Follow up the investigation https://alog.ligo-la.caltech.edu/SEI/index.php?callRep=499
of the glitch present in the H1:ISI-ETMY before the WD trips pointing the ST1 actuator as the cause (Figure 1):

- The glitch spectrum shows that it has a frequency of 10 - 70 Hz, with a peak in 40 Hz. (Figure 2). 
- The L4Cs and GS13s are good sensors for frequencies above 1 Hz, then we may use them for our investigation.
- The ground motion is expected to be low frequency, so the glitch may comes from the ETMY chamber. 
- The transfer function between the sensors and the excitation should be approximately flat.
- For an excitation of the ISI stage 2 in the X direction, the transfer function (TF) and coherence for L4Cs in ST1, GS13s in ST2, and OSEMs in the reaction chain R0 are shown in Figure 3, 4 and 5 respectively. 

Comments:
- Even that the coherence is high, the TF shows a peak near 45 Hz for ST1 L4Cs and ST2 GS13s.