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AN-Cavity-AK-09 April

Solution processes are a common phenomenon in limestone and dolomite formations which can lead to karstification causing subsurface cavities in many cases. These cavities can be air-filled or filled with fine-grained material, which decrease the soil stiffness and thus the load bearing capacity. Weak zones in the subsurface are often caused by dissolution and washing out processes, weathering or geological faults. Weak zones lead to a reduction of the rock strength and often have an increased hydraulic permeability. Washing out and solution processes or insufficient sediment consolidation can lead to sudden or long-term subsidence of the subsoil.

Exploratory drillings, which are generally used for soil investigations, provide only limited information about the subsoil conditions and are not suitable to detect small-scaled structures in a geologically heterogeneous environment. Therefore, geophysical methods are used for a detailed exploration of geological structures and cavities:

• Ground Penetrating Radar (GPR)
• Micro-gravity
• Resistivity imaging method
• Seismic Refraction Tomography
• Multi Channel Analysis of Surface Waces (MASW)
• Refraction Micro Tremor (ReMi)

 
Ground Penetrating Radar signatures of voids


An example of a void infilled with sediment showing top and bottom

 
3D mapping and presentation of cavity


Micro Gravity signatures of voids

 
Micro Gravity signatures of voids

 
Electrical Resistivity Imaging- Water filled cavity

 
Electrical Resistivity Imaging- Weak Zone

 
Electrical Resistivity Imaging- Air filled cavity

 
ReMi Results- Cavity Detection

 
MASW Results- Cavity Detection

 
Crosshole Seismic Tomography for a deep cavity detection

Important Note: Selection of right geophysical tool is critical for a successful cavity detection project. It depends on factors like geology of the area, depth of expected cavity, minimum size of cavity to be detected, presence of water table, etc. Since every site is unique, geophyscial investigation program needs o be customized to cater fir site specific conditions and deliverables