I did some work for a client interested in perimeter detection/identification. He had a magnetic vibration sensor, which was simply a magnet suspended on a spring, inside a coil of wire. It was amazingly sensitive, and could pick up a person walking 50 feet away. There is all kinds of algorythm software to identify vibrational footprints and distinguish between vehicles, wildlife, Al Queda, etc.

Ron

PS...Those deer are probably from Vermont! Civil Union...It's not just a law, it's a Man-date!