NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

enabling us to map the behaviour of subsidence (even
exceeding 20 mm/yr) with high spatial resolution in the
South Eastern Po Plain. The uplifting behavior of the
Apennines chain bordering the Po Plain is identified
together with a narrow zone separating the contrasting
vertical crustal movements (ZERBINI et al. 2007).
Wettzell
Performing comparable investigations at Wettzell
(Germany), no seasonal effect could be clearly identified,
and the long-term trend in gravity is mainly caused by
ground water variations. To take full advantage of the high
capabilities of the modern superconducting gravimeters
(high sensitivity, low and stable drift-rate), hydrological
data have to be incorporated in the gravimetric data processing,
especially if long-term phenomena with small amplitude
are studied (Harnisch and Harnisch 2006a). 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 likely due
to mass changes rather than to tectonic movements
(RICHTER et al. 2004).
Moxa
The continuous observations with the superconducting
gravimeter at Moxa as well as the repeated measurements
with LCR-gravimeters are used to study the benefit of these
data for hydrological studies. This research was supported
by a number of hydrological experiments and modelling
to study the influence of hydrological changes in certain
areas on the SG observations (KRONER and Jahr 2006).
Among other things the gravity effect of a water front
moving downhill could be investigated. In the records of
the superconducting gravimeter typically hydrological
effects of up to several nm/s² are visible. With regard to
time-dependent changes in the gravity differences obtained
on the local network the following was found: Maximum
effects occur between the observation site at the foot of the
hill and the one on the upper hill flank (KRONER et al. 2007,
NAUJOKS et al. 2006). With increasing distance of the
observation point at the valley bottom the variations in the
differences decrease. From the systematics found, which
were confirmed by modelling, it could be shown that
unexpectedly a significant amount of water needs to be
stored in the joints of the hill flank in order to explain the
changes in the gravity differences (KRONER 2006). The
water volume stored as soil moisture and groundwater is
not enough to produce the observed effects in gravity
(KRAUSE et al. 2006). A subsequent consideration of this
influence in the SG data led to a qualitatively good
agreement between the observation at Moxa and a
modelling of the gravity effect based on global hydrological
models (NEUMEYER et al., 2006).
For the catchment area of Moxa observatory hydrological
models were developed (HASAN et al., 2006; KRAUSE et al.,
2005). Simultaneously a 3D gravity model for Moxa
observatory, which takes into account the complex topography
and geological situation, was derived from areal
gravity measurements. Changes in the different compartments
based on hydrological modelling are introduced into
B. Richter: Earth Tides and Non-tidal Gravity Field Variations 85
the gravity model as density changes and the resulting
gravity variation for the different observation points is
computed. By comparison of model-derived and observed
gravity changes additional constraints for hydrological
modelling ought to be obtained. The repeated gravity
measurements have already helped to identify one hydrological
contributor which was not known beforehand to
have a major influence on gravity: changes in the water
storage inside the fissured hill flank.
Time-dependent gravity observations at volcanoes
The studies at Mayon volcano – Philippines were continued
by improved modelling of the gravity variations. Here, not
only elastic deformation was taken into account but also
the gravitational effect of the deformed edifice. With this
model it was possible to explain the obtained gravity variations
much better than with a deformation model alone. The
algorithm was further developed for geodetic data in
volcanic areas (VAJDA et al. 2004, TIAMPO et al. 2004a,
2004b, 2004c).
These micro-gravimetric studies were also applied to the
volcanoes Merapi – Indonesia and Galeras – Colombia
(JENTZSCH et al. 2004).
Verification of ocean tidal models
In order to synthesize the gravity value for points worldwide
from the standard gravity formula and the ocean tide
loading, a comprehensive comparison was performed using
worldwide tidal gravity and tilt results. Three TOPEX/
POSEIDON (T/P) satellite derived models (CSR3.0,
FES95.2 and TPXO.2) beside the classical SCHW80 model
were selected for an accuracy assessment study. The
selected models have been subjected to an intercomparison
test, tide gauge validation test and comparison to 59 tidal
gravity stations. The intercomparison test shows a good
agreement between the T/P-based models for the open
ocean and remarkable disagreement between the selected
models in the coastal regions indicating that such models
are still problematic in these regions. The tide gauge
validation shows that the T/P derived models fit tide gauges
better than SCHW80, with a better fit for the semidiurnal
constituents than for the diurnal constituents. Comparing
the gravimetric ocean-tide loading computed from the
selected models with the residuals from a set of 59 tidal
gravity stations shows that there is an improvement of the
T/P derived models with respect to the Schwiderski model,
especially in M2. However, this improvement is not as
significant as the result of the comparison with the pelagic
data. A procedure developed for the comparison of T/P
derived models with SCHW80 was developed. The results
also provide information about the improvement of
SCHW80, as well as about the properties of the new
models. It is intended to continue this work applying the
very recent models to see how they perform compared to
this study (ZAHRAN et al. 2005, ZAHRAN et al. 2006).
The non-tidal ocean loading effect has been observed in
the height series of four permanent GPS stations, namely
Medicina, Marina di Ravenna, Bologna and Trieste, located
in the northeastern Adriatic area. A validation of the ECCO