مجلة النيلين لعلوم الارض
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Item Calibration of geo-electrical measurements using borehole data for groundwater investigation in basement rocks in Wadi Orshab watershed, Red Sea State, Sudan(Faculty of Petroleum and Minerals, 2019) Mohammed Noor M. H. Hassan, Abdalla E. M. El Sheikh; Khalid A. Elsayed ZeinelabdeinOrshab is a well-known gold prospective area in the Red Sea Hills of NE Sudan. Mining industry is now rapidly growing in this part of Sudan. Orshab watershed represents a main source of safe water for the mining companies as well as for artisanal miners and local people. Despite this fact, the area suffers from acute shortage in fresh water supply, since the area is mostly covered by crystalline basement rocks. Therefore, the overall objective of the present study is to investigate the groundwater occurrence in terms of the depth and thickness of viable aquifer using geophysical methods supported by some borehole lithological data. This requires appropriate and efficient techniques suitable for finding groundwater in such challenging conditions. Traditionally, electrical resistivity methods are generally used in similar investigations. In fact, resistivity measurements cannot be related directly to the lithological type in the subsurface without sampling or support of other geophysical or lithological data. Moreover, depth to the water bearing formation is not accurately estimated through the resistivity methods. Accordingly, the need for calibration of the measured values is crucial in this case. To this end, two VESs were measured close to productive boreholes situated within the investigated area. The extracted information from the interpretation of the two VESs supported by borehole lithological data revealed that the depth of the geo-electric layer is equal to AB/2. Accordingly, this finding will be used in the interpretation of the all the VESs acquired within wadi Orshab. Twenty-five Vertical Electrical Sounding measurements were recorded. The interpretation of these data revealed the presence of different geo-electric layers in the sub-surface. The geo-electric layers are topsoil and alluvial deposits. The top layer was found to have resistivity varying from 1997ohm-m - 27 ohm-m. Its thickness was found to be around 5m. The second and third layers have thicknesses ranging from 2m to 10 m and from 20 m to 45 m, respectively overlying the basement. The present study showed that two types of water bearing formations are present within the watershed: alluvial deposits and weathered basement. The maximum depth of aquifer in the investigated area was found to be 65 m approximately. Therefore, this is the recommended depth for drilling any new boreholes for sustainable water supply in Wadi Orshab. The obtained results from the calibration of the acquired data enhanced the interpretation of the resistivity data and produced depth estimations that are very close to the actual depth. Consequently, this method should be used in area with similar geological setting.Item Quantification of Groundwater Recharge in the Midstream Area-Gash River Basin Using the Water Table Fluctuation Method(جامعة النيلين, 2017) Hago, A. H. AliAbstract Quantification of groundwater recharge differs in their methods of estimation and therefore gives variable recharge estimates whenever a groundwater system is evaluated. Quantifying the fraction or the percentage of infiltrated water which reaches the water table is a key factor in any sustainable planning scheme for groundwater resource management. The water table fluctuation method was used to evaluate the seasonal and annual variations in water level rise and to estimate the groundwater recharge. The method used in this study includes monitoring of groundwater level measurements to estimate groundwater recharge using Water Table Fluctuation “WTF” to determine sustainable withdrawal. The average annual discharge of the River Gash is estimated to be 1,031×106 m3 at El Gera gage station (upstream) and 711× 106 m3 at Kassala bridge gage station (downstream). The annual loss mounts up to 29% of the total discharge. The water loss is attributed to infiltration and evapotranspiration. The present study showed that the monitoring of groundwater level measurements indicates that the water table rises during the rainy season by 9 m in the upstream and 6 m in the midstream areas. The average rise in groundwater table (ΔHn) in the period from 1999 to 2013 found to be 8.36 meters. The average annual groundwater recharge during the period 1999 to 2013 is 107 × 106 m3 whereas average difference in Gash river discharge at Kassala Bridge during the same period is 320 × 106 m3.