Geophones are small, vibration-sensing instruments that are used to measure movement or vibration of the earth, machines, mines, and other applications. Frequency range and application are the most important factors to consider. Short-period geophones can measure frequencies around 1 Hz and are designed to record localized seismic events such as earthquakes. Long-period geophones can measure frequencies around 4.5 Hz and are suitable for oil exploration and teleseismic events. Both types of products work like gravity meters in that the active geophone element consists of a cylinder which hangs from a spring. The cylinder is wrapped in a coil of copper wire and surrounded by a magnet or magnetic housing that is fixed to the earth. The earth’s movement causes the magnet to move around the mass and produce an electrical voltage which is transmitted along the wire, amplified and then recorded. The recorded voltage is proportional to the velocity at which the ground is moving.