Given a set of sparse and heterogeneous outcrop and drilling data, we want to retrieve a complete and coherent model of the underground model, i.e. to determine the entire geometry of the surfaces and volumes of the model as well as the topology of the contacts. The reconstruction method proposed in this article is intended to assist the geologist in his interpretation of 2-dimensional geological sections. It constructs the geological formation= s one by one in an appropriate order. The reconstruction of each formation consists of three main steps. We construct a first model from the Vorono¨{i} diagram of the input data. Then we smooth the boundary of this formation. Lastly, we possibly shift part of the formation along faults. Experimental results on several examples show that the method is very effective. The method extends without major difficulties to truly 3-dimensional reconstructions.