Long-term crustal deformation monitored by gravity and space techniques at Medicina, Italy and Wettzell, Germany

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1 Journal of Geodynamics 38 (2004) Long-term crustal deformation monitored by gravity and space techniques at Medicina, Italy and Wettzell, Germany B. Richter a,, S. Zerbini b, F. Matonti b, D. Simon a a Bundesamt für Kartographie und Geodäsie, Frankfurt, Germany b Dipartimento di Fisica, Università di Bologna, Italy Received 30 January 2004; received in revised form 24 May 2004; accepted 9 July 2004 Abstract Series of gravity recordings at the stations Medicina (Italy) and Wettzell (Germany) are investigated to separate seasonal gravity variations from long-term trends in gravity. The findings are compared to height variations monitored by continuous GPS observations. To study the origin of these variations in height and gravity the environmental parameters at the stations are included in the fact finding. In Medicina, a clear seasonal signal is visible in the gravity and height data series, caused by seasonal fluctuations in the atmosphere including mass redistribution, the ocean, groundwater but also by geo-mechanical effects such as soil consolidation and thermal expansion of the structure supporting the GPS antenna. In Wettzell, no seasonal effect could be clearly identified, and the long-term trend in gravity is mainly caused by ground water variations. The successful combination of height and gravity series with the derived ratio of gravity to height changes indicates that the long-term trends in height and gravity are most likely due to mass changes rather than to tectonic movements Elsevier Ltd. All rights reserved. 1. Introduction Independently from space geodetic techniques like GPS, SLR or VLBI, another way to constrain vertical deformation is to use precise gravity measurements with ballistic absolute gravimeters and superconducting (cryogenic, SG) relative gravimeters. The value of g/ H can exhibit large variations but is mostly between 15 and 35 nm s -2 /cm (Jachens, 1978). The free-air relation of 30 nm s -2 /cm, which Corresponding author. Fax: address: bernd.richter@bkg.bund.de (B. Richter) /$ see front matter 2004 Elsevier Ltd. All rights reserved. doi: /j.jog

2 282 B. Richter et al. / Journal of Geodynamics 38 (2004) mostly occurs locally, corresponds to a vertical surface movement without mass change. For large area variations, the Bouguer relation 20 nm s -2 /cm is frequently found (Torge, 1989). This relation allows the conversion from gravity to height changes and vice-versa. The precision of the most accurate absolute gravimeters is of about nm s -2, equivalent to 10-9 fraction of the Earth gravity field for instantaneous measurements. The SG s, the best realisation of today s relative gravimeters, can measure continuously changes of gravity to a precision better than 0.5 nm s -2 using 1-min averages. Therefore, long-term gravity field variations can be determined with precision better than Consequently, gravimeters are sensitive to height changes of a few millimetres. Gravity variations converted into height changes obtained through the combination of absolute and relative (SG) gravity measurements, can therefore, be compared to those derived from the analysis of space geodetic data (Zerbini et al., 2001). Continuous monitoring and the combination of vertical height and gravity changes allow the separation of the gravity potential signal due to the mass redistribution from the geometric signal due to height changes. This ensemble of different observational capacities constitutes a powerful tool for the realization of integrated multipurpose networks suitable for high-precision monitoring of ground deformation and, more in general, for environmental research. 2. Instruments and location of the experiments One experiment takes place at Medicina, Italy, a fiducial reference station of the space geodesy network near Bologna (Fig. 1a). The station is located in a low-lying sector of the middle-lower Po Plain, where the surficial sedimentary sequences are mostly fined-grained clays and silty clays. A shallow unconfined (phreatic) aquifer is present in the first few meters of the ground. Since the beginning of the Fig. 1. Location of the stations Medicina, Italy (a) and Wettzell, Germany (b), source:

3 B. Richter et al. / Journal of Geodynamics 38 (2004) observations in 1996, the maximum measured fluctuations of the water table are of the order of 2.5 m, according to the seasonal variability in terms of climatic conditions (rainfall, temperature, and relative humidity). Continuous GPS observations acquired with a Leica GPS system 399 receiver have been performed since July The antenna is installed on a specifically designed steel pole screwed into the reference benchmarks. The system and the set-up are unchanged since the installation. To monitor continuously variations of the gravity field, the SG GWR C023 was installed about 600 m apart from the GPS system at the end of October The instrument is operated in a temperature-controlled laboratory on a concrete pier, which is founded 1 m deep into the ground. On another pier, next to the SG, absolute gravity (AG) measurements have been regularly performed by means of AXIS FG5 absolute gravimeters to check the SG data series. These observations are used to eliminate offsets in SG data series as well as the instrumental drift, taking advantage of the AG long-term stability. In addition, relevant meteorological parameters such as air pressure, temperature, relative humidity, precipitation and surficial water table data are collected on a continuous basis. To estimate the atmospheric mass redistribution, data from 12-h balloon radio sounding from a nearby regional meteorological station (approximately 3 km apart Medicina) are also collected. Another case for long-term studies of the gravity field variations are the observations performed at Wettzell, in the Bavarian Forest in Germany, a fiducial reference station for space geodetic applications (Fig. 1b). Here, data of the dual SG GWR DC029 are available since 1999 until This instrument was also episodically checked by absolute gravity observations. Besides the common meteorological parameters, the ground water table is recorded continuously. It should be pointed out that the ground water gauge is located approximately 400 m apart from the gravimeter building. The geological formation in Wettzell is deeply weathered bedrock covered by a thin layer of soil. The ground water is mainly stored in cracks, which have a random distribution in size and depth. So there is no free water table. 3. Data analysis The SG gravity observations acquired at Medicina during the time period October 1996 November 2003 were analysed. For the Wettzell station, the data records are relevant to the period June 1999 December Both stations collect the data with a 10-s sampling rate. The data are then filtered to 1-min samples. The TSoft software package (Vauterin, 1998) is adopted to pre-screen the observations. This allows to remove spikes and offsets between the data blocks and to fill small gaps by means of interpolated data. The ETERNA, version 3.30, software package (Wenzel, 1998) is used for the analysis of the SG gravity data. Details concerning the data analysis procedure can be found in Zerbini et al. (2001). The AG measurements, regularly performed at both stations, allowed to remove the offsets between data blocks of the SG data series. Also the annual instrumental drift of 16.5 nm/s 2 for SG C023 (Medicina) and 19.3 nm/s 2 for the SG D029 (Wettzell) was estimated by these comparisons. The offsets occurred in conjunction with liquid helium refilling and once during an earthquake, so all mechanical shocks. For a better comparison between the GPS height daily solutions and the gravity residuals (tides solid Earth and ocean, polar motion and air pressure effects have been removed from the records), the gravity data are averaged to daily values.

4 284 B. Richter et al. / Journal of Geodynamics 38 (2004) The GPS analysis follows standard procedures using the Bernese Software package, version 4.2 (Beutler et al., 2001) and the IGS orbits to compute daily 3D coordinates. The coordinates are determined in the frame of five fiducial IGS stations (ITRF2000) in the region. Details are provided by Zerbini et al. (2001). 4. Comparison of long-term gravity and height variations The combination of the height variation measurements provided bygps and of the gravity potential changes observed by the SG allows the separation between the geometric signal from that induced by mass changes embedded in the gravity records. This is most important for understanding the signal-producing mechanisms in both the GPS and gravity time series. To study the observed seasonal oscillations, the loading effects in the case of GPS and the loading and Newtonian attraction effects in the case of gravity, due to seasonal variations in the atmospheric pressure and surficial hydrology as well as to non-tidal oceanic processes have been accounted for and modelled. The observed signal in the GPS and gravity time series results from a superposition of various effects (see Table 1, for example). Because only one of the parameters involved is the target of the investigation, the effects of all the others should be eliminated or, at least minimized, by means of theoretical or numerical models. Here, the trend in both series, height and gravity, caused by long-term crustal deformations and/or mass changes will be the focus of the study. The main problem with observed gravity variations is to be able to discriminate in the observed changes how much is due to mass redistribution and how much is due to actual vertical displacements. Already Jachens (1978) derived the bounds for the ratio of gravity to height changes corresponding to different physical phenomena (Fig. 2). For comparison, the ratio obtained for the body tide is added to the figure (Vaníček and Krakiwsky, 1986). Two stations will be investigated, one, Medicina, where seasonal signals are relevant, and a second one, the Wettzell station, where seasonality has not been detected. Both stations, however, are characterized by a remarkable interannual variability. The seasonal signals in the GPS and gravity observation series can be modelled to a high degree of accuracy. The background of the models, specifically for Medicina, is given in Zerbini et al. (2002) and Romagnoli et al. (2003). Here, only a short update concerning modelling of the non-tidal oceanic effects is provided. Table 1 Signals affecting GPS height and gravity measurements Height Deficiencies in the reference system Tidal deformation Short and long-term environmental effects Atmospheric signal (regional, local) Hydrological signal (regional and local) Ocean signal (global/regional) Thermal effect of structures (local) Long-term crustal deformation Gravity Polar motion Tidal effects Short and long-term environmental effects Atmospheric signal (regional, local) Hydrological signal (regional and local) Ocean signal (global/regional) Thermal effect of structures (local) Long-term crustal deformation

5 B. Richter et al. / Journal of Geodynamics 38 (2004) Fig. 2. Ratios of gravity to height changes for different physical phenomena (taken from Vaníček and Krakiwsky, 1986). 5. The Medicina station 5.1. Comparison between the observed and modelled seasonal oscillations The GPS and gravity time series used for this work include data from the beginning of the observations, which started in mid 1996 for GPS and in late 1996 for the SG, till the end of The gravity data collected during 1996 and 1997 were not used because of a gravity anomaly occurred in mid A possible explanation of this event is given in Zerbini et al. (2002). As regards gravity, the effect of the local atmospheric air pressure variations is removed from the SG observations in the course of the data analysis by using an empirically derived transfer function between gravity and the local air pressure values of 2.94 nm s -2 /(h Pa). This is a common procedure in the treatment of the SG data. However, the use of a single transfer function may leave components in

6 286 B. Richter et al. / Journal of Geodynamics 38 (2004) the data, which are not accounted for by using a mean value. An approach using frequency-dependent atmospheric corrections should lead to a better understanding of seasonal variations and would improve the modelling capability. In order to compare observed and modelled seasonal oscillations, loading and mass effects have been calculated. The gravity variations due to the vertical air mass attraction effect induced by seasonal rising of warm air and sinking of cold air have been estimated by using 12-h radio sonde data acquired by the San Pietro Capofiume meteorological station nearby Medicina (Simon, 2003). This effect does not include any loading component because the pressure at ground is not affected. The seasonal non-tidal loading and mass attraction components have been estimated (van Dam, 2003 personal communication) by using the ocean bottom pressure data from the Ocean General Circulation Model used in the ECCO project ( The Green s functions from Farrell (1972) were used to estimate the loading and the mass attraction components. The adoption of the ocean bottom pressure data from the ECCO project has the advantage with respect to the previous used non-tidal ocean models (Zerbini et al., 2002; Romagnoli et al., 2003) that data are available twice daily on a 1 1 grid and this allows to model the short and very short period variations. The combined hydrological loading and mass effect influencing the SG time series was estimated by correlating the simplified climatic hydrological balance, available for the Casola Canina station in the vicinity of Medicina (Zinoni, 2003 personal communication), with the gravity recordings. For the GPS height seasonal variations, the loadings due to air pressure, surficial hydrology, non-tidal oceanic effects and thermal expansion of the structure, which supports the antenna, have been considered. Details concerning the air pressure and surficial hydrological effects and the thermal expansion can be found in Zerbini et al. (2002) and Romagnoli et al. (2003). As pointed out in the preceding discussion concerning gravity, the non-tidal ocean loading has been estimated by means of the ocean bottom pressure data from the ECCO project (van Dam, 2003 personal communication). In this study, a different approach with respect to previously published papers has been chosen to compare observed and modelled seasonal variations. Average observed and modelled seasonal cycles have been computed by stacking the observations as well as the modelled data for the individual years for gravity and for the GPS height series. The scatter of the individual years versus the mean value is used to estimate the variability of the seasonal cycles of gravity and height (Fig. 3). They are displayed as error bars in Fig. 3a and b. The seasonal cycle in gravity, about 50 nm s -2 peak-to-peak (Fig. 3a), is prominent in the observations and in the modelled data; its deformation component, the height seasonal cycle, is displayed in the lower panel (b), the amplitude of the signal is about 1 cm peak-to-peak (Fig. 3b). The observed and modelled gravity seasonal cycles show an excellent agreement in amplitude with maximum differences in the order of 10 nm s -2 (Fig. 3a, yellow line). It is recognizable, however, a shift in phase of about 55 days between the two minima during the summer period. Fig. 3a, shows the stacked observations and the seasonal model, the yellow line is the difference between the two. A crosscorrelation of the gravity residuals (linear trend removed from the data) with the simplified hydrological balance data shows a similar time lag. A backward shifting of 55 days of the hydrological balance contribution to the modelled seasonal cycle would likely bring the observed and modelled cycles to coincide and to further reduce the residuals between the observed and computed seasonal cycles. In Fig. 3b the observed and modelled height seasonal cycles are presented as well as the difference between the two (yellow line). This residual has a peak-to-peak amplitude of about 2 mm, a period of about 40 days and it shows a mod-

7 B. Richter et al. / Journal of Geodynamics 38 (2004) Fig. 3. Observed and modelled gravity and height seasonal cycles and relevant differences. The seasonal cycles were obtained by staking of the observations and of the relevant models. Upper panel (a): Gravity seasonal cycle (observed blue, modelled red), difference between the two cycles (yellow) shifted by 40 nm s -2 for graphical purposes. Lower panel (b): GPS height seasonal cycle (observed blue, modelled red), difference between the two cycles (yellow) shifted by 4 mm for graphical purposes.

8 288 B. Richter et al. / Journal of Geodynamics 38 (2004) Fig. 4. Gravity observations (SG C023 and absolute gravity data FG5 101) at Medicina corrected for the seasonal effect. The remaining linear trend (red line) is 4 nm s -2 /year, converted to height changes 1.2 to 2 mm/year. erate linear trend. It is likely that this residual difference mostly results from modelling of the air pressure and hydrological loadings. A check on the air pressure loading time series has shown that the bumps recognizable in the seasonal cycle and occurring in April and November are, to a large extent, attributable to the modelling of this loading effect. Additional studies are underway to better understand and model the relevant contributions of the local hydrology and of the air pressure to both gravity and GPS height variations Long-term trends in heights and gravity After subtracting the average modelled seasonal cycle from the gravity observations, a long-term trend in gravity of 4 ± 1nms -2 /year can be determined (Fig. 4). However, the signals in the residuals and all the little wiggles on them are in fact not modelled gravity signals and since the SG noise level is less than 0.5 nm s -2, the signal to noise ratio is more like 20:1. It is for this reason that no error bars on the SG data are shown. The error bars are too small to be seen on the SG residual signals. Allowing that the gravity change is due also to mass change, it would be equivalent to 2 mm/year, assuming a conversion of 50 nm s -2 /cm. Latest results from the GPS analysis providing an observed trend in height of 2.57 ± 0.06 mm/year underline the findings with a completely different technique (Fig. 5).

9 B. Richter et al. / Journal of Geodynamics 38 (2004) Fig. 5. GPS height series at Medicina corrected for the seasonal effect. The overall linear trend is 2.57 mm/year (red line). Notwithstanding the existence of natural land subsidence in the area in the order of 1 mm/year (Dante et al., 1997), a possible interpretation could be that the observed subsidence rate in the Medicina area is due to a long-term change in the regional water regime. 6. The Wettzell station At the Wettzell station no seasonal effect is recognizable in the observations or could be separated by stacking. The large non-seasonal gravity variations are mainly driven by ground water variations (Fig. 6a). By fitting the SG time series as well as the absolute gravity observations to the observed water table variations an admittance factor of 67 nm s -2 /m is derived. Assuming the Bouguer plate theory, the admittance factor would lead to a porosity of 17%, which is possible in deeply weathered bedrock. By taking the ground water admittance into account, the gravity variation is reduced by half. The remaining variations could be explained by either near field hydrology by using a closer ground water gauge, not available at the moment, or by other environmental effects not yet modelled for this station. Although, the long-term trend in the water table is representative for the area, it is especially this effect which reduces the trend in the gravity registration significantly. The final calculations lead to an increase in gravity of about 4 ± 1nms -2 /year, which can be interpreted as a subsidence of mm/year (Fig. 6b). Results from space geodetic techniques confirm this rate, e.g. the rate provided by the EUREF time series for Wettzell ( 2.3 mm/year). The gravity to height change ratio of 1.7 nm s -2 /cm would affirm the observed deformation (subsidence).

10 290 B. Richter et al. / Journal of Geodynamics 38 (2004) Fig. 6. Gravity observations at Wettzell. Upper panel (a): Blue is observed gravity, light blue are water table variations with a trend equal to 18 nm s -2 /year (admittance factor 67 nm s -2 /m). Lower panel (b): Dark yellow is gravity corrected for ground water effects and remaining trend in gravity of 4 nm s -2 /year. 7. Conclusions The combination of continuous SG measurements and episodic absolute measurements are the right and only procedure to ensure a high resolution, continuous and instrumental drift free monitoring of local

11 B. Richter et al. / Journal of Geodynamics 38 (2004) gravity variations. To interpret the observed variations additional information derived from space geodesy data and environmental parameters is necessary to set up high-precision seasonal models and long-term regression functions. Besides modelling the seasonal effects, long lasting observation series can be stacked and this work shows that the stacked mean value is close to the modelled one. Both results observed and modelled, can be used to reduce the gravity variations for the seasonal variability. However, none of the environmental effects is strictly seasonal. Water table variations are affected by different factors both of climatological and anthropogenic nature and have most likely a long-term component, which is mapped into the gravity record. To interpret the long-term gravity variations, this mass and load effect shall be separated from possible height induced gravity variations. In both case studies the final environmental free trend in gravity can be confirmed by observed height variations. The combination of the height and gravity series and the resulting gravity height ratio may lead to the conclusion that the observed effects are caused by ground water. Acknowledgements This work has been developed under contracts MIUR 2002 from the Italian Ministry for Education, University and Research and l/r/204/02 from the Italian Space Agency. The authors are grateful to the BKG gravity team and to the staff of the Medicina Radioastronomy station. References Beutler, G., Bock, H., Brockmann, E., Dach, R., Fridez, P., Gurtner, W., Hugentobler, U., Ineichen, D., Johnson, J., Meindl, M., Mervart, L., Rothacher, M., Schaer, S., Springer, T., Weber, R., In: Hugentobler, U., Schaer, S., Fridez, P. (Eds.), 2001 Bernese GPS Software Version 4.2. Astronomical Institute, University of Berne, Switzerland, 515 pp. Dante, A., Gonella, M., Teatini, P., Tomasi, L., Geographic Information System (GIS) and Data Management and Retrieval System (DMRS) in the CENAS Project. In: CENAS: Study on the Coastline Evolution of the Eastern Po Plain due to Sealevel Change Caused by Climate Variation and to Natural and Anthropic Subsidence. Environment Programme Padova, Settembre 1997, pp Farrell, W.E., Deformation of the earth by surface loads. Rev. Geophys. Space Phys. 10, Jachens, R.C., The gravity method and interpretive techniques for detecting vertical crustal movements. In: Mueller, I.I. (Ed.), IAG/IUGG and COSPAR, Columbus, USA, Proceedings of Ninth Geodesy/Solid Earth and Ocean Physics (GEOP) Research Conference, An International Symposium on the Application of Geodesy to Geodynamics. Department of Geodetic Science Report 280, The Ohio State University, Columbus, USA, pp Romagnoli, C., Zerbini, S., Lago, L., Richter, B., Simon, D., Domenichini, F., Elmi, C., Ghiotti, M., Influence of soil consolidation and thermal expansion effects on height and gravity variations. J. Geodyn. 35, Simon, D., Modelling of the Gravimetric Effects Induced by Vertical Air Mass Shifts [Mitteilungen des Bundesamtes fur Kartographie und Geodasie], Band 21. Verlag des Bundesamtes für Kartographie und Geodäsie, Frankfurt am Main, Germany, 100 pp. Torge, W., Gravimetry. de Gruyter, Berlin, New York, 465 pp. Vaníček, P., Krakiwsky, E., Geodesy: the Concepts. North-Holland, Amsterdam, New York, Oxford, 697 pp. Vauterin, P., TSoft: graphical and interactive software for the analysis of Earth Tide data. In: Proceedings of 13th International Symposium on Earth Tides, Brussels, Observatoire Royal de Belgique, Serie Geophysique, pp

12 292 B. Richter et al. / Journal of Geodynamics 38 (2004) Wenzel, H.-G., Earth tide data processing package ETERNA 3.30: the nanogal software. In: Ducarme, B., Paquet, P. (Eds.), Proceedings of the 13th International Symposium Earth Tides, Brussel, pp Zerbini, S., Richter, B., Negusini, M., Romagnoli, C., Simon, D., Domenichini, F., Schwahn, W., Height and gravity variations by continuous GPS, gravity and environmental parameter observations in the southern Po Plain, near Bologna, Italy. Earth Planet. Sci. Lett. 192 (3), Zerbini, S., Negusini, M., Romagnoli, C., Domenichini, F., Richter, B., Simon, D., Multi-parameter continuous observations to detect ground deformation and to study environmental variability impacts. Global Planet. Change 34,