Geophone

A type of seismic receiver most often used in seismic exploration.

Most geophones are small, cheap, and intended for rapid deployment in large numbers. The one illustrated on the stamp, like the modern cut-away example shown here, would be about 4 × 20 cm, with a total mass of about 400 g. The design has barely changed in decades. The long spike on the base is to try to ensure good contact with the ground (coupling).

Inside the geophone, a frame-mounted magnet is surrounded by a proof mass affixed to a copper coil. This analog instrument measures particle velocity, not acceleration, because the instrument is operating above the resonant frequency of the coil. Because of the small proof mass, the lower practical frequency limit is usually only about 6 Hz, the upper about 250 Hz (5 octaves).

Geophones are used on land, and on the sea-floor. If repeatability over time is important, as with a time-lapse survey, phones like this may be buried in the ground and cemented in place.

Three-component geophones, which measure particle velocity along three axes, contain three orthogonally arrange coils. Since the coils cannot detect a 0 Hz (DC) signal, they cannot detect down and so must be planted perfectly vertically (usually within 2&deg;).

Arrays
Geophones are often not evenly distributed along a receiver line, but grouped into small clusters of 3–6 geophones.

Agile links

 * Geophysical stamps 3: Geophone — blog post