GNGTS 2014 - Atti del 33° Convegno Nazionale

Data acquisition along profile 1. The performed measurements were based on passive seismic techniques, both exploiting the ambient seismic noise: the ESAC and Re.Mi. methods. Both techniques are able to acquire the vertical sequence of shear waves velocity. Along this profile we performed 20 measurements, in particular, 13 acquisitions according to the ESAC procedure and 7 with the Re.Mi. one. In Tab. 1 are reported both kilometric and geographic coordinates of all investigated sites. The Re.Mi. ( Refraction Microtremors ) method is based on a linear layout of vertical geophones commonly used in seismic refraction surveys. The data analysis is performed via Fourier transform, after a preliminary ‘slant stack’ operation, and followed by 1D inversion of the fundamental phase. A conceptual limit of this method is due to the fact that only the waves propagating parallel to the geophones layout are properly analysed, while for all other directions the inversion procedure tends to overestimate the shear velocity. The ESAC ( Extended Spatial AutoCorrelation ) method is characterised by a 2D layout of the geophones located along T- or L-shape profiles. This assures a better coverage of the seismic waves record and avoids the above mentioned directionality problem of the Re.Mi. technique. Inversion of ESAC data is performed in the distance domain for each frequency. The use of the ESAC method is crucial for obtaining information on the velocity distribution at greater depths with respect to Re.Mi. results, while the latter technique is more easily applied in the field for sites with reduced accessibility. In each site a layout of 24 3-component geophones (velocimeters), spaced 8 m, was developed and the vertical (vertical Rayleigh component) and one horizontal (Love) components of the particles velocity were acquired using a commercial seismograph, characterized by a 24 bits dynamics. Inversion of the data was based on a 1D scheme, i.e. variation of geophysical parameters only along the vertical axis, using a commercial software (SeisOpt tm , http://www.optimsoftware.com/index. php/seisopt-remi-by-optim-software), and inverting simultaneously Rayleigh and Love velocity versus frequency dispersion spectra. Smooth inversion option was selected, based on the Simulated Annealing (SA) algorithm. The relatively long spacing between geophones produced aliasing, but this noise disturbed spectra at frequencies higher than 10-15 Hz, corresponding to the shallowest depths (no more than 10 m), is irrelevant to our analyses. In Fig. 2, the 1D shear wave velocity profiles obtained by inversion of the fundamental mode dispersion curves of Fig. 2 – The 1D shear-waves velocity profiles obtained using the ESAC (top) and Re.Mi. (bottom) techniques. Site coordinates are reported in Tab. 1. GNGTS 2014 S essione 1.2 119