Use of electrical Resistivity tomography (ERT) and electromagnetic induction (EMI) methods to Characterize Karst Hazards in north-eastern of Algeria

Sinkholes are natural phenomena caused by the dissolution of limestone rocks due to the infiltration of acidic surface water. Initially, this infiltration creates voids, and then a sudden collapse occurs when the roof structure fails to support the weight load of the overlying deposits. In Algeria, a sinkhole occurred in February 2015 after a severe snowstorm, on an agricultural field near the city of El Ouldja (South-Eastern Setif). The sinkhole occurred only after the dissolution of a quaternary lacustrine limestone. This dissolution created a karstic cavity, covered by an alluvial deposit. These alluvium deposits progressively pulled down from the base to the top through a sediment removal phenomenon at the deep karstic cavity opening. The sinkhole is circular with 16-m diameter and 16-m deep. This phenomenon represents a risk for local populations living with the fear of a worsening and expansion of the existing cavity or the formation of new sinkholes under the same circumstances. This work aims at identifying high-risk zones in the entire site surrounding the karstic collapse phenomenon through geological analysis of the sinkhole walls as well as two geophysical methods, namely, electrical resistivity tomography (ERT) and electromagnetic induction (EMI). Results revealed the existence of developing superficial karst cavities, in the NNW and SSE part, beneath the Quaternary alluvium of the study area, bordered by a fracture network. Such structures are a natural risk that needs to be considered not only for agricultural land management, but also for rural construction.