Reports 1-1 of 1 Clear search Modify search
koki.okutomi - 0:42 Friday 22 December 2017 (3893) Print this report
Geophone characterization

[Okutomi with Miyo-kun's help]

I characterized three geophones for Type-A ETMX control test.

The geophones have better noise level than the expected accelerometers!


To characterize the assembled geophones with measurements of

  • Efficiency: a conversion from raw outputs to data in velocity or displacement
  • Noise: sensor noise in frequency domain


  • The geophone L-4C has each inspection sheet which gives the information of the conversion efficiency
  • But their individual parameters seems not to be valid any more
    • Because a simultaneous measurement with the geophones and PEM seismometers has different level of ground motion to each other using the parameters on the inspection sheet
  • So I estimated the parameters of geophone with a free run measurement to get the conversion efficiency 
  • Then the converted sensor noise of the geophones is obtained

Free-run measurement

  • Geophones
    • In-pod x3: with pre-amp circuit, set on each aluminum plate on the floor (for tilt adjustment)
    • Naked: without pre-amp circuit, set on a table 
  • Seismometers
    • Trillium Compact x2 (JGW-S1707526): 45dB amplification with whitening filter, set in a corner of EXV room (outside of the clean booth)
    • Trillium 120QA (JGW-T1707554): 45dB amplification with whitening filter, set in a corner of EXV room (outside of the clean booth)

  • Geophone parameter
    • Generator constant [V/(m/s)]: assumed that the values in the inspection sheet are correct
    • Damping ratio: to be estimated
    • Resonant frequency (f0) [Hz]: to be estimated
  • Geophones' parameters are adjusted so that the geophones and the seismometers have same spectra of ground motion in the range of 0.1-10 Hz
  • In this parameter estimation, the seismometers are regarded as reference because of
    • broadband flat response (0.01-10 Hz)
    • the fact that the 3 seismometers have a consistent level of displacement spectrum even with a simple calibration method

  • Estimated geophone parameters
  S/N Generator const. [V/(m/s)] Damping ratio (nominal) Damping ratio (estimated) f0 (nominal) [Hz] f0 (estimated) [Hz]
GEO H1 #11 282.26 0.299 => 0.34 0.99 => 1.07
GEO H2 #12 282.30 0.276 => 0.315 0.99 => 1.065
GEO H3 #13 286.47 0.293 => 0.27 1.0 => 1.31

  • Notes:
    • The pre-amp circuit (JGW-D1301466) seems to have a gain of 375.53 * 5.0 / 2 = 938.8, not 375.53 * 5.0 = 1877.7
      • Sekiguchi-san's PhD thesis (p.114 in JGW-P1504155) missed to mention that the differential opamp at the final stage outputs a half amplitude of the input voltage
      • You can check that in the data sheet of the differential opamp => product page
    • Trillium Compact: 749.1 [V/(m/s)] (measured, cf. JGW-D1707542)
    • Trillium 120QA: 1202.1 [V/(m/s)] (nominal, cf. JGW-T1707554)
    • The disagreement between the geophones and the seismometers in heigher frequency (> 10 Hz)would come from the accuracy of the pod assembling, the base plate where the geophones are put and so on
      • This disagreement may not be a problem because the frequency band above 10 Hz is out of the range for the geophone to monitor velocity of the IP stage

Noise measurement

  • Sensor noises are measured using the conversion efficiency obtained above
    • Putting the geophone pod upright makes the internal proof mass fixed

  • Geophone sensor noise
  Displacement noise [m/Hz1/2] @ 0.1 Hz Displacement noise [m/Hz1/2] @ 10 Hz
GEO H1 8 x 10-9 3 x 10-13
GEO H2 8 x 10-9 3 x 10-13
GEO H3 1 x 10-8 3 x 10-13
  • Accelerometers which we "were" going to use for Type-A are expected to have a noise level of ~ 3 x 10-8 m/Hz1/2 @ 0.1 Hz (according to a measurement in TAMA?)
  • The geophones now we have can achieve smaller noise level than the expected accelerometers

Current situation

  • Now the geophone pods are installed on the IP of Type-A ETMX
  • The response we obtained needs to be implemented into the input filter on the digital system
  • Sensor configuration on the ETMX IP:

Images attached to this report
Search Help