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4.3 Magnetic Observatory Distribution
It should be noted that the global models such as BGGM and even HDGM, can only measure longer wave length effects of the E h’s magnetic field distribution and cannot be expected to take account of very localised crustal effects caused by magnetic minerals, typically found in deep basement formations in the vicinity of drilling. See chapter 6 for a discussion of In Field Referencing (IFR), a technique for measuring the local field to a higher accuracy.
Figure 30: Magnetic Observatory distribution.
4.4 Diurnal Variation h m u n lsmplym ns‘d ly’ ndfo many centuries it has been noticed that the magnetic field seems to follow a rough sine wave during the course of the day. Here is a graph of field strength observations taken in Colorado over a 2 day period.
It can be seen that the field strength is following a 24 hour period sine wave.
See chapter 7 for a discussion of ‘Interpolated In Field Referencing’, a method of correcting for diurnal variation in the field.
Figure 31: Diurnal field variation.
These variations may be small but for high accuracy MWD work especially at high latitudes, they may need to be corrected for. We now know that this effect is due to the rotation of the Earth and a varying exposure to the solar wind. The sun is constantly emitting ionized plasma in huge quantities across the solar system. These winds intensify during magnetic storms and the material can be seen on a clear night at high or low latitudes, being concentrated at h m gn cpol s ndfo mng h ‘Au o o ls’ nd h ‘Au o Aus ls’. During such storms the measurements taken from magnetometers and compasses are unlikely to be reliable but even in quiet times, the diurnal variation is always present .
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