Fig. 7

Examples of results for three different masking thresholds of magnetic field spectral intensity. a Masked where the magnetic field spectral intensity was less than \(10^{-3}~ \mathrm {pT^{2}/Hz}\), b masked where magnetic field spectral intensity was less than \(10^{-1}~\mathrm {pT^{2}/Hz}\) and c masked where magnetic field spectral intensity was less than \(10^{0}~\mathrm {pT^{2}/Hz}\)