3.04 Gravimetric Methods – Superconducting Gravity Meters

108 Superconducting Gravity Meters
(a)
nm s –2
(b)
mm
(c)
mm
11.0
7.0
3.0
–1.0
4/18/00
8.0
6.0
4.0
2.0
4/22/00 4/26/00 4/30/00
0.0
4/18/00 4/22/00 4/26/00 4/30/00
590.0
570.0
550.0
530.0
510.0
490.0
4/18/00 4/22/00 4/26/00 4/30/00
Figure 26 Gravity residual observations (a),
accumulated precipitation (per hour) (b), and groundwater
table level (c) at Moxa, Germany, from 18 Apr. to 2 May 2000.
Reproduced from Kroner C, Jahr T, and Jentzsch G (2004)
Resultsfrom44monthsofobservationswitha
superconducting gravimeter at Moxa/Germany. Journal of
Geodynamics 38(3–5): 263–280, with permission from Elsevier.
3.04.3.7 Earth Rotation
At the long period end of the observable spectrum of
gravity (Figure 15), there are two isolated theoretical
periods of the Earth’s normal-mode spectrum. One is
the inner-core wobble (ICW), whose period is of the
order of years. For model PREM, wobble programs
yield a value of about 700 days, but the theory is
hardly suitable for such a long-period motion.
Mathews et al. (2002) quote a value of 6.6 yr that is
derived from their theory and with VLBI observations
and models, but at either period the predicted
small amplitude makes this a difficult target (e.g.,
Guo et al., 2006).
The other mode is much more accessible, and is
of course the CW, with a period of about 435 days in
the mantle reference frame. This is one of the two
components of polar motion normally seen in gravity
studies, the other being the smaller amplitude forced
annual wobble that is seen in combination with
the annual tide and other seasonal effects. Note that
at much longer periods there is an 18.6 lunisolar
year tide that will be extremely difficult to
identify in gravity. Discussion of Richter’s (1983)
observation of the CW has been mentioned earlier
and almost every SG in the GGP network has
reported a clear signal of the CW – see, for example,
Richter et al. (1995b) and Sato et al. (1997b) for the
Japanese Antarctic station. Here, we review only a
few results.
Loyer et al. (1999) showed the importance of using
a long data set when trying to infer the transfer
function of the polar motion; clearly more than
nm s –2
Cantley (CA)
120
Gravity residuals
100
Hydrology
Snow
80
60
40
20
0
–20
–40
–60
–80
1997 1998 1999 2000 2001 2002 2003 2004
Year
nm s –2
100
80
60
40
20
0
–20
–40
Wettzell (WE)
Gravity residuals
Hydrology
Snow
–60
1999.5 2000 2000.5 2001 2001.5 2002 2002.5 2003 2003.5
Year
Figure 27 Gravity residuals, hydrology (soil moisture þ snow), and snow-modeled contributions at stations Cantley
(Canada) and Wettzell (Germany). Reproduced from Boy J-P and Hinderer J (2006) Study of the seasonal gravity signal in
superconducting gravimeter data. Journal of Geodynamics 41: 227–233, with permission from Elsevier.