To improve the performance and safety of tunnel constructions, seismic predictions methods can be used to detect relevant geological structures ahead of the tunnel face (e.g. faults, lithological boundaries). We present a simple and robust processing method that can automatically calculate the distance of such a geological inhomogeneity from the seismic response of only a few receivers mounted on the tunnel wall. The method works fully automatic and does need much computational resources which is ideal under tunneling conditions. Our approach has been develop on 3D synthetic finite difference and tested on real tunneling data. In both cases, the distance of a fault zone has been determined accurately and without any a priori information.