Overview:
- When the subsurface vector crustal magnetic field together with the reference field model used in the computation are delivered in the volume format and the volume contains all foreseeable wells
- these two sets of magnetic field volumes and the future reference field model are enough to determine accurately the crustal and baseline field values along any future (or past) well path at a future (or past) time
- It also implies that it doesn’t matter to use IGRF, BGGM or HDGM as the reference model in the creation of the local crustal magnetic field volumes, and what is critical is to calculate and apply the difference while drilling
Is the vector crustal magnetic field relative to a current reference field model for a current date still valid in the years (5, 10, 20) to come?
Given the following facts
- The main field itself changes with time (secular variation)
- The degree/order of the spherical harmonic (SH) expansion used to define the reference field model increases
- Directional drilling involves the main field at the aeromagnetic survey time and the main field at the drilling time
How about the reference field model used at the drilling time has different SH degrees for different years (higher for recent years)?
- This means that different SH degrees are used to define the crustal fields at different years (the main field is sufficiently crustal fields at different years (the main field is sufficiently defined by SH degrees of up to 15)
- This should and can be avoided (the crustal field doesn This should and can be avoided (the crustal field doesn t’ change with time, and may be updated when the crustal magnetic anomaly database has significant updates)
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View the entire Presentation:
An Accurate Determination of Crustal Magnetic Field
Xiong Li and Benny Poedjono - Schlumberger