An attempt to deduce the ground angular motion from tiltmeter signals at low frequency has been performed. In order to do that, the mechanical tf, the loop correction and the calibration factor have been taken in account. The result has been compared to a combination of top stage susp sensors, which should be equivalent to an accelerometer placed on the basement, in acc/g units (fig 1). The data are taken in low wind condition, so that the susp signal below 0.1 Hz is quite low (likely limited by the sensor noise). We can see that the tiltmeter sensitivity is still not comparable.

Above 0.2 Hz, where the longitudinal ground motion become dominant on the susp sensor, the tiltmeter shows a quite low signal. At the microseismic peak there is some coherence (fig 2), but the coupling is about 2e-3 (fig 3).

Taking in account data in high wind condition, the expected tilt at 0.1 Hz start to be slightly above the tiltmeter sensitivity (fig 4). In fact the coherence around 0.1 Hz is not zero (fig 5), and the transfer function is about 1 (fig 6).