Ugalde, A.

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.

The experience acquired from more than ten years of operation of an Antarctic geomagnetic observatory is described along with the development of data transmission facilities. The observatory was deployed at the Spanish Antarctic Station in 1996. The main instrument was an Overhauser magnetometer deployed in dual axis Helmholtz coils, a δD/δI configuration. The site is only manned during the summer, with the magnetometer left recording throughout the rest of the year. During the 2007-2008 survey the observatory instrumentation has been upgraded with a DMI suspended triaxial fluxgate magnetometer, new sampling hardware and data logging software. Both sampling and timing are carried out under the control of a PIC based microcontroller and GPS receiver. Data presentation, transmission and archiving are performed under the control of a low power embedded PC. For real time access to the data two options have been provided and rigorously tested during the last 10 years: METEOSAT and GOES Data Collection Systems, and recently, a high frequency (HF) digital radio-link, using ionospheric propagation between Antarctica and Spain, has been developed. This latest transmission system is being continuously upgraded, and it would be possible to extend its application to other remote stations. Measurements have been made during the last four years in order to determine the channel characteristics and its variability, mainly the multipath and Doppler spread and the link availability for a given SNR in the receiver. These measurements are being used to design the physical layer of a radiomodem intended to maximize the link capacity keeping the emitted power low.