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

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1 Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany T. Jahr, G. Jentzsch, A. Weise To cite this version: T. Jahr, G. Jentzsch, A. Weise. Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany. Journal of Geodynamics, Elsevier, 2009, 48 (3-5), pp.126. < /j.jog >. <hal > HAL Id: hal Submitted on 20 May 2011 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Title: Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany Authors: T. Jahr, G. Jentzsch, A. Weise PII: S (09) DOI: doi: /j.jog Reference: GEOD 905 To appear in: Journal of Geodynamics Please cite this article as: Jahr, T., Jentzsch, G., Weise, A., Natural and manmade induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany, Journal of Geodynamics (2008), doi: /j.jog This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

3 Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa / Germany T. Jahr*, G. Jentzsch, A. Weise Institute of Geosciences, Friedrich-Schiller-University Jena, Germany 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 manmade hydrological variations. Since 1999 tilt observations have been taken at the Geodynamic Observatory Moxa in Thuringia/Germany. In two 50m and one 100m 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. * Corresponding author: Thomas Jahr, thomas.jahr@uni-jena.de Keywords: Tilt observations, ground water, pore pressure, Earth tides, free modes of the Earth 1 Page 1 of 18

4 1. Introduction The ASKANIA borehole tiltmeter (ABT) was designed and constructed as a vertical pendulum in the 60 th and 70 th of the last century (Jacoby, 1966; Rosenbach and Jacoby, 1969; Flach et al., 1971). With a resolution of better than 0.2 msec (~ 1nrad) 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 30m (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; Kümpel et al., 2006; Jahr et al., 2008) 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 (approx. 10t) observable by the ABT installed in 50m 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). Figure 1 (Moxa bird view photo) 2 Page 2 of 18

5 2. ASKANIA tiltmeter and installation The ASKANIA borehole tiltmeter is 1.6m long and has a diameter of 14cm and a total weight of approx. 80kg (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 0.6m, has an eigenfrequency of 0.7Hz. Two orthogonal capacitive read-out systems provide two independent channels, which can be calibrated by two inherent calibration systems each consisting of small balls to be moved between two well defined positions. The installation of the tiltmeters in the boreholes at Moxa observatory requires a nonoptical azimuth estimation, because the boreholes do not allow direct visibility of the light emitting diodes (LED), due to the bending of the drill holes. Therefore a method, based on seismic signals had to be developed. Two 3-component geophone sets (4.5Hz) were installed: one on the top of the tiltmeter with a well-known orientation to the sensor components. The other one is placed on the surface with horizontal orientations to northsouth and east-west directions. A seismic signal recorded with both geophone sets can now be used without knowing the exact location of its source. The horizontal seismic recordings (sample rate: 100Hz) from the geophones on the tiltmeter are transformed from the actual orientation to all possible values, from 0 to 360 in 0.1 -steps. All resulting 3600 times series are correlated with the time series recorded at the surface. The true orientation is met for the orientation of the transferred record which yields the highest correlation factor. This procedure was pre-investigated with a seismic shake-table. The results show, that the azimuth estimation is better than ±1. The first ABT installation in Moxa was realized in 2002 in a 50m borehole (Fig. 2), however reinstallations were necessary twice due to lightning damages in 2003 and Page 3 of 18

6 In December 2004, eight days before the big Sumatra-Andaman earthquake occurred, a second ABT was installed in the 100m deep borehole (Fig. 2). Figure 2 (ground view) Therefore, the free modes of this huge seismic event could be recorded with two ABTs at different depths (Fig. 3). Both spectra show a lot of free modes, and a slight increase of the signal to noise ratio for the deeper installation of the ABT (100m depth). This confirms the expectation that the noise level esp. for high frequencies is reduced for deeper installations. The toroidal and some spheroidal modes were also observed with high quality by the tiltmeter array at the super deep drilling site KTB (Jentzsch et al., 2005; comp. Jahr et al., 2008). Figure 3 (Free modes, 50m +100m) 3. 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. 4 Page 4 of 18

7 3.1 Natural sources Between January and April 2005 the ground water level, recorded close to the tiltmeters in 50m depth, shows three significant increases (Fig. 4a), which are caused by strong precipitation and snow melt events. The east-west components of the tiltmeters in 50m and 100m depth show clearly that only the 50m deep tiltmeter is affected by these ground water events. But the hodographs, showing the tip movement above ground, indicate that also the tiltmeter in 100m depth observed the induced ground water effects, but as expected with much smaller amplitudes (Fig. 4b). The hodographs, esp. for the 50m deep tiltmeter, show that the induced deformation is mainly oriented east-west, although the main cleft striking is NNW SSE in this area (Naujoks, 2008). This is a hint that the process is strongly controlled by the north-south striking slopes with the main topographic gradients in east-west direction. A detailed investigation shows, that the induced tilt signals occur simultaneously, which indicates that the tilt effect is caused by the same pore pressure changes and not by flow processes of the ground water. Figure 4 (natural hydrological effects) 3.2 Man-made sources A local load of more than 10,000kg caused by a large truck yields an increase of ground water level of > 2mm (Fig. 5), observed in 50m depth. The question is: Are this pore pressure change and the associated small deformation also detectable in the high resolution tilt data? The hypothesis about the underlying process of the truck load effect is illustrated in Fig. 6: Before the truck enters the area close to the borehole the situation is an undistorted equilibrium of fluid and pore pressure, fluid in the borehole and in the surroundings (Fig. 6a). Under the load the fluid is pressed into the permeable part of the 5 Page 5 of 18

8 rocks and the water level increases in the ground water borehole (Fig. 6b). When this situation is stable after ~20 minutes a new, but now loaded equilibrium is reached (Fig. 6c). When the truck leaves this area, then the fluid moves back to the equilibrium as at the beginning (Fig. 6d). Figure 5 (ground water level during truck load) Figure 6 (loading hypothesis) Up to now it is not determinable, whether the small load induced ground water level change (Fig. 5) is also detectable in the tilt record at the depth of 50 m or not. There is a weak correlation between ground water level change and observed tilt components shown in Fig. 7a, but it is not significant due to the very small load. Nevertheless, for eight load events a systematic change of the main noise orientation is observed (6-9 ) in the tilt data in direction to the load point, which is shown exemplarily for one load event in Fig. 7b (event shown in Fig. 5). Figure 7 (induced tilt effects) 4. 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. 6 Page 6 of 18

9 barometric pressure loads. For further investigations we propose a loading experiment, whereby a truck is moving systematically several times close towards the boreholes and back. The simultaneously observed tilt data can be stacked, which enables an increase of the signal-to-noise ratio for the induced signal. In order to decide about installation depths of such tiltmeters and possible disturbing effects, further investigations should also be supported by numerical modelling to allow a better estimation of the expected signals. Although some of these induced signals are small, they may throw a light on the fluid-induced processes taking place in the vicinity of the tiltmeter installation site. Applications of such local tilt observations are e.g. the monitoring of building, ground and slope stability in tectonic active or man-made influenced areas close to reservoirs or mining regions. The use of several tiltmeters in a local array seems to be most promising (Jahr et al., 2008). 5. Acknowledgment 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 about all hydrological induced signals at Moxa observatory. Both reviewers, H.-J. Kümpel and G. Mentes, carefully checked and improved with their comments our manuscript which is gratefully acknowledged. 7 Page 7 of 18

10 6. References Baker, T.F., Tidal Tilt at Llanwrst, North Wales: tidal loading and Earth structure. Geophys. J. R. astr. Soc., Vol. 62, Flach, D., Jentzsch, G., Rosenbach, O., and Wilhelm, H., Ball Calibrations of Askania Borehole Tiltmeter (Earth Tide Pendulum). Zeitschrift für Geophysik, 37, Große-Brauckmann, W., and Rosenbach, O., Influence of Geological Structures on the Interpretation of Tilt Measurements. Proc. 9 th Int. Symp. Earth Tides, New York, 1981, Jacoby, H.D., Das neue Bohrloch-Gezeitenpendel nach Graf. ASKANIA-Warte, 67, Jahr, T., Jentzsch, G., and Kroner, C., The Geodynamic Observatory Moxa / Germany: Instrumentation and Purposes. J. Geod. Soc. Japan, 47(1), Jahr, T., Letz, H., and Jentzsch, G., Monitoring fluid induced deformation of the earth s crust: A large scale experiment at the KTB location/germany, J. Geodyn., 41(1-3), Jahr, T., Jentzsch, G., Gebauer, A., Lau, T., Deformation, seismicity and fluids: Results of the 2004/2005 water injection experiment at the KTB / Germany. J. Geophys. Res., 113, B11410, doi: /2008JB Jentzsch, G., and Koß, S., Interpretation of long-period tilt records at Blå Sjø, Southern Norway, with respect to the variations of the lake level. Phys. Chem. Earth 22, Jentzsch, G., Jahr, T., Letz, H., and Gebauer, A., Erdeigenschwingungen nach dem Sumatra-Erdbeben vom 26. Dezember 2004 beobachtet mit 5 ASKANIA-Bohrloch- 8 Page 8 of 18

11 Neigungsmessern an der KTB. Mitteilungen der Deutschen Geophysikalischen Gesellschaft 1/2005, Kroner, C., and Jahr, T., Hydrological experiments around the superconducting gravimeter at Moxa Observatory. J. Geodyn., 41, (1-3), Kümpel, H.-J., Verformung in der Umgebung von Brunnen. Habilitationsschrift, Univ. Kiel, 198 p. Kümpel, H.-J., Erzinger, J., and Shapiro, S., Two massive hydraulic tests completed in deep KTB borehole. Scientific Drilling 3, Naujoks, M., Hydrological information in gravity: observation and modelling. Dissertation at the Chem. and Geosci. Faculty of the Friedrich-Schiller-University Jena, 108 p. Naujoks, M., Weise, A., Kroner, C., Jahr, T., Detection of small hydrological variations in gravity by repeated observations with relative gravimeters. J. Geodesy, 82, , doi:101007/s Rosenbach, O., and Jacoby, H.D., First experiences with the Askania borehole tiltmeter (earth tide pendulum). Proc. 3 rd Int. Symp. on Recent Crustal Movements, Leningrad. Weise, A., 1992: Neigungsmessungen in der Geodynamik Ergebnisse von der 3- Komponenten-Station Metsähovi. Dissertation, TU Clausthal, 180 p. Weise, A., Jentzsch, G., Kiviniemi, A., and Kääriäinen, J., Comparison of longperiod tilt measurements: results from two clinometric stations Metsähovi and Lohja, Finland. J. Geodyn., 27, Zschau, J., Tidal Sea Load Tilt of the Crust, and its Application to the Study of Crustal and Upper Mantle Structure. Geophys. J. R. astr. Soc., Vol. 44, Page 9 of 18

12 Figure captions: Figure 1: The Geodynamic Observatory Moxa in Thuringia/Germany (Photo: W. Kühnel, 2005). Three ABT boreholes, two 50m and one 100m deep, are situated adjacent to the observatory building close to the uppermost left corner (north, Fig. 2). Figure 2: Sketch of the ground plan of the Geodynamic Observatory Moxa (Fig. 1). In front of the buildings (north) the boreholes of the tiltmeters, the ground water borehole and the location of the truck during the refill of the gas tank are marked. Figure 3: Two spectra of the 2-days tilt records after the Sumatra-Andaman earthquake, Dec , observed in depths of 50m (tiltmeter ASKANIA P109) and 100m (tiltmeter ASKANIA P108). The results look similar, however, for the main modes the signal-to-noise ratio is slightly higher for the deeper installation. Figure 4: Correlation of ground water level (dotted line) and tilt records (EW-components with positive to east, tides removed): a) for three hydrological events a strong correlation is observed for the tilt in 50m depth (black), but only weak correlation for the 100m deep installation (grey); b) Movement of the tip of the pendulum above ground. The three hydrological events shown above are clearly detectable in the tilt records for the two different depths and they occur simultaneously (left: 50m, right: 100m deep installation). Note: the tilt signals are oriented nearly east-west. 10 Page 10 of 18

13 Figure 5: Ground water level variation near the observatory, observed in 50m depth (x-axis in hours): For about 20 minutes the surface was loaded by a truck (10t). During this time a clear ground water level increase is observed. After departure of the truck the signal returns to the level before loading. This means: this observed signal is due to a mostly elastic reaction of the upper layers due to pore pressure increase, but the observed delay points to poro-elastic effects under locally drained conditions. Figure 6: Hypothesis about the truck load effect: a) the unloaded equilibrium; b) an increase of the ground water level caused by the load (above); c) the loaded equilibrium; d) relaxation after the end of the loading. The water level reacts on the increasing pore pressure due to a mostly elastic deformation of the rocks. Figure 7: a) Ground water level (dotted line) compared with tide removed unfiltered tilt record (P109) of both components (X: black; Y: grey at 50m depth); b) changed noise orientation during the load period (before loading: grey, during loading: black) for the same event in direction to the load point (6-9 ). This characteristic change of orientation was also estimated for other seven load events. 11 Page 11 of 18

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