P-wave velocity, density and vertical stress magnitude along the crustal Po Plain (northern Italy) from sonic log drilling data

摘要

In order to better characterize P-wave velocities for the shallow crust of the Po Plainand surrounding regions, we have selected 64 deep wells mainly located in the plainand also along the Apennine belt and Adriatic coast. In particular, we have analyzedthe stratigraphic profiles for all wells, and the available sonic logs (37 out of 64).From these data we have examinated the P-wave velocity trend with depth andestimated rock density following an empirical relationship between sonic velocity anddensity in sedimentary rocks. Then we have calculated, notably for the first time in alarge area of Italy, the overburden stress magnitude for each well. For instance at adepth of 5 km we have found values varying from 105 to 130 MPa moving from theAdriatic coast to the Apennine belt. Consequently, the Apennines belt shows amaximum regional lithostatic gradient of around 26 MPa/km while the Po Plain andAdriatic region have values of around 21 MPa/km. The maximum density value thatcan be considered for the Apennine crustal belt corresponds to 2.65 g/cm3; in the PoPlain the mean density is around 2.25 g/cm3, while in the Adriatic area the averagedensity has the lowest value in the region at 2.13 g/cm3. Although in this area a 2Dcrustal P-wave velocity model does not adequately constrain the complicated anduneven tectonics, we have nevertheless established a shallow model consisting of fiveseparate layers. The strength of this paper lies in the possible use of these direct data,together with other derived geological and geophysical information, to build a 3Dmodel of the area.