Source: INGV ambiente, 10/09/26
by Caterina Zei/INGV Einstein Telescope Team*
A window onto the Earth: why do we measure gravity?
Gravity may seem constant, but it is not. Its tiny variations over time and space can reveal how mass is redistributed within the Earth system and provide information on processes taking place in the atmosphere, hydrosphere, and inside the planet. Measuring such small variations requires instruments capable of detecting extremely weak signals. A superconducting gravimeter can record minute changes in gravitational acceleration and track how they evolve over time.
What do variations in gravity tell us?
The data provided by this technology can support research in several fields related to the environment and geophysics, including:
- Detecting fluid circulation underground: from the movement of aquifers and deep groundwater resources to the dynamics of magma and hydrothermal fluids.
- Studying the Earth’s elastic response: by monitoring Earth tides, i.e. deformations of the solid Earth caused by the gravitational pull of the Moon and the Sun.
- Detecting tectonic deformation: by recording variations associated with stress accumulation or seismic and volcanic processes.
Gravity measurements, however, are not only used to observe changes over time: they are also essential for describing the Earth’s gravitational field and defining the geoid.
The true shape of the Earth: how is the geoid measured?
The geoid is an equipotential surface of the Earth’s gravitational field that ideally coincides with mean sea level and its continuation beneath the continents. To better understand what an “equipotential surface” means, let’s think about water: the geoid is the surface on which water does not flow in any direction because it is all at the same “energy level”. If we were to drop an object from this surface, it would fall straight down, perfectly perpendicular to the surface, without deviating to the right or left. Because the Earth is not a perfect sphere and its internal masses are distributed unevenly, the geoid (Fig. 1) has “depressions” and “elevations” relative to the reference ellipsoid, the mathematical model used to represent the shape of the Earth in simplified form. The colours in Fig. 1 represent “geoid anomalies”, that is, the differences between the mathematical model and the actual shape of the geoid.
Fig. 1 – The summit of Chimborazo, in Ecuador, is the point on Earth farthest from the planet’s centre because the Earth bulges at the equator. The distorted, potato-like shape of the Earth shown in the image is highly exaggerated due to the strong vertical scale amplification. In reality, the maximum difference between the geoid and the reference ellipsoid is approximately 100 metres.
The extremely high-precision measurements provided by the superconducting gravimeter, combined with satellite data and global terrestrial networks, make it possible to continuously update geoid models. This is essential both for studying environmental changes on our planet and for improving the accuracy of satellite navigation systems (GPS/GNSS).
How does a superconducting gravimeter work?
To understand the innovation behind this instrument, let us first imagine a traditional gravimeter. To measure gravity, it uses a conventional spring with a mass attached to it. If gravity increases slightly, the mass is pulled downward and the spring stretches; if gravity decreases, the spring contracts. Mechanical springs, however, wear out over time, are affected by temperature variations, and may gradually lose performance, compromising measurement accuracy. The superconducting gravimeter (Fig. 2) completely eliminates springs and mechanical components under tension, instead exploiting a principle of quantum physics: superconductivity. Here, in a nutshell, is how the instrument works:
- Extreme cooling: the inside of the instrument is cooled to around -269 °C using liquid helium. At this temperature, superconductivity is activated in the metals, allowing electric current to flow without resistance.
- The sphere that “floats” in the air: instead of a traditional spring, the magnetic field generated by the current levitates a tiny niobium sphere in a vacuum, with no friction or physical contact.
- Microscopic measurement: every tiny change in gravity causes the sphere to move. The instrument measures the force required to keep it in place, recording minute variations, down to one billionth of the normal value.
Fig. 2 – The GWR iGrav#070 superconducting gravimeter.
Sos Enattos, the Einstein Telescope and the global scientific network
At the Sos Enattos mine in Lula (NU), researchers from the National Institute for Geophysics and Volcanology (INGV) have successfully completed the installation, testing and commissioning of the GWR iGrav#070 superconducting gravimeter (Fig. 2). The instrument is part of Sardinia FABER (Far Fault Observatory), a low-noise observatory funded by the Italian National Recovery and Resilience Plan (NRRP) as part of the MEET project, coordinated by INGV to strengthen the Italian geophysical research infrastructure.
The installation is part of the characterisation of the site proposed to host the Einstein Telescope (ET), Europe’s future third-generation observatory for gravitational waves. The geological stability and seismic quietness of the Sardinian area will allow the iGrav to measure micro-variations in gravity with extremely high sensitivity, while isolating environmental disturbances that could otherwise limit the performance of gravitational-wave detectors.
The data collected at Sos Enattos will feed into international networks, including IGETS (International Geodynamics and Earth Tide Service), and will contribute to strengthening the National Fiducial Gravimetric Network established by INGV in 2023. The infrastructure is part of the activities of the National Gravimetric Service Centre (CSGN), established in 2025 to coordinate national observations, and complements the other superconducting gravimeters already operating in Italy, including those operated by INGV on Mount Etna and the one operated by the University of Naples Federico II at the Phlegraean Fields.
*Spina Cianetti, Domenico Di Mauro, Carlo Giunchi, Filippo Greco e Marco Olivieri.