Overview
- The differences in the crustal declination and inclination perturbations processed by the latest versions of IGRF and BGGM are insignificant
- The differences in the crustal TMI anomalies due to different main (reference) field models at different years, and at different depths are almost a constant and can be easily estimated and corrected
- Crustal declination and inclination perturbations computed for a current date may be valid for up to 20 years in the offshore eastern Canada example
Vector Crustal Magnetic Field
- Is the vector crustal magnetic field relative to a current main field model for a current date still valid in the years to come?
- main field itself changes with time (secular variation)
- degree/order of the spherical harmonic expansion used to define the main field model increases
- directional drilling involves the main field at the aeromagnetic survey time and the main field at the drilling time.
Conventional Surface Magnetic Surveys
- Conventional magnetometers measure the Total Magnetic Intensity, a scalar quantity not the vector field
- However, directional drillng requires
- the vector magnetic field
- at depths
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Xiong Li - Fugro Gravity & Magnetic Services
Benny Poedjono - Schlumberger