The geophone is an instrument used i9n seismology to investigate rock structures and types. It consists of a mass connected to a coil and suspended over a magnet by a set of springs.
When a small explosion is set off some distance away, a clock is simultaneously started in the geophone. When shock waves reach the geophone, they make the mass and coil vibrate up and down about the magnet. A voltage is induced, the magnitude of which depends on the frequency and amplitude of the vibrations. The voltage is used to stop the clock. The time for the vibrations to travel through the rock from the explosion to the geophone can then be measured and the speed of the vibrations through the rock found.
Because the speed of the vibrations depends on the rock type and structure. It is about 2,000 m/s in soft soil to over 4,000 m/s in deposits of ore.