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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    Application of Electrical Resistivity and Electromagnetic Prospecting Methods for Ground Water Studies around Dembi near Debrezeit
    (Addis Abeba 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. Tw~ty 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 rE:lsult of inversion, a four layer geoelectric section is constructed for all - .-------~--.~------.. ---~- ---~---~ . . - - profiles in the s.tudied area. Th~ 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 weatbered 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 resisffvity value is interpreted to be highly saturated volcanic ash and clayIt 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
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    Application of Electrical Resistivity Method for Ground Water Exploration in south Western Butajira area Guraghe zone
    (Addis Abeba university, 1997-05) Mebrahtu Bereket; Haile Tigistu (PhD)
    The basic mathematical foundation for the vertical electrical sounding method has been presented by deriving the formula for the potential distribution at the surface of a horizontally stratified earth. The apparent resistivity and its relation with resistivity transform function has been shown particularly for . Sclumberger Configuration. The theoretical background for resistivity inversion with the ridge regression technique has been explained following the method of Inman. On the basis of the above theoretical consideration the electrical resistivity method has been applied for ~ound water expluratinn and possible site for borehole location in South Western Butajira Area, Southern Ethiopia. To do this survey along three profile lines were taken with a total of 20 YES points out of which 15 were analyzed in detail.
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    Application of Geophysical Methods to Delineate Fresh Water Saline and Water Zones in Amibara Irrigation Farm Lands Amibara, Eastern Ethiopia
    (Addis Abeba university, 2009-06) Zebene Abebayehu; Haile Tigistu (PhD)
    With the aim to mapping the saline and fresh water interface and verifying the application of geophysical methods to study problems of salinity at the Amibara irrigation farm; electrical methods were applied over two parallel traverses. The methods employed included Schlumberger sounding surveys at ten YES points on the two traverses with YES points spacing of 450 meters to 1100 meters and dipole-dipole surveys over four profiles of approximate length of 250 meters over the same two traverses. The dipole surveys were carried out with three levels expected to give depth of investigations of about 5, 7.5 and to meters. The results of these field surveys are interpreted and presented in the form of geoelctricsections and pseudo sections. A general geological section of the survey area is given from the interpretation of the sounding data. The results have clearly mapped the different geologic horizons of the subsurface and delineated areas of low resistivity due to effects associated with highly saline formation fluids. The work has verified the effectiveness of the electrical methods of prospecting in studying problems associated with salinity and shows the potential of these methods in planning current and near future farm land utilization and monitoring of current pilot desalination programes. Surveys carried out over properly selected sample areas could minimize the cost of obtaining such information from expensive piezometric wells and soil test pits
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    Application of Integer a Ted Geophysical Methods For the Evaluation of Thermal Centers and Their Structural Controls in Boku ,Nazareth, Main Ethiopian Rift
    (Addis Abeba university, 1999-05) Mengistu Berhanu; Haile Tigistu (PhD)
    In this work combined Vet1ical Electrical Sounding (VES), Dipole-Dipole profiling and magnetic surveys are carried out around the Boku thermal area near Nazareth town, The analysis of the results obtained from the integrated geophysical methods has provided useful infOlmation regarding the electrical characteristics of the subsurface, which is related to the thickness and depth ranges of the different subsurface layers. The physical propelty distribution, which is obtained by these surveys, is analysed with pal1icular significance for thermal water and structural controls. Nine Vel1ical Electrical Sounding (VES) measurements using the Schulumberger configuration along two pm'aBel profiles were calTied out. The field apparent resistivity data is plotted against electrode separation and initial model parameters for each sounding station are obtained using two layer master curves and the auxilimy point charts. For quantitative evaluation of the resistivity soundings, the field data were fed to an iterative computer program, In this program the apparent resistivities obtained as a function of electrode spacings are converted to true resistivities as a function of depth, Using the initial model parameters obtained from curve matching, the computer program iteratively modifies the resistivities to obtain the best fit with the field data for the layer thickness chosen for the model. Based on the inversion, a five-layered geoelectric section is obtained along the first profile except at YES 2, which has got a three-layer em1h stmcture. A unique distribution of subsurface resistivity has been obselved at YES 4 and on the second profile. This unique subsurface resistivity is charactelised by a velY low resistivity value for the large electrode separation. This behaviour may actually be associated with the high conductance of the deep layers. The location of the Baku thelmal centre, which is between the position of YES 5 and 6, has a four and five layered earth stmcture. From the geoelectric section it is expected that a fault zone may be thestructural contact zone, A pattem of velY l~w resis:tivity: V~IJe' IS' observ'ed at' th~ position' of YES2, which is bounded by a velY high resistivity value at the position ofVES 1 and YES 3. A lithological contact zone may be expected around the location of YES 1 and YES 2. The same resistivity difference is observed between YES 5 and YES 6. YES 4 has shown a typical nature, which is characterised, by a velY low resistive zone. To ensure the real elech'ical behaviour of the area an additional geophysical method such as gravity methods may be more valuable. The second profile consists of YES 7,8 and 9 and YES 7 has a seven layered earth sh'ucture, YES 8 has a tlu'ee layered ealth structure and YES 9 had a four layered eatth structure. Along profile 2 the interesting behaviour is the velY low resistive zones for the large electrode separation which indicates the deep structural and geological features of the study area. The dipole- dipole profiling psedosection section has been carded out only along the first profile. Here also, very low resistive zone is investigated around the location of the first segment, which is the position of YES 1, 2 and 3. Also velY low resistive zone is investigated around YES 5 and·6 that is the Boku thelmal centre. Segment two of the profiling pseudosection generally indicates a high resistive zone. The magnetics survey shows generally qualitative information. A velY low magnetic response is observed around the location of YES 1 for the first profile. Along the second profile a velY low magnetic response is observed around the location of YES 2 and YES 5, which might be associated with the fault or fracture in the area. Again profile 3 has shown a velY low magnetic response around the position of VES