With Takahashi-san.

Summary: we changed  the geophone together with the pod; the readout is as expected now.

We opened the vacuum chamber and checked the resistance between pins 4 and 5 at the cable directly connected to the geophone pod. The resistance was about 7.5 Ohm, indicating that the problem was not  at the extension cable. Next we checked  the resistance measuring directly at the electrical feedthrough at the pod. The resistance was low again, which implied that the problem was inside the pod.

Therefore, we changed the geophone together with the pod. The amplitude of the readout is now as expected.