The noise increased at 0.5 Hz and is pretty big. This might explain the loose of coherence at low frequencies.
This is because spectrum analyzer did not plot the real noise spectrum at the first few points at low frequencies. I've remeasured the noise at 1mHz - 3Hz at "output -" (TP9) and compared it to the seismometer signal
The noise seems to be much less then the signal. I've measured the noise several times and once I got a huge amount of noise
I made another measurement in some time and got the low noise again. A circuit might have a bad contact somewhere.
The plan is to change AD620 adjustable resistor (R2) from 5.49kOhm to 500Ohm to increase the gain from 20 up to 200.