Fiber Bragg gratings (FBG) seismic activity sensors coupled in sensor arrays provide increased sensitivity to seismic activities, lower strain values and an absence of EMI allowing detection of troop and vehicle movements based on ground vibrations generating seismic signals. The FBG sensor head features a spring-mass configuration with a head surface, fiber optic cable and the FBG sensor attached to a spring that stretches during the acceleration of the mass. Waves propagating inside the earth and along the ground arriving at the sensor locations cause movement of the seismic activity sensors embedded in the ground and this movement induces an acceleration-dependent change in the FBG gratings placed on the sensors. In multiple sensor configurations, the cable will be connected to the other sensors in the network. FBG sensors, sensor arrays and sensor methods are provided in several embodiments.