GNGTS 2022 - Atti del 40° Convegno Nazionale

372 GNGTS 2022 Sessione 2.2 Hz, noise attenuations of even 40 dB are observed at depths of a few tens of meters (Greig et al., 2014; Withers et al., 1996; Hutt et al., 2017) Among the sensors used for installation in wells, the “borehole” and “posthole” seismometers are generally distinguished. While the former are designed for deep drilling (hundreds and thousands of meters) and are equipped with lateral anchoring systems to the well casing, the latter are designed for installation at the bottom of shallow drillings (<50 m), even without coating (Hayman, 2014). The posthole sensor is the model Trillium Compact PH from Nanometrics. It is a compact (3.2 kg, 97x160 mm) sensor with bandwidth up to 120 s. When connected with Nanometrics Centaur digital recorder, a graphical interface allows to set the sensors and also to access the digital level integrated within the sensor. The installations have been carried out either in already existing wells and specifically drilled perforation. Of course, in both cases, some characteristics should be preliminarily evaluated. Devoted geophysical investigation (seismic tomography, MASW, ambient noise characterization) have been carried out to fully characterize the sites. Before the installation, the well is inspected by means of a borehole camera to verify the integrity of the casing and of the bottom (Fig. 1). Fig. 1 - Installation operation at various sites. a: visual inspection by means of a borehole camera; b: the lowering of the sensor inside a drilled perforation; c: details of the pulley aligned on the top of a pre-existing well; d: final set-up of the earthquake monitoring site: note the posthole sensor placed at surface next to the well and the accelerometer for its later replacement; e: lab testing for the anchoring system. The metered cable of the camera also helps to estimate the depth of the well and therefore to design the waterproof extension supply cable and the stainless steel retaining cable. Then, the well is filled some tenths of meters of sands to constitute a bed for the sensor. The sensor, along with its cable, is loweredwithin the hole bymeans of the retaining cable (diameter 3mm). The two cables are tightened together with plastic ties at distance of about 1.5 m during the descent. The descent is controlled with a hand winch screwed into a portable wooden tripod and a pulley which is aligned to the top of the hole (Fig. 1). A digital dynamometer provides the indication of the load under the pulley, so that the operators are able to detect exactly the touchdown of the sensor and arrest its lowering. After a final camera check, another layer of sands is then released in the well to cover the sensor. Now the sensor can be connected to the data logger and powered to check the digital leveling system. By means of gentle maneuvers