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Abstract
Earth's gravity and magnetic fields are used as natural reference frames in directional drilling. The azimuth of the bottomhole assembly is inferred by comparing the magnetic field measured-while-drilling (MWD) with a geomagnetic reference model.

To provide a reference of sufficient quality for accurate well placement, the US National Geophysical Data Center (NGDC), in partnership with industry, has developed high-definition geomagnetic models (HDGM), updated regularly using the latest satellite, airborne and marine measurements of the Earth's magnetic field. Standard geomagnetic reference models represent the main magnetic field originating in the Earth's liquid core, but the new models additionally account for crustal magnetic anomalies, which constitute a significant source of error in directional drilling. NGDC maintains a public archive of global ship and airborne magnetic field measurements. These are compiled into a global magnetic anomaly grid and expanded into ellipsoidal harmonics. The harmonic expansion coefficients are then included in the high-definition models to accurately represent the direction and strength of the local geomagnetic field. The latest global model to degree and order 720 resolves magnetic anomalies down to 28 km half-wavelength, achieving more than an order-of-magnitude improvement over previous models.

A side-by-side comparison of different on- and off-shore regions shows the high level of local detail represented in the new model. Accounting for a larger waveband of the geomagnetic spectrum significantly improves the accuracy of the reference field. This directly benefits the reliability of the well azimuth determination. We further demonstrate that model accuracy is a prerequisite for applying drill string interference corrections. Finally, an accurate reference model facilitates the validation of MWD surveys by keeping the field acceptance criteria centered on the true downhole magnetic field. Together, these factors improve well placement, prevent and mitigate the danger of collision with existing wellbores and enable real-time steering to save rig-time and reduce drilling costs.

Introduction
Magnetic field sensors are widely used in navigation systems and in determining the orientation of devices such as satellites, solar panels and antennas. Such electronic compasses play a particular important part below the sea and earth surface where the global positioning system (GPS) is unavailable. Measurement while drilling (MWD) employs a combination of gravity and magnetic field sensors to determine the inclination and azimuth of the bottom hole assembly (BHA). Conversion from magnetic azimuth to true azimuth requires knowledge of the direction and strength of the ambient magnetic field, which is provided by a geomagnetic reference model. Such a model specifies the declination angle (measured in degrees positive east of true north), the dip angle (measured positive downward), and the total field strength (measured in nT).