Elsevier

Journal of Geodynamics

Volume 48, Issues 3–5, December 2009, Pages 126-131
Journal of Geodynamics

Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany

https://doi.org/10.1016/j.jog.2009.09.011Get rights and content

Abstract

It is well known, that high resolution borehole tiltmeters are able to observe deformations, caused by hydrological variations. The quantitative coherence is often unexplained, especially if the sources of deformation can be based on both natural as well as man-made hydrological variations. Since 1999 tilt observations have been taken at the Geodynamic Observatory Moxa in Thuringia/Germany. In two 50 m and one 100 m deep boreholes the ASKANIA tiltmeters are installed. The high quality of the recorded tilt data can be proved by the analysis of well known geodynamic signals like the tides of the solid Earth and the free modes of the Earth. Here we focus on investigations of induced tilt signals caused by pore pressure changes due to precipitation and/or ground water level changes and, in addition, on man-made induced pore pressure variations. The correlation of natural ground water level changes with the observed tilt data can be shown by different events of precipitation and snow melting. However, also the load effect of a big truck yields a small elastic deformation which is clearly detectable in the ground water level recording. The correlated tilt effect is discussed regarding changes of the tilt amplitude and the orientation of the induced pendulum tip movement during the load phase.

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

References (18)

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