LockInsidePipe

Lock inside pipe
Place a solid steel rod inside pipe posts on an entrance. A normal padlock is used to lock the rod inside the pipe. Thus the lock can't be broken with a bolt-cutter since there is no leverage available.

Angle grinder cutting detection
A vibration sensor will detect any attempt at cutting through the rod with an angle grinder. An ultra-violet sensor will detect a cutting torch.

Make steel rod part of electronic loop
Run a small current through the 2cm steel rod, measuring its impedance in real-time. Any flame cutting of the steel rod will influence the resistance of the rod because of its change in temperature. The colder a piece of metal the less its resistance.

The whole post rod becomes an electronic circuit which must be isolated from ground so that the measurement current isn't shorted.

PIR inside steel pipe
To unlock the rod you must place your hand inside the steel post ancored on one side of the entrance. A PIR/Ultrasonic distance measurement circuit will detect this.

Keypad deactivation
For added security and to prevent the unlocking of the rod under duress, place a keypad besides the post. A correct code must be entered or the Ultrasonic/PIR combination will send an alert to the base via a GpsGprs, MeshNetworking, RS485 or RadioModems combination.