Investigating karst collapse geohazards using magnetotellurics: A case study of M'rara basin, Algerian Sahara

The region of M'rara, which is located in the northeast Algerian Sahara basin, is seriously affected by several natural hazards. This region is exposed to risks such as the collapses and fissures around the water borehole that is situated in the center of the small town of M'rara. The present study was conducted in order to investigate the ground structures under eighteen (18) sites located near the above mentioned water borehole using the magnetotelluric (MT) method to collect the required data. The main objective of this research is to obtain a realistic three-dimensional (3-D) resistivity model of the structures, based on the 3-D inversion method. This was done using the ModEM package. The preferred model presented in this paper resulted from the total impedance as well as the vertical magnetic transfer functions were jointly inverted. Moreover, the dimensionality of the data was analyzed by two different methods, namely the Bahr method and the phase tensor method, which revealed that data at periods >0.5 s are relatively 3-D model. The outcomes of the 3-D inversion are sufficient to identify the M'rara basin; where reveal the folded resistive structure with an anticline form below the Senonian level and the great fractures of axial and transverse types that extend in the hinge zone of this structure. Moreover, the model was presented regarding the fault which is conductive zone. Finally, the interpretation results represent the reasons for the appearance of these collapses and fissures which are due to the process of karst dissolution of the limestone beds. (C) 2018 Elsevier B.V. All rights reserved.