I've moved GUR1 seismometer from MC2 to the working tables in order not to disturb the MC while working with the seismometer box. The new place for the GUR1 for a few days is near the printer, cables and blue boxes. I've cleaned all mess and wires from the floor, so that seismometer now looks like that
I've connected 2 inputs of the N/S 1 circuit of the seismometer box with a 50 Ohm resistor and measured the noise at the output. The comparison with the seismic signal is
The noise increased at 0.5 Hz and is pretty big. This might explain the loose of coherence at low frequencies.