Integrated gravity and resistivity investigations of the deep Hammam Bradaa aquifer, Northeast Algeria: Implications for groundwater exploration

Abstract

Recently, the Guelma region, located northeast of Algeria, has been facing an unusual water shortage. To bypass this situation, a geophysical study using gravity and resistivity has been initiated to enhance the understanding of the geology and hydrogeology of the deep structures of the study area. In order to accomplish this, a residual gravity anomaly map has been constructed by removing from the Bouguer anomaly map a regional gravity gradient using the upward continuation technique. For the purpose of validating the Vertical Electrical Sounding(VES) interpretation, two soundings were conducted near boreholes with known lithology. The Hammam Bradaa deep aquifer of the Guelma region is composed of limestones that belong to the Constantine neritic domain. The Constantine neritic domain is covered by thrust nappes and outcrops in some places of the study area as tectonic windows. The exact geometry such as depth and lateral extent of the aquifer are not well known. Since limestones are characterized by high densities and resistivities relative to the host rock, the combining of gravity and VES was able to accurately map their extension among the other lithologies composing the geological section. Multiscale edge detection of gravity data known as "worms" made it possible to highlight the principal lineaments across the study area which may contribute to the recharge of the Hammam Bradaa aquifer. The 3D Gravity inversion enabled the mapping of the spatial configuration of limestones that are characterized by positive gravity contrast relative to the host rocks. A vertical density slice extracted from the 3D gravity inversion, running from north to south, reveals a significant thickness of limestone beneath Hammam Bradaa which suggests a strong potential for groundwater resources. The geo-electric section, constructed from the interpretation of VES data and calibrated with well lithology, enabled the mapping of the depth to the top of limestone beneath conductive alluvium and marls.