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Stress-induced anisotropy: the effects of stress on seismic wave propagation

The dynamic nature of the Earth’s stress field is a result of tectonism, uplift and extension. However, stress-induced phenomena are also influenced and complicated by remnant stresshistory effects. The resultant intrinsic anisotropic sedimentary layers present within the Earth allow their interpretation using the seismic reflection and transmission methods. Interpretation objectives include the determination of hydrocarbon reservoirs, fracture presence/orientation, and in situ stress directionality. Stress has the potential to affect most petrophysical rock properties. The effects of stress history on the acoustic and elastic properties of artificially manufactured sandstones have been investigated using ultrasonic techniques. Homogeneous mixtures of quartz sand and epoxy resin were allowed to harden under the application of different forming stress magnitudes. This was followed by unloading under anisotropic stress conditions satisfying uniaxial strain criteria. The resultant sandstones exhibited azimuthal velocity, amplitude and Poisson’s ratio changes with a 90-degree periodicity. Pronounced, well-defined shear-wave splitting was also prevalent. As the manufacturing (forming) stress was increased the average velocity of all body waves decreased, attenuation increased and the percentage anisotropy increased. The azimuthal anisotropy was consistent with the symmetry of anisotropic stresses during unloading. The acoustic trend supported the concept of shear-wave splitting resulting from the formation of intergranular microcracks in the plane orthogonal to the maximum stress during unloading. As the forming stress was increased, so too did the length of the unloading path. This in turn induced a higher density of aligned microcracks, inducing a larger, controllable, acoustic anisotropy. This method requires little more than a car jack and ultrasonic transducers. It is a method to provide useful knowledge of the effects on seismic wave propagation as a result of varying rock matrix parameters. These developments offer a new approach to research into stress-induced phenomena and their implications for seismic interpretation.