Geophysics,Space Science and Astronomy(IGSSA)

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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
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.
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.
Application of Electrical Resistivity Method for Ground Water Exploration in South Western Butajira Area, Guraghe Zone
(Addis Ababa University, 1997-05) Mebrahtu, Bereket; Hale, Tigistu
The basic mathematical fowldation 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 . Sci umberger Configuration. The theoretical background for resistivity inversion with the ridge regression technique has been explained following the method of IIIDlan. On the basis of the above theoretical consideration the electrical resistivity method has been applied for ground water exploration and possible site for borehole location in South Western Butajira Area, Southern Ethiopia. To do th is survey along three profi le lines were taken with a total of 20 YES points out of which 15 were analyzed in detail.
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
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
Application of Integrated Geophysical Methods for Graphite Exploration in Gara-Gedemsa Area (Moy Ale, Southern Ethiopia
(Addis Abeba university, 1997-12) Jaleta Alemna; Mewa Getnet (PhD)
The thesis research was aimed at stud)ing the theoretical and practical aspects of the Radiometric, 'vIagnetic, Electromagnetic, Induced Polarization (IP) and Resistivity methods and apply the integrated methods for graphite exploration in the Gara Gedemsa area. TIle physical plinciples and mathematical foundations, fields of applications and Plinciples of data acquisition and interpretation of each method are studied in detail. Integrated geophysical survey, comprising the Radiometric, Magnetic, GENlE-EM, IP and Resistivity methods, was conducted in the Gara Gedemsa area to map the lateral and vertical distributions of the graphitic mineralization. The area is located in Southern Ethiopia some 6 km northlv<;:st of ;Vloyale to\Yn. Geologically, tllis area is mostly covered by mafic-ultramafic rocks (talc-tremolite. talc-tremolite-actinolite and ampllibole schists), whereas quartzite, quartz-utica sellist and pegmatite veins that are closely distributed \\1th the graphite scllists are found to have rebtive1y smaller di,tribution. The geophysical survey results have delineated one major grapltitic zone at the southern margin of th" survey gJld with shike length more than 2 km along W30~ and an average 1\ idth \'arying from 25m to 50 m. From the pole-dipole IP, Resislh lty and GENlE amplitude ratio data. tlus gJ'aphitic body is found to be dipping towards not1h at about 55° and extends to a depth of not kss than 50 m. TIus grapltitic zone is almost equally demarcated by ,Ill methods employed for lh~ SUl'\"~'. HO\\ever. the GE:-"1E-E:\I survey with the 50 III transnlitler-receiver spacing has shOI\n better resolution in di~criminaling the paralld gJ'aplulic lenses closely separated by the nonconducth'e quartz-utica schists . .-\ less prominent gJ'apltitic zone is outlined at the central part of the gJid, and from exploration point of view, it deserves less interest due to its smaller lateral distlibution and low anomaly-to-backgJ'ound contrast. Based on the geophysical survey results, a generahled physicogeological model of the Gara Gedemsa gJ'apllite OCCUITence is generated. Moreover, two bore holes are recommended over the main gJ'apllitic zone in order to corul!:m its depth extension and then:by evaluate the econoutic significance of the deposit
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.
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.
Engineering Geological Characterization of Gerbi Dam Site, Reservoir and Catchment Area
(Addis Ababa University, 2004-06) Gashaw, Rediet; Ayenew, Dr. Tenalem
Constructions of dams require the input of thorough engineering geological investigation. To fully understand and characterize the soils and rocks of the study area a detailed investigation has been carried out taking into account many disciplines such as geomorphological, geological, hydrogeological and geophysical investigations.
Engineering Geological Study of Omorati Area
(Addis Ababa University, 1990-06) Mehari, Germaye; Olsz ewska, Prof . B. Grabowska
Engineering Geological Study
Engineering Geological Study of the Proposed Entoto Tunnel
(Addis Ababa University, 1999-06) Wolderfuel, Girma; Dr. Tenalem, Ayenew
For the devolopmant of Addis Ababa city' s water supply, the re needs to des ign a project of constru cting IwO darns and a 4.8 kill transmission tunnel through the Entoto ridge, by the Add is Ababa Water and Sewrage Authority (AA WSA
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.
GIS Based Groundwater Vulnerability Assessment in Akaki River Catchment, Addis Ababa (Central Ethiopia)
(Addis Ababa, 2003-11) Nigussa, Dereje; Ayenew, Tenalem (PhD)
The studied area, Akaki River Catchment, is Iocaled at tile headwaters of Ihe Awash River Basin. The Int010 Mountain Ranges formed by Ambo-Kassam regional fault form Ihe hydrographic divide between Awash and Blue Nile Rivers. The Akaki River Catchment has an 3pp1'oximale surface area 01 aboull464 km2 and perimeter 01216 km, and approximately bounded between SO 46' 5T' and go 13' 00" N and JaG 35' 00" and 39005' OO"E. It lias an elevation range of less than 2040 to 3,200 m above mean sea level (amsl). The studied area is made up of different volcanic rocks ranging in age from Ouaternary to Miocene, induding recent alluviaillesidual deposits. There are also regional and local faults. Differenllype 01 day soil and lac\Jslrine deposits formed from Ihe weathering 01 volcanic rocks are the dominant soil type that characterizes Ihe area. AI some localities these clay soil are either Ihin or totally absent. The mean yearly rainfall depth of the Akaki River Catchment was 1150 mm (from 1964• 1998). The arithmetic mean monthly rainfall ranges from 6 mm (in December) to 276mm (in August) for the three stations (see chapter 2) in too catchment for 35 years. The highest and icMtest meilfl monthly maximum temperature occurs in months 01 March (25 Oc) and the lowest is in the month of August (20 Oc), while the minimum mean monthly values were 8 Oc (in December) and 12 Dc ~n March) for Addis Ababa Observatory from 1951- 1998 respectively. The calculated mean annual temperature was 16 Dc. Studies show both surface and groundwater pollution iD the Akaki River Catchment; and associated it to the following major sources: industrial activities, agricultural practices, munidpall domesticl wastes, fuel stations, garages, and health centers and also to graveyard (cemeteries), quarry mining, and market areas (see chapter 4). In this chapter an accotJnt was made on possible potential pollution sources. The over view include general potential pdlutionfcontamination sources and existing potentiaigrourKtwater polluting sources identified by earlier wor1c:s in the studied area. To see spatial distribution oIgfoundwater contamination situation in the catchment the discussion was supported by groundwater quality analysis data at some selected sites in the catchment extracted from recent study. For these purpose three impol1ant groundwater quality indicators (CI, NOl, and TOS) was seleded. The population density of Addis Ababa and its environs vary from more than 400 persons per hectare to less than 25 persons per hedare. The existing land use pattems in the catchment were broadly divided into four groups as: agriOJlture/open area (69%), forest land (1 5%), urban area &its associated lISes (15%) and water body and wetland covers (0.98%) 01 the total land use/cover. The general objective of the study is to identify and map the aquifer vulnerability to pollution in the Maki River Catchment by using an empirical modeIIsystem known as DRASTIC (Aller et. ai, 1987; Evans and Meyers, 1990) 10 assess relative groundwater pollution susceplibility using hydrogeologic factors (intrinsic ralher Ihan SpecifIC or integrated approach) was adopted, with GIS there by to prepare vulnerability index maps. One of the Specific objectives of this thesis was the application or use for the first lime in Ethiopia 01 an index method of aquifer vulnerability assessment with Geograp/lic Information System (GIS). Practically and academically, the research is of interest since there was no earlier comprehensive study concerned with groundwater conlaminatiol1 proIfJdiol1. Thus, to supplement the policy makers on grovndwater rasource management and protectioo in this catchment, there must be map-based informatiol1 that indicates spatial distribution of relatively vulnerable areas that is useful in major aspects of planning (chapter 1). YVhere as, previous researchers dealt with general hydrogeology, hydrOChemistry, pollution condition, etc. in this catchment. Basic concepts and over views/definitions 01 groundwater aquifer vutnerabiUly mapping was given in chapter 5 as it is necessary to make de
The Gravity Field of the Southern Ethiopian Rift Systems Between 3.30 To 7.150 Latitude and 360 To 420 Longitude
(Addis Ababa University, 1997-05) Asrat, Estianos; Alemu, Abera (PhD)
1800 gravity data covering the research a rca are reduce to Bouguer and free air values and interpreted based on the Bouguer anomaly map and a pair of short wave length (SWL, residual) and long wave length (LWL, regional) anomaly maps. All data are reduced to sea level wth uniform density of 2.67 glcm'- Effect of Bouguer masses were calcu lated applying the simple Bouguer corrections. Terrain correction "vas not applied. The Bouguer anomly map of the rift zone between 3.3° N to 7.15° Nand 36° E to 42° E is presented. The geologic and tectonic setting of the area is di scussed based on the compilation of gravity data and synthesis of publi shed papers. A positive Bouguer gravity anomaly of - 165 mGal occur over the rift floor of the Abaya - Chamo rift and is flanked by negative anomalies that res ide on its shoulders. Gravity anomaly of magnitude -135 mGal is observed over the Chew Bahir ri ft floor and flanked by relative positive anomalies on its shoulders, which is different from the MER. This could be due to lacustrine deposits of the rift floor. The free - air anomaly map is si mply relatcd to topographic relief. The uplifted blocks and horsts are characterised by high free - air gravity anomaly and depressions defined by free - air minimum (-40 to - 75 mGal ). The transition from grabens to horsts is marked by sharp free - air gravity gradients (0 to +40 mGal). After removal of the long wave length components of the Bouguer gravity anomaly the residual gravity anomaly is interpreted. The residual gravity field exhibits rapid changes of gravity gradients over short distance appears to show the presence of fa ults which bound major rift basins. Anomalous areas are corelated to their respective geologic in te rpretation. The regional gravity anomaly map produced by trending analysis shows the deep structural features of the study area. It shows the transition from negative to positive gravity fie ld from MER to SER and negative towards the Kenyan rift indicating the distinction between the SER and the other two rift systems (MER and Kenyan rifts).
Gravity Field Studies In fuel Main Ethiopian Rift and Southern Afar
(Addis Abeba university, 1997-06) ZemIchael Lijam; Alemu Abera (PhD)
All available data, over 3000 stations, obtained from the EIGS and the Geophysical observatory (Dr. Abera's Ph.D work) were reduced to sea level with a uniform crustal density of 2.67 gmicm3 • Effects of Bouguer masses were calculated applying the simple Bouguer cOITection. Terrain correction was not applied in which case its effect was treated as systematic error in computing the over all mean square elTor of the simple Bouguer anomalies at each station. Theoretical gra\·ity field was computed by means of the intemational gravity fonnula of 1967 (GRS67) and tied to the IGS};71. The accuracy of the Bouguer anomaly at each station is calculated to about ± 2.5 mga!. The regional and residual gravity field were calculated from the Bouguer anomaly map by mea~ of a computer software. The compiled Bouguer anomaly map shows a strong con'elation between the regional pattern and topographic feature of the survey area. Owing to higher elevations, the adjacent plateaus are marked by negative Bou"ouer anomalies. the minimum of magnitude -270 mgals being located around Debreberhan for the study area. The maximu'm gravity values occur in the Afar de]Jression conesponding to the relatively lower elevations. TIle main Ethiopian rift including Afar is characterized by a relatively positive anomaly as compared to the plateaus. The high on the rift accounts for the excess mass at depth on which the crust lies relative to the plateaus where a mass deficiency is assumed. Gravity values on the main Ethiopian rift are generally less than those in the Afar. TIils accounts for the th. inning of crust material towards the .-\far along the rift, or it can equall.y be explained as by a thickening of a denser material beneath the crust (probably the anomalous mantle). The margins separating the rift £i'om the plateaus are marked by steep ~ gra\ity gradients with their magnitude' representing the slope of the escarpment. Steep gradients characterize the transition of gra\.ity fidd from the plateaus to the rift floor
Lake Sediment Records of Environmental and Climatic Changes from Northern Ethiopia (Lake Ashange)
(Addis Ababa, Ethiopia, 2005) Kubsa, Zelalem; Umer, Mohammed (PhD)
Lacustrine sediments can be used to study the palaeoenvironmental evolution of a particular catchment. This is because they reflect developments in the Lake Ecosystem and productivity as weU as changes in the rate and type of processes of the catchment such as weathering, erosion and sediment transport. Moreover, they are often deposited in undisturbed sedimentary environments and give time-integrated infonnauon on the evolution of the basin and its surroundings. The aim of the present work is to contribute to the understanding of the Holocene evol ution of climate variability and environmental changes due to both climatic and human causes in Northern Ethiopia from the sediments of lake cores and shorel ines as well as from sections collected in and around Lake Ashange in the year 2003 under the Lake Tana project. The studied core is dominantly organic and measures 8.1 m and is recovered under 9m of water depth. The basal radiocarbon age shows a date of 11 ,920 ± 40 years BP. More radiocarbon dates, from samples corresponding to important shifts in the studied proxies, are under analysis. Because of this, linear extrapolation of age versus sediment depth has been applied in this study by considering the top of the core as modem. Sediments were also analyzed for lithology. magnetic susceptibil ity, X-ray diffraction [(XRD) Mineralogy] and X-ray fluorescence [(XRF) (Elemenllil analysis)]. Moreover water samples were collected for chemical analysis and secondary information was also gathered from hydrometeorological data as well as from aerial photographs of 1965, 1980 and 1986. The integrated analyses gave the first evidences of continuous and high resolution climatic and environmental changes for the late Pleistocene and Holocene of Northern Ethiopia. The core was subdivided into four zones corresponding to important changes relative to the studied parameters.
Velocity Structure of the Upper Crust Across the Main Ethiopian Rift
(Addis Ababa,University, 2004-06) Melak, Mehari; Mammo, Tilahun (PhD)
Refraction t~e is one of the geophysical investigation methods used 10 map the subsurface layers based upon the velocity variation of the seismic wave as it propagates through different geological materials. As a part of the Ethiopia Afar Geoscientfic Lithospheric Experiment (EAGLE) project conducted in January 200\ the detail study of the crustal seismic P-waves has been carried out to produce the seismic model down to the depth of 10 la)1 ·from Chefodonsa (E5 13296, N99264 10 to Kula (E575823, N886441) for about 120Km profile length across the Main Ethiopian Rift. Five shot points with inter shot separation of about 46Km were used as seismic source and the data were recorded on 120 geophones deployed in the study area at I km spacing. The first arrival times were picked, processed and inverted using the generalized reciprocal method (ORM); and gave the four major seismic layers up to the depth of 10 km across the rift. There is a steady increase in P-wave veloc ity from about 3000mls in the first layer to about 6365m1s in the last layer. Lateral Velocity vari ations are also observed in the layers .. Attempts have been made to correlate the seismic layers with the known geological fonnations. Boarder faults and other major faults have been clearly identified.
Velocity Structure of the Upper Crust Beneath The Northern Main Ethiopian Rift
(Addis Abeba university, 2005-06) Kassie, Gennetu; Nurie, Kimemu
Seismic and geologic sections are of the vital importance to the various professional bodies, institutions and industries. They give us an insight on the grounds we are literally standing on. This study has conducted the surface imaging of the earth to a depth of 10 ~--:,----'~"---."'- km on a 92.5 km long seismic profile line. The study was conducted starting from the northern most of the Main Ethiopian Rift to the southern most of Afar to give us results, which are archetype to the study area. For the generation of seismic waves explosives were detonated inside boreholes and programmable seismic energy recorders called "Texans" were j_epl()yed on the sutface at about one kilometer spacing. Reduced first arrival P-waves were picked and converted to actual time prior to inversion. The geo-seismic model was inverted by the methods of Generalized Reciprocal Method (GRM). Four layers at an avcrage depth of 2 km, 4.5 km, and 8.5 km were identified with corresponding average P-wav~ velocities of2875 mis, 4000 lnlS, 5700lnlS, and 6400 mls ~,-~----- ~--::-~ ~~-"-'-'-- .-----"-'--., respectively. The geological section of the studied area has been identified to rest on a clystalline basement overlain by Meta-sediments and Meta-volcanic rocks. The first layer has been identified as Miocene-qpaternary sediments; Cenozoic volcanic flows

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