Abstract. The Ivrea-Verbano zone and the adjacent Serie dei Laghi, in the inner arc of the western Alps (NW Italy), display an upended cross section through much of the thickness of the continental crust as it existed in that region in Triassic/Jurassic time. We collected a suite of oriented rock samples that represent most of the volume of the rocks of the region. P and S wave velocity measurements were made in three orthogonal directions, related to the mesoscopic fabric of the rocks, at room temperature and up to 550 MPa confining pressure. Combined hightemperature and high-pressure measurements were made up to 700øC on a subset of the samples. The lithologic units were divided into 23 different principal rock types, and we present velocity data averaged in terms of these groups. Vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section. These show that Vp and V, both increase and anisotropy decreases systematically with depth of burial, reflecting variations in rock type and metamorphic grade with depth. Vp and V, anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding, with the slowest direction tending to be normal to the foliation or banding. The Vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust. These correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution.

Cross‐hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available. A strong correlation between electrical resistivity anisotropy and seismic compressional‐wave velocity anisotropy has been observed. Analysis of core samples from the site reveal that the shale‐rich rocks have fabric‐related average velocity anisotropy of between 10% and 30%. The cross‐hole seismic data are consistent with these values, indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures. One region with velocity anisotropy greater than 30% has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross‐hole resistivity tomography data. However, in general the study highlights the uncertainties that can arise, due to the relative influence of rock fabric and fluid‐filled fractures, when using geophysical techniques for hydrological investigations.

Experimental acoustic measurements on sandstone rocks at both sonic and ultrasonic frequencies show that fluid saturation can cause a noticeable change in both the dynamic bulk and shear elastic moduli of sandstones. We observed that the change in dynamic shear modulus upon fluid saturation is highly dependent on the type of saturant, its viscosity, rock microstructure, and applied pressures. Frequency dispersion has some influence on dynamic elastic moduli too, but its effect is limited to the ultrasonic frequency ranges and above. We propose that viscous coupling, reduction in free surface energy, and, to a limited extent, frequency dispersion due to both local and global flow are the main mechanisms responsible for the change in dynamic shear elastic modulus upon fluid saturation and substitution, and we quantify influences.

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A B S T R A C TThis paper describes the measurements of the acoustic and petrophysical properties of two suites of low-shale sandstone samples from North Sea hydrocarbon reservoirs, under simulated reservoir conditions. The acoustic velocities and quality factors of the samples, saturated with different pore fluids (brine, dead oil and kerosene), were measured at a frequency of about 0.8 MHz and over a range of pressures from 5 MPa to 40 MPa.The compressional-wave velocity is strongly correlated with the shear-wave velocity in this suite of rocks. The ratio V P /V S varies significantly with change of both porefluid type and differential pressure, confirming the usefulness of this parameter for seismic monitoring of producing reservoirs.The results of quality factor measurements were compared with predictions from Biot-flow and squirt-flow loss mechanisms. The results suggested that the dominating loss in these samples is due to squirt-flow of fluid between the pores of various geometries. The contribution of the Biot-flow loss mechanism to the total loss is negligible. The compressional-wave quality factor was shown to be inversely correlated with rock permeability, suggesting the possibility of using attenuation as a permeability indicator tool in low-shale, high-porosity sandstone reservoirs. I N T R O D U C T I O NAn understanding of the relationships between seismic (velocity and attenuation) and fluid-flow properties (porosity and permeability) of reservoir rocks under various stress regimes and with different pore fluids is the key to successful exploration and exploitation of hydrocarbon reservoirs. Obvious applications are the potential improvements in sonic wireline data interpretation, modelling and inversion of time-lapse seismic, and amplitude-versus-offset surveys. Variation of the in situ effective pressure in hydrocarbon reservoirs (for example, during production phases) directly affects the velocity and amplitude data determined from surface seismic and sonic log surveys. A good estimation of the magnitude of these changes over the life of a reservoir is important for better reservoir management. and others have investigated the effects of pressure and fluid type on the acoustic properties of sandstone rocks. This research project explored the effects of pressure and fluid type on the acoustic compressional-and shear-wave velocities (V P and V S ) and quality factors (Q P and Q S ) of two suites of sandstone samples from North Sea offshore reservoirs. These two suites of samples are referred to as PEGASUS Well 2 and PEGASUS Well 8. In total, the compressional-and shear-wave velocities of 19 sandstone samples were measured at ultrasonic frequency (about 0.8 MHz) under simulated in situ conditions of reservoir pressures (confining pressure and pore-fluid pressure). The measurements were conducted on air-dry and fully C 2005 European