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.