A seismic sensor is also called seismometer and measures the motion of the ground when it is shaken by a perturbation such as an earthquake, be it a large displacement or a microquake. The physical variable associated to the measurement of a seismometer is dynamic. It can be either the amplified ground motion, the velocity or acceleration. Current seismometers transform one of these three parameters into a voltage measurement. Usually, three seismometers are needed to retrieve the three components of the displacement. As for other sensors, there exists many types of seismic sensors, and they can be distinguished in active and passive sensors as well.