Laboratory measurements of the physical properties of Triassic Evaporites from Central Italy and correlation with geophysical data

The Triassic Evaporites (TE) of the Umbria–Marche Apennines, a sedimentary succession made up of a
sequence of alternating sulphates (anhydrites and gypsum) and dolostones, represent a key lithology in
terms of sealing properties and earthquake triggering. Here we: (1) report laboratory measurements of
density, porosity, Vp, Vs, seismic anisotropy and permeability at effective confining pressures from 0 to
100 MPa, conducted on samples of TE collected from both outcrops and boreholes; and (2) attempt to
upscale the laboratory results to larger scale geophysical investigation, such as Vp sonic logs, seismic
tomography and in situ measurements of pore-fluid pressure. The average laboratory P-wave velocity is
6.0 km/s for dolostones, 4.6 km/s for gypsum–dolostones and 5.8 km/s for anhydrites, at ambient pressure.
As effective confining pressure is increased up to 100 MPa, the average P-wave velocity increases to 7.0 km/s
for dolostones, 5.3 km/s for gypsum and 6.4 km/s for anhydrites. Vp/Vs ratios appear to be independent of
confining pressure, with average values of ∼1.8 to ∼2.2 for sulphates and ∼1.9 to ∼2.2 for dolostones,
respectively, for dry and saturated conditions. All samples are characterized by very lowpermeability (10−18m2
to 10−21m2), with the higher values for gypsum–dolostones and fractured dolostones samples. The Vp profiles
obtained fromultrasonic laboratory measurementsmatch well the in situ Vp profilesmeasured using sonic logs. In
the laboratory, the Vp/Vs ratio increases when pressurized pore fluids are present, in agreement with 4D seismic
tomography that relates the increase in Vp/Vs ratio to the migration of fluids during the 1997 Umbria–Marche
seismic sequence (Mmax∼6.0) that nucleates within the TE. Our low-permeability values are consistent with deep
borehole measurements of high pore-fluid pressure trapped within the Triassic Evaporites.