Assessing the Absolute and Relative Accuracy of Magnetic Variometers Compared to Observatory Data for IFR2

The Earth’s magnetic field changes constantly on short timescales (minutes to days) due to the Sun’s influence on the magnetosphere and ionosphere and on longer timescales (months to decades) due to processes in the outer core. Accurately estimating the static and changing fields at a drilling location, along with associated uncertainty, can be achieved with IFR2 using real time data from an absolute magnetic observatory. In 2022, British Geological Survey installed three new magnetic variometer sites in the UK: in western Northern Ireland (FLO), central England (LEI) and on the south coast (HTX) to monitor space weather activity across Britain. These consist of a vector fluxgate magnetometer buried in the ground on a levelled plinth attached to a digitizer/logger and modem to transmit real time data. The systems are powered by a solar panel with batteries. The sites have been running for several years and offer a long-term experiment to determine the relative and absolute quality of the data when compared to the UK INTERMAGNET-standard observatories (LER, HAD and ESK). Using the IFR2 technique, we estimated the magnetic field values expected at each variometer site and compared it to the measured data. We show results from the three variometers, outline the issues arising and compute uncertainties for each site. We find that, in general, while the variometer systems capture the short-period magnetic field variation well (which they are designed to do), action is needed to remove erroneous data such as spikes and man-made interference. However, we also find that even with great care taken to align the systems initially, the absolute accuracy is not well constrained due to temperature, instrument drift, subsidence and/or man-made disturbances. This pushes the uncertainties well beyond the ISCWSA-recommended limits of 0.1° in Dec, 0.05° in Magnetic Dip and 50 nT in Total Field.