Observatories and Research Facilities for EUropean Seismology
Volume 2, no 3 December 2000 Orfeus Newsletter


BFO and Moxa: two observatories for seismological broadband observations

C. Kroner1, Th. Jahr1, G. Jentzsch1, W. Zürn2, R. Widmer-Schnidrig3 and B. Heck4,
1 Institute of Geosciences, Burgweg 11, 07749 Jena, Germany
2 Geophysical Institute, Hertzstr. 16, 76187 Karlsruhe, Germany
3 Institute of Geophysics, Richard-Wagner-Str. 44, 70184 Stuttgart, Germany
4 Geodetic Institute, Englerstr. 7, Postfach 6980; D-76128 Karlsruhe, Germany

Introduction - Black Forest Observatory Schiltach -
Geodynamic Observatory Moxa - Note - References

Introduction

During the last decades a number of seismological stations were established around the world thus improving the data basis for seismological studies. Some of these observatories are equipped with additional instruments besides 3-component broadband seismometers to monitor deformations. Data from these observatories allow integrated studies of the deformation field from seismic frequencies up to tidal bands. When meteorological sensors are installed at these seismological stations, from the monitoring of the environmental conditions conclusions can be drawn whether changes in those affect the noise contents of the seismological observations. Comparisons with i.e. gravity, tilt and strain data then can point to possible physical mechanisms of noise transfer, a first step towards a noise reduction in the seismological wave form data. This belongs to one of the activities of the working group 7 dealing with environmental influences on gravity, tilt, and strain observations, established 1997 by the Earth Tide Commission.

Two observatories operating a 3-component STS2 seismometer and being equipped with further instrumentation for deformation monitoring as well as a set of meteorological sensors are described in the following. Both, the Black Forest Observatory Schiltach and the Geodynamic Observatory Moxa, are located in Germany (Fig. 1) in a distance of about 400 km from each other.


Figure 1. Observaories BFO and MOXA

Black Forest Observatory Schiltach

The Black Forest Observatory (BFO) is also known as Schiltach Observatory and is located in the Central Black Forest in SW-Germany (approximately in the centroid of a triangle with its corners at the cities of Stuttgart, Karlsruhe and Freiburg and about 70 km due east of Strasbourg, France). It is operated jointly by the Institutes of Geophysics and Geodesy at the Universities of Karlsruhe and Stuttgart. The WGS84-coordinates of the STS-2 seismometer are 48.3301 degs N, 8.3296 degs E and 639 m ellipt. elevation (589 m amsl). A brief description of the observatory in English can be found in Emter et al. (1994), more extensive descriptions in German in Emter et al. (1998, 1999). The sensors are located in an old silver and cobalt mine in a hillside, horizontally between 400 to 700 m from the adit and 150 to 170 m below the surface. The rock is granite. An air-lock with a time constant of more than 12 hours stabilizes the environment of the sensors in temperature, air pressure and air convection. Temperature is slightly less than 10 degrees centigrade and is stable to better than 5 mK unless this part is entered. It has not changed by more than 0.05 degrees since 1972. The distance to the nearest small scale industry is a minimum of 4 kilometers. The next railroad with only very little traffic and no freight trains and major road are 5 km to the South. More details on BFO can be found at the following Web-sites:
www-gik.bau-verm.uni-karlsruhe.de/~bfo
www-gpi.physik.uni-karlsruhe.de/pub/widmer/BFO
At present (Dec. 2000) the following instruments are operating at BFO:

Instrument Sampling Rate (Hz) Archive
1 STS-2 (ZNE) 80, 20, 1 SZGRF, ORFEUS, IRIS
3 STS-1 (ZNE) 20, 0.2 IRIS, BFO
1 Geotech S-13 (Z) 125 (PCM) GLA
2 Kinemetrics SH-1 (NE) 125 (PCM) GLA
1 LCR-ET 19 tidal gravimeter with electrostatic feedback 0.1, 0.2 IRIS, BFO
1 ASKANIA tiltmeter (NE) 0.1, 0.2 IRIS, BFO
1 DPFT (110 m, N 331 E) differential pressure fluid tiltmeter 0.2 BFO
3 Cambridge-strainmeters 10 m Invar wires (N 2, 60, 300 E) 0.1, 0.2 IRIS, BFO
3 magnetic variometers 0.0167 FUR
In addition
2 Barometers (rel.) inside, outside 0.1, 0.2 IRIS, BFO
2 Thermometers (rel.) 0.0167 BFO
1 rain gauge 0.0167 BFO
1 Anemometer analog BFO

(GLA stands for: Geologisches Landesamt Baden-Wuerttemberg, Freiburg; FUR for: Magnetic Observatory Fuerstenfeldbruck)

The mine and some of the instrumentation is also described in Widmer et al. (1992) and Richter et al. (1995). The quality of some of the data from BFO was mentioned in Beauduin et al. (1996), Freybourger et al. (1997), Banka and Crossley (1999) and Masters et al. (2000). Emter et al. (1999) contains a list of relevant publications from the personnel operating BFO. A recent paper showing free-mode spectra is by Zürn et al. (2000). (For more information contact R. Widmer-Schnidrig or W. Zürn at BFO)

Geodynamic Observatory Moxa

Moxa observatory, located in the eastern part of Thuringia (Germany), has a more than 30 year long tradition of seismological observations. It was established in 1964 when the city of Jena became to noisy as a location for a seismological station. The site of Moxa was chosen because it was close enough to Jena (about 30 km south) but at the same time sufficiently far away from industrial plants, major roads, and towns. The given criterium was none of those should exist within a radius of 10 km. Another decisive factor was the existence of the Silberleite valley which allowed to build the observatory partly into a hill. Regarding the geology the observatory is situated in the Ziegenruecker Kulmmulde in the Thuringian-Franconian Slate Mountains that consists of intensively folded and fractured basement rocks. In the surroundings of the station thick series of slates and graywackes of Lower Visean Age are found. During the years 1996 to 1999 the seismological station was modernized and extended into a geodynamic broadband observatory. The general objective of the observatory is to monitor, analyze and interpret geodynamic signals due to mass shifts and deformations at the Earth's surface ranging from seismic frequencies up to long-term variations. Besides studies of global geodynamic signals research interests focus on the influence of environmental variations on the geophysical data monitored and their reduction. Moxa observatory is included in two larger networks: Like the BFO it is a station of the German Regional Seismic Network (GRSN) and with its superconducting gravimeter it belongs to the Global Geodynamics Project (GGP; Crossley et al., 1999), a global network of these instruments (see GGP superconducting gravimeter data base). At the observatory a seismogram archive exists with the first seismogram dating back to 1904. The WGS84-coordinates of the STS-2 seismometer are 50.6447 degs N, 11.6156 degs E and 501 m ellipt. elevation (455 m amsl).

At present (Dec. 2000) the following instruments are operating at the observatory:

Instrument Sampling Rate (Hz) Archive
1 STS-2 (ZNE) 80, 20, 1 SZGRF, ORFEUS
1 STS-1 (ZNE, loan BGR) 0.1 MOX
2 quartz tube strainmeters (EW, NS; 25 m long) 0.1 MOX
1 laser strainmeter (diagonal, 38 m long) 0.1 MOX
1 LCR-ET 18 tidal gravimeter with electrostatic feedback 0.1 MOX
1 GWR superconducting gravimeter CD-034 with a double sensor system 1 MOX, GGP (Jan. 2001)
ASKANIA borehole tiltmeters 0.1 MOX

The rock and soil coverage of the observatory chambers ranges from 2 to 3 m in the front area up to 35 m for the observatory parts inside the hill. All strainmeters as well as the STS-1 and STS-2 seismometers are operating in the area with maximum coverage. The gravimeters are installed in the covered front area in a temperature-stabilized room. The tilt measurements are carried out in the boreholes (depth 50 and 100 m) in front of the observatory. For the observation of environmental variations several sensors are available. Most of these sensors are mounted on a pole at a roof corner of the observatory building. The following meteorological parameters are continuously monitored:

  • barometric pressure
  • air temperature
  • air humidity
  • wind velocity
  • wind direction
  • precipitation
  • illuminance
  • soil temperature/humidity
  • groundwater table variations

The sampling rate of these parameters is 0.1 Hz. In addition a temperature and a barometric pressure sensor are installed in the gallery and in the temperature-stabilized gravimeter chamber. The pressure sensor in the gravimeterchamber has a sampling rate of 1 Hz. The monitoring of the water table variations is carried out in a 50 m deep borehole in front of the observatory.

More details on Moxa observatory can be found at www.geo.uni-jena.de/moxa/ or contact Th. Jahr or G. Jentzsch. Some recent publications about the instruments operating at the observatory and results of some noise studies in the long periods are by Jahr et al. (2001) and Kroner et al. (2001).

Note
Station height is here measured according to two definitions:
ellipt: is the distance along the ellipsoidal normal from the reference ellipsoid (representing an idealized earth figure) to the observation point. The ellipsoid used is defined in the World Geodetic System 1984 (WGS84) by the U.S. Department of Defense.
amsl: is short for 'above mean sea level'. The mean sea level is derived from tide-gauge registrations carried out over many years of one or more coastal stations. As mean sea levels of different regions do not coincide, heights are additionally refered to a global height datum as e.g. given by WGS84 at the National Imagery and Mapping Agency (NIMA) in the US.

References

  • Banka, D., Crossley, D., 1999. Noise levels of superconducting gravimeters at seismic frequencies. Geophys. J. Int., 139, 87-97.
  • Beauduin, R., Lognonne, P., Montagner, J.-P., Cacho, S., Karczewski, J. F., Morand, M., 1996. The effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station. Bull. seismol. Soc. Am., 86, 1760-1769.
  • Crossley et al., 1999. Network of superconducting gravimeters benefits a number of disciplines. EOS, 80, no. 11, March 1999.
  • Emter, D., Wenzel, H.-G., Zürn, W., 1994. The Black Forest Observatory, Schiltach. Soil Dynamics and Earthquake Engineering, 16, 73-75.
  • Emter, D., Wenzel, H.-G., Zürn, W., 1998. 25 Jahre Geowissenschaftliches Gemeinschaftsobservatorium Schiltach. Z. Verm. 123, 2, 61-67.
  • Emter, D., Wenzel, H.-G., Zürn, W., 1999. Das Geowissenschaftliche Gemeinschaftsobservatorium Schiltach (BFO). Mittlg. DGG, 2, 4-15.
  • Freybourger, M., Hinderer, J., Trampert, J., 1997. Comparative study of superconducting gravimeters and broadband seismometers STS-1/Z in seismic and subseismic frequency bands. Phys. Earth planet. Inter., 101, 203-217.
  • Jahr, Th., Jenztsch, G., and Kroner, C., 2001. The geodynamic observatory Moxa: instrumentation and purposes. to be publ. in J. Geodetic Soc. Japan.
  • Kroner, C., Jahr, Th., and Jentzsch, G., 2001. Comparison of data sets recorded with the dual sphere superconducting gravimeter CD 034 at the Geodynamic Observatory Moxa. submitted to J. Geodetic. Soc. Japan.
  • Masters, G., Laske, G., Gilbert, F., 2000. Autoregressive estimation of the splitting matrix of free-oscillation multiplets. Geophys. J. Int., 141, 25-42.
  • Richter, B., Wenzel, H.-G., Zürn, W., Klopping, F., 1995. From Chandler Wobble to Free Oscillations: Comparison of Cryogenic Gravimeters and Other Instruments in a Wide Period Range. Phys. Earth Planet. Inter., 91, 131-148.
  • Widmer, R., Zürn, W., Masters, G., 1992. Observation of Low Order Toroidal Modes from the 1989 Macquarie Rise Event. Geophys. J. Int., 111, 2, 226-236.
  • Zürn, W., Laske, G., Widmer-Schnidrig, R., Gilbert, F., 2000. Observation of Coriolis coupled modes below 1 mHz. Geophys. J. Int., 143, 113-118.


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