1. A security system for protecting perimeters from an approaching human or vehicle comprising: a seismic sensor system configured to sense ground vibration caused by human footsteps, a vehicle, quadrupedal animal footsteps, and other activity; and a trained biologically realistic neural network system configured to receive information indicative of the ground vibration sensed by the sensor system and to discriminate between ground vibration caused by each of the following based on this information: human footsteps; a vehicle; quadrupedal animal footsteps; and other activity.