Infrasound can be used to investigate the internal structure of our planet, just like ultrasound is used for foetal scanning. Earthquakes produce very powerful seismic waves that can be classed as infrasound waves.
Seismic waves from large earthquakes are detected around the world. Their paths are curved as the waves refract due to the gradually changing density of the layers.
S-waves only travel through solids. S-waves are not detected on the opposite side of the Earth – this suggests that the mantle has solid properties, but the outer core must be liquid.
P-waves can travel through solids and liquids. P-waves are detected on the opposite side of the Earth. Refractions between layers cause two shadow zones, where no P-waves are detected – the size and positions of these shadow zones indicate there is a solid inner core.