Design and Implementation of Low Noise Downhole Fluxgate Magnetometer Based on Cobalt-based Amorphous Strips

Downhole long-term geomagnetic observation is a new way to replace surface station observation and plays an important role in geohazard monitoring and early warning. To meet the demand for low-noise application of magnetometer, this paper establishes a low-noise theoretical model of circuit and magnetic Core material, prepares a cobalt-based amorphous soft magnetic core, and develops a fluxgate magnetometer applicable to the special downhole environment. The developed instrument was installed in a 1036m deep seismic observation well in Xichang, Sichuan Province, China, in January 2022, and has been working continuously for several months. The measured RMS noise is less than 0.01nT (DC~0.3Hz), which exceeds the relevant national instrument entry standard by one order of magnitude, verifying the reliability and advancedness of the instrument.