Seismic: These sensors contained seismic detectors. That is, they detected movement in the earth -- similar to earthquake detectors -- such as vehicle(s) moving past or footsteps. Seismic devices required a small spike to be driven into the ground; this spike contained the sensing element. If the ground moved, the sensing element moved and activated a radio transmitter that transmitted a beep, alerting the monitor to the fact that something was moving near the sensor location.
Seismic: These sensors contained seismic detectors. That is, they detected movement in the earth -- similar to earthquake detectors -- such as vehicle(s) moving past or footsteps. Seismic devices required a small spike to be driven into the ground; this spike contained the sensing element. If the ground moved, the sensing element moved and activated a radio transmitter that transmitted a beep, alerting the monitor to the fact that something was moving near the sensor location.