Mapping the spatial variability of subsurface resistivity by using vertical electrical sounding data and geostatistical analysis at Borena Area, Ethiopia

Vertical electrical sounding survey has been done to map and visualize resistivity distribution at Borena basin. The area is situated in Southern Main Ethiopian Rift, where Rirriba rift, Mega rift and associated fractures define the structural setting of the area. It is covered by Quaternary deposits, Quaternary and Tertiary basaltic rocks and Precambrian metamorphic rocks. Bullal basaltic formation outstretch in the Rirriba rift and it is thought to be potential groundwater aquifer. About 288 Vertical Electrical Sounding data were collected. Inflection and extreme points were used to identify characteristic points. Variograms are modeled and kriging interpolation is used to map distribution of resistivity, determined from characteristic points, in the area. Very low to low resistivity variations are mapped in northern end of the study area, whereas medium to moderately resistive ground are mapped in the middle and southern part of the area. The low resistivity horizon at the shallow subsurface could be due to salinity since the area occupy numerous saline craters and maars. Approximate mapping of large sets of Vertical Electrical Sounding data with geostatistical treatment has facilitated the interpretation and provided a sound picture of the subsurface.