Integrated 2D Gravity and Vertical Electrical Sounding Modeling for Groundwater Potential Assessment in the Megado Area, Southern Ethiopia

<title>Abstract</title> <p>Groundwater is the principal source of potable water in the arid lowlands of southern Ethiopia, yet its occurrence in complex volcanic–crystalline terrains remain poorly constrained. The present study integrates twenty Schlumberger Vertical Electrical Soundings (VES) with 119 gravity stations to develop a quantitative two-dimensional subsurface model of the Megado area, Borena Zone, Oromia Regional State. VES measurements were acquired at current-electrode half-spacings (AB/2) ranging from 1.5 m to 1000 m, while gravity data, sourced from the Borena Groundwater Project, were processed through Bouguer reduction, terrain correction, and second-degree polynomial filtering. One-dimensional inversion in WinResist (with RMS error ≤ 4%) and two-dimensional forward–inverse gravity modelling in GM-SYS were performed iteratively to refine the layer geometries. Six geoelectric layers were delineated, with the aquifer horizon characterized by resistivity values of 60–90 Ω·m and thicknesses of 97.4–113.6 m, predominantly hosted by fractured and saturated basalt. The complete Bouguer anomaly ranged from − 3.71 to + 1.56 mGal, whereas residual anomalies (–0.97 to + 1.33 mGal) revealed NE–SW-trending normal faults consistent with the regional Mega Rift system. The basement, identified as banded gneiss, occurred at depths of 104–405 m and is dissected by a high-resistivity intrusive body (&gt; 1000 Ω·m, density 2.95 g/cm³). A statistically significant negative correlation (R² = 0.71, p &lt; 0.001) was observed between basement depth and saturated-aquifer thickness, indicating that fault-bounded basement lows constitute primary groundwater accumulation sites. Two boreholes drilled based on the integrated interpretation yielded 22 L/min, validating the predictive value of the methodology. These findings provide a transferable framework for groundwater exploration in volcanic–basement terrains across the East African Rift System.</p>