Geologists can look at rocks deep beneath the surface using sound, just like a doctor uses an ultrasound machine to see a developing baby? Ultrasounds and seismic waves are both types of sound waves, and they need a medium like air, liquid or rock to travel through. Because of that, when they come to a boundary between two substances, some of the sound gets reflected. With medical ultrasound, the waves reflect off the surface of the baby’s body. There are two types of seismic waves. P-waves travel through liquids and solids, and S-waves only travel through solids. When they hit boundaries between different substances they change directions, showing the earth’s structure.