Geophysics,Space Science and Astronomy(IGSSA)
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Browsing Geophysics,Space Science and Astronomy(IGSSA) by Author "Haile, Tigistu (PhD)"
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Item 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.Item Application of Integreated Geophysical Methods for the Evaluanon of Thermal Centers and their Structural Control in Boku,Nazareth, Main Ethiopian Rift(Addis Ababa University, 2001-06) Mengistu, Berihun; Haile, Tigistu (PhD)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.Item Application of Resistivity Methods for Groundwater Exploration Around Chuko, Southern Region of Ethiopia.(Addis Ababa University, 1998) Bunaro, Eyasu; Haile, Tigistu (PhD)Geophysical surveys and studies were carried out in southern Ethiopia around Chuko town for ground water exploration by the hydrogeological tearn in the Ethiopian Institute of Geological Surveys (EIGS) during 1984 - 1985. These data are reanalyzed from the scientific point of view. It is observed that the apparent resistivity data at a horizon having sufficient thickness can be descnbed by the linear equation where the slope is the same as tangent of the dip angle of the fault. The causes of the principles of equivalence for this particular survey area are aquifers (and possibly aquicludes ) and this problem can be managed by fixing the resistivity value of aquifer known from other information sources such as weDs. The results of the inversion shows that the resistivities of all layers associated with aquifers converges towards this fixed value without any change in the depth range indicating the fact that it is not the number of layers but the resistivity value of aquifer that is in the interpretation sounding curves for the ground water exploration. From type curve analysis the assumed faults are investigated which are identical with the ones obtained using profiling results. The type curve mapping is powerful in zone as weD as fault mapping. The stacked graphic plot, pseudoelectric and geoelectric section analysis shows that the resistivity of the water bearing formations are in general greater than 40 ohm-m and these formations, which are dominated by fractured volcanic rocks and sands of varying grain size, are most available in the eastern part of the base line with varying depth ranges.Item Geophysical Investigation for Ground Waterpotential Assessment and Mapping Structures at Alidege Plain, South Afar, Ethiopia(Addis Ababa University, 2011-06-06) Mulugeta, Tewodros; Haile, Tigistu (PhD)Vertical Electrical Sounding data collected over the Alidege Plain, located in the Zone 2, of the Afar Regional State has been used in this work. The plain is an extensive flat northeast- southwest running plain in the middle Awash valley with an average width of 26-30Kms and length of more than 70kms. The objective of the VES survey was to make an assessment of the groundwater potential of the plain, with a view to developing the plain both for agricultural and pastoral uses. A total of 90 VES, collected at different field campaigns by the Water Works Design and Supervision Enterprises, has been used. The VES are well distributed over the plain and also a good number of the VES are aligned to enable one to construct a number of sections to depict the subsurface geology of the area. All the VES were individually first interpreted, and to aid in the interpretation of the data, the data were further presented in the form of apparent resistivity pseudodepth sections, sliced-stacked pseudodepth depth sections and true resistivity 2D geoelectric sections. A number of available boreholes distributed over the area were also used to calibrate the VES interpretation. From the combination of these interpretations, it is found that the Alidege plain has adequate groundwater potential with the thick layers of moderately to highly fractured basalts and ignimbrite bearing the good portion of the groundwater resource. A number of structures have also been mapped at varying depths that could control the movement of groundwater and also aid in the location of productive boreholes. From the sliced-stacked pseudodepth sections, it has also been possible to show the direction of regional groundwater movement in the subsurface to be from northeast direction towards southwest direction.