GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.2 391 to 45 meters, and equipped for the Downhole geophysical test. Below are the description data related to drilling: Tab. 1 - Parameters for field data acquisition. Survey Acquisition detalis Geognostic survey Lat: 42°,402415, Long: 12°,866859 Material: PVC pipe with a 3” diameter and 0.5 cm thickness; Cementation: consisting of water, cement and bentonite Near-surface VSP Geophones: 8 + 2 hydrophones (4.5 Hz) 1 m spaced Source: 6 kg hammer weight Device: 1 seismographs Geode Geometrics For the attenuation analysis, a geophone downhole string was used to perform a near-surface, VSP survey, consisting of n°8 single-channel hydrophones (the hole was previously fill of water) with a 4,5 Hz natural frequency, spaced 1 meter apart and progressively lowered down 5 meters for each measure, resulting in an overlap of three traces between two subsequent acquisitions. Two additional geophones was mounted on the ground to assure control over the absolute time. The seismic source consists of a 6 kg sledge-hammer operated manually on an aluminium plate (30 cm of radius) fixed on the ground. The sampling interval is 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. Data processing. The assessment of seismic attenuation values from VSP data is done essentially by measuring changes in amplitude and broadening of compressional P-wave first arrivals corresponding to direct travel path from source to receivers position. Fig. 1 - (a) One shot traces visualization (shot n. 9, depth 39-47 m.). (b) Vertical Seismic Profile after conditioning operations (c) Complete Vertical Seismic Profile.