Near real-time filtering of high precision borehole strainmeter signals for volcano surveillance

Volumetric strain signal recorded by the Sacks-Evertson strainmeter has a very high resolution and can measure small strain changes (down to 10–9) accompanying volcanic processes. However, different disturbing components perturb the recorded strain signal. These disturbances can mask ultra-small strain changes related to volcano activity and thus need to be filtered in order to accurately monitor volcano deformation. We developed the software STRALERT (STRain and wArning signaLs in nEar Real-Time) to provide both the recorded and the filtered strain signals in near real-time to the Surveillance Room of the “Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Etneo” which monitors the Etna activity. The software encloses a modified version of the code BAYTAP-G for filtering the incoming signal from the continuously running strainmeter at Etna. Thanks to the appropriate and robust signal filtering, STRALERT is capable to provide anefficient estimate of transient strain changes, which concur in tracing volcano deformation and detecting the onset and the conclusion of eruptive events. Our findings demonstrate the important contribution of STRALERT for volcano surveillance operations.