Observatories and Research Facilities for EUropean Seismology
Volume 3, no 2 December 2001 Orfeus Newsletter


The Netherlands Seismic Broad-band Network

Reinoud Sleeman
Seismology Division, Royal Netherlands Meteorological Institute (KNMI), P.O.Box 201, De Bilt, The Netherlands

Introduction - Network of broad-band seismometers
Data communication and archving - Data availability

Introduction

The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) deploys a network of seismometers and accelerometers (Figure 1, Table 1) to monitor global, regional and local seismicity. The seismicity in the Netherlands can roughly be divided into tectonic seismicity in the southern part of the country and induced seismicity in the northern part. Seismic hazard studies show that the maximum expected Richter magnitude (ML) for the Netherlands is about 6.3 for the southeast region. The largest earthquakes in that region, recorded by seismometers, occurred at Uden in 1932 (ML = 5.0) and at Roermond in 1992 (ML = 5.8). Earthquakes in the northern part of the Netherlands have been recorded since 1986, and are related to the extraction of gas from the subsurface. Recently, an ML = 3.5 earthquake near Alkmaar was recorded on 9 September 2001. The maximum expected Richter magnitude for induced earthquakes in the Netherlands is 3.8.

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.

click for large image

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.

HGN WIT WTSB OPLO
Location Heimansgroeve Witteveen Winterswijk Oploo
Latitude 50.764 52.814 51.966 51.589
Longitude 5.932 6.668 6.799 5.812
Elevation (m) 135 17 43 24
Sensor STS-1 STS-1 STS-2 STS-2
Bandwidth 360 sec - 10 Hz 20 sec - 5 Hz 120 sec - 50 Hz 120 sec - 50 Hz
Gain 2400 V/m/s 2400 V/m/s 1500 V/m/s 1500 V/m/s
Installation Sensor Feb 1993 Feb 1994 Feb 2000 Sep 1998
Quanterra data-logger Q4120 Q4120 Q730 Q4120
Continous recording 40 sps 40 sps 40 sps 40 sps
Triggered recording 100 sps 100 sps 100 sps 100 sps
Local storage HD, tape HD, tape PC HD, tape
Installation Quanterra Jun 2001 Dec 2000 Early 2002 Sep 1998
Data transmission Continuous
Real-time
Continuous
Real-time
Continuous
Real-time
Event based
On request
Protocol TCP/IP, Comserv TCP/IP, Comserv TCP/IP, SeedLink Dial-up, Kermit

Table 1. Seismic broad-band stations operated by the KNMI.

Network of broad-band seismometers

The first digital data-acquisition system for the Netherlands broad-band network dates back to around 1990 and is based on a conventional analog to digital converter. This system uses gain ranging and has a resolution of only 16 bits and a dynamic range of about 140 dB. As our broad-band seismic sensors (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.

Data communication and archiving

The real-time data communication between the KNMI and stations HGN and WIT is managed and controlled by comserv (Figure 3), a software package developed by Quanterra Inc. and UC Berkeley. Comserv runs on a Sun Ultra 5 system under Solaris 7, and is configured to communicate with the data-loggers over TCP/IP. The package enables remote acquisition of the data, system monitoring and user access. In our configuration the 40 sps continuous data-streams and the 100 sps triggered data are collected in real-time by comserv. The data are collected in mini-SEED format, using STEIM-2 compression.

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.

Data availability

The broad-band waveform data are open for the seismological community in different ways. First, data may be requested by e-mail. Both AutoDRM (autodrm@knmi.nl) and NetDC will be interfaced to the waveform data from HGN, WIT and WTSB, as to exchange data by e-mail or ftp. AutoDRM provides the data in the GSE2.0 or SEED2.3 format, NetDC ships data in SEED2.3. Secondly, triggered earthquake data are currently collected by the Spyder system at the ODC and made available through the 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.

Acknowledgements

The KNMI coordinates the EC funded project MEREDIAN (EVR1-CT-2000-40007). The purpose of MEREDIAN is to improve the existing European infrastructure for seismological data exchange.

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