Natural cavities or cavities caused by mining can collapse what lead frequently to sudden subsidence of the subsurface material (sinkholes). Especially in urban areas, considerable damages on buildings can be observed due to the collapse.
A detailed investigation of the geological conditions below buildings is limited if only drillings are used. Therefore, seismic tomography is applied between the prepared boreholes to measure the seismic velocity which provides information about the rock strength. The measurement example shows the seismic tomogram with a sinkhole structure at the surface.
Collapse of the subsoil
Tomogram indicates a sinkhole close to the building
Cracks and fissures in buildings are often caused by settlements of the subsoil. Settlements are the result of vertical displacing of soil so that material can subside downwards by either mining-induced or naturally formed cavities. If these processes are noticed early enough a focused cause study can be started to find a sufficient remediation measure.
Two different examples show high-resolution tomographic studies using geophysical instruments which were placed in vertical and inclined boreholes. The results of the investigations can be seen in the adjacent images. Loose areas are characterized by low seismic velocities. The red dashed line in the tomogram indicates geologically induced subsidence. The other tomogram shows an ancient and poorly backfilled shaft (blue colours).
First tomogram shows geologically induced settlement (red dashed line) and second indicates an ancient poorly backfilled shaft (blue colours)
A vertical seismic profiling (VSP) survey was carried out in an ancient mining region because the ground below a shopping centre and the adjacent area had been settled. Weak zones or possibly cavities were suspected. The survey focused on detecting potential weak zones to derive information about the ground stabilization.
Seismic velocities and depth migrated seismic sections illustrated in 3D