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Instrumentation and Processing

The seismic network is one of the first real-time monitoring systems that were installed at Åknes. In 2004 we conducted a pilot monitoring experiment recording the seismic activity for a 2-month time period using a very rudimentary system of 6 vertical geophones and local data acquisition and storage. Based on these observations we decided to install a permanent seismic monitoring system.

The figure shows the current monitoring systems and measure points in the vicinity of the top scarp. Green symbols denote the geophone positions.

Above the scarp a concrete bunker has been constructed, which houses the power supply and communication system and the radio-link to Hellesylt. Its roof serves as helicopter pad. Several measuring points equipped with prisms/reflectors have been set up.

The photo shows the bunker as it was in 2007.




Continuous and real-time data are recorded by a weather station, two laser ranging systems, extensometers, and the microseismic network. Additionally, four webcams can be accessed on demand. The seismic network consists of 8 3-component geophones 4.5 Hz, GS11D, Geospace) installed in an area of about 250 x 150 meters. Each geophone is connected individually by an armored cable to the central acquisition system in the bunker.

The photo shows the view from the bunker towards Geiranger and Hellesylt.



The acquisition system which has been customized by NORSAR engineers includes a 24-channel digitizer (Geode, Geometrics), a GPS clock, an industry computer with low power consumption and a GSM telephone relay. It is powered by a set of batteries which in turn is charged by a diesel generator.

The photos shows the lightning protection system and one of the 3-component geophones semented to the solid rock.


 
The system is connected to a radio link that bridges the 13 km distance to the village of Hellesylt, and from there a secure VPN connection to NORSAR has been established. The data are transferred in real-time to NORSAR and recording parameters of the acquisition system can be modified at any time remotely. The system became operational in November 2005 and is performing very stable. So far only two complete outages happened: one due to a failure of the central power supply, the other due to a failed software update. Partial outages have been caused by cable damages from rock falls and water leakage into the geophones. The robustness of the system is a very important issue, because the Åknes site is remote and can only be accessed by helicopter. The bunker can be reached under good weather conditions even in winter, but cable maintenance is only possible during summer and early autumn.
 
We continuously check the state of health of individual channels as well as the connection to the site. For each incoming data file we compute average amplitude levels of the channels to assess the current noise conditions.

Until August 2006 we recorded in continuous mode with 8 ms sampling to gain experience on the different types of seismic signals and the ambient noise conditions. After that period we switched to triggered mode monitoring at 1 ms. The data are automatically transferred from Åknes to NORSAR using ftp, and the average daily data volume is about 250 MByte.

The figure shows uptimes (black) and downtimes (red) of the Åknes microseimic network.


The data are submitted to an automatic event detection (S/N > 2 on at least 10 channels within a common time window of 1s). The results are displayed in near real-time (~10-15 minutes delay) on the Lastest data web-page.

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