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kouseki.miyo - 16:57 Monday 26 April 2021 (16572) Print this report
Comment to Preparation for noise evaluation of ITMX geophones (16559)


Summary: Today we calibrated the signals of the three Geophones (H1, H2, H3) used at ITMX against the IXV seismometer. In the 0.2 - 1 Hz band where the ground is shaked by micro seismic, the three geophones agree with the seismometer to within +/-2%.

As reported in klog#16559, the three Geophones used in ITMX are installed with their connectors aligned with the +X axis of the IXV seismometer, so we decided to calibrate the Geophone signals against the seismometer signals. The method is as follows. First, we measure the transfer function between the IN1 signal of the Geophone and the EW signal of the seismometer. Note that the Geophone's IN1 is the count value of the Geophone's output signal, and the seismometer's EW signal is the count value of Trillium 120 seismometer amplified 30dB. The measured transfer function was fitted by eye ball to an error of +/-5% in the 0.2 - 1 Hz band. A comparison between the fitted transfer function and the measured data is shown in Fig. 1,2. The Python code used in the fitting was placed in my working directory "/users/Miyo/dropbox/LOG/210426/calibration.pynb".

The transfer function of the Geophone and seismometer calibrated to velocity is shown in Fig. 3. The error was about +/-2% in the band of 0.2-1Hz, where the ground shakes greatly due to the micro seismic and there is enough coherence. The Geophone filter used in this calibration was committed in the following commit:

What to do next
We will calibrate the out-of-loop Geophone called H4 in the same way. Once that's done, we'll use the differential signal between the Geophones to evaluate the noise level.

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