The Swedish National Seismic Network is a large new broadband digital seismic array recently installed in collaboration with Uppsala Universitet.
The installed sensor base constitutes the largest network of fully digital Guralp instruments currently in use: 45 CMG-3TD digital broadband sensors with exceptionally high sensitivity.
The enhanced 10 000 V/m/s response of the sensors will allow scientists to study in detail deformation processes in this low-seismicity area, and to discover new structure in the Swedish crust. Teleseismic events will also be monitored to provide information on the local crust and upper mantle.
Guralp Systems was briefed to provide instruments capable of detecting very small (−2 to 0 mag.) tremors without reducing the overall sensor performance, and our engineers have been working with Reynir Böðvarsson and his group at Uppsala to meet these stringent requirements through a series of site visits, discussions and the exchange and modification of sensors.
Initial testing using instruments with a 3000 V/m/s response showed that these tremors were being lost in background noise, especially at frequencies above 10 Hz.
The smallest signal theoretically detectable by a digital seismometer can be obtained by dividing the resolution of the digitizer by the ideal sensor response. We measured the resolution of the digitizers by determining their shorted-input noise over the frequency band.
As can be seen, the resolution of the digitizers at this response cuts the Peterson NLNM at around 10 Hz. Knowing this information, and the magnitude of the tremors to be measured, we were able to tune the sensors’ voltage response to a suitable level.
Each sensor in the array now has a 10 000 V/m/s response over the full 30 s − 50 Hz frequency range, placing the noise curve well below the NLNM and allowing low-magnitude events to be detected with ease.
The sensor housing includes a built-in CMG-DM24 digitizer module, which digitizes and timestamps all seismic signals using a local GPS receiver. Power is obtained through a cable running from a nearby recording station.
The instrument at each of the 45 stations is placed within a 3 m deep tubular polyester enclosure, buried to allow the instrument to stand direct on the bedrock. The installation requires no special precautions, and is extremely simple and cost-effective whilst giving excellent results.
The sensor output is digitized within the instrument, bringing several important advantages:
The SNSN project uses Linux computers to administer data flow around the network, with data provided from enclosures to recording stations over optic fibre links, which are converted to RS232 by fibre modems at each end.
The Linux PCs run the SIL acquisition system, developed as a joint venture among the Nordic nations and already in use for earthquake prediction research in Iceland. This system can automatically select data of interest and forward it on for further study.
For a full description of the data layout of the Swedish National Seismic Network, please see the SNSN site.
