Geophysical Investigation for an Assessment of Sinkhole Risk in Schleswig-Holstein/Northern Germany

On the sports ground in the village of Münsterdorf small scale (about 2 m wide) sinkholes occur in a regular time interval of about 2 years. Origins of the sinkholes are cavities formed in a Cretateaous chalk layer covered by about 20 m of unconsolidated sediments. Geophysical investigations were carried out to delineate the area of sinkhole risk.

Criteria were established to define sinkhole risk following the “dropout sinkhole” theory of Waltham and Fookes”: a) weakened chalk surface with fissures, b) sandy layers covering the chalk surface, c) cohesive layer in the sedimentary cover leading to a “soil cavity” after sandy material is washed into the fissures of the chalk.

Seismic, resitivity and GPR methods were applied in this area. In the area of sinkhole occurance reduced s-wave velocities and a diffuse reflection image of the chalk surface were found (in contrast to the clear chalk reflections outside the sinkhole area). Resistivity measurements (2D ERT and AEM) verified a sandy layer (high specific resistivity) on top of the chalk layer and a low resistivity layer (till or clay, cohesive) in the sedimentary cover. A 3D GPR survey covering the sports ground found evidence of former sinkholes in the area.