The knowledge and characterization of fissures is of central importance for deep geothermal energy. The earlier information on the fracture system is available, the sooner decisions can be made regarding borehole construction and completion, which will positively influence the efficiency and thus the profitability of geothermal drilling. The aim of the project is therefore the development of a low-cost, high-temperature measurement technique, which is suitable for describing fracture systems with regard to fracture distribution, direction, length and thickness of the fractures during drilling. The proposed project directly continues a partial aspect of the already funded project 'Acoustic exploration of the subsurface for deep geothermal energy with the drilling set' (FKZ 0325622). An acoustic measurement technique for application in geothermal drillings is to be developed, which is based on fibre optics. Fibre optics is particularly suitable for operating sensors in extremely hot environments. At the same time fibre optics offers the possibility to integrate many sensors in a common optical fibre. Such sensor arrangements are necessary for acoustic receivers of measuring instruments used in the borehole to characterize the type of rock drilled through during drilling and to determine the position of the drill string in the geological formation.