Browsing by Author "Gashaw, Ewenet"

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    Application of Electrical Resistivity and Elt:ctromagnetic Prospecting Methods for Ground water Studies Around Dembi near Debrezeit
    (Addis Ababa University, 1998-06) Gashaw, Ewenet; Haile, Tigistu (PhD)
    Combined electrical resistivity and electromagnetic prospecting methods were carried out around Dembi area near Debrezeit town. The analysis of the results obtained from both geophysical methods has provided useful information regarding the electrical characteristics of the subsurface which is related with the thickness and depth ranges of layers. The physical property distribution which is obtained by the two methods is analyzed with particular significance for ground water. Twenty vertical electrical sounding (yES) measurements using Schulumberger configuration along four parallel profiles were conducted. The field apparent resistivity data is plotted against electrode separation and initial model parameters for each sounding station is obtained using auxiliary point method and two layer master curves. For quantitative evaluation of the resistivity soundings, the field data was submitted to a curve matching computer program. In this program the apparent resistivities obtained as a function of electrode spacings is converted to true resistivities as function of depth. As result of inversion, a f().LJr_.I~l'~_~!J.~()e l~<:;!~c._ ~ection is constructed for all profiles in the studied area. The investigation has made it possible to establish the layer stratification along all the profiles. Generally, this geophysical investigation around Dembi area has indicated a four layer earth structure. The first layer is described by a more or less uniform thin layer of alluvial deposits. Underlying the top layer is a second layer characterized by a very low apparent resistivity value which represented the bottom part of the soil section with considerable moisture and clay content. The third geoelectric layer is made up of a vast portion of weathered and fractured basalt section which is assumed to be the most favorable site for the accumUlation of subsurface water. From hydrogeological point of view, this layer satisfies the physical condition to be favorable host for ground water. Underlying the third layer a section identified by comparatively low apparent resistivity value is interpreted to be highly saturated volcanic ash and clay.It was also possible to observe a generally decreasing apparent resistivity values towards the Western direction along each profile over the investigated area. A decreasing trend of apparent resistivity values were also observed due to the conductive nature of the ground towards North. The results obtained from electromagnetic interpretation also showed anomalously high conductivity responses around the third and the fourth profiles. An increasing tendency of conductivity was also observed towards the Western direction coinciding with the result obtained from the resistivity survey. The results of both geophysical methods has shown strong correlation in that, a decrease in apparent resistivity values in the sounding survey is associated with an increase in electrical conductivity obtained from electromagnetic survey towards North and West of the studied area.