The BOF-EX is best described as a multiple-point single tube extensometer. It consists of four main components: the mechanical anchor, the measurement module, the extension tubing and the centralizers. An installation consists of a number of measurement modules, in series in a borehole, each one being mounted on the length of extension tubing required to span a lower and upper mechanical anchor. Displacement measurement is therefore made in the hole, in sections distributed along the borehole length.