An Automatic High-precision Absolute Magnetic Field Vector Measurement System for Geomagnetic Observatories

Abstract:

Nowadays, most geomagnetic observatories measure the absolute magnetic field vectors (absolute total field intensity and absolute direction including inclination and declination according to a fixed georeference frame) with DI (declination-inclination) magnetometers and total field magnetometers. However, the measurement process is usually manually operated and time-consuming; the efficiency of the DI magnetometer is very low (30 minutes per data point manually). Other magnetometers, such as Gem dIdD (delta inclination delta declination) magnetometers, are based on Overhauser sensors and use bias fields to resolve angular variations of the geomagnetic field. However, dIdD magnetometers require baseline values determined from an absolute measurement similar to the DI magnetometer measurement process, which is also time-consuming. To make the measurements efficient and fully automatic, we improved the system with new designs and measurement procedures. Our improved system consists of 3 pairs of Helmholtz coils and 4 Overhauser sensors. The 3 Helmholtz coils are in a series circuit so that the current to generate bias fields are the same. The intensity of synthetic fields of bias fields and the geomagnetic field can be measured using Overhauser sensors, along with an intensity measured without bias fields, we can solve the absolute values of geomagnetic vectors. This automatic high-precision absolute geomagnetic field vector measurement system is based on Overhauser sensors, which can reach an absolute accuracy of 0.1 nT for intensity and 0.004° for direction. The 4 Overhauser sensors operate simultaneously at a sampling frequency of 1 Hz, which provides a full-vector absolute geomagnetic field solution every second. The high sampling rate high-precision absolute geomagnetic field data that this system provides can reveal more detailed variations of the geomagnetic field. The absolute geomagnetic data can provide a foundation for cross-analysis of different observatories, particularly for regional earthquake-related geomagnetic field variation analysis.