Seismic structure of gneissic/granitic upper crust: geological and petrophysical evidence from the Strona-Ceneri Zone (northern Italy) and implications for crustal seismic exploration

Abstract

We present a stochastic seismic model of gneissic/granitic upper crust by estimating the amplitude probability distribution and the autocorrelation function of the inferred seismic velocity field of the Strona-Ceneri Zone (northern Italy). Unlike previous investigations of the adjacent Ivrea Zone and the Lewisian gneiss complex (Scotland), the lithologies mapped in the Strona-Ceneri Zone cannot be directly correlated with laboratory-measured seismic velocities. The amplitude probability distribution of the extensive petrophysical database and the relatively uniform mineralogical composition of the lithologies indicate that the velocity distribution in the Strona-Ceneri Zone can be approximated as being continuous and Gaussian. Based on this assumption and using the spatial autocorrelation function measured from a digitized geological cross-section, we have modelled the fine-scale structure of the Strona-Ceneri Zone as a self-affine medium with a fractal dimension of 2.50 ± 0.15 and horizontal and vertical characteristic scales of 2.6 ± 0.4 km and 1.4 × 0.3 km, respectively. On near-offset shot-gather seismic data, Strona-Ceneri-type upper crustal heterogeneity produces a reflected field with the appearance of incoherent scattered noise characteristic of non-reflective, ‘transparent’ upper crust found in extended Phanerozoic provinces. This type of heterogeneity has a considerable effect on seismic imaging of underlying reflective lower crust: individual reflection segments are disrupted and shifted in time by up to several tenths of a second, and the boundaries of the reflective zone become distorted and blurred. At far offsets seismic scattering from heterogeneity of this type reproduces the commonly observed rapid decay and scattered distribution of Pg amplitudes. Comparable amplitude decay patterns result from layered deterministic upper crustal velocity structures with velocity inversions.