Refraction Seismic Studies of the Shallow Underground below a Historical Church in Western Germany

The historical Christus church in Schwelm, Western Germany, shows structural damages at its front side and its southern tower, which are supposed to be due to the geological situation in the area. To describe the shallow underground below the church and to detect locations of anomalies, a refraction seismic program which uses the complete information of the refracted waves consisting of the travel times as well as the phase and amplitude characteristics in the recorded seismic data, was performed. At first a joint application of Common Midpoint- (CMP-) refraction seismics, which uses first break phases and the GRM was applied. In a second step the resulting intercept time sections were depth converted using the results of the GRM and the refraction tomography, which was performed as well. With the combination of These 3 different refraction seismic processing techniques anomalous zones below the church were detected and classified. These anomalies were due to weak zones and karstified cavities in the limestone below the church, as well as to anthropogenic structures. The anomalous zones were located horizontally and vertically, interpreted and thus, together with the results of refraction seismic, a model of the shallow underground below the church was developed.