Instruments and Methodologies for Measurement of the Earth’s Magnetic Field

Abstract

In modern magnetic observatories the most widely used instrument for recording magnetic field variations is the triaxial fluxgate magnetometer. For absolute observations, the declination-inclination magnetometer, in conjunction with a proton precession or an Overhauser magnetometer, is the norm. To meet the needs of users, a triaxial fluxgate must have a resolution of 0.01 nT. It must also have good temperature and long-term stability. Several sources of error can lead to degradation of the data, temperature variations and tilting of the sensors being among the most important. The declination-inclination magnetometer consists of a single-axis fluxgate sensor mounted on a nonmagnetic theodolite. With care, most sources of error can be eliminated, and an absolute accuracy of better than 0.1 arcmin is achievable. Proton precession and Overhauser magnetometers make use of the quantum-mechanical properties of protons and electrons to determine the strength of the magnetic field. The Overhauser magnetometer is rapidly supplanting the proton magnetometer (0.1 nT once per second sensitivity) because it can sample the field much more rapidly and precisely (0.01 nT once per second). Potassium magnetometers, which belong to the family of optically pumped magnetometers, are an attractive alternative to Overhauser magnetometers, especially when used in a dIdD instrument.