3.04 Gravimetric Methods – Superconducting Gravity Meters

110 Superconducting Gravity Meters
Δg (μGal)
Δg (μGal)
10
5
0
–5
Membach: 4 Aug. 1995–31 May 2000
–10
0 365 730 1095 1460
t (days)
10
5
0
–5
–10
Potsdam: 30 Jun. 1992–8 Oct. 1998
0 365 730 1095 1460 1825 2190
t (days)
Figure 30 Comparison between gravity residuals
(solid line) and polar motion gravity prediction (dashed line)
at stations Membach (Belgium) and Potsdam (Germany).
Reproduced from Xu J-Q, Sun H-P, and Yang X-F (2004b) A
study of gravity variations caused by polar motion using
superconducting gravimeter data from the GGP network.
Journal of Geodesy 78: 201–209 (doi: 10.1007/s00190-004-
0386-1), with permission from Springer.
long-term gravity measurements to solve tectonic problems
is however significant, as clearly demonstrated
by a recent study (Lambert et al., 2006) devotedto
PGR in North America. In particular, they could
show that the admittance of gravity to height changes
(in mGal per millimeter) deduced from combined GPS
and AG measurements is indeed close to the theoretically
predicted value by Wahr et al. (1995) and by
James and Ivins (1998) for a viscoelastic model.
In fact, combining gravity and height changes
measurements is very efficient in discriminating
PGR effects from present-day ice melting contributions
as shown by a recent investigation of the longterm
gravity changes in Svalbard (Sato et al., 2006a,
2006b). Another study based on 7 years of collocated
gravity measurements of SG and AG at Membach
(Belgium) by Francis et al. (2004) indicates that there
is a small decrease in gravity connected to uplift seen
by GPS at the same location. A longer data set is
required to determine if this gravity decrease is due
to PGR or active tectonics in the Ardenne mountains
(see also Van Camp et al., 2002).
Such comparisons of AG and GPS measurements
are much easier to do when there are continuous SG
measurements at the same location. In fact, the continuity
of the SG record is often important to check
the integrity of the AG measurements (discussed
earlier in connection with the SG calibration) and
to model effects such as hydrology that increase the
AG scatter. Amalvict et al. (2004, 2006) analyzed the
long-term gravity changes of the Strasbourg SG with
regular AG measurements and collocated GPS and
interpreted the results in terms of hydrology and
tectonics of the Rhine graben. Figure 31 shows the
gravity trend which seems to be present in an 8 year
data set (1997–2004) and how hydrological contributions
partly explain some of the gravity features.
3.04.3.9 Ground/Satellite Gravity Field
Comparison
There is a major international effort in the present
decade to measure variations in the Earth’s global
gravity field using low-orbit satellites. The first satellite
CHAMP was launched in 2000 and was followed 2
years later by GRACE. In the near future, there will be
a third mission called GOCE that will orbit even closer
to the ground and hence be even more sensitive to
smaller-scale gravity changes. The primary goal of
these missions is to use the temporal changes of the
Earth’s gravity field to infer changes in regional and
continental water storage, and ocean circulation (see
Wahr et al., 1998; Rodell and Famiglietti, 1999).
A major concern with satellite measurements of
time-varying gravity is how to calibrate and validate
such observations. In addition to comparisons with
models (the primary technique used to date), there
are several possibilities using actual measurements at
the ground (GPS, gravity) or at the ocean bottom
(water pressure). This problem is important because
if successful, it would produce an independent method
of validation that does not rely solely on modeling. We
will show hereafter that surface measurements from
the GGP network provide a useful additional constraint
on space gravity data. The validation signal is
related to seasonal gravity changes that are coherent
on length scales appropriate to satellite altitudes (typically
a few hundreds of kilometers).
A first study directly comparing CHAMP data to
six SG ground observations was done by Neumeyer
et al. (2004b) and has led to satisfactory results for all
the stations in the 1 year analysis period (from