Further case studies:
  • Gravimetric control of compaction success  
  • Gravimetric 3D modelling  
Gravimetry
Case study 2: Investigation of karst phenomena

After demolition of an old building, the drilling investigation on this site was started for planning the construction of a new building. In one of the drillings an approx. 10 m wide opening (in the vicinity of anomaly "A" in fig. 2) was found in 15 m depth. A geophysical investigation should provide information about the exact location and dimensions of this opening and, in addition, about possibly existing further karst phenomena at the site.


For the investigation of the karst phenomena a gravity survey was performed on a grid of 4 m x 4 m. Beside the usually measured parameters like coordinates and elevation of every gravimetric point, it was necessary in this case to record the position and elevation of the adjacent buildings too because they have an influence on the gravity mesurements which were done in the immediate vicinity.


As a result of the gravity measurements, areas with gravimetric minima (green coloured) give indications of openings or loosening zones at the site the new building should be built on. Besides the probable geological reasons for anomaly A (where conclusions about the dimension of the opening and the associated loosening zones could be drawn) and the minimum situated south-east, influences of former or existing buildings were seen. Thus, anomaly C was caused by a road tunnel that crosses the area.


With this gravity survey a delimitation of the presumed karst regions in the investigation area could be obtained. Even if not all anomalies could be assigned to a definite cause, the gravimetric result is a good base for a targeted approach of subsequent direct investigations (e.g. drillings) and for the planning of possibly necessary security measures.


The surveys are performed with our gravimeter AutoGrav CG-5 of Scintrex Ltd., Kanada.

Fig. 2: Gravimetric local field