Integrated Geophysical Approach for the Detection of Underground Voids in a Construction Site

During the excavation for the construction of a multi-store building in the centre of Athens, underground voids were encountered in a compact limestone with karstic zones. Some voids are full of loose material. Three geophysical surveys (GPR, ERT and microgravity) applied for a quick detection of the underground. Analyzing the results of each method, for the detection of voids, GPR is a quick method to start locating areas of interest, with the resistivity tomography defining all features of the underground with the correct positioning if a 3D approach is applied. The identification of a void is better applied with a microgravity survey correlating with the results of the previous surveys. In order to diminish the equivalence in the geophysical models, to eliminate the artifacts and generally to improve the reliability and the accuracy of geophysical interpretation, an overall geophysical approach was followed using the three different geophysical methods. Karstic zones with loose material and possibly filled or small opened voids inside the compact rock were outlined. Specific areas were outlined, where all three geophysical methods indicate that opened voids possibly exist.