|Observatories and Research Facilities for EUropean Seismology|
|Volume 2, no 3||December 2000||Orfeus Newsletter|
Networked Seismographs: GEOFON Real-Time Data DistributionWinfried Hanka1, Andres Heinloo2 and Karl-Heinz Jaeckel1
Therefore the idea to develop an open, flexible, comfortable, and low cost seismological data acquisition, processing, and communication system with real-time networking capability was born. The Seismological Communication Processor (SeisComP) project is an initiative of the GEOFON Program of GFZ Potsdam with contributions of several other institutions. It relies on commercial VBB digitizers, cheap, reliable and worldwide available hardware (mainly PCs with Linux operating system), the Internet as major communication channel and - as much as possible - on proven open software. The present version of SeisComP consists of four individual software packages for data acquisition, recording and monitoring (station or network processor functionality), real-time communication and user access tools (data center processor functionality).
Figure 1. The basic components of a SeisComP system.
Acquisition, recording and user access of the data from one or more digitizers can take place on the same physical computer (stand-alone mode) or on different SeisComP nodes connected by the real-time communication system SeedLink (networked mode). The network communication is based on dedicated or dial-up TCP/IP links or Internet. At each node of the network, locally acquired data and/or data from other nodes can be jointly recorded and accessed. So the configuration of arbitrary "virtual" real-time networks is easy to achieve.Quanterra Inc and UC Berkeley to provide acquisition functionality for data from Quanterra dataloggers on Sun computers. This package was ported to Linux OS to operate on PC hardware. Therefore, all presently available Quanterra dataloggers are supported (Q680, Q4120, Q730). At GEOFON stations, the data are normally acquired over a RS-232 serial link (hard link, direct cable or arbitrary modem link) using the standard Quanterra comlink protocol. The Quanterra master console is usually connected by a second serial link. The server of the original ComServ package is modified such, that it is possible to acquire data also securely via TCP/IP using the SeedLink protocol (soft link, see below). That allows a combined acquisition of locally and remotely connected digitizers, even over large distances using the Internet. It is also possible to connect to the remote station in dial-up mode on arbitrary schedules.
Beside the original Quanterra dataloggers also other types of digitizers or even arbitrary data sources can be supported by SeisComP. This is done by implementing plug-ins specially designed for each new digitizer. Presently, in the public SeisComP version, this is the case for the Earth Data PS2400. The actual plug-in talks to the digitizer in its specific transfer protocol and transmits the data and logging information to another program called digiserv, which provides the data in MiniSEED packets to the modified ComServ server. Down-sampling and the generation of multiple data streams for those digitizers which do not support this, can also be performed within digiserv. Normally, it is expected that the digitizers provide correctly time-tagged data already. Lennartz Electronics has just finished a plug-in for their M24 digitizer which does not have GPS timing on board. In this implementation, a GPS corrected real-time clock on the Linux PC is used instead.
Local data recording is essential for stand-alone station processors. The original ComServ datalog client supports only disk storage. A new client, datadump, records on any tape medium and keeps track of disk and tape space. An extended version of the recording and monitoring package, used by SZGRF for the German Regional Seismic Network (GRSN), also supports disk shadowing and automatic CD-R recording.
Data acquisition and recording can be monitored within SeisComP by a number ComServ clients and other tools implemented in the Station Operation Manager (SOM). Among others, the Qplot client provides online monitor plots on screen, files (including web browser loadable gif files) or printer. A Java applet for real "live seismograms" is planned in future versions. Also all kinds of logging and state-of-health information can be displayed in special windows.
Figure 2. Example data flow in a SeedLink network using direct and dial-up connections and chain and star type communication models.
The SeedLink server is designed to support unreliable connections. Some number of recent packets are kept in the main memory for very efficient access, older packets are temporarily stored in the disk buffer. So it is possible to completely recover the connection even if the network is down for a long time. The size of memory and disk buffers can be specified by configuration. No error correction is implemented by SeedLink, however, because the TCP protocol guarantees error-free communication. The SeedLink concept is not only useful for real-time data exchange between stations and data centers - providing SeedLink access to users in addition to traditional data request methods makes it possible to run applications which require real-time datastreams. Each user can in principle create his own virtual VBB real-time network.Swiss AutoDRM (by ETH Zurich and GI Stuttgart) and the IRIS breq_fast. The user access package can also be used independent from a SeisComP station or network setup as an automated data center if the data base is provided in a defined flat-file structure.
So far only high-power standard office-type or industry PC hardware is used in SeisComP installations. Presently a low power version based on embedded PC104 boards is being developed and will become commercially available soon. Together with low-power digitizers (such as the Earth Data PS2400 or the new Q330 by Quanterra) and modern radio communication equipment (wireless LAN, "bluetooth"), this development will allow to use SeisComP in remote field installations where minimizing the power consumption is critical, but where advanced recording or networking capability is still important. Since the new GFZ field datalogger, manufactured by Earth Data, is using a newer version of the PS2400 digitizer unit and also a PC104 based Linux system for recording, it can easily be supported by the SeisComP software as well. That will allow in future temporary experiments with low power equipment following the same networking, monitoring, processing and archiving strategy as used for modern permanent real-time networks (e.g. in connection with a mobile satellite hub for the field data collection center).
A major future SeedLink improvement will be the support of reverse Internet connections. That means, the data center will not connect to a station as in standard mode, but the station will connect to the data center via a local Internet provider. Thereafter, the data center will request the next batch of data in the dialup mode as usual. This would allow to perform near-real-time data distribution at lowest possible costs, even take advantage from Internet flat rates. Only this measure will open SeedLink communication forthe majority of GEOFON stations, where direct Internet access is not possible and long-distance dial-up calls are too expensive to be able to download all available data.
A special SeedLink plug-in will be designed to support the transfer of arbitrary data files. That will e.g. allow to access non-SeisComP stations with DRM or AutoDRM functionality and feed conventionally retrieved data records into the SeedLink near-real-time distribution scheme. This feature can also be used to replace ftp data transfer of arbitrary files where unreliable Internet connections or security problems will not allow standard ftp usage.
Java based web tools are planned for easy SeisComP installation, configuration and monitoring over the web and across the network.
An event detection client will be implemented in the recording and monitoring package to provide event triggering capability for those digitizers, which do not provide this feature onboard.
The IRIS NetDC networking protocol for DC-DC data exchange will be integrated in the user access package.
Intensive discussion and several developments regarding real-time data distribution are going on within the IRIS community (Ahern, 2000) as well as in Europe. It is planned to closely coordinate with these efforts and to integrate the SeisComP concept in an even broader context.
The SeisComP concept has been proven to provide an open, flexible and comprehensive solution for a manufacturer independent low-cost networked VBB seismological communication system. It includes software packages for data acquisition, recording, monitoring, real-time networking across all kinds of TCP/IP networks and sophisticated user access. It has been successfully installed in the GEOFON and co-operating networks and provides a number of new features, including real-time data distribution. Besides its advantages in terms of improved network operation - easier maintenance, more complete data basis, faster data quality checks and failure detections - SeisComP offers new possibilities in the usage of regional or even global VBB data for automated near-real-time applications like quick epicenter localization and magnitude determination as well as quick moment tensor computation. This would substantial improve the rapid warning capability of the public after larger earthquakes. Substantial improvements of the package are planned within the MEREDIAN EC project.