COMPARISON BETWEEN NATURAL FRACTURES AND FRACTURE PARAMETERS DERIVED FROM VSP

The feasibility of resolving a subsurface fracture system using P-wave and cross-polarized shear-wave vertical seismic profiling (VSP) was examined. In situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer (BHTV) and by measurements of core sample velocities. The BHTV images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part. Two sets of dominant parallel fractures are observed by BHTV in the fracture-rich depth interval of the shallower part; the orientations of fracture strikes in the two sets are almost the same directing to NW, whereas the average dip angles of the fractures in the two sets are 73-degrees and 46-degrees, respectively. The fracture orientations in the fracture-poor depth interval of the deeper part are found to be random. The degree of anisotropy of the core sample velocity is so small that the present VSP measurement cannot resolve the expected anisotropy from the core sample measurement. P- and two shear-wave velocities (V(P), V(S1) and V(S2)) are determined from the downgoing phases of three-component VSP with a zero-offset P-wave source and with zero-offset shear-wave sources polarized in two orthogonal directions, respectively. The VSP results are as follows. (1) The shear-wave polarization anisotropy is not detected within the accuracy of 4 per cent. (2) V(P) and V(S) (average of V(S1), and V(S2)) for the fracture-rich interval are 94 per cent and 86 per cent, respectively, of those for the fracture-poor interval. The VSP results are consistent with expected velocity changes due to an isotropic distribution of cracks. However, a biplanar crack model cannot satisfy the VSP results, whereas the BHTV images show two such sets of parallel fractures. It is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present VSP experiment.