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ELECTRONIC SEISMOLOGIST January/February 2001 Steve Malone FOREWORD Almost two years ago the Electronic Seismologist (ES) hosted a column by Havskov and Ottemöller (SRL 70:5) on the SeisAn earthquake analysis system. Since then the ES has received a number of e-mails asking questions about this package and how to get it. A common question is, "What about acquiring the seismic data in the first place?" There are many different seismic data-acquisition packages available, and the SeisAn system is well adapted to use data from some of them. However, some questions received indicated dissatisfaction with what the questioner knew or could easily find out about, while other questioners wanted recommendations. The ES, while expert in many useless things, felt at some disadvantage making such recommendations. What better way to get good recommendations than to go back to the source of the SeisAn package? It turned out that this group in Bergen, Norway (one of the few places in the world with a climate colder, darker, and wetter than the home of the ES) has a package exactly suitable for this and other uses. The ES is happy to host this month's column by Utheim, Havskov, and Natvik on a simple but versatile seismic data-acquisition system which would seem to be ideal for the small to medium-sized seismic network. SeisLog DATA ACQUISITION SYSTEMS
T. Utheim, J. Havskov, and Y. Natvik Introduction Currently there are few well documented and tested public-domain seismic data-acquisition systems available. Probably the best known is the W. Lee system (Lee, 1989), distributed by IASPEI software libraries. The system works on a PC and can work with both a local A/D card and recently also a stream of digital data arriving in real time from remote data loggers. Although limited by the MS-DOS operating system, the system has seen widespread use due to being appropriate for many users and being well documented. The newer Earthworm is now gaining wide popularity (Malone, 1996). Earthworm is being developed by several institutions, is very flexible, and works equally well on two operating systems, NT and Solaris. It is clearly the most sophisticated public domain system available. However, Earthworm is not as well documented as the W. Lee system and requires good computer knowledge to set up. This note describes SeisLog, which was initially developed on the DEC PDP for the Norwegian National Seismic Network (Havskov et al., 1992). SeisLog was later moved to the Motorola 6800 VME platform under operating system OS9 (Havskov and Utheim, 1992). The VME version was the first used outside Norway, and several of those systems are still in operation today. For the last few years, SeisLog has been running on Intel PC's under the commercial Unix-like real-time operating system QNX. A recent development is a completely redesigned version for Windows. SeisLog has been in use for fifteen years and is installed at about 100 sites in twenty countries. The system capabilities and sophistication are probably somewhere between the W. Lee and the Earthworm systems. Documentation quality is similar to that of the W. Lee system. All three systems have been used by commercial companies for low-cost installations. The purpose of this note is to describe the current capability of SeisLog and to indicate its development status. SeisLog for QNX SeisLog is intended to be a general-purpose data-acquisition system for recording earthquakes. It uses hardware components which are readily available. The software consists of several separate C programs communicating through a shared memory common block. The software design is divided into a driver part, which is dependent on the hardware used for digitization, and the processing and recording part. For complete details, see Utheim and Havskov (1999). Data flow and recording: The driver part delivers time-stamped data to a multiplexed memory ring buffer where all data have the same sample rate and data are synchronized in time. The data in the memory buffer are then used for performing a traditional STA/LTA trigger. Event files are written directly from memory. All channels are also written into a disk ring-buffer system from which any time segment can later be extracted. Optionally, resampled data can be written in a separate ring buffer. The trigger parameters include standard parameters like filter, pre- and postevent memory length, and an array propagation window. Up to five different trigger sets can be used simultaneously, so the system can be set up to trigger independently under different conditions using different stations. Each trigger set can select a subset of particular channels to write out in event files. Digitizers: SeisLog has several drivers for different A/D devices. Metrabyte A/D cards are supported. Other digitizers used are external and send their data to SeisLog via a serial line. Digitizers from Nanometrics, Terra, GeoSig, Earth Data, and Guralp are supported. Timing: Timing can be done in two ways. If the digitizer has built-in timing, such as most external data loggers, no other time stamping is done. For digitizers without timing, a Garmin GPS with a 1 Hz output is used to synchronize a SeisLog software clock. Data are then time-stamped when they arrive, and appropriate delays in the transmission are accounted for. The overall accuracy is better than 10 ms. Processing: The system has several independent programs for monitoring operation, plotting data, reading phases, computing locations, plotting epicenters, etc. These are simple and fast to use and are intended only for preliminary processing. The data are recorded in a simple binary format, but event files can optionally be written in SAC. A conversion program is available to convert to SeisAn format (Havskov and Ottemöller, 1999). Communication: QNX is a multiuser system, and data can easily be downloaded from the Internet. There is also a facility to download CMP6-compressed data by modem. Automatic data collection can be performed with SeisNet (Ottemöller and Havskov, 1999) using a modem and/or the Internet. SeisLog can also automatically send out e-mail to notify users about new events or notify operators of potential problems. Since the system provides normal login via telnet or modem connections, all normal system operations can be done remotely. Use of SeisLog: SeisLog is currently used for single broadband or short-period stations with external digitizers as well as centrally recording networks using A/D cards. Both uses can be combined into a regional network, such as has been done in Central America (Alvarenga et al., 1998). The QNX operating system has proven to be a very stable operating platform, and systems have been in operation for many months with frequent user logins and without any interruption in operation. SeisLog for Windows More and more digitizers or data loggers now have built-in timing, so there is less need for a real-time operating system. Many users are familiar with and have Windows 95/98/2000/NT, so it is convenient to have a simple data-acquisition system using these standard operating systems. Similar to the QNX version, SeisLog for Windows (Natvik and Havskov, 1999) consists of one or several digitizers connected to a PC via serial lines and/or an A/D board. Each digitizer (maximum 16) can have up to 64 physical channels, but a total maximum of 64 channels can be handled by the system. The digitizers normally provide time-stamped buffers. However, if no external trace time-stamping is available, timing is done by the CPU, but less accurate timing is obtained even though the CPU time can be synchronized with a GPS clock. The digitizers need not use the same sample rate nor be time-synchronized. Currently the system can work with Nanometrics, Earth Data, and GeoSig external digitizers and a Computer Board A/D card. The data from each or selected physical channels enter a memory ring-buffer system where STA/LTA calculations are performed and single-channel detections are declared. Each channel can optionally be written to a disk ring buffer. Using the detection states of the memory ring buffers, network detections are declared using up to five different sets of detection criteria, such as minimum number of channels for detecting an event among a given set of channels. On declaration of an event, the waveform data file for selected channels is written directly from memory. In addition, the trigger times for each channel are written in a separate file. Both files are written in SeisAn format and can optionally be written directly into a SeisAn database for ease of later analysis or automatic location and magnitude calculation. Since SeisAn is used for the main data inspection and processing system, SeisLog has only one monitor window where real-time signals can be shown and many real-time parameters monitored. Both event files and ring-buffer files can be plotted with SeisAn. This means that complete analysis can be done directly on the recorded data; however, a simple preliminary analysis is now more involved than under the QNX system. If the system is to be used remotely, an FTP server can be installed and data downloaded via FTP. This interface will also work with SeisNet. However, in order to set parameters or check the system operation remotely, a commercial software package such as PCAnywhere must be used, and the system cannot operate with SeisNet via a modem. Discussion SeisLog for QNX is a very well tested, robust system with a proven record. It has very good communication capabilities that are easily set up. It lacks some flexibility in using a mix of digitizers and sample rates. However, that is rarely a problem in our experience. A drawback has been that the user has to pay for a QNX license; however, QNX will soon be free for noncommercial use. SeisLog for Windows has some of the flexibility that SeisLog for QNX lacks: a Windows user interface and the advantage of recording directly into the SeisAn database. It does not have very accurate internal timing (~50 ms), the system is less well tested with only a couple of systems in operation, it has fewer options generally, and the communication ability is poorer. However, it is easy to set up and use, and it is particularly well suited for local operation. Both systems are under continuing development. New digitizers are being added as need arises. A prototype Web interface to the systems is now being tested. This will make it possible to inspect and download data using a browser. For the QNX system, it will also be possible to monitor real-time data and set parameters with a browser. There has so far not been a need for more than 64 channels, so there is no plan to expand that capability. However, this is not a programming problem. How to Get SeisLog SeisLog for QNX (version 8.1) and SeisLog for Windows (version 0.99) are available via FTP at ftp://ftp.ifjf.uib.no/ or on the Web at http://www.geo.uib.no/seismo/SOFTWARE. SeisLog is distributed as a compiled version (source code included). Both systems have complete manuals. Programs are written in C. SeisLog for QNX requires a QNX operating system license, which can be obtained at http://www.qnx.com/qnx2000/index.html. For graphics printing, a PostScript printer is needed. SeisLog is also distributed on a CD Seismological Software Collection from the University of Bergen, which among other programs also includes SeisAn and SeisNet. REFERENCES Alvarenga, E., R. Barquero, I. Boscini, J. Escobar, M. Fernández, P. Mayol, J. Havskov, N. Gálvez, Z. Hernández, L. Ottemöller, J. Pacheco, C. Redondo, W. Rojas, F. Vega, E. Talavera, W. Taylor, A. Tapia, C. Tenorio, and J. Toral (1998). Central American Seismic Center (CASC), Seism. Res. Lett. 69, 394-399. Havskov, J., L. B. Kvamme, R. A. Hansen, H. Bungum, and C. D. Lindholm (1992). The Northern Norway Seismic Network: Design, operation and results, Bull. Seism. Soc. Am. 82, 481-496. Havskov, J. and T. Utheim (1992). SeisLog and SeisAn: A complete system for seismic data acquisition and analysis, Cahier du Centre Européen de Géodynamique et de Sismologie 5, 67-74. Havskov, J. and L Ottemöller (1999). SeisAn earthquake analysis software, Seism. Res. Lett. 70, 532-534. Lee, W. H. K. (1989). Toolbox for Seismic Data Acquisition, Processing and Analysis, IASPEI Software Library, Vol. 1, published by IASPEI in collaboration with the Seismological Society of America. Malone, S. (1996). Near-real-time seismology, Seism. Res. Lett. 67, 52-54. Natvik, Y. and J. Havskov (1999). SeisLog: A Seismic Data Acquisition System for Windows 95/NT, Version 1.0, Manual, Institute of Solid Earth Physics, University of Bergen, Norway, 30 pp. Ottemöller, L. and J. Havskov (1999). SeisNet: A general purpose virtual seismic network, Seism. Res. Lett. 70, 522-528. Utheim, T. and J. Havskov (1999). The SeisLog Data Acquisition System, Version 8.1, manual, Institute of Solid Earth Physics, University of Bergen, Norway, 105 pp. SRL encourages guest columnists to contribute to the "Electronic Seismologist." Please contact Steve Malone with your ideas. His e-mail address is steve@geophys.washington.edu.
Posted: 21 January 2001 |