Three-dimensional measurements of horizontal bore-hole

The exact positioning of horizontal boreholes in underground research laboratories is important for planning in-situ experiments in view of evaluating the performance of waste matrices and container materials in underground disposal facilities. In this report, a technique for the three-dimensional positioning of bore-holes in a clay formation in the underground laboratory of the Mol site (Belgium) is described. This technique allows to position a borehole with altimetric and planimetric accuracies of respectively 1.5±0.5 millimeters and 17±5 millimeters at the extreme end of a 20 metres deep borehole in the Boom clay formation, which is a potential host rock for the underground disposal of radioactive waste in Belgium. (A.S.)