The Swiss Digital Seismic Network (SDSNet)
M. Baer, P. Zweifel, D. Giardini| Swiss Seismological Service, ETH-Hoenggerberg, CH-8093 Zurich, Switzerland. |
Introduction
The main tasks of the Swiss Seismological Service are monitoring of the seismicity of Switzerland and its adjacent areas, rapid notification of the authorities and the public, and providing reliable data for earthqauke research. In the early 1970's, a telemetered seismic network was installed and the data were recorded on microfilm until 1983, when the data started to be digitized on a central computer system and to be processed automatically. In the 1990's, the Swiss national strong motion network was installed with 60 free-field stations and 34 dam related stations. Both networks make up the Swiss national earthquake monitoring system.In early 1997 it was decided to replace the outdated short-period FM telemetry system by a new digital network. The new network should be homogeneous with high dynamic range, fully digital with automatic detection and processing of local and teleseismic events with at least the performance of the analog network, and supply the research community with state of the art seismic data. By the end of the year 2000 this network will be fully operational.
Instrumentation
In order to locate local events with the same accuracy as with the analog network, the number of stations was set to 28. All stations are equipped with Streckeisen STS-2 seismometers covering a period range from 120 s to 0.02 s with nearly constant response. For reliability reasons we requested that all other equipment at the station sites must be solid state with no moving parts. The digitizers, communication and data acquisition systems were ordered from Nanometrics (Kanata, Canada).The HRD24 digitizers, synchronized by GPS, are configured to sample the seismic signals at 120 Hz, thus making use of the full period range of the STS-2 seismometers. They store digitized data for about two hours in solid state memory and transmit the data in packets over serial lines to an RM4 bridge. The serial link can be as short as a few centimeters or up to several hundred kilometers when using modem-to-modem connections. The RM4 bridge wraps the serial packets into UDP/IP packets which are broadcasted to two recording NAQS32 computers over the communication network of the Swiss government. Each RM4 can process up to five serial links at 38.4 kbps. The two redundant NAQS32 systems store the seismic signals for about ten days on disk.
Site Selection
Former analog sites were equipped with the new instruments if the local infrastructure, like power supply and communication permit for continuity reasons. In other cases new sites had to be selected. Furthermore, some new sites were added to obtain a better coverage of the Swiss territory. Continuous seismic noise measurements were taken over a period of at least four days at potential sites. The noise spectra were compared to the low noise earth model. In the Alpine foreland where the sedimentary layers are substantially thicker than in the more alpine regions, we had to accept higher noise levels at the lower frequencies due to the oceanic microseisms as well as at higher frequencies due to higher civilization noise.Four new stations were installed in declassified former military bunkers or caverns, which are usually cut into solid rock and exhibit excellent infrastructure. Figure 1 shows a map with the new digital seismic network.
Figure 1. The new digital seismic network (SDSNet) of the Swiss Seismological Service. Blue triangles: Station in operation; Yellow Triangles: Site under construction; Red triangles: Site evaluation under way.
Data Acquisition and Processing
Reliable seismic monitoring can be achieved only by avoiding single points of failure. Wherever possible, we install redundant hardware and ensure alternate data flow paths as can be seen from figure 2.
Figure 2. Block diagram of data acquisition and processing.
Data acquisition is performed on two redundant PC's running the Windows/NT
operating system. The UDP/IP packages sent by the RM4 bridges are
stored in a ringbuffer of about ten days. Whenever the NAQS32 system
detects that a package is missing it issues a retransmit request to the according
HRD24 for as long as availability base on the HRD24's memory can
be expected.
By using UDP instead of TCP no continuous connection needs to be maintained
thus eliminating potential dead lock situations
upon inadvertent interruption of the communication line.
The NAQS32 software performs signal detection (triggers) based on STA/LTA in programmable
frequency bands and event detection based on coincidence criteria
of triggers.
Programs written purely in Java are available on the NAQS32 system
to monitor and control the RM4 bridges, the state of health of the HRD24's,
the GPS receivers and the mass position of the STS-2 seismometers.
Thus, there is full control over the stations from the central site.
All data transmission and acquisition is within the wide area network (WAN)
of the Swiss federal government which is well protected from the
general Internet. The advantage of using this network is always enough bandwidth
for the transmission of the seismic data and very unlikely network congestion.
On the other hand it requires that communication
between our UNIX-data processing system and the NAQS32 system
needs to run through firewalls, which is solved by call back
options of the NAQS32 software.
The data processing system consists of two physical UNIX computers
('wave' and 'front') forming the virtual machine 'seismo'.
Continuous data are transferred from the NAQS32 system in segments
of five minutes duration. The Nanometrics Y-file format is then converted
to the standard format used by the SED.
The 120 Hz continuous waveforms are stored for seven days on hard disks and
thereafter they are available from the archive on the magneto-optical
jukebox. This jukebox currently stores about one year of continuous data.
In addition, for events detected by the NAQS32 system, a waveform segment of three minutes
with twenty seconds pre-event time is transferred from the NAQS32 computer
to 'seismo' and automatically processed in much a similar way as the former
analog data.
All the seismic signals are available within ten minutes through our
AutoDRM.
Conclusions
The Nanometrics hard- and software has proved to be very reliable and flexible. Especially the combination of serial and IP communication allows to position the RM4 at the optimal place in the network. Through the implementation of a re-transmit mechanism by NAQS32 software, virtually no seismic data is lost. Field trips to the stations are reduced to a minimum due to the software giving complete control over each station from the data acquisition center.We will continue to run the analog network for performance and tuning purposes until the end of 2001; thereafter it will be closed.