Currently, our institute is performing a research program supported by the Federal Ministry of Economics and Technology (BMWi). The main objective of this research program is titled “experimental investigation for the verification of a Finite-Element-Multiphase-Model for heat transport processes in the ground” whereby the subsoil is analyzed as a three-phases-model with separate consideration of conduction, convection and their subsequent interaction. Extensive experimental field tests as well as laboratory tests are conducted at the Technical University Darmstadt. In addition to the extensive field tests a geothermal laboratory device has been developed. With the help of this device the heat transport processes in different geological and hydrogeological conditions can be simulated. Furthermore, it is possible to determine the increase of the effective thermal conductivity of a line source with rising groundwater flow velocities. The ratio of conductive and convective energy transport to the whole transported energy can be investigated in laboratory. After all the different types of heat transport processes in geothermal systems can be characterized clearly. With the extensive geothermal data of the laboratory tests common numerical programs can be verified and optimized. Therefore, all measured data will be reconsidered by numerical back analysis. Detailed description of the laboratory apparatus and first results of the numerical sensitivity and back analysis will be given in this paper.