Adelheid Weise

Comparison of long-period tilt measurements: results from the two clinometric stations Metsähovi and Lohja, Finland

Abstract The records of an ASKANIA borehole tiltmeter and two water tube tiltmeters are compared. The borehole tiltmeter was operating at the Geodetic Observatory Metsahovi in a 63 m deep borehole and the long water tube tiltmeters are installed in a limestone mine at Lohja, both observatories are some 15 km apart west of Helsinki, Finland. Regarding the tides the comparison yields a coherence within 0.5% . . . 3% for the diminishing factors, which proves their regional validity within the error bars. Air pressure induced tilts in both stations reach about 10 msec of similar signals, pointing into the same direction of about 150°N. They are caused by the horizontal air pressure gradients as well as by the local air pressure variations in interaction with the coast line of the nearby sea (Gulf of Finland). In the borehole, hydraulic induced tilt signals are reaching the amplitudes of the tides (about 20 msec), even 20 m below an aquifer in solid granite. Reproduceable short-period (minutes to hours) pore pressure induced tilts are caused by pumping ground water. These effects can be explained by poroelastic modeling and thus, they can be partly corrected. They are corresponding to the main directions of permeability in a system of clefts. The seasonal drift directions in the borehole point to pore pressure variations in the aquifer interacting with a nearby swamp, correlated with the freeze. As the physics of the hydrological and air pressure induced tilts can be explained, instrumental effects are excluded at both stations. Thus, continuous tilt measurements are suitable not only for the investigation of small scale but also for regional scale crustal dynamics. But in order to avoid local disturbances the borehole tiltmeter needs to be installed deeper than now, in boreholes of 100 . . . 500 m depth.

Hydrological signals in gravity — foe or friend?

Although hydrological effects on gravity are known nearly as long as the influence of barometric pressure, they are not as well understood as the latter. The improvement of gravity data quality during the last years adds weight to the importance of understanding the hydrological influence. Moxa observatory is one station at which studies regarding hydrological effects are carried out. From soil moisture, water level and meteorological observations the effects of different hydrological contributors including snow can be modelled and compared to the gravity residuals of the superconducting gravimeter (SG). The total peak-to-peak amplitude amounts to 35 nm/s2. Contributions from the various areas around the observatory partly compensate due to the hilly morphology. The comparison between residuals and computed total hydrological effect yields a good agreement, but also shows that not all hydrological influences have been taken into account. A significant additional hydrological influence is due to the hill flank near the SG.

The Improved Hydrological Gravity Model for Moxa Observatory, Germany

The gravity variations observed by the superconducting gravimeter (SG) CD-034 at Moxa Geodynamic Observatory/Germany were compared with the GRACE results some years ago. The combination of a local hydrological model of a catchment area with a 3D-gravimetric model had been applied successfully for correcting the SG record of Moxa which is especially necessary due to the strong topography nearest to the SG location. Now, the models have been corrected and improved considerably by inserting several details in the very near surrounding. Mainly these are: the observatory building is inserted with the roof covered by a soil layer above the gravity sensor where humidity is varying, snow is placed on top of the roof and on topography (steep slope), and ground water is taken into account, additionally. The result is that the comparison of the corrected gravity residuals with gravity variations of the satellite mission GRACE, now using RL5 data, shows higher agreement, not only in amplitude but also the formerly apparent phase shift is obviously not realistic. The agreement between terrestrial gravity variations (SG) and the GRACE data is improved considerably which is discussed widely.

Pumping Induced Pore Pressure Changes in Tilt Measurements Near a Fault Zone in Mizunami, Japan

Tilt meters are a widely used tool for monitoring long-term and small-scale deformations. Therefore, they are perfectly suited to test potential subsurface waste repositories. Since November 2005 a high sensitive tilt meter of the ASKANIA-type is recording at a distance of 300 m from Mizunami Underground Research Laboratory, where two vertical shafts are under construction. During the construction of those shafts large pumps were used to reduce the groundwater level, leading to two large- and several small-scale pore pressure induced tilt signals. Due to the fault system nearby, the tilt direction does not coincide with the direction towards the pump as would be expected in homogeneous media. In this study we analyze the main surface tilt direction caused by pore pressure induced deformation. Our results show two main directions which are both nearly perpendicular to the fault. Also, the long-term signals show a high correlation with the short-term pore pressure induced tilt signals.

Local Hydrological Information in Gravity Time Series: Application and Reduction

Hydrological variations of up to some 10 nm/s2 are significant and broadband signals in temporal gravity observations. On the one hand they need to be eliminated from the data as they interfere with geodynamic signals. On the other hand they can be used to improve the understanding of hydrological process dynamics and to evaluate distributed hydrological models. Compared to satellite observations which are affected by global and regional hydrological variations continuous recordings from superconducting gravimeters (SGs) additionally may contain extractable information on local changes. To compare terrestrial data to satellite observations and to regional/global hydrological models, a local hydrological impact on the observations must be quantified and appropriately reduced first.

Micro-Gravity Measurements in Northern Victoria-Land, Antarctica: A Feasibility Study

Within the Italian Antarctic research programme repeated GPS-measurements are being carried out at well installed points in the area of Northern-Victoria-Land (NVL), Antarctica, called VLNDEF: Victoria Land Network for Deformation Control. Although the deformations obtained over a period of 4 years are quite small we consider micro-gravity measurement a suitable complement for geodynamic research. Since the German Federal Institute for Geosciences and Natural Resources (BGR) is active in that area as well, until the year 2010 performing nine expeditions comprising geological and geophysical work, we used the offer to join expedition GANOVEX X during the season 2009/2010. With three gravimeters 13 points of the Italian network near the German station Gondwana were observed. The advantage of such measurements is that they do not require topographic corrections. Thus, the instrumental resolution and the measurement conditions, resp., are the limits for the resolution and accuracy of the measurements. In order to receive a reliable data base we used three well calibrated gravimeters in parallel and repeated the measurements several times. Local effects due to changes of the ice cover and snow fall are below the achieved measurement accuracy, because the points are so exposed that local effects are negligible. Although the elevation differences up to more than 2,700 m are quite strong, the results reveal the feasibility of these measurements in that area: The obtained standard deviations are in the order of ±10 to ±20 μGal.