Structural analysis of gravity and magnetic data: implication for groundwater study in the Ziway, Abijata and Langano lakes corridor and surroundings, central main Ethiopian rift

Abstract

SUMMARY The study area that constitutes the Ziway–Abijata and Langano lakes corridor and its surroundings is located in the central part of the main Ethiopian rift. This region is characterized by active tectonics, volcanic activities, varied geological and structural history, geothermal activity and dotted lakes on the lower lands of the rift floor. The residual gravity and DRTP (differential reduction to pole) magnetic anomalies were employed in this study. This integrated approach included modern data enhancement techniques, including tilt derivative combined with horizontal gradient analysis and curvature analysis of the horizontal gradient to identify suburface lineaments influencing groundwater flow in the study area. Moreover, the gravity and magnetic techniques are employed to determine anomalous depth sources through spectral analysis and Euler deconvolution tecniques that are thought to provide vital information for groundwater studies. Interpretation of the residual gravity and DRTP magnetic anomalies revealed distinict patterns linked to subsurface physical properties (density and susceptibility) showing positive correlations with the existing surface geology. Depth ranges deep ($\approx\!2\,\,{\rm{km}})$, intermediate ($\approx\! 0.93\,\,{\rm{km}})$ and shallow ($\approx$0.3 km) were estimated using spectral depth analysis and Euler deconvolution from the compiled residual gravity and DRTP magnetic anomalies. These estimated depth ranges have been considered to investigate various subsurface geological features from the surface down to about 2 km, where tertiary ignimbrite reserviors (identified in this study and previous studies by drilling) have been encountered. Curvature analysis of horizontal gradient, a combined approch involving horizontal gradient and tilt derivative for both gravity and magnetics data, were used to identify subsurface lineaments. The study has identified NNE–SSW, NE–SW and N–S trends possibly linked to Quaternary faults, and NNW–SSE and NW–SE trends related to the pre-existing Mesozoic structures. Moreover, the E–W to NW–SE trends identified in this study are thought to correspond to the oblique-slip faults intersecting the major rift faults. The findings of this study are believed to contribute to the overall attempt for the assessment of the groundwater resources of the study area and its surrounding regions.