The geophones or hydrophones measure the seismic wavefield, and their result is electrical signals. The voltages, which are the data from the geophones, are initially amplified at the preamplifier and may then pass through an analogue low-cut filter (a low-cut-off filter applied to a seismic signal attenuates frequencies more minor than the cut-off frequency).

Geophone instruments are based on an inertial mass suspended from a spring or coil. The response of a geophone is directly proportional to the ground velocity. They function more like a microphone or loudspeaker with a magnet surrounded by a coil of wire. The system uses electromagnetic induction.

A primary feature of geophones is that they can only monitor frequencies above their natural frequency, up to a particular bogus frequency. A typical frequency of the order, 10 Hertz, and a spurious frequency on the order of 250 Hz are specific. This frequency range precludes using geophones for many applications, such as monitoring pile driving, etc. However, geophones are one of the most common sensors employed for monitoring blasting and construction activities. Geophone seismic surveys are also used to monitor velocities.