Microgravity Methods to Characterize the karst structures in north-western Algeria

A gravimetric study was conducted on a site located at Tlemcen, a city of western Algeria, where it was intended to build residential buildings. During the excavation for the preparation of foundations, cavities were discovered. This is confirmed by visual inspection. The study area is geologically composed of dolomitic limestone jurrassique, characterized by strong pérméabilité caused by cracking and karstic formation.

The geophysical method more appropriate in this case is the microgravimetric. The gravimetric campaign which lasted 15 days is composed of more than 1000 stations measures and was realized on several zones at the site with a step of 2.5 m. All these stations have been identified topographically.

            The Bouguer anomaly map presents a short-wavelength gravity low which reaches a minimum value up of - 33.190 mGal. A qualitative analysis of this map showed that the relative gravity lows is related to the mass deficit. Some of the anomalies detected by microgravimetric are well correlated with cavities observed on the surface. Mass deficits have been assimilated as underground cavities and that can present a danger to the stability of buildings.

The 3D modelling has been realized using software based on the algorithm of Talwani (Talwani & all 1960), it has allowed us to determinate location and dimensions of the cavities detected.

Key words: Cavities, karst, Microgravimetic, Anomalies, Talwani, Tlemcen, Algeria.