4D Hybrid Microgravity Measurements: Two Case Studies of Monitoring at Mt. Etna Volcano and at a Gas Storage Reservoir in Northern Italy

Detection of clear gravity signals associated with the renewal of the volcanic activity and the emerging
need of characterizing the dynamic changes of subsurface systems have led to increased application of the
microgravity method in time-lapsed monitoring, also known as 4D gravity approach. Conventionally,
microgravity measurements have been carried out using relative gravimeters, which measure spatial changes
with respect to a fixed reference site. Since 2007, with the aim of comparing relative microgravity
measurements routinely acquired on Etna with absolute gravity observations, we performed repeated surveys
using transportable absolute gravimeters.