Application of Geophysical Method for Delinatig Shallow and Deep Ground Water Flow System Along Ziway-Butajira and Meki-Midrekebd Traverses, Central Mer, and Thier Implication for Flouride Concentration
No Thumbnail Available
Date
2017-05
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa Universty
Abstract
An integrated geophysical survey using Vertical Electrical Sounding (VES) and Audio Magneto Telluric (AMT) method was conducted to separate shallow and deep ground water flow systems and the role geological structure play in the movement of these waters from Rift Floor to Western Escarpment sectors, specifically through geophysical work along the Ziway-Butajira and Meki-MidreKebd traverses. These methods were selected for their resolution and ability to map the shallow to intermediate depth (VES) and the deeper (AMT) horizons in the subsurface.
Geographically the Ziway-Butajira and Meki- Midrekebd areas lie in the Central Main Ethiopian Rift within Oromia and South Nations, Nationalities and Peoples Regional State in Ethiopia. The areas also lie on dominantly volcanic and lacustrine deposit terrain. The geophysical survey included twenty three (23) VES point using Schlumberger array with maximum half current electrodes AB/2 =750m and twenty seven (27) 2DAMT measuring points. These data were interpreted both qualitatively and quantitatively in order to infer the subsurface geoelectrical stratification of the area and hence the geology as well as the major geologic structures over the transects. It was also possible to extract information on the presence of groundwater bearing horizons in the subsurface including their location and depth.
The qualitative analysis of VES data were performed by using apparent resistivity psuedodepth sections constructed for VES points oriented in a near collinear direction, while the qualitative interpretation of the 2D AMT data were also performed by using 2D MT geoelectric sections. For quantitative interpretation, the individual VES data were interpreted using the ResixIP and Win Resist modeling software to obtain the layer parameters beneath each VES and constructing the geoelectrical sections along selected survey lines. The interpretation of the VES during modeling was constrained by lithological logs from two nearby boreholes. The information from VES data and the borehole were also used to constrain the depth information in the 2D MT modeling which are conducted on four survey line. In general, the VES result revealed four to five main geoelectrical layers that differ in degree of weathering and fracturing, composition and depth of burial. The geoelctrical sections further enabled the identification of the depth to the aquifer along the survey line. On the other hand, the 2D AMT help to identify low resistivity zones and structures that extend to larger depth- which has easily mapped the subsurface to depths of about 1200 m. Finally the overall interpretation was done by integrating all the above result together with the topographic maps and bore hall information. As a result of these shallow and deep ground water systems and geological structures are identified. It is found that vertical and near vertical structures that extend to larger depth is mapped along the traverses showing that there is a flow of shallow groundwaters to the deeper horizons, with no major separation between the two groundwater systems. It has also been possible to delineate drilling sites for potential productive boreholes.
Description
Keywords
Geophysical work along