Page | 29
4.2 V ons n h E h’sM gn cF ld
n p obl mw h h E h’sM gn cF ld s h wllno s nds ll. h cou s ofhso y, h m gn cco of the Earth has been turbulent with the result that the magnetic vector is constantly changing. In geological time scales this chang s y pd. s f dosh‘ cul ’ on.
Figure 27: Tracking the magnetic North Pole.
In order to keep track of this movement, several global magnetic models are maintained to provide prediction models. For example, an international organization called INTERMAGNET collates data from observatories scattered throughout the world to model the intensity and attitude of the E h’s m gn c f ld. E y y , h d s s n o the British Geological Survey in Edinburgh where it is distilled to a computer model called the British Global Geomagnetic Model (BGGM). Historically this has been the most commonly used model for magnetic field prediction for the drilling industry but there are others. The United States National Oceanic and Atmospheric Administration (NOAA) also produce a model known as the High Definition Geomagnetic Model from their National Geophysical Data Centre in Boulder Colorado. This takes account of more localized crustal effects by using a higher order function to model the observed variations in the Earth field. In practice, when higher accuracy MWD is required, it is increasingly popular to measure the local field using IFR (see chapter 6) and to map the local anomalies as corrections to one of the global models. In this way, the global model takes care of the secular variation over time and the local effects are not dependent on a mathematical best fit over long wavelengths.
LINKS
BGGM
NOAA
Made with FlippingBook flipbook maker