ANNALS OF GEOPHYSICS, 61, 1, SE114, 2018; doi: 10.4401/ag-7496 Improvement and development of the tiltmetric monitoring networks of Neapolitan volcanoes Ciro Ricco 1 , Ida Aquino 1,* , Vincenzo Augusti 1 , Luca D’Auria 2 , Carlo Del Gaudio 1 , Giovanni Scarpato 1 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV) – Osservatorio Vesuviano, Napoli, Italy 2 Instituto Tecnológico y de Energías Renovables (ITER) Tenerife, España Article history Received July 18, 2017; accepted February 23, 2018. Subject classification: Ground tilt; Volcano monitoring; Borehole sensors; Neapolitan volcanoes; Raspberry Pi 3. SE114 ABSTRACT The goal of this work is to illustrate the technological development of the tiltmetric monitoring network of Osservatorio Vesuviano, since 1986, retracing the technical steps that led to the current network configura- tion. This network is devoted to the monitoring of the three Neapolitan volcanoes: Vesuvius, Phlegrean Fields and Ischia Island, which are widely recognized among the volcanic areas with the highest risk world- wide. Ground tilt monitoring had to face numerous technical obstacles, mainly due to background noise characterizing densely urbanized areas like the Neapolitan volcanoes. The network started with the first instal- lations of horizontal optical pendulums that recorded analog signals. In the following years, the network was equipped with electronic sensors more handy and easy to install. A technological leap forward occurred when the first digital sensors were installed in deep boreholes, allowing them to record tilt signals not affected by thermal disturbances. These data are digitally acquired and managed by a datalogger for the data storage and exchange. Currently the network consists of 10 sensors in Phlegrean Fields, 8 in Vesuvius and 3 in Ischia Island, making it one of the densest tiltmetric volcano monitoring network worldwide. This net- work can boast a database containing thirty years of data acquired by both analog and digital stations. Data consists of both ground tilt and meteorological signals, such as temperature and atmospheric pressure. In these areas they allow a continuous monitoring of the temporal varia- tions of the ground tilt with important implications in understanding the dynamics of these active volcanoes. 1. Introduction Ground tilt monitoring is the continuously mea- surement of changes in the slope of the ground sur- face. It is able to detect slight tilt variations, in both direction and amplitude, of the volcano edifice caused by the volcano dynamics (e.g. overpressure in a magma chamber, ascent of magma, perturbation of the hydrothermal system). During an inflation episode, the flanks of volcanoes deform reaching vari- ations of inclination of the order of about ten micro- radians [Dzurusin 2006]. Jointly with other geodetic methods, the study of the angular component of strain allows scientists to better understand the nature of geophysical phenomenon going on in a certain vol- canic area [Ricco et al. 2003]. Tilt data, together with tensor strainmeters are used to measure the non-diagonal components (i≠j) of the deformation tensor (pure tilt in a solid half- space), while the diagonal components (i=j) of it are filled using data recorded by volumetric strainmeters or dilatometers [Zadro and Braitenberg 1999]. Even the knowledge of the ground tilt alone of- fers valuable data for modeling of the strain field. In practice it provides a parameter used to immediately integrate the displacements observed (i.e. by optical levelling, GPS or SAR) on a restless volcanic area [Ricco et al. 2007]. 2. Evolution of ground tilt monitoring instru- ments. At the end of 1985, two of authors (C.D.G. and C.R.) set up the first tiltmetric network at Phlegrean Fields. The instrumentation was installed in collabo- ration with the Institute de Physique du Globe de Paris (I.P.G.P.) [Luongo et al. 1998]. The first station, called DMB, was located in an underground tunnel aligned NS about 15 m deep, in Pozzuoli Town (Figure 1) and two more stations (DMA and DMC) were installed later. The second sta- tion, called BAI, was installed in a gallery below the