The Use of Gravity Prospecting to Help the Seismic Microzonation Studies of the Leonessa Basin in Rieti, Italy

A research approach that integrated different geophysical methods for local subsoil evaluation using for seismic microzonation studies was carried out in this research paper. The gravity prospecting were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits of the Leonessa basin. The Leonessa basin is one of the major intermontane tectonic depressions of Central Apennines boarded on the southern side by the great normal fault, here referred to as the Leonessa fault. The study involved a test area of 32 km2 occupied by 250 gravity stations. The stations were accurately located with differential GPS (Ashtech Z-Xtreme dual-frequency GPS) that provided centimetric accuracy in elevation. The gravity measurements were performed using a LaCoste & Romberg gravimeter mod. D60 that has a reading resolution and an accuracy of 0.01 mGal. Measurements have been done in the depressions and also over the basement in order to determine the regional anomalies. Various corrections have been considered for calculating the Bouguer anomaly, the Regional gravity and the Residual gravity. On the basis of the Residual gravity, we carried out three 2D gravimetric models. These models have allowed us to reconstruct the arrangement in the depth of the sedimentary top of bedrock, to evaluate the thickness of the Quaternary deposits and to recognize the paleo-morfology arrangement of the basin. The models match quite well with the information determined from well logs and seismic methods. On the basis of gravity anomalies, we reproduced the Structural Geological Map and the 3D Model of the buried morphology of the Leonessa basin. Gravity methods have been used most extensively in the search for oil and gas particularly in the early twentieth century but we think that such methods can be employed very widely in mapping the thickness of unconsolidated deposits, where ground motion amplification can occur.