves the potential for a further magnetic field bias on the z axis. In practice the z axis is often the most significant direction of influence since the significant magnetic material in the drill string is always above or below the MWD sensors. Clearly it is not possible to flip the z axis so the clean z axis is derived from the following formula:
In this formula, Bt means the total background magnetic field strength and Bx By and Bz mean the sensor axes readings from the magnetometer. This formula relies on the accuracy of Bt and ideally should have an IFR survey to accurately measure Bt. This simple technique allows the surveyor to calculate bias values for all three axes and remove the majority of the magnetic interference from the subsequent observations.
One caveat that should be kept in mind is the stability of the calculation. The value for Bz is very sensitive to azimuth and inclination.
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