|Observatories and Research Facilities for EUropean Seismology|
|Volume 3, no 2||December 2001||Orfeus Newsletter|
The Netherlands Seismic Broad-band NetworkReinoud Sleeman
The network of seismic instruments may be divided into broad-band instruments, short period surface and borehole instruments, and accelerometers. This article gives an overview of the network of broad-band stations deployed by the Seismology Division of the KNMI.
Figure 1. Network of seismic stations in the Netherlands. The four labeled broad-band stations are deployed by the KNMI. Stations HGN, WIT and WTSB transmit data continuously to the KNMI.
Table 1. Seismic broad-band stations operated by the KNMI. STS-1 and STS-2) require high-resolution digitizers we are replacing the acquisition systems with broad-band data-loggers from Quanterra Inc. These data-loggers are based on an oversampled delta-sigma digitizer, and have a resolution and dynamic range of about 145 dB. A Quanterra Q4124 was installed at station OPLO in 1998. At the end of 2000, station WIT was upgraded with a Q4124 data-logger, followed by the installation of a Q4126 data-logger at station HGN in 2001. The upgrade of station WTSB with a Quanterra Q730 system is planned in early 2002.
The Quanterra data-loggers are equipped with a GPS receiver for accurate timing of the sampling process. Different sample rate streams are derived from the oversampled output of the digitizers by digital filtering and downsampling. The systems at OPLO, HGN and WIT are recording continuous time-series sampled at 40 sps, as well as segmented (triggered) time-series from earthquakes and other seismic sources digitized at 100 sps. WTSB will be configured in the same way.
The data-loggers in HGN and WIT are connected to the wide area network (WAN) of the KNMI using routers and a dedicated telephone line (Figure 2) to transmit data in real-time. Data from OPLO is stored locally on hard disk and tape, and collected by dial-up. The Q730 (WTSB) does not have local data storage, as opposed to the Q4120. Therefore a PC, running Linux and SeisComP, will be installed at WTSB for local data storage. SeisComP is developed at GEOFON as an open, manufacturer independent data aquisition, recording and distribution system. The system communicates with the Q730 over a serial line, retrieves the waveform data and stores it locally on hard-disk. The PC will be connected to the WAN of the KNMI to transmit data in real-time using 'SeisComp/SeedLink' protocols.
Figure 2. Block diagram of data acquisition and infrastructure.
Station HKB (Herkenbosch) is operated by the Seismology Division of the Royal Observatory of Belgium (ROB) since the beginning of 1997. The station is equipped with a Guralp CMG-40T sensor, and a PC based data aquisition system which was developed by the ROB. The station will be connected to an ISDN line in early 2002. The ROB makes the data available through AutoDRM.
Comserv stores the incoming data in a shared memory ringbuffer, which enables multiple (software) clients to have simultanuous access to the data. Our configuration currently uses two clients. One client (datalog) writes the waveform data and log-records to hard disk, one file per channel per day. Another client (comserv-plugin) forwards the waveform data to a SeedLink server. SeedLink is a robust and reliable communication protocol developed for the GEOFON network to collect data over the Internet or other TCP/IP links. We use a SeedLink server to forward the real-time data from HGN, WIT and WTSB to the real-time acquisition system (Antelope) at the ORFEUS Data Center (ODC). Also, the real-time data communication between station WTSB and the KNMI will be managed and controlled by SeedLink once the Q730 is installed.
A PC running real-time Linux will be used to display the waveform data in real-time on a drum recorder. The SeedLink protocol will be used to collect data on this PC over the network. The digital data is stored in a ring buffer from which it flows smoothly, controlled by a real-time process, to the digital-to-analog converter (DAC) to drive the drum recorder.
The waveform data-files from HGN, WIT and WTSB are archived automatically on the mass storage system (DLT tapes) at the KNMI, once a day. In addition, data from HGN are automatically gathered in a SEED volume and send to the IRIS-DMC (Seattle).
Figure 3. Software block diagram for real-time data flow.
WILBER system. These data are currently in SAC format. Thirdly, the real-time waveform data are fed into ORFEUS' real-time acquisition system Antelope. This system will start to produce full SEED event volumes shortly after an earthquake, which will be made available through WILBER-2. This system is planned for 2002. MEREDIAN (EVR1-CT-2000-40007). The purpose of MEREDIAN is to improve the existing European infrastructure for seismological data exchange.