3.04 Gravimetric Methods – Superconducting Gravity Meters

74 Superconducting Gravity Meters
0.1 nGal. This is in effect the quantization noise of
the SG þ data acquisition system, but not necessarily
its precision. There is no reference gravity more
accurate than the SG itself, so the precision of the
SG can only be obtained by inference. In the frequency
domain for studies of tides or normal modes,
it is common for the SG to measure small periodic
tidal signals and long-period seismic signals with a
sensitivity of 1 nGal and better. Therefore, 1 nGal is
generally referred to as the nominal precision, or
sensitivity, of the SG.
The determination of the instrumental noise of
the SG is complicated by the fact that sources of the
Earth noise (signals from the atmosphere, oceans, and
hydrology) are generally much larger than instrumental
noise for frequencies ranging from 3 10 8
Hz (1 cycle per year) to 1 Hz. The SG noise is higher
than Earth noise only in the small subseismic band
between 1 and 20 mHz where the noise level is
typically from 1 to 3 nm s 2 Hz 1/2 (0.1–0.3 mGal
Hz 1/2 ) for most SG stations (Rosat et al., 2004).
This is more than 2 orders of magnitude lower than
the noise level of the AG as reported by Crossley et al.
(2001) and Van Camp et al. (2005).
It is common practice to filter and decimate data
to 1 min samples to look at small temporal gravity
variations; assuming white noise, the above noise
level translates into a precision of 0.01–0.03 mGal.
This level is consistent with common experience.
For example, Meurers (2000) easily observed gravity
signals of magnitude 0.1–0.3 mGal over 10–30 min
intervals and Imanishi et al. (2004) identified reliable
coseismic offsets of a similar magnitude.
The best way to determine real-world accuracy in
the temporal domain is to compare different instruments
operating side by side. Two comparisons have
been done – one in Miami (Richter, 1990; Klopping
et al., 1995, Harnisch and Harnisch, 1995) and one in
Boulder (Harnisch et al., 1998), with the result that
different pairs of instruments agree to 0.1 mGal, or
better. For a dual-sphere instrument, the data streams
from the two spheres are largely independent,
and the difference signals between two sensors of
the four dual-sphere SGs are typically 1 mGal
over record lengths of years. Additionally, the residual
curves from the dual SG in Moxa agree within a
few tenths of a mGal to the polar motion modeled
using data provided by the International Earth
Rotation Service (IERS). These results suggest that
0.1 mGal is the time-domain accuracy of the SG for
long periods.
3.04.1.5 Recent Developments
3.04.1.5.1 Ultralong hold time dewar
Although Compact Dewars were extremely successful,
the goal remained to further decrease helium consumption
and annual disturbances from helium transfers. By
1997, the first commercial (and somewhat) practical 4 K
refrigeration systems became available. The Leybold
Vacuum Products KelKool 4.2 GM coldhead produced
cooling power of 50 W at 50 K at its upper stage and
0.5 W at 4 K at its lower stage. It cooled well below the
4.2 K liquefaction temperature of helium and produced
more than 5 times the cooling power of the APD
DE202. Soon afterward, GWR produced an ultralong
hold time dewar (ULHD) based on the 125 l Compact
Dewar design with its neck modified to accommodate
the much larger 4.2 GM coldhead. This is a closedcycle
system, because the helium gas condenses in the
neck on the lower stage and drips back into the storage
volume of the dewar (Richter and Warburton, 1998).
This success pointed to the future in which SGs could
operate indefinitely without transferring liquid helium
or consumption of liquid helium. The first ULHD
system is shown in Figure 5.
Figure 5 One of the first dual-sphere instruments (CD029,
now at Wettzell, Germany) with a ULHD using a Leybold
KelKool 4.2 K GM coldhead. The support cranes used to
insert and remove the coldhead are shown in the background.