electric signals in two narrow frequency bands arising from ground or airborne vibrations incident on one or more geophones at the perimeter of an area to be protected are monitored and an alarm given if signal components that are indicative of human footsteps near the geophones occur in both frequency bands two or more times within a predetermined period . these signal components comprise bursts of oscillations having steep leading edges and durations of some thirty to one hundred milliseconds , and there are provided means to rectify these bursts and to detect resulting pulse waveforms .