The buried lines themselves are secure unless you dig them up with a backhoe or something similar. What's unsecured are the shut-off valves and pigging stations which are generally surrounded by chain-link fence and barbed wire. Also the pumping stations are manned by only a small crew at night if at all. Most operational decisions are made, using digital data via remote sensing, in a central control center located in urban areas away from the pipelines themselves.
Gas lines are usually quite shallow. Most of ours are only about 2-3 feet down and we have a Call Before You Dig system which I believe is Nationwide.