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Geophysical Investigation of the Triassic Salt Material Hazard: El Fahs Case Example (Northern Tunisia)

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Document pages: 16 pages

Abstract: TheTunisian territory (area of diapirs) is exposed to the risks of groundmovements linked to water, some of which are related to the phenomenon ofdissolution of gypsum, allowing the appearance of underground cavities whichpresent natural risks and set people in danger. The analysis of the hazard wasdetermined by the field study coupled with the application of geophysicalmethods to locate and map the cavities and identify their dimensions and theirpositions in the subsoil. In the region of El Fahs (40 km NW of Tunis): we useda non-destructive method, georadar (GPR) with a 200 MHz antenna band-width, andan electric method, by using the electric tomography of which we haveapplicated the sequence of dipole-dipole measurement. The results ob-tainedwere examined and interpreted according to 2D profiles. The geophysi-calmethods of GPR and electrical tomography aim to detect many calvities indifferent depths. However, the geological radar was able to identify severalcavities and the zones of dissolution whose investigation depth did not exceedthe first 3 meters. On the contrary, the electric tomography method allowingthe presence of several deeper underground cavities with larger dimensionsex-tended to 64 meters. To conclude, the geological and hydrogeologicalcontexts of the terrain studied and on the state of the soil and subsoil allowus to better understanding the mechanisms of the establishment of thedissolution phe-nomenon and the appearance of cavities in the basement. Theanalysis of the predisposing factors present in the study area shows that thegeodynamic con-text of the Triassic ascent takes place according to a precisethermodynamic process that favors the phenomenon of dissolution of the gypsum.

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