The best pictures we have of the Earth's interior come from the field of SEISMOLOGY, in which we analyze the way in which vibrations propogate through the Earth.
A foreshock-mainshock-aftershock sequence
Examples of Geophones
Modern Seismometers in China
This model of an ancient Chinese Seismograph is in the shape of a watchtower in a Han period frontier village.
Helicorders, or Drum Recorders, which have been the standard for many years.
A RefTek Data Acquistion System installed in Kyrgyzstan
Seismometers work just like a phonograph needle; or, if you like, a speaker in reverse: Inertial mass on a spring with a magnet moving through a coil of wire.
The waves produced by an earthquake arrive at a point as distinct "phases". We can use these phases to tell us a lot about the earthquake source and the earth structure.
Example: location of an epicenter can be accomplished by the S-P time.
Aside: We know now that most earthquakes are shallow (<15 km depth) but there are belts of deep earthquakes. How can this be?
More sophisticated processing can reveal the nature of the rupture process.