3.04 Gravimetric Methods – Superconducting Gravity Meters

66 Superconducting Gravity Meters
3.04.2.3.2 Frequency-dependent admittance 89
3.04.2.3.3 Green’s functions and nonlocal pressure corrections 90
3.04.2.3.4 3-D atmospheric corrections 91
3.04.2.4 Calibration Issues 92
3.04.2.4.1 Basics 92
3.04.2.4.2 Amplitude calibration, relative methods 93
3.04.2.4.3 Amplitude calibration, absolute methods 94
3.04.2.4.4 Phase calibration 95
3.04.2.5 Other Corrections to Residual Gravity 96
3.04.2.5.1 Polar motion 96
3.04.2.5.2 Instrument drift 96
3.04.2.5.3 Hydrology 96
3.04.2.5.4 Residual gravity 97
3.04.3 Scientific Achievements Using SGs 97
3.04.3.1 The Global Geodynamics Project 97
3.04.3.2 Seismic and Subseismic Signals 100
3.04.3.3 Atmospheric Loading 103
3.04.3.4 Tides and Nearly Diurnal Earth Wobbles 104
3.04.3.4.1 Resonance effects in diurnal tides 104
3.04.3.4.2 Ocean loading 105
3.04.3.5 Nontidal Ocean Circulation 106
3.04.3.6 Hydrology 107
3.04.3.7 Earth Rotation 108
3.04.3.8 Tectonic Effects 109
3.04.3.9 Ground/Satellite Gravity Field Comparison 110
3.04.3.10 Future Possibilities 112
3.04.4 Conclusions 113
References 115
3.04.1 The Superconducting
Gravimeter
3.04.1.1 Historical
3.04.1.1.1 Early years at UCSD
The superconducting gravimeter (SG) was first introduced
by Prothero and Goodkind (1968) as part of
Prothero’s (1967) thesis work on the design and development
of the instrument at University of California at
San Diego (UCSD). The SG broke new ground in
geophysics instrumentation, and was an elegant realization
of the principles of superconductivity. Although
the basic sensor configuration has remained unchanged
for nearly 40 years, continuous improvements in all
other aspects of the original design have successfully
converted the SG from a prototype laboratory instrument
to a reliable research tool. (Note that in this
article, the traditional gravity abbreviations are frequently
used: 1 microgal ¼ 1 mGal ¼ 10 mm s 2 ,
1 nanogal¼ 1nGal¼ 0.01 nm s 2 , and cpd ¼ cycles
per (solar) day.)
In 1970, Richard Warburton became a postdoctoral
student with John Goodkind, and this
collaboration was the foundation for the eventual
line of commercial SGs. Richard Reineman, an
undergraduate laboratory assistant working with
William Prothero in 1969, was integral to the effort
as a development technician with Goodkind and
Warburton. Prothero and Goodkind (1972) published
the first observations taken over a 4 month period and
obtained precise tidal amplitude and phases, new
information on ocean tide loading, and a recording
of seismic normal modes following the 7.1-magnitude
Kamchatka earthquake from 1969. Within a few years,
the UCSD group generated significant papers using
SG data on ocean tide loading (Warburton et al., 1975)
and the effects of barometric pressure on gravity
(Warburton and Goodkind, 1977). This phase of the
SG research culminated with a detailed tidal analysis
of 18 months of data that included the first estimate of
the effect of the nearly diurnal wobble on the resonant
amplification of small diurnal tidal phases (Warburton