Analysis of different geophysical methods in the detection of an underground opening at a controlled test site
This study involves the application and the analysis of four geophysical methods (resistivity tomography, microgravity, magnetic, M.A.S.W.) for the detection of a tunnel under controlled site conditions. Rresistivity tomography provided satisfactory information both for the target and the near surface geological formations. Dipole-Dipole and Pole-Dipole are the most suitable arrays for the detection of the investigated void, especially when the later combines forward and reversed measurements. The time consuming and laborious microgravity method is suitable for the delineation of the tunnel. Prior information is necessary for the inversion of the microgravity data. The pseudo-section deduced from Multi-channel Analysis of Surface Waves showed two geological layers and imaged a shallow smoothed heterogeneity, attributed to the underground target. However, the target limits are not well defined due to the low lateral resolution. The gradient magnetic method accurately delineates the tunnel due to the increased magnetic susceptibility contrast between the target and the host rock. This condition is usually satisfied when the target is a contemporary man made structure.