GNGTS 2019 - Atti del 38° Convegno Nazionale

366 GNGTS 2019 S essione 2.2 For the attenuation analysis, a geophone downhole string provided by the Politecnico di Torino (DIATI) was used to perform a near-surface, VSP survey, consisting of n°8 three-channel geophones with a 4,5 Hz natural frequency, connected in series and spaced 1 meter apart and progressively lowered down 5 meters for each measure, resulting in an overlap of three traces between two subsequent acquisitions. Controls over the absolute time was performed on real time. For P-wave investigation the seismic source consists in an Aluminum plate of 30 cm diameter. For S-wave generation we employed an Aluminum source ground built by DICEA’s Geophysics Area (Fig 1b). Both sources were striked with a 6kg sledgehammer for seismic wave generation. The sampling was 0.03125 ms, so that the sampling rate is 32000 Hz. The number of samples recorded is 16384 for a total recording length of 512 ms. The n-traces recorded can be combined into n-1 non-redundant pairs. Table 1 - Parameters for field data acquisition. Survey Acquisition detalis Lat: 42°,402415, Long: 12°,866859 Geognostic survey Material: PVC pipe with a 3” diameter and 0.5 cm thickness; Cementation: consisting of water, cement and bentonite Geophones: 8 geophones (4.5 Hz) 1 m spaced Source: 6 kg hammer weight Device: 1 seismographs Geode Geometrics Near-surface VSP Sampling: 0.03125 ms Sampling rate: 32000 Hz Number of samples: 16384 Record length: 512 ms Fig. 1 - (a) Reconstructed seismic profiles of P and SH waves. (b) Views of the experiment.