GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 2.2 371 POSTHOLE BROADBAND SENSORS FOR THE UPGRADE OF THE SEISMIC NETWORK IN SICILY WITHIN THE GRINT (ITALIAN RESEARCH INFRASTRUCTURE FOR GEOSCIENCES) PROJECT A. D’Alessandro 1 , S. Bongiovanni 2 , P. Cosenza 3 , A. Di Benedetto 1 , A. Figlioli 2 , S. Scudero 1 , S. Speciale 1 , G. Vitale 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Nazionale Terremoti, Rome, Italy 2 Università di Palermo, Dipartimento di Scienze della Terra e del Mare, Palermo, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy Introduction. GRINT Italian Research Infrastructure for Geosciences” is a project in the framework of the “Programma Operativo Nazionale” (PON) of the Italian Ministry for the University and Research. This plan envisages both the upgrade of monitoring systems in the already existing nodes, and the integration of new nodes. The main objective is to build a multiparametric network which integrates the main seismic network, with the monitoring of other geophysical signals, aimed at recognizing the phases of an earthquake: from the preparatory processes, to the trigger and arrest. Such upgrade will provide also the improvement of a series of seismological products: waveforms, seismological studies, bulletins for the seismic surveillance. Sicily (Italy) is one among the target areas of the GRINT project. Here, the plan for the modernization started some years ago (D’Alessandro et al. , 2021) and the existing multi- parametric monitoring infrastructure will benefit of GRINT project. The activity is mainly of technological nature. It consists of the technical enhancement and upgrade of existing systems, an upgrade of seismic stations with installation of sensors covering the entire bandwidth of the seismic signal, both velocimeters and accelerometers. New nodes will be integrated in the existing network. The new seismic sensors are posthole broad-band velocimeters. They are installed in apposite perforations or wells that intercept the bedrock. Such solution reduces the level of seismic noise whose sources are almost always superficial (i.e. anthropogenic sources, wind, atmospheric pressure, vegetation) and makes as stable as possible parameters such as temperature, pressure and humidity. Together with sensors, power-supply systems, data loggers, real-time data transfer systems will be updated to optimize the performances, reduce the costs and the maintenance, and ensure the full operational services also during possible crisis. In addition, radon sensors will be installed in some seismic stations of the network. In detail, this contribution introduces the experiences gained by the group Osservatorio Nazionale Terremoti (ONT, INGV) which operates in Sicily, and concerns the installation of post- hole broad-band sensors. This activity at such extended scale (35 sensors to be installed) can be considered as a pioneering one in Italy. We introduce the adopted procedures and the technical challenges faced for the optimal and effective deployment of the borehole sensors. Post-hole sensors: installation and signal characterization. Sicily (Italy) is one among the target areas of the GRINT project. Here, the plan for the modernization started some years ago (D’Alessandro et al. , 2021) and the existing multi-parametric network will benefit of GRINT project. The installation of broadband posthole sensors thanks to GRINT project will allow the acquisition of high quality data directly usable for all the aims of the earthquake network. The main reason for installing a seismometer in a well is to search for an environment in which the seismic-environmental noise is lower than the surface and consequently the signal-to-noise ratio (SNR) is maximized. The reduction of noise for well installations is strictly dependent on the frequency and is also variable on the different components. At frequencies of a few Hz, reductions of the order of 20 dB are observed already at a depth of a few tens of meters and reductions of up to 40 dB at a few hundred meters. For frequencies higher than 10