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 *

The seismic visibility of fractures depends on the magnitude of their normal and shear compliance and how these quantities vary with the scale of the fracture and depth of burial. Reliable estimates of fracture compliance as a function of confining pressure, in the range 10-13–10-14 m Pa-1, have been obtained from laboratory measurements on core samples. The possibility of fractal scaling of fracture parameters has been proposed, in which case fracture compliance might be expected to increase with the scale of the fracture. Laboratory and field estimates of fracture compliance are presented covering a range of fracture sizes. Compliance is shown to increase with the scale of the fractures. Results obtained are broadly consistent with the magnitudes predicted from linear slip theory, in which the displacement discontinuity across a partially sealed interface is linearly related to the traction on the interface.

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.

A B S T R A C TA field measurement of fracture compliance is described. The aim was to determine how compliance scales with fracture size and, specifically, how laboratory measurements of fracture compliance compared with field estimates from sonic and seismic data. A test site was constructed, consisting of three 40 m vertical boreholes drilled in the floor of a Carboniferous Limestone quarry. Detailed knowledge of the rocks in the test area was obtained from core analysis, wireline logging and local area fracture mapping. Seismic cross-hole surveys were performed using a sparker source with a dominant frequency of 2000 Hz and hydrophone receivers. The rocks had a compressional-wave velocity anisotropy of 10%, which was attributed to the presence of predominantly horizontal, partially open fractures. Estimates of normal fracture compliance within a range from 2.5 × 10 −13 m/Pa to 3.5 × 10 −12 m/Pa were obtained from both the cross-hole data and the sonic-log data. This is an order of magnitude greater than values obtained from laboratory experiments which are reported elsewhere.

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