Abstract: A laboratory calibration and a downhole online calibration method for the JLU-FG03A downhole fluxgate magnetometer are designed for the application needs of geomagnetic field observation under high pressure and humid environment, and the construction of the experimental test system and the related test methods are presented. The laboratory calibration method tests the range, bandwidth, noise and sensitivity of temperature drift of the magnetometer, while the downhole online calibration method tests the reliability of the magnetometer data and noise levels when working in the well. Laboratory calibration results show that the instrument has a range of +- 100 000 nT, a bandwidth of DC-10 Hz, RMS ( Root Mean Square ) noise less than 0. 01 nT (DC-0. 3 Hz) and a temperature drift of 23. 39 ppm when the adaptive feedback function is on, which meets the application requirements for downhole geomagnetic observation. The downhole online calibration experiments show that the instrument is in normal working condition downhole and the noise measurement results indicate a good downhole magnetic environment, which is conducive to the best performance of the magnetometer.

Key words: downhole, fluxgate magnetometer, laboratory calibration, online calibration