Gravity and tidal observatories of the Geophysical Institute The tidal observatories of the Geophysical Institute have been recording tidal data since 1952 (Příbram, later Římov, Kašperské Hory, Lazec and Jezeří-Jezerka). In 1965-68 the first international comparative measurements of various models of tiltmeters took place in Příbram observatory in a deep mine. In the 1990s another two sites were selected for further observations with respect to favourable conditions, one of them for the research of tidal effects (Lazec - a part of the world network) and the other one for applied research at a coal mine gallery (Jezeri - observation of rock massif stability). In 2003 we decided to install various instruments into the seismological station Skalna with the aim of establishing a new complex geodynamic observatory in the seismoactive region of Western Bohemia.
The recorded data provide information on temporal variation of the Earth's gravity field and on the deformation of the Earth's body induced by the tide generating forces of the Moon, the Sun and the planets. The effects of other geological phenomena and processes (earthquakes, fluids, etc.) are of interest as well.
The world-wide observations were promoted by the Earth Tide Commission of IAG (Intern. Assoc. of Geodesy) in order to obtain data for the assessment of local, regional and global deformation models of the Earth.
PŘÍBRAM, Central Bohemia [49.6861N, 13.9972E]
The tidal group is working at this observatory located on top of an abandoned deep mine (depth 1300 m). At this site all istruments are maintained and undergo long-term tests in the mine gallery (tiltmeters, gravimeter, barographs, etc.). In the mine gallery there is also a seismological station.
The data from all other sites are transmitted to this tidal centre for control and processing. Besides a number of in-house software, the Preterna and Eterna packages (Germany) are used for data processing and analysis.
The photos show a typical digital/analog recording setup and a PC screen view of recorded tiltmeter signals with obvious Earth tide curves.
 LAZEC, South Bohemia [48.8367N; 14.2828E]
The tidal observatory was established in a graphite mine gallery as a research station. Special chamber was cut for the installation of two perpendicular self-produced extensometers with 20 m long pure quartz tubes, tested for both capacity and inductance recording systems. Two tiltmeters ASNS with capacity sensors and resolution of 0.0001" recorded tilts in both N-S and E-W directions. Gravity meter BN27 (adjusted Askania GS11) with additional control of internal temperature recorded continuously the gravity acceleration. All instruments were connected to digital recording system with modem data transfer to the processing centre in Příbram. The data were being sent to the World Earth Tide Centre in Belgium. This observatory is now in stand-by due to the end of the mine operation.
JEZEŘÍ, North Bohemia [50.5553N; 13.5052E]
The observatory was established in 1982 when a complex investigation of the Krušné hory slopes began with the aim of controlling the stability of the slopes of an open-pit coal mine. The castle Jezeri is located just above the gallery. It is composed of two sites in a horizontal gallery, equiped with the Ostrovsky's and ASNS tiltmeters with permanent recording of tilts. Station 1, operating since 1982, is located 410 m deep in the crystalline massif of the adjacent mountains. The target is to record the stability of this deep block of the massif suspected to rotation or sliding movements.
Station 2 was built in 2001 in order to monitor the weak zone of crystalline basement close to the contact with Tertiary sediments containing a coal seem. This station is a part of the fast geomechanic processes monitoring system of the mining company. It has clearly recorded e.g. the 2002 flood effect on block stability with anomalous tilts caused by a huge water infill in a support pillar of sedimentary formation.
This observatory has been technically upgraded for automatic data transmittion and nowadays is being prepared for real-time monitoring system establishment. The aim of the system is to provide actual continuous information on the stability of the mine slopes suspected of possible sliding. Day-to-day evaluation of the data, together with all other observed parameters, should provide indications of slope instability and warning signals for the mine authorities.
At the same time, the data are a subject to long-term earth tide effects investigation.
The photos show the entrance to the Jezeří gallery and the result of landsliding at the edge of the mine (destroyed road, Jezeří castle at the back).
SKALNÁ, West Bohemia [50.1688N, 12.3606E]
This new complex geodynamic observatory is located in the West Bohemia seismoactive region in an underground gallery in a granite block in the town of Skalná. The observatory should contribute to the monitoring of the ongoing geodynamic processes. It is furnished with a seismograph, a couple of tiltmeters, a barometer and a strainmeter. Except the seismograph, the other equipment is still in a testing period, especially due to stabilisation of tiltmeters in local temperature conditions and their stable setup on the rock. Ocassionally, continuous measurements of gravity are performed to test local changes of the gravity field. Significant signal was recorded during the Sumatra earthquake in Dec. 2004, when the trend of the gravity variations has changed for a long time.
The photos show the maintenance of tiltmeters electronics, dumping the data from the LCR Graviton EG gravimeter and a general view of the interior of the observatory Skalna (gravimeter, tiltmeters, seismograph).
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