GEOSCOPE station noise levels

The noise level at GEOSCOPE seismograph stations operating in 1995 has been studied in order to quantify the quality of stations for periods ranging from 0.2 to 8000 sec. The power spectral density curves presented in this article are a useful tool for selecting stations as a function of signal-to-noise ratio in the frequency band of interest. Seismic-noise level is the lowest for continental stations in the entire frequency band. It is similarly low at most coastal stations (stations located less than 150 km away from the coast). Finally, the noise level, is low for island stations at long periods but increases significantly for periods smaller than 20 seconds, and in particular in the period range of the microseismic peak. The noise level on horizontal components varies, in most stations, as a function of local time for periods greater than 20 sec, being higher during the day than during the night. Only stations located in cold areas with little daily temperature variations and stations installed in a long tunnel do not display these daily variations. There is no seasonal variations of short-period noise (periods less than 5 sec). For some continental stations, we observe variations in the amplitude of the 7-sec microseismic peak during the year. For all three components, the peak amplitude is higher and shifted toward longer periods in fall and winter than in spring and summer. This phenomenon can be explained by the increase of the number and the size of oceanic storms in fall and winter. Long-period seismic noise (periods greater than 30 sec) also varies for some stations as a function of the season; however, no systematic characteristics have been observed.