Case study of June 25, 2020: Spectrograms of the electric and magnetic PSD computed on the ground from the snapshots of waveform. The line visible at 38 Hz on the electric field components is a spurious interference line. The weaker horizontal lines visible in the background of the magnetic field components are also stray lines, the same as we have already seen in Fig. 1. Strong parasitic vertical lines are also observed on the magnetic field components: they are caused by the activation of the SCM survival heater.