Spring-based relative gravimeters, first introduced in the 1930s, are the most widely used for geophysical applications. They measure the change in the equilibrium position of a proof mass suspended from a spring, resulting from a change in the gravity field. Superconducting gravimeters (SGs; first introduced in the 1970s) offer an alternative to spring-based instruments, although still provide relative measurements only. Instead of a mechanical spring, SGs exploit the magnetic levitation of a superconducting sphere in a field of superconducting persistent coils. They have high power requirements and occupy a large physical footprint; therefore, they are not well suited for field deployments. State of the art absolute gravimeters exploit the free-fall of macroscopic test masses. Transportable free-fall absolute gravimeters (FFAGs ) were developed starting in in the late 1960s. They employ a laser interferometer to precisely monitor the free-fall trajectory of the test mass. FFAGs are not well-suited for field use in harsh conditions because of large size and high power requirement. Furthermore, they cannot record gravity data continuously for extended intervals. All of the above gravimeters come at very high costs and no new methods for measuring gravity have been introduced for nearly five decades, so all existing gravimeters are based on older technologies and principles. Given the current state of the art, it is clear that the development of new instruments represents a fundamental step that can move gravimetry from a niche field into a cornerstone resource for geophysical monitoring and research.

Through the MEMS gravimeters that will be produced as part of NEWTON-g, most barriers that have limited the development of continuous gravimetry for geophysical applications will be broken down. Indeed, thanks to small size and low power consumption, MEMS devices are especially well suited for use under rugged environmental conditions. Furthermore, thanks to the low cost, it will be possible to install extended arrays of gravity sensors, thus detecting underground mass changes with high spatial and temporal resolution, including in hazardous areas (like the summit region of an active volcano) where gravimeters are rarely installed owing to the high risk of losing expensive instrumentation to eruptive activity.
The quantum gravimeter that will be developed under NEWTON-g will be the ideal complement to the MEMS devices. Compared to standard FFAGs, we expect that the new quantum gravimeter will be (i) easier to install, with the ability to complete a high-quality absolute measurement within 1–1.5 hours of arrival at the installation site; (ii) less affected by ground vibrations thanks to an innovative technology involving active compensation of ground vibration noise; (iii) able to measure continuously for extended periods of time and (iv) cheaper, if not in terms of production costs, then in terms of maintenance costs.
The methodological approach of NEWTON-g, based on the joint use of MEMS and quantum gravimeters, represents a paradigm shift in the use of gravimetry for geophysical applications. The deployment will involve an array of MEMS devices anchored to the quantum gravimeter; the latter will provide a high-resolution reference to calibrate the signals from the MEMS devices.

Our specific target for testing the new gravity measurement technologies developed under NEWTON-g will be Mount Etna (Italy), a persistently active volcano that has been a UNESCO world heritage since 2013. Owing to its persistent activity, with a wide spectrum of eruptive styles, large volcano-related gravity changes often develop at Mt. Etna, over different time scales. The volcano has also been well studied and is the site of a comprehensive existing monitoring infrastructure, including some continuous gravity stations. Furthermore, the active crater zone of Etna can be reached by car by authorized personnel. This combination of factors provides a unique and attractive scenario for testing and benchmarking the new gravity instruments.

Volcanic eruptions threaten several countries across Europe, including Greece, Italy, Iceland, Russia and overseas territories of EU countries. Historical records show that some of these volcanoes have the potential to produce destructive eruptions. A recent example of volcanic eruption impact on civil society is the 2010 eruption of the Eyjafjallajökull volcano (Iceland), that forced most countries in northern Europe to close their airspace for some days to weeks, due to volcanic ash. Volcanic hazards with the potential of affecting urbanized areas and economic sectors in the near and far field, imply a demand for the associated risk to be mitigated and, in turn, imply that emergency response agencies must have easy access to critical information for early warning and volcanic hazard assessment. Given the unique insights into subsurface fluids provided by gravity, the technique has exceptional utility for characterizing the hazard potential at a given volcano.
The specific objectives of NEWTON-g include the design and development of a new prototype gravity measurement system and its field application at Mount Etna volcano, including incorporation of the data produced by the new system into early warning systems, hazard reporting and crisis management plans for the detection and assessment of short-term (days-months) volcanic hazard. Such an effort may serve as a model for how volcano observatories around the world might use the tools and methods developed by NEWTON-g for their own monitoring programs.

There is currently no capability in Europe for the manufacture of gravity monitoring equipment, despite the widespread application of gravity in a variety of fields. Indeed, all the gravimeters available on the market are produced in the USA and Canada. With the realization of the gravity imaging system, jointly exploiting MEMS and quantum devices, proposed by NEWTON-g, the center of production for both absolute gravity technology, as well as less expensive relative instrumentation, would move from North America to Europe.