As part of the Indonesian German joint project MERAPI (Mechanism Evaluation, Risk Assessment, Prediction Improvement) an active seismic experiment is carried out in order to derive a 3-D image of the velocity structure of the strato-volcano Merapi. Such a model will be a fundamental basis for a successful localization and modeling of earthquakes and tremor sources inside the volcano. As a seismic source a 2.5 l mudgun is shot in 3 water basins. The basins are located on a circle, nearly 120 degree far from each other. Around 100 shots from every source point are measured by 32 MARK L-4C-3D seismometers on 3 km long profiles on the flanks of the volcano and recorded by a radio triggered 96 chan. SUMMIT instrument.