Abstract Accurate knowledge of porosity is essential for understanding the links between basic petrophysical parameters, such as diffusion coefficients, permeability and conductivity. Standard methods used to determine porosity quantify the bulk porosity and the distribution of pore sizes. Crystalline rocks are rarely monomineralic, and the porosity of polyphasic rocks is considered heterogeneous at the mineral grain scale. Calculation of bulk petrophysical parameters must take into account porosity and mineral-phase microstructures, as well as connectivity. The polymethylmethacrylate (PMMA) method uses radioactively ( 14 C)-labelled methylmethacrylate ( 14 C-MMA) liquid to impregnate the rock sample, which is then polymerized by irradiation, cut and autoradiographed. Porosity is quantified by digitizing the autoradiograph and subsequent densitometry. Staining of the same rock surface uses chemical agents that rapidly reveal the primary minerals of unaltered and altered crystalline rocks: mainly quartz, K-feldspar, plagioclase and dark minerals. The images of PMMA autoradiographs and stained rock surfaces are combined to quantify mineral-specific porosities. The methodology has been applied here to a granite core from Palmottu (central Finland) representing coarse-grained granite adjacent to a potential water-conducting fracture. Imaging of the porosity relative to mineralogy is presented and complemented by mineral specific porosities.

Abstract The growth of shallow sills is studied in analogue experiments performed in polymethyl methacrylate (PMMA) and glass. The experimental fractures curve towards the surface to become saucer-shaped, which is consistent with many field observations of dolerite sills. The curvature of the saucer is shown to decrease as the in situ stress acting parallel to the surface increases relative to an estimate of the strength of the fracture-induced stress field. The initially circular fractures also elongate in plan view to become egg-shaped, a tendency that decreases with increasing importance of viscous dissipation in the growth process. Sill emplacement is further examined mathematically by considering a shallow, circular, fluid-driven fracture propagating in a homogeneous brittle elastic material. The fractures are shown to undergo three transitions related to the mechanics of sill growth. Each transition is associated with a characteristic time that is derived from analysis of the governing equations using scaling methods. These characteristic times provide an estimate of how long viscous flow is the dominant energy dissipation mechanism, how long significant lag between the fluid and fracture fronts is expected to persist, and how long the sill will take to attain an extent that is of the same order as its depth.