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Nearly 60 % of the Earth surface is composed of basaltic rocks which form the upper part of the oceanic crust. This paper discusses the elastic properties of basalt at several spatial scales, from the sample and its microstructure to the reservoir scale. Firstly we present experimental data obtained on the frequency relating to measurement of the bulk modulus of a fresh and young basalt sampled in Iceland. We show that a gap of about 20 % exists between the bulk modulus calculated at 0.01 Hz and the one measured at 1 MHz. We relate this effect to the microstructure and to a squirt-flow effect. Secondly we investigate the modes of deformation of basalt by performing triaxial tests and recording acoustic emissions. The most significant result is the one obtained for high confining pressure. Indeed for such pressure, an axial loading induces a localization of deformation under the form of subhorizontal bands in the sample. In these bands, focal mechanisms of acoustic emissions reveal the co-existence of two processes, pure compaction and pure shear. It is the first time that such a mode of deformation is observed in a low-porosity rock (< 10 %). Finally, we studied the elastic properties of the upper Icelandic crust by using a mechanical approach on tomography data derived from microseismicity activity. We interpreted the evolution of Pand Swaves velocities in terms of crack density and fluid state according to depth. Our results show that at 6 km depth a large part of the Icelandic crust is saturated with a supercritical fluid phase, while at 2 km depth, only the most hydrothermal areas present such a fluid signature. Moreover, crack density tends to increase with depth under these areas leading to the assumption of a hydrofracturation process due to supercritical fluid overpressures. Introduction Iceland is an offshore part of the mid-Atlantic spreading ridge. It is a natural ideal laboratory to study the interplay between rocks and fluids in an oceanic ridge [Geoffroy and Dorbath, 2008]. The present work focuses on the Reykjanes Peninsula, which is one of the most active regions in Iceland. The oceanic crust is about 15-20 km thick beneath the Reykjanes Peninsula [Foulger et al., 2003], and a large part of the upper crust is made up of basaltic rocks. Basalts are generally considered as low-porosity and low-permeability rocks [Vinciguerra et al., 2005]. Due to thermal cooling, a lot of cracks and air bubbles are present in their microstructure. From a theoretical point of view, such a dual porosity can lead to fluid flow which can involve a frequency effect on the elastic velocities [Adelinet et al., 2010a, 2010b]. Indeed, velocities recorded on the field, i.e. in the low-frequency range, are generally lower than velocities recorded in the laboratory, in the MHz frequency gap. The specific microstructure of basalts should also play a role in their deformation. Some experimental data exist on this topic. They always show a brittle behavior under increasing axial stress, whatever the conditions of confining Petrophysical and Geophysical Characterization of an Icelandic Geothermal Reservoir, Case Study on the Reykjanes Peninsula Mathilde Adelinet1,2, Jérôme Fortin2, Alexandre Schubnel2, Mickaële Le Ravalec1, Catherine Dorbath3, Yves Guéguen2, and Laurent Geoffroy4 1IFP Energies nouvelles, Rueil-Malmaison, France 2Laboratoire de géologie, ENS, Paris, France 3IRD-EOST, Strasbourg, France 4LGPN, Université du Maine, France Figure 1. Bulk modulus obtained from HF velocities inversion and LF oscillation tests on the 0-100 MPa interval (A : frequency effect, B : fluid effect). ‘sat’ stands for saturated