Fig. 3.

Synthesised Gradiometry (empty room noise). A) Magnetic fields plotted in time (left) and frequency (right), measured in an empty room. Red and blue traces show the OPM data with and without gradiometer correction respectively. The black trace shows a SQUID magnetometer and the green trace shows a SQUID gradiometer. Note that 97% of variance in the OPM signal was explained by the simultaneously measured SQUID reference sensors and their temporal derivatives. Here, gradiometry was applied to the full (100 s) time window and full (2–80 Hz) bandwidth. B) Left hand side shows a noise amplitude spectrum when gradiometer correction is applied to individual frequency bands. Uncorrected (blue) and corrected (red) OPM mean noise levels across 28 bands are shown. Centre and right panels show time-frequency spectra in fT/√Hz, before and after gradiometer correction.