The geophone is an instrument that was originally invented by the French during WWII for detecting German underground operations.
A geophone is a receiver for picking up sound waves that propagate within the earth or solid structures. It is made with a housing that contains a heavy weight, mounted elastically between two thin, flexible metallic plates. Any acoustic vibrations that propagate through the ground will cause the housing (which is in contact with the ground) to move, while the heavy inertia weight remains stationary. Piezoelectric quartz crystal geophones that are used to detect acoustic waves of a specific frequency length, are known as piezo-geophones.
Earlier examples of the geophone design had the inert mass attached to a diaphragm that divides the inside of the housing into two different parts. The movement of the diaphragm in relation to the outer housing produces a differing compression and displacement to both sides, that are reflected through tubes to the operators’ ears. Modern day geophones (including prospecting seismographs) are built using electro-mechanical transducers that can convert received ground vibration into an electrical current oscillation. By the addition of an amplifier and a recording loop oscillograph, excellent definition can be recorded
Common uses of geophones include acoustic prospecting for minerals, military operations, and underground rescue operations. Geophones that utilise the vibro-graph principle are a common preference and frequently used.
Other applications of a geophone could also be for pressure sensors, school projects, botanical, sport or natural science applications. Examples include: burrowing animal observation, ant nest recordings, amplification of a cricket ball striking a wicket, detecting water leaks in swimming pools, underground pipes or even a security device to detect foot fall in a restricted area. There are likewise multiple uses even in television sound recording and production. All of these are derived from the hydrophone principal.