Comparison of Two gPhones During Quiet Seismic Period

• 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

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

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