We have measured the velocities and attenuations of compressional and shear waves in 29 water‐saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high‐pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Qp and Qs for compressional and shear waves respectively. The results show that Qs is strongly correlated with Vs, that Qp is weakly correlated with Vp, and that Qp is strongly correlated with Qs. Qp is strongly dependent on the volume percentage of the assemblage of intra‐pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra‐pore mineral assemblage, which is a result of their very different elastic moduli.

Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (Vp and Vs, respectively) decrease with increasing porosity and that Vp decreases approximately twice as fast as Vs. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.

A B S T R A C TWe measured in the laboratory ultrasonic compressional and shear-wave velocity and attenuation (0.7-1.0 MHz) and low-frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elasticelectrical properties of reservoir sandstones. P-and S-wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi-logarithmic scale for both clean and clay-rich sandstones; P-and S-wave attenuations showed a bell-shaped correlation (partial for S-waves) with apparent electrical formation factor. The joint elastic-electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co-located seismic and controlled source electromagnetic surveys.

A B S T R A C TLaboratory estimates of the normal (B n ) and shear (B t ) compliance of artificial fractures in samples of Jurassic and Carboniferous limestone under wet and dry conditions are presented. The experiments were performed over a range of confining pressures (from 5 MPa up to 60 MPa), at ultrasonic frequencies in a Triaxial Hoek cell, using the pulse-echo reflection technique. The results of this study confirm that the B n /B t ratio of a fracture is dependent on the fluid fill. A value of B n / B t of less than 0.05 was obtained for our wet (honey saturated) sample which is consistent with the prediction that this ratio should be close to zero for fluid saturated fractures. Values of B n /B t for the dry sample are significantly higher and increase with confining pressure from 0.2 to 0.5. It is suggested that a Bn/Bt ratio of 0.5 is probably a more representative value to use in modelling studies of gas filled fractures than the common assumption that Bn ≈ Bt. I N T R O D U C T I O NWe present laboratory estimates of the normal (B n ) and shear (B t ) compliance of artificial fractures in samples of Jurassic and Carboniferous limestone under wet and dry conditions for a range of applied confining pressures. The main aim of the study was to determine the ratio of normal to shear compliance (Bn/Bt). There is increasing interest in the numerical modelling of seismic wave propagation through fractured media in which the fractures cannot be assumed to be small relative to the seismic wavelength. Vlastos et al. (2006) have modelled seismic wave propagation in a fractured network including the effects of changes in pore pressure, which is relevant to the interpretation of time-lapse data. Other recent modelling studies include Willis et al. (2006), Will, Archer and Dershowitz (2005), Daley et al. (2002) and Zhu and Sneider (2002). For models of gas filled fractures, it is usually assumed that Bn = Bt, principally based on theoretical arguments since experimental data are very sparse.Liu, Hudson and Pointer (2000) have shown that if a dry (gas filled) fracture is modelled as a planar distribution of small *