3.04 - Gravimetric Methods – Superconducting Gravity Meters

Abstract

This chapter is devoted to the superconducting gravimeter (SG) and its wide use in Earth sciences. The first part is merely instrumental: the history of the technical developments is first given followed by a precise description of the basic principles of operation. The instrument performances in terms of long-term drift, calibration stability, and precision are provided. Recent technical developments and user requirements for operating in an optimal way an SG station are reviewed. The second part deals with the SG data analysis. The various steps going from the raw observed gravity signal to the residual signal ready for geophysical investigation are described in detail: pre-processing (gaps, offsets), corrections for solid and ocean tides, atmospheric pressure effects. The importance of calibration issues (amplitude and phase) is emphasized. This second part ends with other corrections to the residual gravity due to polar motion, instrumental drift, and hydrology. The third part presents the scientific achievements using SGs. The Global Geodynamics Project (GGP) of the existing international network of SGs is first introduced and we point out the broad spectral range of geophysical phenomena (from 1s to several years) that can be investigated with SGs. We show how the extremely low noise in the long-period seismic band can be exploited to retrieve seismic normal modes. Special attention is paid to the search for the Slichter translational mode of the solid inner core in the subseismic band. The various approaches to model atmospheric loading in gravity are reviewed, as well as solid Earth tidal effects including the free core nutation resonance in the diurnal band. Ocean loading can also be investigated on a wide spectral range through the detection of nonlinear ocean tides (harmonics of the classical diurnal and semidiurnal tides) and the observation of long-period tides (fortnightly and monthly). The signature in SG data of nontidal ocean circulation, continental hydrology (soil moisture and groundwater), and Earth rotation effects all demonstrate the ability of these instruments to study seasonal and multiyear geodynamical processes. Some specific applications related to tectonics are shown. We also comment on the possibility of using surface gravity measurements with a regional network of SGs to validate satellite-derived gravity missions such as GRACE. A prospective of future possibilities using SGs in various fields ends this third part.