P-wave Velocity and Density: Borehole Data for a Better Shallow Crust Model in the Po Plain (Italy)

Abstract:

Density and seismic wave velocity are fundamental properties of rocks that are often measured only through laboratory experiments. In northern Italy the major concern in crustal models is due to the presence of the Po Plain, a huge basin filled with a Plio-Quaternary sedimentary sequence whose thickness varies from few hundred meters to some kilometers, bounded by the northern Apennines and the Alps. In order to better characterize the first few kilometers of the crust in the Po Plain and surrounding regions we have used wireline borehole logs in 64 selected deep wells. We have analyzed all available stratigraphic profiles, geophysical logs and particularly 37 sonic logs. From these latter data we have characterized the P-wave velocity trend with depth and for each recognized lithology. Densities have been estimated following an empirical relationship between sonic velocity and density in sedimentary rocks: in the Po Plain the mean density value is 2.25 g/cm³, whereas the maximum density of the shallow crust in the Apennine is around 2.65 g/cm³. We have also calculated the vertical stress magnitude in each well: the Po Plain shows a maximum regional lithostatic gradient around 21 MPa/km while in the Apennines belt the values are around 26 MPa/km. Although in this area a 2D crustal velocity model does not adequately constrain the complicated and uneven tectonics, we have established a shallow crust model consisting of five layers. These direct data, together with other derived geological and geophysical information, can contribute to build a 3D model of the area.