GRAVITY AND ELECTRICAL RESISTIVITY ANALYSIS OF DEEP AND SHALLOW STRUCTURES RELATED TO AQUIFERS WITHIN THE JEFFARA PLAIN, SOUTHEAST TUNISIA

The aquifers under the Jeffara Plain in southeastern Tunisia is a complex aquifer system consisting of interconnected shallow and deep aqufiers within Triassic and younger sediments. The extent of the aquifers is only basically known, so a gravity and electrical resistivity analysis was undertaken to aid in determining the aquifers extent. The gravity analysis consisted of creating a residual gravity anomaly map, a power spectrum analysis to determine the depth to density sources and a 3D Euler deconvolution analysis to locate lineaments and the depth of density sources. The gravity analysis was used to infer the location and depth of faults and thicker portions of the aquifer containing sediments The electrical resistivity survey was used to characterize the geometry and the depth of deep and shallow aquifers surrounding Medenine. The residual gravity anomaly map identified a series of northwest-trending gravity minima related to grabens. The Euler deconvolution determined faults can be classified into deep faults (˃ 2.5 km depth) that these associated with the Medenine fault and nearly determined offshoots that trend north-northeast, shallow (0.5-1.0 km) and near surface faults (< 0.5 km). The shallow and the near-surface faults involve Upper Cretaceous and younger sediments and trend in a variety of directions. The electrical resistivity models indicate that the shallow aquifers are approximately 20 m thick and the deep aquifer is found below 25 m in depth and is at least 30 m thick. The shallow aquifers were found on all four models but the deep aquifer was found only two profiles. The location of both the shallow and deep aquifers were affected by fault imaged by the electrical resistivity data and and these faults were confirmed by the gravity analysis. This shows that the shallow/near surface faults are important conduits between the shallow and deep aquifers. The most important fault was the Medenine and its newly deduced offshoots as the electrical resistivity models indicate that these faults act as a barrier for the deep aquifer. Based on both the gravity analysis and the electrical resistivity models, the extent of the deep aquifer can be extended further to the east than was previously known.