By collecting seismograms from many earthquakes and many places, we have been able to unravel the internal structure of Earth. Seismic waves are influenced by Earth geology in several ways. Two important interaction between the rocks and the waves are refraction and reflection. Refraction is the process of a wave changing direction when it travels from one region with a certain rock type to another region with a different rock type. Reflection is the process where a wave bounces off the boundary between two regions with different rock types.
Many observed waves are combinations of reflections and refractions.
Travel Times Two ways that we study Earth using seismic observations are travel time and normal mode analyses. That process is called seismic tomography. The time it takes a normal mode to complete an oscillation tells us information about Earth. We live on the continental crust and we can see variations in rock type on the surface. Thus, it is not surprising that we see variations beneath the surface as well, but the average properties of the crust are important.