Geomagnetism

High Definition Geomagnetic Model (HDGM)

High Definition Geomagnetic Model 2013

Directional drillers use the Earth's magnetic field as a natural reference frame to orient the drill bit. The azimuth of the bottom-hole assembly is inferred by comparing the magnetic field measured while drilling (MWD) with a geomagnetic reference field. Crustal magnetic anomalies constitute a significant source of error in directional drilling, if not accounted for in the geomagnetic reference field. To meet increasing demand for accurate geomagnetic referencing, the U.S. National Geophysical Data Center (NGDC) produces the High Definition Geomagnetic Model (HDGM) which accounts for long-wavelength crustal magnetic anomalies. HDGM significantly reduces geomagnetic referencing errors.

Fig.1: (click to enlarge) Geomagnetic declination maps for offshore Brazil. The standard approach (left) does not account for crustal anomalies and therefore is unrealistically smooth. The advanced approach (middle) shows the long-wavelength crustal distortion accounted for by the global HDGM model. The best solution (right) makes additional use of an aeromagnetic survey to fully account for local distortion of the geomagnetic field.

Further case studies:

US Shale Plays: Declination anomaly map Dip anomaly map Field strength anomaly map North Sea: Declination map Dip map Field strength map

The HDGM is available for purchase from NGDC.

2013-May-06, Stefan Maus