Investigation of the near-surface seismic structure of the Thuringian Basin using 3D traveltime tomography
The objective of the project INFLUINS is to understand the coupled dynamics of near-surface and deep fluid patterns in sedimentary basins using the Thuringian Basin (Central Germany). To reveal subsurface structure and material parameters, in 2011 seismic measurements were carried out in the center of the Thuringian Basin. Data acquisition consisted of three reflection seismic profiles and a seismic array aiming to characterize the 3D elastic properties of the basin using refraction seismics. Based on first arrival traveltimes obtained from the seismic data, P-velocity models of the Triassic in the Thuringian Basin are constructed using traveltime tomography. The seismic data indicates that refracted wave propagation is characterized by two refractions at different depths. P-velocity models are constructed using a sequential inversion strategy to suppress small-scale P-velocity variations. For refractions observed in the seismic data, the Lower Muschelkalk acts as a barrier for ray penetration depth due to its high P-velocity confirmed by reference data. The comparison of the results from tomography with reference data shows, that major velocity trends are in accordance, however, tomography does not image low-velocity layers properly. The P-velocity models are used to investigate the heterogeneity of elastic properties within stratigraphic units. Therefore, depths of strata boundaries are estimated using reflection seismic and geological reference data. Whereas in most parts of the models, P-velocity is preserved within strata, for Keuper and the Upper Muschelkalk significant P-velocity variations are observed, especially at the near-surface. Two NW-SE striking fault zones can not be imaged properly by tomography, however, the reflection data indicates strata displacements of 200 m caused by faults. A 3D P-velocity model including the seismic array data allows to improve an a priori reference model of the basin, as the Lower Muschelkalk is imaged 200 m deeper.