Overview:
• Providing a single answer on the uncertainty associated with the geomagnetic reference field is hard
• The uncertainty associated with a particular reference value, depends on what sources are included in the estimate; location on the earth’s surface; and often on time of day, season and solar cycle
• Including all sources will reduce the uncertainties; and importantly, provide a more robust estimate of the uncertainties by removing any hourly, seasonal and solar cycle variations
The Earth’s Magnetic Field
• Most of the field is from the Earth’s core
• varies slowly with time (months to years)
• Local fields from magnetized rocks in Earth’s crust
• relatively stable with time
• Fields due to currents in the ionosphere and the magnetosphere
• variations from seconds to years
Global Magnetic Field Models
• Spherical harmonic model of the core field and the long wavelength crustal field
• Uses selected satellite and ground based observatory data
• Prediction into future
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Estimating The Geomagnetic Field as a Reference for Wellbore Surveys
Ellen Clark - British Geological Survey