Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany
Introduction
The ASKANIA borehole tiltmeter (ABT) was designed and constructed as a vertical pendulum in the 60th and 70th of the last century (Jacoby, 1966, Rosenbach and Jacoby, 1969, Flach et al., 1971). With a resolution of better than 0.2 msec (∼1 nrad) this tiltmeter type belongs to the most sensitive tilt sensors worldwide (Baker, 1980, Weise, 1992). In order to avoid meteorological influences as far as possible the ABTs need to be installed in boreholes deeper than 30 m (Große-Brauckmann and Rosenbach, 1983). These instruments have been used for the observation of deformations in the nano-rad-scale in the upper crust, which is not possible by air- or satellite-borne methods up to now. Especially pore pressure changes cause disturbances, which has been shown for installations in sediments by Kümpel (1989) and in solid rocks by Weise et al. (1999).
Meteorological effects like precipitation, snow melt and seasonal or local ground water level variations as well as loading effects, e.g. due to ocean tides can affect the tilt signals in the milliarcsecond range (Zschau, 1976, Baker, 1980). However, induced tilt effects can also be caused by artificial sources, like pumping, injection (Jahr et al., 2006, Jahr et al., 2008, Kümpel et al., 2006) or man-made loadings, e.g. filling of a reservoir (Jentzsch and Koß, 1997). Also very locally acting loads can be taken into account, but are load effects due to a big truck (approximately 10 t) observable by the ABT installed in 50 m depth? These signals but also naturally induced tilts are observed and investigated at the Geodynamic Observatory Moxa (Fig. 1), located in Thuringia/Germany (Jahr et al., 2001). In the surroundings of the observatory the hydrologic situation, although complicated, is fairly known from gravity investigations and modelling (Kroner and Jahr, 2006, Naujoks et al., 2008).
Section snippets
ASKANIA tiltmeter and installation
The ASKANIA borehole tiltmeter is 1.6 m long and has a diameter of 14 cm and a total weight of approximately 80 kg (Jacoby, 1966). It consists of a casing in which a pendulum carrier (outer pendulum) is installed, enabling the adjustment of the sensor inside (inner pendulum). The inner pendulum, with a length of 0.6 m, has an eigenfrequency of 0.7 Hz. Two orthogonal capacitive read-out systems provide two independent channels, which can be calibrated by two inherent calibration systems each
Hydrological induced tilt signals
Hydrological disturbances can affect the observed tilt signals due to pore pressure changes in the close vicinity of the tiltmeter. Such disturbances can be natural events like precipitation or snow melting or they can be of artificial origin like pumping or injection. Moxa observatory allows to study both types of induced tilt processes in detail because high quality meteorological and groundwater records are available.
Discussion and conclusion
The high resolution tilt records at the Geodynamic Observatory Moxa show two types of hydrologically induced tilt signals: The natural ones are detectable and they are strongly correlated with signals in ground water level change. The man-made effects due to loading are observable, however the very small contributions in amplitude and orientation are superimposed by other tilt signals and noise, mainly due to meteorological effects, e.g. barometric pressure loads. For further investigations we
Acknowledgments
The technicians M. Brunner, W. Kühnel and M. Meininger installed and maintained the tiltmeters and the meteorological station at Moxa Observatory during the whole observation period and during the loading experiments; this is gratefully acknowledged. We also thank our colleagues of the “Arbeitskreis Geodäsie/Geophysik” (Working Group Geodesy/Geophysics) and the hydrologists, esp. P. Krause, of the Institute of Geography of the Friedrich-Schiller-University Jena, for the fruitful discussion
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