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Points to Ponder
The E h’s M gn c F ld s n xc ll n f nc fo us because lbl b n h h E h’scus. However, some common mistakes made when using it as a reference may be avoided using the guidelines below. s s ll
1. Do not assume that the declination will be the same from year to year. The Magnetic Field changes rapidly and today the Magnetic North Pole is moving at a rate of nearly 1km per week. 2. When using an IFR map (see section 6) be aware that it will have been published as a set of crustal corrections to the field model values, NOT as a correction to apply to azimuth. For example, if the declination correction is +0.25, the global model says the declination prediction is -5 degs, and the well is heading 90 degs magnetic, the correct sequence is: a. Apply the correction to the predicted declination; new declination is -4.75 degs b. Add the new declination to all observed magnetic azimuths to get true azimuth. In this case 90 + -4.75 = 85.25 degs True. The effect is the same but the declination and should be reported as such. 3. Always use a licensed, high-definition global model. Never use a free one downloaded from the internet, which are great for yachtsmen but not for directional drilling.
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