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Abstract

A new calibration facility was developed, tested, and installed at the premises of the Geomagnetic observatory in Niemegk to test induction coil magnetometers (search coils) in the frequency range 100Hz to 1 MHz. The new calibration facility is based on a Braunbeck coil, which extends existing installations based on a three-dimensional Helmholtz coil and a cylindrical coil which cover only the frequency range from a few mHz to 100 kHz. The new calibration coil is primarily needed to test magnetic field sensors used by the Geophysical Instrument Pool Potsdam for the so-called Radio Magnetotelluric (RMT) method. The RMT method uses electromagnetic fields transmitted by radio stations to infer the electrical conductivity distribution of the shallow subsurface (~1 m to 50 m). We present theoretical calculations to determine the parameters of the new calibration coil, which could be confirmed with measurements using an OMICRON BODE 100 spectrum analyser. The new coil system was also tested by calibrating two existing magnetic field sensors (METRONIX) MFS07 and SHFT-02e) with known response functions. We infer an overall accuracy of the new system of <0.05nT in amplitude and <0.5° in phase for frequencies <700kHz and ≤0.05nT and ≤3° degrees for frequencies > 700kHz. We also developed, tested and calibrated a new magnetic field sensor, using a set of three orthogonal toroidal coils (based on the antenna AN 200). The initial results look promising. The overall noise figures surpass those of the Metronix SHFT-02e sensor. The amplitude characteristic is slightly non-linear though, varying between 5 and 75 mV/nT over the frequency range 10 kHz to 1 MHz.