Abstract: The experimental study on problems in the synthetic observation of apparent resistivi-ty and geomagnetism at Nanjing seismic station is conducted when its observation system for apparent resistivity was reformed last year. It indicates that the short distance (50 m) between the geomagnetic observation room and the current line connected to the current electrodes would be the main reason for the short term pulse 5.7 nT in the vertical compo-nent of geomagnetic records, as power supply provides the current of 1.8 A to the ground through the line for measuring apparent resistivity. The numerical modelling of the mag-netic field caused by long straight conducting wire current with 1.8 A gives very good ex-planation for all the observed facts, which leads to an adequate suggestion for the new de-sign of apparent resistivity monitoring at the station. These experimental and theoretical results could be applied at those stations with synthetic observation similar to Nanjing sta-tion.

Key words: Geomagnetic field, Geoelectric field, Nanjing seismic station