1. A security system for protecting perimeters from an approaching human or vehicle comprising: a seismic sensor system configured to sense ground vibration, including ground vibration caused by each of the following: human footsteps, a vehicle, and quadrupedal animal footsteps; 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 different causes of the ground vibration based on this information, including between ground vibration caused by each of the following: human footsteps; a vehicle; and quadrupedal animal footsteps, wherein the trained biologically realistic neural network system includes a plurality of trained biologically realistic neural networks in a parallel configuration.