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 100MPa, 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 6.0km/s for dolostones, 4.6km/s for gypsum-dolostones and 5.8km/s for anhydrites, at ambient pressure. As effective confining pressure is increased up to 100MPa, the average P-wave velocity increases to 7.0km/s for dolostones, 5.3km/s for gypsum and 6.4km/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 low permeability (10-18m2 to 10-21m2), with the higher values for gypsum-dolostones and fractured dolostones samples. The Vp profiles obtained from ultrasonic laboratory measurements match well the in situ Vp profiles measured 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. © 2010 Elsevier B.V.