Seismic Tomography to Investigate Geologic Structures underneath Highway Bridges

In Germany approx. 2500 bridges have to be inspected for the need of renovation. The knowledge about the underlying geological formations is partially very poor, especially for older bridge buildings. By the use of geophysical methods additional information can be achieved to minimize the risks and the renovation costs. This presentation is focussed on seismic tomography investigations in connection with traffic construction projects.

Generally the tomographic method uses the information of the curved propagation of refracted waves (refraction tomography) or it is applied between two boreholes (crosshole tomography). All results are calculated by an SIRT inversion approach and show the distribution of seismic velocities along a cut or a plane between the boreholes. The seismic velocities are correlated with material properties like density, shear strength and compressibility. Low velocity zones are indications to weak areas or cavities.

The potential of these non destructive tomography methods are shown in this presentation by several projects. The method was applied in the planning phase for new bridge constructions as well as in the construction phase of a bridge foundation in a karst area.