The instrument used to transform seismic energy into an electrical voltage; a seismometer, seis, detector, receiver, jug, bug, or pickup. Geophones ordinarily respond to only one component of the ground’s displacement, velocity, or acceleration that is involved in the passage of a seismic wave. Three mutually orthogonal phones are used to record all three components; see triphone. Most land geophones are of the moving-coil type; see Figure G-5.
A coil is suspended by springs in a magnetic field (the magnet may be integral with the case of the instrument). A seismic wave moves the case and the magnet, but the coil remains relatively stationary because of its inertia. The relative movement of a magnetic field with respect to the coil generates a voltage across the coil, the voltage being proportional to the relative velocity of the coil with respect to the magnet (when above the natural frequency of the geophone). Below the natural frequency, the output (for input of constant velocity of magnet motion) is proportional to frequency and hence to the acceleration involved in the seismic wave passage. Compare hydrophone and streamer.