Gravimetric monitoring detects changes in subsurface mass by measuring tiny variations in the local gravitational field. At volcanoes, these signals can indicate magma intrusion or withdrawal, movement of hydrothermal fluids, or changes in the density of material beneath the surface. When combined with GNSS measurements of surface deformation, gravity data help distinguish between scenarios that produce similar surface signals — for example, differentiating uplift caused by magma injection from uplift driven by pressurised fluids.
How it works
A gravimeter measures the acceleration due to gravity at a fixed point. Subsurface mass changes — such as the arrival of new magma or the draining of a hydrothermal reservoir — alter the local gravitational pull. By tracking these variations over time and correcting for known effects such as Earth tides and atmospheric pressure changes, volcanologists can infer what is happening at depth.
Gravity changes at volcanoes are extremely small, typically on the order of
a few micro-Gal (millionths of the standard gravitational acceleration).
Detecting such signals requires highly sensitive instruments and careful
environmental corrections.
AVERT deployment
At Poas volcano, a MEMS-based Wee-g micro-gravimeter developed by the University of Glasgow was installed at the VPMI hub site during the 2024 field workshop. This represents the first continuous MEMS gravimeter deployed on any volcano worldwide. The instrument outputs continuous data via a serial communication interface and was integrated into the AVERT platform as a test of the system’s extensibility to new sensor types.
Gravimeter data at Poas are used together with GNSS and gas observations to constrain processes in the hydrothermal system and shallow magmatic reservoir, capturing mass changes not evident in surface deformation alone. 
