Integrated electrical resistivity and gravity geophysical investigation of structurally controlled fractured basalt aquifers for sustainable groundwater development

Groundwater scarcity is becoming an emerging problem in fast-growing urban centers in Ethiopia where population explosion, agricultural developments, and industrialization are posing pressure on scarce water sources. This research focuses on the hydrogeology features and groundwater potential assessment of the study area using the combination of Vertical Electrical Sounding (VES), borehole lithology logging, and gravity surveys. Based on the VES inversion results with relatively small root mean square errors (1.65–4.27%), the subsurface geology of the study area is characterized by heterogeneous volcanic layers composed of topsoil, clay, fractured basalt, and massive basalt. From the identified formations, fractured and vesicular basalt layers with relatively medium resistivity value (45–239 Ωm) are considered as the major aquifer zones due to increased secondary porosity and permeability while clay layers with relatively low resistivity value (6–20 Ωm) and massive basalt layers with very high resistivity (> 322 Ωm) act as aquitards and impermeable layer, respectively. The borehole lithology logs have shown that porous scoriaceous and fractured basalt layers are the groundwater-producing zones. The low Complete Bouguer anomalies (− 219 to −204 mGal) and residual gravity anomalies (− 7.09 to 7.11 mGal) are related to fractured areas with favorable conditions for water collection; conversely, the high anomalies represent dense massive basalt without any groundwater potential. The Derivative gravity methods, such as First Vertical Derivative, Total Horizontal Derivative, and Analytical Signal, have been used to delineate weak zones and fractures that increase the groundwater permeability. The overall interpretation shows that groundwater presence in the study area depends on fracture intensity, connectivity, and aquifer thickness, not only on the lithology.