Appendix C – Axial and Cross Axial Magnetic Correction

Axial correction or Short Collar Correction (SCC) is used with magnetic survey tools when there is insufficient non-magnetic material above or below the sensor. A magnetic survey tool has magnetometers measuring the Earth’s magnetic field in three axes, X, Y and Z. The Z axis is assumed to be aligned in the along hole direction of the drill string. The sensor must be surrounded by non-magnetic material which shields the X and Y magnetometers from magnetic interference. In normal drilling operations usually enough non-magnetic material above and below the sensor is incorporated to magnetically isolate it from the steel components within the drill string.

This however is not always possible, resulting in magnetic interference in the Z axis magnetometer. This can be corrected by using a mathematical algorithm. This algorithm assumes that the value for the X and Y axis magnetometers are correct and using values for dip angle and HL for the surface location, the Z axis magnetometer can be corrected. There are, however, limits to when this correction can be used and these are listed below.

Inclination Range (Deg)	Azimuth Range (Deg)	Limitations
0-60	0-360	No Limitations
60-90	0-60, 120-240, 300-360	No Limitations
60-90	60-70, 110-120, 240-250, 290-300	See Note (1)
60-90	70-110, 250-290	See Note (2)

• Note 1 – If possible, SCC should NOT be used and the BHA should be correctly spaced with non-magnetic tubulars as required.

• Note 2 – SCC must NOT be used if the well path is known to be within these parameters. Again, the BHA should be correctly spaced with non-magnetic tubulars as required.

If the above parameters are encountered, then the SCC algorithm will not function correctly and erratic corrected data will be seen. It is therefore necessary to discuss beforehand with the Directional Driller / Company Representative as to an agreeable plan of action if the above criteria are expected to be encountered during drilling.