GNGTS 2015 - Atti del 34° Convegno Nazionale

varies between 4.90 m and 22.27 m depth. During drilling we have been performed in situ permeability tests. The diagram summarizing the stratigraphic exploratory drillings carried facies divided into 4 groups: the lithoid Flysch, the altered Flysch, sediments of marine origin and soils of continental and anthropogenic origin. Data processing. Electrical Resistivity Tomography (ERT). The geoelectrical tomographic data inversion was performed with the method proposed by Loke and Barker (1996) through the use of the code Res2dinv. This program automatically determine a 2D pseudosection resistivity model obtained from electrical imaging survey (Fig. 1). The plotting of data against position along a traverse line, to produce a display of resistivity data in which values are given to the intersection point of 45° lines drawn from mid-points of the current and potential electrode pairs. Depths in the resulting ‘section’ below a transverse bear no simple relationship to the true geology; a pseudosection shows the variation of the measured parameter with position and with effective depth of penetration, rather than with true depth. Refraction seismic and tomographic inversion. For each line we have been executed a tomographic inversion, using the tomographic software Cat3d. The algorithm used is the reverse SIRT (Simultaneous Iterative Reconstraction Technique), based on an iterative process that minimizes the residue of the times. Considering the type of dromocrone, the first arrivals were associated to diving wave. After analyzing individual lines 2D, it was performed tomography joint of the first arrivals of the times corresponding to all 2D on a 3D model (Fig. 2). We applied the travel time tomography of first arrivals on all the acquired seismic lines in order to define a precise velocity model of the shallower part of the investigated area. As tomographic volume (350x250x75 m), we used a regular grid (21x15x30 voxels) on which we applied the staggered grid method (Vesnaver and Böhm, 2000), obtaining a more precise final model (63x45x60 voxels) with lateral and vertical resolution of 5.5 m and 1.25 m respectively. We used the SIRT method (Simultaneous Reconstruction Technique) as inversion algorithm and the diving ray paths in the ray tracing approach (Böhm et al. , 1999). The picked arrivals of all the lines were jointly inverted in order to preserve the coherency of velocity fields in correspondence of the cross points of the seismic lines. Then, we extrapolated the 2D velocity sections to the whole volume to complete the velocity information of the investigated area. The reliability of the inversion was estimated by the time residuals computation (difference between picked and computed travel times): 1.6 ms rms residual (equivalent to 4.4 % on picked travel times). Results. Data interpretation and definition of the geological and hydrological model. The geological model was established by integrating the information obtained with the geophysical surveys and those derived from the geological survey of the land and the stratigraphy of the boreholes (Fig. 3). The anthropogenic fill is over imposed on the marine clays while in the SE these are emerging to the portion of the ground surface. The calibration of the data with the ERT survey has allowed Fig. 2 – Tomographic inversion of refraction seismic survey of of the Ex-Esso industrial site (Trieste, Italy). GNGTS 2015 S essione 3.2 73