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Comparison of Two gPhones During Quiet Seismic Period

The purpose of this study is to compare two gPhones (SN# 52 & 57) during a quiet seismic period (i.e. no earthquakes):
  • Day: 26 June 2008 (Midnight to 2PM GMT ~ 14 Hours)
  • Check synchronization of the two timing units that use GPS-slaved rubidium clocks
  • To determine the background seismic noise
  • To determine the instrument noise by differencing the two outputs

Data


The plot below is a 5 minute zoom of the above data. Note the very good correlation between the two instruments


Click on buttons to view different sections of the data

Next we focus on a quiet section of data during the evening

Below is a data plot from gPhone #52 in Lafayette, CO during the evening.
Standard deviation 6-8 microGal on one second samples


Frequency spectrum of the seismic noise measured with gPhone #52



Frequency spectrum of the instrument noise (gPhone #57 subtracted from gPhone #52)



Conclusions


  • The correlation between the instruments is high even during quiet periods meaning the gPhone is sensitive enough to measure quiet seismic noise
  • The GPS-slaved rubidium timing provides a good way to synchronize different instruments
  • The seismic noise takes place at about 5Hz (~0.2Hz) with another peak near 20s
  • The background seismic noise is about 3 microGal/sqrt(Hz) between 0.0005 to .03Hz
  • The seismic noise is removed when one takes the difference between the two gPhones
  • The background instrument noise appears to be about 3 microGal/sqrt(Hz) between 0.001 to 0.1Hz