Soil thermal conductivity from early TRT logs using an active hybrid optic fibre system

At the Molinella test site in Italy, a monitoring well has been equipped with a hybrid active optical fibre cable down to the depth of 100 m sealed with geothermal grouting. The cored borehole provided a full stratigraphic sequence of unconsolidated alluvial deposits. The cored material has been classified from a geotechnical point of view, and the thermal parameters of the most relevant lithologies have been directly measured. Active heating of the optical fibre cable has been provided by a constant heating power injected through copper wires contained within the cable structure. This way, not only the equivalent thermal conductivity of the entire stratigraphy but also the thermal conductivity at a spatial resolution of 1 m have been obtained. For each investigated layer, the thermal conductivity obtained from the distributed temperature measurements and the ones acquired using direct measurements are interpreted and compared.