Possible railway effects in magnetic measurements at geophysical observatory Novosibirsk

After renewing the Geophysical Observatory Novosibirsk (NVS) from analog photorecording to digital magnetometers, noise in the form of spikes and short jumps of records with an amplitude of up to several nT, which were not previously visible, appeared in the measurement results. The main effect was observed in the vertical intensity Z and total field intensity F. A direct current railway running 12 km west of the observatory was considered as a possible source of these disturbances. In 2006-2012, route surveys of F were carried out along two profiles of different lengths, approximately perpendicular to the railway. It has been shown that the noise amplitude decreases linearly with the distance from the source. An attempts were made to identify the noise origin, linking them with the schedule of trains. A review of recent work on the problem of the railways and subways effects on the results of magnetic measurements at observatories, field measurements and magnetotelluric sounding is made. Magnetic effects in the vertical and horizontal components of the field are calculated within the framework of a simple leakage current distribution model that takes into account the presence of a high-resistance foundation in the geoelectric section in the observatory area. The model provides only a partial explanation of the observed effects. There has been a tendency to decrease the intensity of interference in recent years.