Characterization of a marine intrusion using electrical sounding and seismic refraction: case of the the Oued Nador plain (Tipaza, Algeria)

The present study aims to charcaterize, by geophysical methods (electrical sounding and seismic refraction), the downstream part of the Oued Nador alluvial layer or deposits (Tipaza, Algeria) and the morphology of its bedrock to better locate the freshwater-saltwater limit. This will allow to better understand the marine intrusion.Indeed, the aquifer is defined by a clay and marl substratum of middle to lower Pliocene Piacenzian age rich in recent Neogene and Quaternary filling (Quaternary alluvium composed of sand, gravel and pebbles). During Spring of 2015, 2016, and 2017 three electrical surveys were conducted. The results indicate that the origin of the salinity is due to progressive marine intrusion inside the aquifer (Bechkit et al, 2017; Bechkit et al, 2018a; Bechkit et al, 2018b). However the presence of clay makes the contrast between the saltwater and the clays less important so that it is difficult distinguish electrically the two mediums. In May 2018, we combined both electrical survey and a refraction seismic acquisition on the area of study. This work was carried out in order to separate, through the ranges of wave propagation velocity, the saltwater formations from the clay ones. The presence at a certain distance from the coast of an elevation of the clay bedrock has an advantage. Indeed, it acts as a natural barrier against any intrusion on a large scale of saltwater at depth. The study carried out in this context showed that the combination of two geophysical methods is very useful in environmental study for the evaluation of the salt water intrusion in an alluvial ground in the presence of clay formations.