Modernizing the Danish Network
Tine B. Larsen and Peter Voss
The Danish seismological network is currently highly heterogeneous with seismometers
ranging from WWSSN over S-13 to STS-1 that are connected to a mixture of digital and
analog recording systems. In early 1998 extra funds were made available for upgrading the
stations in Denmark and Greenland in order to obtain a more homogeneous, fully digital
network. The objective of this letter is to outline the process of modernizing an entire,
although small, network with the hope to inspire and encourage other network operators
planning similar undertakings.
Fig. 1. Stations in Denmark
The very first step of the process was to envision the look and feel of the upgraded
network. This lead to the formulation of the following list of requirements:
- Broadband seismographs. The added possibilities offered by broadband
seismographs over short-period instruments, combined with a long Danish tradition for
surface wave studies made this an easy choice.
- Large local ringbuffers. Data must be saved locally at each
station, even if the data is transferred continuously to the main office. Too many times
data has been lost due to problems with the phone lines, and the price of large hard
disks has become quite manageable.
- Local time stamping. In the current system data from several
stations are transferred over dedicated phone lines to the KMS office, where the time
stamps are added. This procedure is incompatible with local storage of the data.
- Dial-up access to all stations. It is not feasible to transfer data
from all stations in the network over phone lines on a daily basis, but it should be
possible to call up any station and transfer chunks of data whenever desired.
- Continuous data. Local Danish earthquakes usually do not stand out
significantly from the background noise and could too easily be overlooked by an
automatic event picker. It is therefore necessary to save continuous data.
- Historical link. A connection should be established between the old
data and the data from the new instruments. It is important to know how the new
registrations relate to the old ones, otherwise the data will be less useful.
- Instrument response. Only companies willing and able to provide
complete information about data formats and instrument response ahead of the purchase
would be considered.
Based on these requirements, a call for proposals was sent out to a handful of companies.
The companies were asked to delineate how their equipment would fit into our upgrade
strategy, which made a final purchase decision easier.
The Streckeisen STS-2 seismometer was chosen due to its wide frequency band, good
reputation and widespread use among colleagues. To digitize the data the
Nanometrics HRD24 was chosen owing to
considerations of price, quality and customer support. The HRD24 also has a built-in
GPS for timing and it is compatible with the
University of Bergen SEISLOG
acquisition software, which we decided to use. The SEISLOG system has the advantage that
it is free, and it is used by some of our close neighbors, both in Norway and at the
British Geological Survey. The SEISLOG and SEISAN packages will be supplemented by some
KMS developed PC software.
Data transfer between the seismological stations and the KMS is complicated by the fact
that the KMS computer systems are protected by a firewall. It is therefore impossible to
make direct dial-up connections between the stations and the office computers. Instead we
are setting up a system where we can call the stations in Denmark over ISDN lines and
transfer the data through a central KMS dial-up point to the main computer system for
processing. Two stations, COP and MUD, will keep on sending data continuously to the KMS
office over dedicated phone lines, while the remaining stations will be used on a dial-up
basis. Continuous data from the dial-up stations will be saved on CD-ROM or DAT and
mailed to the KMS on a regular basis.
The situation in Greenland is more complicated. While ISDN is available in Greenland, the
fast transfer rates can be obtained only within Greenland and not across the Atlantic to
Denmark. Given the high cost of phone calls to Greenland, it is not feasible to transfer
data on a regular basis over phone lines. Instead we are working on an Internet solution,
where the stations in Greenland would connect locally to the Internet, so that we may ftp
the data from Greenland at the cost of a local phone call. The technical details of this
solution have not been worked out yet.
Data from the Danish network (old system only) is available by autodrm
(email@example.com), and the weekly bulletins can be
found on the
KMS seismology web page
under Seismic service.
Fig. 2, stations in Greenland.
Before the Danish seismological network is fully upgraded and the old instruments are
phased out, a final calibration will be performed on all the current seismographs. In
this way the older data is well-documented and can be used later. The method chosen is
developed by Prof. Erhard Wielandt and was presented at the ORFEUS workshop in Prague,
November 1998. For our calibration we used his software,
method is a relative calibration of each seismic sensor where the calibration signal is
compared with a known reference signal. The calibration has been performed on the STS-1
seismometers at the COP station, but not without difficulties, as it turned out that the
old Nanometrics RD3 digitizer had been modified with a high-pass filter. This experience
emphasizes the importance of calibrating old equipment, when no-one can remember how it
Four STS-2 seismometers and four HRD24 digitizers were delivered in December 1998, and
the instruments are planned to be in operation at MUD, BSD and SCO by fall 1999. The new
station in Narsarsuaq will be slightly delayed due to a field project, but the station is
expected to be fully operational sometime in 2000. The old systems will be shut down by
the end of 1999.
We would like to thank our colleagues at KMS who participate very actively in this
process, Jens Havskov and Terje Utheim, University of Bergen, Winfried Hanka, GFZ,
Erhard Wielandt, University of Stuttgart, Jure Ravnik, Geophysical Survey of Slovenia,
and ORFEUS for invaluable help and numerous discussions.