Geophysical investigation for Groundwater Southern Gezira (Central of Sudan)Geophysical investigation for Groundwater Southern Gezira (Central of Sudan)Geophysical investigation for Groundwate

A regional gravity survey and vertical electrical sounding measurements, in addition to a hydrogeological study were carried out in the Gezira area between latitudes 130 30’–140 48’ E and longitudes 320 18’ –330 36’ N. The aim of the study is to investigate the geology of the area and subsurface basinal structures and to indicate water-bearing zones and their characteristic. A Bouguer Anomaly map of scale 1: 400,000 was produced. The amplitude of the area ranges from -27mgals to -72mgals. The Bouguer anomaly map of the study area displays several anomalous zones, recognized as contour closures of local gravity highs and lows. In the Bouguer map there is broad zone of low gravity anomaly which is elongated in a general NE-SW direction with maximum depth of 1900m and a length of 40m. Several depression zones are shown on the map. The interpretation of the observed Bouguer anomalies, totals of five profiles are selected which approximately cover the most of the area. A density contrast of (-0.47 g/cm3) between the Basement complex rocks and the overlying sediments, and associated structures are assumed for the models. The study area is surrounded by the White Nile rift to the west which is formed by the junction of two major grabens; the Umm Rawaba graben extending in a NW direction and the White Nile graben extending in N to NW direction. The Basement rocks and the overlying Mesozoic deposits are block-faulted at Rabak. A total of 24 vertical electrical sounding points using a Schlumberger configuration were made in the area. The quantitative interpretation of the VES data was performed using an automatic inversion computer programme; Resix Version 3.08 interpex Limited 1994. The result of the interpretation is given in form of geoelectric sections which provide information about the geology down to depth of about 130 m. According to the results of the vertical electrical sounding measurement, the study area is characterized by rapid facies changes and thickness variation. There are two main water – bearing formations in the Gezira. These are the Nubian Sandstone and Gezira Formations. Both aquifers contain large quantities of water. Where the Gezira Formation is thin and clayrich, the main aquifer is in the underlying Nubian Sandstone Formation. Very often the two aquifers are interconnected and their waters are mixed. The standing water level is in general higher near the river Niles indicating that the rivers are the main source of recharge to the Gezira area. The total dissolved solids of he groundwater in the study area ranges between a 180 ppm at Umm Asalah well to 2184 ppm at El Burgan well. About 80% of the wells tap water with a T.D.S of less than 500 ppm which indicates good quality water