On the importance of accounting for instrumental effects when applying gravimetry to volcano monitoring

Repeated and continuous gravity measurements have long been performed to monitor active volcanoes and study the processes that may lead to unrest and eruptions. Possible instrumental effects must be accurately accounted for, since they can be behind apparent gravity changes even stronger than the real (i.e., volcano-related) ones.

At tall volcanoes, where, due to the rough topography, the difference between the gravity field values at external and summit stations can be hundreds of mGal, strong time changes in gravity may arise from changes in the calibration factor of the device used to perform campaign measurements. For example, a difference in the value of the gravity field between reference station and summit active area of 300 mGal implies an apparent time change of 60 µGal, if a shift of 100 ppm occurs in the calibration factor of the gravimeter. To avoid this, the calibration factor of the instrument used to perform campaign measurements should be regularly checked.

The instrumental drift of relative gravimeters can make it difficult to detect long-term (months to years) gravity changes through continuous measurements. This shortcoming may also affect data from superconducting gravimeters (much more stable than spring gravimeters), especially if relatively small gravity changes are to be detected over time scales of several months, or longer. To address this issue, absolute gravity measurements must be performed at the same site where the continuously recording gravimeter is installed. Here we present some examples from Mt. Etna volcano, where instrumental effects, potentially leading to apparent time changes of some tens of µGal, were detected through suitable measurement strategies.