High resolution geophysical data in areas with evidences of fossil and active fluid seepage (Northern Apennines, Italy).

The University of Modena and Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) collaborated to acquire two geophysical data set in two areas located near Sassuolo (Modena; Northen Appenines). The first area is characterized by fossil fluid expulsion structures forming in the Upper Miocene, while the second one is actually interested by expulsion of mud and free gas. We used seismic 3D surveys to reconstruct in detail the shallow structures in both areas. Ground Penetrating Radar (GPR) and geo-electrical survey are used only in the second area to better characterize the first meters. The fossil structures located in the first area (Sasso Strega) have a horizontal dimension of about 10 meters, and a vertical extension of few ten. To investigate the presence of these buried structures a 3D seismic acquisition was performed, in which additional shots are located across the investigated area by using the natural topography to have the same effect of a deep source. In this way, the structures are lightened directly from the bottom. The source was the mini-bang and the receivers are vertical geophones (10 Hz). The receiver inline and cross-line intervals were 5 m, while the source interval was equal to 5 m (for inline shots) and 10 m (for cross-line shots). After the processing of the data to increase the signal/noise ratio, the tomographic inversion of the first breaks was applied to determine the geometry and the compressional velocity of buried structures. In the second site (near Nirano), a 3D seismic acquisition was acquired using three component geophones. In this case, the source was a Vibroseis source, that generates both compressional and shear waves. So, the acquisition allowed us to obtain information about both compressional and shear velocity fields by using the inversion tomography of the first arrivals. Of course, the comparison between the two fields is an useful tool to investigate the fluid presence in the investigated area. In fact, it is well known that the compressional velocities are influenced by both matrix and fluid, while the shear velocities detect only the matrix change.In this site we also collected geoelectrical and GPR data to recostruct the shallow features of the subsoil. In particular the geoelectrical data are inverted using tomographic approach to obtain the resistivity and chargeability models.We present the preliminary results of the collected data in the two areas.