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variations from the recorded total magnetic field intensity data. At mid latitudes, one or a few base stations are typically
located within or near the survey area and are used to monitor and remove time dependent variations. These
are usually assumed to be of external origin and uniform throughout the survey area. Here we investigate the
influence on the magnetic base station correction of the time varying magnetic field variations generated by internal
telluric currents flowing in anomalous regional 2D/3D conductivity structures. The study is based on the statistical
analysis of a data set collected by four magnetovariational stations installed in northwestern Italy. The variometer stations
were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. They recorded
the geomagnetic field from the beginning to the end of April 2005, with a sampling rate of 0.33 Hz. Cross-correlation
and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods
longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different stations.
This indicates spatial uniformity of the source field and of the induced currents in the 1D Earth. Additionally,
the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic signature
of the Sestri-Voltaggio line, interpreted as a major regional tectonic boundary. Induced telluric currents flowing
through this 2D/3D electrical conductivity discontinuity affect mainly the vertical magnetic component at the
closer locations. By comparing this component at near (32 km) and far (70 km) stations, we have found that the mean
value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band ranging
from 2.5×10?3 to 5.5×10?5 Hz. This energy, folded in the spatial domain of an hypothetical survey in this region
produces unwanted noise in the dataset. Considering a fifth of nyquist frequency the optimal tie-line spacing to assure
complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and
100 knots (aeromagnetic survey) respectively. Similar power spectra analysis can be applied elsewhere to optimise
tie-line spacing for levelling and filtering parameters utlilised for microlevelling.
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