Nonlinear interactions of P and S waves under uniaxial stress

Fractures play a significant role in nonlinear wave interactions in rocks. These same fractures are fundamental to the production of geothermal energy as well as unconventional oil and gas, so understanding them has significant practical value. In this study, we examine the nonlinear interactions of P and S waves as a function of uniaxial stress. We perform a set of experiments on two sandstone samples known to have aligned fractures taken from the same quarry but cut so that the experiment is oriented differently with respect to the fractures. Complementary measurements show that the velocity decreases with stress indicating that the applied stress opens the fractures in both samples. For most of our data, we observe a significant decrease in the nonlinear response as a function of the applied stress independent of the orientation of the fractures and the experiment. An interesting exception is the coupling of two S-waves where we observe an increase in the nonlinear response at lower stresses before a decrease as the load is increased.Fractures play a significant role in nonlinear wave interactions in rocks. These same fractures are fundamental to the production of geothermal energy as well as unconventional oil and gas, so understanding them has significant practical value. In this study, we examine the nonlinear interactions of P and S waves as a function of uniaxial stress. We perform a set of experiments on two sandstone samples known to have aligned fractures taken from the same quarry but cut so that the experiment is oriented differently with respect to the fractures. Complementary measurements show that the velocity decreases with stress indicating that the applied stress opens the fractures in both samples. For most of our data, we observe a significant decrease in the nonlinear response as a function of the applied stress independent of the orientation of the fractures and the experiment. An interesting exception is the coupling of two S-waves where we observe an increase in the nonlinear response at lower stresses before a d...