Ultrasonic properties of sedimentary rocks: effect of pressure, saturation, frequency and microcracks

Seismic velocities and attenuations are influenced by lithology, porosity and permeability as well as the kind and quantity of the pore fluids. The microstructure of rocks is another important factor influencing seismic properties. This influence can be used to investigate the pressure-dependent closure of microcracks. For this purpose, velocities and attenuations of P and S waves have been determined at ultrasonic frequencies in three different rocks (vacuum dry and partially saturated with water) as a function of hydrostatic pressure up to 200 MPa. A new combined model has been developed. With this model an explicit description of the microstructure [local fluid flow (LF)] and macrostructure (Gassmann effect, global fluid flow) is possible. Assuming a patchy saturation (i.e. inhomogeneous distribution of the fluid in the pores) and modulus reduction, both the saturation-dependent measurements and the pressure-dependent data can be explained. The modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears: in high-porosity sandstones (Bentheimer and Obernkirchner Sandstone) a strong closure of cracks already with a small increase of pressure must be concluded. In a low-porosity Harzer Greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0-100 MPa. It is shown, however, that there must be open cracks in all rocks even under elevated hydrostatic pressure, resulting in a measurable effect due to LF.