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3-d geoLogicaL-geophysicaL modeL and preLiminarysynthetic seismic refLection modeLLing aLong crop18a Linein the LardereLLo areaR. de Franco1, L. Petracchini2, G. Caielli1, D. Scrocca2, A. Santilano3,4, A. Manzella3

1 Istituto per la Dinamica dei Processi Ambientali - CNR, Milano, Italy2 Istituto di Geologia Ambientale e Geoingegneria – CNR, Roma, Italy3 Istituto di Geoscienze e Georisorse – CNR, Pisa, Italy4 DIATI - Politecnico di Torino, Italy

Exploration strategies of geothermal reservoirs may significantly benefit from the development of synthetic seismic reflection profiles by confirming the possibility to detect prospective features on acquired seismic reflection data and to calibrate geological-geophysical interpretation and model reconstructions. To be elaborated a synthetic seismic reflection profile requires a conceptual geological model of the subsurface structure and physical properties, which is one of the tasks of the IMAGE FP7 European project (Integrated Methods for Advanced Geothermal Exploration).

The Larderello geothermal field is characterized by a shallow and by a deep reservoir. The latter is hosted in the metamorphic basement (Batini et al., 2003; Bertini et al., 1996). In seismic reflection profile, the deepest reservoir is characterized by a strong amplitude reflective signal, the well-known K-horizon, widely observed in several seismic lines (Batini et al., 1978; Accaino et al., 2005) and probably drilled by the San Pompeo 2 well (Gianelli et al., 1997).

In this study, geological and geophysical available data have been integrated to develop a new 3D geological-geophysical model of the portion of the Larderello geothermal field drilled by the San Pompeo 2 well. The geological-geophysical 3D modelling was performed using Petrel software.

The 3D model has been used to generate a 2-D model for the synthetic seismic modelling of the main seismic units up to the k-horizon along the CROP-18A seismic reflection line acquired within the CROP project. (Scrocca et al., 2013).

The exploding reflector approach, developed in Matlab by the CREWES consortium and partly modified by us in this project, has been used to generate the synthetic seismic sections.

The exploding reflector generates the seismograms for a velocity model defined by pixel (25x25 m) with constant velocity value. The positions of receivers were located at the CDP position of the line. The wavefield is propagated in depth using a finite difference algorithm, and is then convolved with the input wavelet (Ricker wavelet with 25 Hz of central frequency) to produce the seismogram at the receiver. The finite difference algorithm uses a nine-point approximation of the Laplacian operator and assumes the absorbing boundaries on the three sides of the model (bottom, right and left).

The geological units defined for the velocity model are respectively the Neogene Unit, the Ligurian Flysch Complex, the Tuscan Units plus Tectonic Wedge Complex and the Metamorphic Units. Using the velocity ranges of the previous units reported in literature (Batini et al., 1978, Accaino et al., 2005), we have assigned to these units Vp values of 2700 m/s, 3850 m/s, 5500 m/s and 4800 m/s respectively (Fig. 1).

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Fig. 1 – Interpolated interval velocity (Vp) model along the trace of the CROP18A line derived from the 3D-geological-geophysical model based on velocity ranges reported in literature (Batini et al., 1978, Accaino et al., 2005). Neogene Unit (in violet) 2700 m/s, Ligurian Flysch Complex (in turquoise) 3850 m/s, Tuscan Units plus Tectonic Wedge Complex (in orange) 5500 m/s, and Metamorphic Units (in green) 4800 m/s.

Fig. 2 – Synthetic seismic section along CROP18A. Superimposed the line drawing of the base of Neogene (blue line), the top of Tuscan Units (black line) and the top of Metamorphic Units (yellow line).

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The unit interfaces derive from the 2D interpolation with a mesh-grid along the line and depth directions of 50x50 m of the 3D-geological-geophysical model along the trace of the CROP18A line. Besides the model derived from the reconstructed 3D-geological geophysical model, we have also calculated the seismic response of the velocity model along the line obtained by Accaino et al. (2005) with the tomographic inversion.

The preliminary results (Fig. 2) indicate that the seismic modelling along the CROP18A is mainly influenced by the geometries and by the impedance contrast between the first three units. The effects of the shallow units seem contaminate the synthetic section up to 4 sec of TWT times.

Although the preliminary state of our work, we can affirm that in the processing phase and during the seismic interpretation of this line these effects should be accurately considered in order to avoid misinterpretation of the deeper portion of the recorded seismic section.Acknowledgment. The research was funded by Project IMAGE in the framework of the European Community’s Seventh Framework Programme under grant agreement No. 608553.

ReferencesAccaino F., Tinivella U., Rossi G. & Nicolich R. 2005. Imaging of CROP-18 deep seismic crustal data. Boll. Soc.

Geol. It., Volume Speciale n. 3, 195-204.Batini F., Burgassi P.D., Cameli G.M., Nicolich R. & Squarci P. 1978. Contribution to the study of the deep lithospheric

profiles: «deep» reflecting horizons in Larderello-Travale geothermal field. Mem. Soc. Geol. It., 19, 477-484.Batini F., Brogi A., Lazzarotto A., Liotta D. & Pandeli E. 2003. Geological features of Larderello–Travale and

Mt.Amiata geothermal areas (southern Tuscany, Italy). Episodes 26 (3), 239–244.Bertini G., Gianelli G. & Battaglia A. 1996. Risultati ed interpretazione delle datazioni radiometriche (metodo 230Th/

234U) dei campioni di minerali idrotermali presenti nelle rocce attraversate dai sondaggi geotermici (Larderello e Monteverdi) e negli affioramenti di rocce mineralizzate (Sassa e Canneto-Malentrata), ENEL-CNR-CISE internal report, Pisa 1996, 18 pp.

CREWES - Consortium for Research in Elastic Wave Exploration Seismology. CREWES MATLAB Toolbox. University of Calgary. https://www.crewes.org/ResearchLinks/FreeSoftware/

Gianelli G., Manzella A. & Puxeddu M. 1997. Crustal models of the geothermal areas of Southern Tuscany. Tectonophysics 281, 221 – 239.

Scrocca D., Doglioni C., Innocenti F., Manetti P., Mazzotti A., Bertelli L., Burbi L., Doffizi S. (Eds) 2003. CROP Atlas: seismic reflection profiles of the Italian crust. Memorie Descrittive della Carta Geologica d’Italia, 62, 1-194, ISBN: 88-240-2548-X.