The role of high precision geodetic survey in ground water exploration: A case study from Borena Zone, Southern Ethiopia

Elias Lewi¹, Yelebe Birhanu² and Shimeles Fisseha^1
1 Institute of Geophysics Space Science and Astronomy, Addis Ababa University, Addis Ababa, Ethiopia.
² Department of Geosciences, University of Montana, Missoula, Montana, USA

Gravity survey has long been considered as an expensive operation and a secondary method in hydrogeological explorations because of the precision required in height control and its weakness in vertical resolution. However, technological progress in geodetic measurement such as the GPS technique have addressed the first concern and opened a new era, for the cost effective utilization of gravity survey in many exploration works.

In this work, we present a high precession detailed gravity survey, carried out as part of an integrated geophysical survey to map potential areas for ground water resources in the arid Borena zone of the Oromia Regional State, Southern Ethiopia. The target area is situated on the eastern margin of the southern part of main Ethiopian rift. Being bounded by the volcanic mountains to the west and elevated grounds of basement rocks to the east, greater percentage of the study area is characterized by submerged grounds of volcanic plains underlain by thick basalt flows with fine to coarse columnar joint structures. The primary objective of the detailed gravity survey, in this framework, is to map geological structures and the Precambrian crystalline basement undulations, in those areas where other methods such as Vertical Electrical Sounding (VES), failed to prevail.

As a result, the subsurface is mapped in sufficient detail and adequate depth information is inferred from the gravity survey results. Moreover, low residual Bouguer gravity regions, reflecting regions of deeper basement and/or structural weakness in the overlying Cenozoic volcanics, have been found to be favorable targets to sink productive wells. Hence our result proved that employing high precision gravity survey, in conjunction with other geophysical methods such as VES and Magnetics in hydrogeophysical investigations, has not only delivered accurate basement depth constraint but also proved effective in identifying regions of high ground water potential.

Keywords: high precession gravity survey, basement depth, ground water potential