Integration loop of of ‘global offset’ seismic, continuous profiling magnetotelluric and gravity data

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In this paper an approach aimed at integrating many different geophysical/geological data is discussed. It is based on a recursive process of forward and inverse modelling of seismic and non-seismic data. The experimental data set of the Enhanced Seismic In Thrust belts (ESIT) research project, funded by Eni E&P, Enterprise Oil Italiana and the European Union, was used in order to apply the approach to a real case of complex geological setting. Near-vertical reflection seismic, long-offset seismic, high-resolution magnetotelluric, gravity, borehole and surface geological data were involved in the process. We demonstrate how a ‘self-feeding’ integration loop is an efficient way to produce a unique geophysical model that responds to several basic requirements, such as optimized inversion/modelling in each parameter space, best fit in each geophysical domain, best seismic imaging, reliable geological meaning and cost/benefit ratio optimization. Introduction Integration of multiple data sets in complex geological settings represents one of the most challenging objectives in geophysics. This is true especially in the case of geophysical projects based on the acquisition of highly redundant data sets and characterized by many different sources of information. In fact, particularly in complex areas where the quality of standard seismic is poor, alternative non-seismic approaches are required. An exploration strategy based on many different and complementary methodologies always produces a complex data set, and integrating all the information in consistent and reliable models can fail if an appropriate integration strategy is not applied. In previous work (Dell'Aversana & Morandi 2000, 2002; Dell'Aversana 2001), we introduced an integration approach based on recursive forward and inverse modelling of seismic, magnetotelluric and gravity data. We showed that the so- called ‘global offset’ seismic approach (which also includes high-fold, long-offset data) can improve significantly the process of building reliable models by a quantitative integration with MT and gravity data and with the support of borehole information. In a subsequent paper (Dell'Aversana et al. 2002b), we demonstrated how, by applying prestack depth migration to global offset data, it is possible to improve the depth imaging in difficult geological settings, also when the S/N ratio of the conventional near-vertical reflection data is very low. This result can be obtained especially if non-seismic data are used for defining accurate multi-parametric models. These models can contribute to the definition of an appropriate velocity field for seismic data migration, as will be clarified in this paper. In fact, recent experiments and applications showed how the continuous profiling magnetotelluric method can produce reliable resistivity sections that can support both the velocity field definition and the geological interpretation in case of low-quality seismic sections (Zerilli & Dell'Aversana 2002). Here, we continue the discussion about the integration of many data sets, but also introduce several important additional concepts. We take the opportunity offered by the large multiple data set collected during the ESIT research project (Buia et al. 2002; Dell'Aversana et al. 2002b). We demonstrate that an appropriate quantitative integration of global offset seismic, continuous profiling high-resolution magnetotelluric (HRMT), gravity, borehole and geological data is a reliable and cost-effective process. Each source of information contributes to different aspects of the process, due to the varying benefits and limitations of each of the different methodologies used. Based on many different geophysical parameters, the final result is a well-calibrated model that is consistent with the best seismic imaging. The geological consistency is considered as a fundamental requirement at each step of the process.