The Ebre Observatory seismological station: past and present instrumentation and noise conditions

Abstract

A review of the past and present instrumentation at Ebre Observatory seismological station (EBR) is presented in this work. In order to assess the quality of EBR from the point of view of modern seismic instrumentation site requirements, the noise conditions at EBR are also investigated by means of velocity power spectral density analysis in the frequency band 0.01< f <15 Hz by exploiting the three component broadband and digital data processing current capability. A correlation between noise levels and meteorological conditions at the site is observed, using the data of a complete meteorological station located a few tens of meters from the seismic cave. Results show that for long periods ( f < 0.04 Hz) and wind speeds ranging from 0 to 15.5 m/s, seismic noise levels may vary up to 45 dB for the horizontal components. These variations are related to tilts due to wind-generated pressure fl uctuations. Secondly, the seasonal variation of noise levels at the frequency band 0.04 < f < 0.3 Hz is characterized, obtaining that the amplitude of the seasonal variation is higher for secondary (up to 52 dB) than for primary (up to 44 dB) microseisms. On the other hand, a seasonal variation of the frequency of the three-component main noise peak is also suggested in this site and its variation is opposite to the velocity power spectral density amplitude evolution. Finally, a correlation between wind speed and seismic background noise is also observed at high frequencies.