Obtain data on the motion of the Earth's surface after an earthquake. The data is taken from monitoring equipment that has been set up at different points across the globe. Seismologists use seismometers to measure motions of the ground. Considerable information is available from the Incorporated Research Institutions for Seismology.
Analyze the data to investigate the time taken for a waves to arrive at various points after an earthquake. The velocity of waves depends on a variety of factors, but generally increases in denser materials. When moving from a less dense medium to a denser medium, waves change direction. This is known as refraction. Waves passing from the crust to the mantle will travel faster on average. Geometry can then be used to calculate the depth at which this change of direction takes place.
Analyze the data to see at which point S-waves stop being transmitted. When earthquakes happen, two kinds of waves travel through the inside of the Earth: primary waves (P-waves) and secondary waves (S-waves). P-waves are longitudinal, which means the direction of vibration is parallel to the direction in which the waves travel, and they can travel through solid and liquid rock. S-waves vibrate at right angles to the direction of the wave, and cannot travel through liquids. Geometry can then be used to calculate at which depth the Earth's liquid core is reached, from the point at which S-waves stop being transmitted.
Analyze the P-wave data to see at what points on Earth some P-waves are arriving faster than others. These are the ones that have passed through the solid inner core, which has a higher density than the outer core, and therefore waves travel faster though this as well as changing direction.