Magnetic Base Station Deceptions, a magnetovariational analysis along the Ligurian Sea coast, Italy

Reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal
 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