Application of Integreated Geophysical Methods for the Evaluanon of Thermal Centers and their Structural Control in Boku,Nazareth, Main Ethiopian Rift

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Addis Ababa University


Geophysical methods provide the tools for solving various geological problems. In the case of this work, vertical electrical sounding (YES) and magnetic methods are carried out in the vicinity of Lake Beseka. The site is situated at about 200 km east of Addis Ababa near the town of Methahara. The study is performed in connection with M.SC research training to appreciate the effectiveness of the methods used and on the one hand it contributes additional data to obtain the subsurface information that has been contributing to the Jake level rise. The variation of resistivity with depth is studied by a progressive increase of the Schlumberger current electrode configuration using P ASI -16GL earth resistivity meter and PASI-P300 energizer. In order to get a reasonable subsurfuce information, the apparent resistivity curve plotted in the field had been compared with a set of theoretically calculated master curves. The layer parameters, resistivity and thickness, obtained by iteration processes were used to construct geoelectric sections for each profile to show different lithological units in the vertical direction. In addition to YES, the magnetic survey was carried out using scintrex made proton precession magnetometer (lGS-MP-3/4) and monitored with a selected base station for diurnal correction. The magnetic survey is applied to delineate subsurface structures (fuultS/shear zones), which have been created due to the tectonic activities taking place in the area. As shown from the total field magnetic map (fig. 19), the northwestern part of the lake is characterized by exposed or shallow depth volcanic rocks. But the northeastern part of the Jake is generally seems to be magnetically quite. The NNW and SSE inferred fault may intersect the NNE--SSW trending fault through which the thermal springs apparent in the area may come to the surface. The results of the Vertical electrical sounding surveys show that the resistivity of the different acquifer systems is low in the vicinity of the Jake and increases away because of the intrusion of the saline lake water. It has been found that no input of water to the lake is possible from the adjacent farmJands, as the water table gets deeper as one goes away from the lake.



Geophysical Investigations for Lake Level