Geophysical Exploration of a Buried River Valley for Groundwater in Wukro, Tigray Region

摘要

The study was conducted with the aim of delineating a buried river channel with good ground water potential on the area and evaluating the usefulness of the two geophysical methods in such geological situation.The geophysical method used was electrical resistivity (vertical electrical sounding and profiling) and gravity survey to locate favorable areas for ground water development by delineating the extent and trend of buried channels.A total area of 1,000 m × 800 m =0.8 (m2) was covered by the method.A base line of 1 km long is laid out parallel to the buried channel,5 lines each 1 km long are cut perpendicular to the base line,with 200 m profile interval and readings were taken every20 m.The gravity survey consisted of 246 stations carried out on six lines each 1 km long at an average line spacing of 200 m,and a station interval of 25 m along the profiles.The buried channel is characterized by undulations with shallows and deeps.The geophysical results were well correlated with drill holes and the local geology of the area.Low values of apparent resistivity and low gravity correlated with high yielding of the well and relatively high values of resistivity correspond to the sandstone formation.Based on the results,drilling was conducted on the area.The three drill holes intersected the buried channel streams and each well in the area are yielding 20 ～ 25 L/s.Since buried stream channels usually contain water saturated sand and/or gravel which are good aquifers and potential sources of ground water which acts as conduits for water flow.The depth to the water table is 10 m from the surface.Maximum thickness of sand and gravel could be intercepted.The use of electrical resistivity and gravity method can be taken for ground water development program in the future to help and locate more appropriate locations for drilling.Because of the presence of similar geological conditions the methods are successfully used to locate buried channel aquifers as potential sources of ground water.