10.4.1 Borehole probes
One of the simplest sub-surface monitoring methods is the borehole probe comprising a length of reinforcing steel about 2m long that is lowered down the drill hole on a length of rope. If the hole intersects a moving slide plane, the hole will be displaced at this level and it will no longer be possible to pull the bar pass this point. Similarly, a probe can be lowered down the hole, and in this way both the top and bottom of the slide plane can be located. The advantages of the probe are the low cost and simplicity, but it provides little information on the rate of movement.