It is now well-established that earthquakes change the seismic velocity of the near surface. There is certainly some understanding of what mechanisms are responsible for these changes, but there remain many questions. One of these open questions is how cracks and other microstructures within the rock control these changing velocities. Here we look at the nonlinear interaction of two waves, one of which (the PUMP) simulates the effect of an earthquake and the other (the probe) senses the changes in the travel time caused by the passage of the PUMP wave. We use a sandstone sample that is established to have a nonlinear response that depends on the orientation of the sample layering. We study two samples with different orientations of this layering, which we infer to be different orientations of the micro-structure. We show that the dependence of these changes on applied load are exponential, with a characteristic load of 11.4-12.5~MPa that is independent of sample orientation and probe wavetype (P or S); this value agrees with results from the literature.