Seismic velocities in fractured rocks: An experimental verification of Hudson`s theory
- Univ. of Reading, Whiteknights (United Kingdom)
Flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures. These cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir: several theories, including Hudson`s, claim to predict the seismic effects of cracks. Although Hudson`s theory has already been used to calculate crack densities from seismic survey`s, the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution. This paper describes an experimental verification of the theory. The rock used, Carrara marble, was chosen for its uniformity and low porosity, so that the effect of cracks would not be obscured by other influences. Cracks were induced by loading of laboratory specimens. Velocities of compressional and shear waves were measured by ultrasound at 0.85 MHz in dry and water-saturated specimens at high and low effective pressures.The cracks were then counted in polished sections of the specimens. In ``dry`` specimens with both dry and saturated cracks, Hudson`s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated. The theory made poor predictions for fully saturated specimens. Shear-wave splitting, caused by anisotropy due to both crystal and crack alignment, was observed. Cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading. The results demonstrate that Hudson`s theory may be used in some cases to determine crack and fracture densities from compressional- and shear-wave velocity data.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 143804
- Journal Information:
- Geophysical Prospecting, Vol. 42, Issue 1; Other Information: PBD: Jan 1994
- Country of Publication:
- United States
- Language:
- English
Similar Records
Changes in compressional and shear wave velocities and dynamic moduli during operation of a hot dry rock geothermal system
Pressure and fluid saturation prediction in a multicomponent reservoir, using combined seismic and electromagnetic imaging