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Report on testing the reliability and the signal to noise ratio of networks of the optical seismometer prototypes in the field

The described system was tested for noise at Cheia seismic site (Romania). The tests were performed using a reference electrodynamical sensor Teledyne Geotech type S-13 (USA), belonging to NIEP, and 6 tests were performed using the vertical HiRes optical sensor of the setup described above (because it is the most sensitive). The acquisition time window for all tests was 0.5 s. For each test the signal spectrum (FFT) was determined in the range 0-500Hz.

The noise tests performed on the reference sensor S-13 reveal micro-seismic noise in the frequency range up to 75-100Hz, but some of them also show weak components in the range 200-250Hz. Also, the noise floor is visible.

The HiRes micro-seismic spectra are somehow different compared to the ones obtained from the reference sensor S-13. For HiRes the low frequency region is usually narrower (dominant line), while for S-13 it is more non-selective. The HiRes optical sensor clearly reveals better spectral components in the range 150-200Hz, which are hardly visible with S-13. For HiRes the noise floor is nearly zero in the region 350-500Hz, while for S-13 it is visible and constant.

As already known, it is difficult to precisely characterize a measuring device concerning noise. In order to see the extent in which noise affects weak signals, we imagined a distant shock test performed simultaneously with S-13 and the vertical HiRes optical sensor, using the same shock energy, but various distances. The signal spectra reveal that in the shock test at 15 m the signal delivered by S-13 begins to be affected by noise, while the HiRes spectrum is still clean.

In the test at 20m the HiRes spectrum begins to broaden, indicating the presence of noise, which can be also seen in the spectrum of S-13. In the test at 30m the spectrum of HiRes broadens a little more, but the low frequency components are still clearly separated, while the spectrum of S-13 degrades dramatically.

In our opinion, these tests clearly indicate the lower noise of the HiRes optical sensor, at least in the low frequency range (up to 100-150Hz), compared to the reference sensor S-13. Combined with the much higher sensitivity of the HiRes sensor, this noise behavior leads to their ability to see weaker seismic signals, i.e. to a higher signal-to-noise ratio (SNR). We point out that few commercial classical seismic sensors contain the SNR value in their datasheets.

Reported by

University "Politehnica" of Bucharest (UPB)
Spl. Independentei 313
060042 Bucharest