The calibration of a seismograph establishes knowledge of the relationship between its input (the ground motion) and its output (an electric signal), and is a prerequisite for a reconstruction of the ground motion. The parameters of the transfer function must be determined from the response of the system to a known input signal. Precisely known ground motions are however difficult to generate; one therefore makes use of the equivalence between ground accelerations and external forces (Eq. 25), and calibrates seismometers with an electromagnetic force generated in a calibration coil. If the factor of proportionality between the current in the coil and the equivalent ground acceleration is known, then the calibration is a purely electrical measurement. Otherwise, the missing parameter - either the transducer constant of the calibration coil, or the responsivity of the sensor itself - must be determined from a mechanical experiment in which the seismometer responds to a known mechanical displacement or a known tilt. This is called an absolute calibration. Since it is difficult to generate precise mechanical calibration signals over a large bandwidth, one does normally not attempt to determine the complete transfer function in this way.
The present section is mainly concerned with the electrical calibration although the same methods may also be used for the mechanical calibration on a shake table. Specific procedures for the mechanical calibration without a shake table are presented in sections 8.2 and 8.3.