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The formula is identical to a Pythagoras formula for calculating one side of a right angled triangle where Bt would be the hypotenuse and the base would be the cross axial field as follows.
If the value of Bz is very small, which will happen when drilling at high angle heading East or West, the resolution of clean Bz will be extremely sensitive to the accuracy of the other two sides. The slightest error in Bt can produce a very exaggerated effect on Bz and, in many cases, produce a value in Bz more erroneous than the magnetic error. In other words, this technique should not be used in such geometries in case the correction is more erroneous than the original reading.
For this reason and because the Bz value can be positive or negative it is not recommended that this technique be employed n h follow ng ‘ no go zones ’ - above 70 degrees inclination within 20 degrees of East / West (magnetic).
Figure 64: ‘No go zone’ for axial correction.
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