Application of Integer a Ted Geophysical Methods For the Evaluation of Thermal Centers and Their Structural Controls in Boku ,Nazareth, Main Ethiopian Rift
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Date
1999-05
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Addis Abeba university
Abstract
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
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Keywords
Thermal Centers