Seismic Sensors and their Calibration

There are two basic types of seismic sensors: inertial seismometers which measure ground motion relative to an inertial reference (a suspended mass), and strainmeters or extensometers which measure the motion of one point of the ground relative to another. Since the motion of the ground relative to an inertial reference is in most cases much larger than the differential motion within a vault 9of reasonable dimensions, inertial seismometers are generally more sensitive to earthquake signals. However, at very low frequencies it becomes increasingly difficult to maintain an inertial reference, and for the observation of low-order free oscilla-tions of the Earth, tidal motions, and quasi-static deformations, strainmeters may outperform inertial seismometers. Strainmeters are conceptually simpler than inertial seismometers al-though their technical realization and installation may be more difficult (IS 5.1). This Chapter is concerned with inertial seismometers only. For a more comprehensive description of iner-tial seismometers, recorders and communication equipment see Havskov and Alguacil (2002). An inertial seismometer converts ground motion into an electric signal but its properties can-not be described by a single scale factor, such as output volts per millimeter of ground mo