4.2 Variations in the Earth’s Magnetic Field

One problem with the Earth’s Magnetic Field is that it will not stand still. Over the course of history, the magnetic core of the Earth has been turbulent with the result that the magnetic vector is constantly changing. In geological time scales this change is very rapid. It is referred to as the ‘Secular’ variation.

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 Earth’s magnetic field. Every year, the data is sent to 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 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.

The model below is a combined effort between NOAA and the BGS called the World Magnetic Model which is updated every 5 years. This is a lower order model, as is the International Geomagnetic Reference Field produced by IAGA but these are freely accessible over the internet whereas the higher order models require an annual license.

The higher order world models (BGGM and HDGM) are considered to be better than 1 degree (99% confidence) at most latitudes. This may be less true at higher latitudes above 60 degrees but at these latitudes, IFR techniques are frequently used.