GNGTS 2014 - Atti del 33° Convegno Nazionale

indicating that during the corresponding time interval of 20 years occurred not any vertical movement and/or mass redistribution in the underground significant for the gravity value at the Earth’s surface. As changes in time of the calibration factor of the gravimeter can occur as a consequence of perturbations of different origins (e.g. Bonvalot et al. , 1998; Riccardi et al. , 2002), four �� sessions of on site calibration of the gravimeter have been carried out at the station of Cosenza. The results are shown in Fig. 2 together �� those of the set of 18 calibrations carried out at a station in Naples, where the same instrument had been operating for several years before to be moved to Cosenza. Fig. 2 – Calibration factors measured at the stations of Naples and Cosenza. The global set of 24 calibration sessions shows randomly scattered results without any significant trend versus time. �� In fact the slope of the regression straight line turns out to be -0.01 ± 0.03 (r = -0.08). To further check the long term trend of the calibration factor, the latter has been computed on temporal windows of 48 hours, with reference to the gravity tide predicted by the DDW99/ NH tidal model (Dehant et al. , 1999). Not even the resulting values by this second method show any significant temporal trend; effectively they result randomly scattered around a straight line having the slope of (3 ± 0.4) · 10 -3 (r = 0.3) nm/s 2 /mVolt/day. The gravity data recorded have been decimated to 1 hour �� after the application of a low- pass filter �� (cut off frequency: 12 cycles/day) to avoid aliasing effects. The ETERNA software (ver.3.3; Wenzel, 1996b) �� was used to compute amplitudes, phases and amplification factors δ of the spectral components of the gravity tide. �� The HW95 (Hartmann and Wenzel, 1995), the most recent and widespread catalogue among several others describing the tidal field (Wenzel, 1996a), was adopted to process the gravity record� �� . The catalogue �� consists of 12935 tidal waves, containing 19300 adjusted coefficients computed using the JPL DE200 numerical ephemeris of the Solar System. A rms error of 0.0015 nm/s 2 on the tidal field was estimated at intermediate latitudes. �� The obtained values of amplitude, amplification factor δ and phase shift of the main tidal waves are given in Tab. 1. �� According to the recommendations of the Working Group on Theoretical Tidal Model (SSG of the Earth Tide Commission, Sec. V, of the IAG), the δ factor is defined as Earth’s transfer function between the body tide observed at a station and the amplitude of the vertical component of the gradient of the tidal potential (Dehant, 1989). 124 GNGTS 2014 S essione 1.2