A field and laboratory investigation of the compliance of fractured rock
Compressional and shear wave velocity and attenuation measurements were obtained in the laboratory from 50 mm diameter, cylindrical, limestone core samples over a confining pressure range of 5 – 60 MPa. Normal and tangential fracture compliance values, as a function of confining pressure, were calculated for a single fracture cut perpendicular to the long axis of the core. The ratio of the normal to tangential compliance was approximately 0.4 and was independent of the applied stress. Values of normal and tangential fracture compliance calculated were of the order 10-14 m/Pa, and decreased with an increase in confining pressure. Both Q-1P and Q-1S1/Qs were shown to be small for these samples. A borehole test site was constructed in a Carboniferous limestone quarry, at Tytherington, situated north of Bristol, UK. This quarry was chosen because the rock type was fairly homogeneous and the fractures could be mapped in the quarry walls as well as down three, 40 m vertical boreholes drilled in-line in the quarry floor. Wireline logs were obtained in all the holes and a seismic crosshole survey was carried out between the two outermost boreholes. An estimate of in-situ normal fracture compliance, ZN, was obtained from the log and crosshole data, in 4 different ways, using effective medium theories as well as the displacement discontinuity theory. An additional estimate of ZN was obtained from a separate borehole test site constructed in fractured Devonian meta-sediments at Reskajeage, Cornwall, UK. These fractures were much larger in size than those observed at Tytherington quarry. From the above field and laboratory measurements, fracture compliance was shown to increase approximately linearly with the size of the fractures. In addition, a study of crosshole seismic attenuation was performed at Tytherington quarry. Q was found to be frequency dependent. This frequency dependence was interpreted as being due to scattering rather than intrinsic attenuation.