Download GEOELECTRIC STRUCTURE AND LITHOLOGY BENEATH THE

GEOELECTRIC STRUCTURE AND LITHOLOGY BENEATH THE
OLOBANITA WELL-FIELD, KENYA-RIFT
ABSTRACT
A study review on the geology within Olobanita area which approximately covers
an area of about 400 sq.km was carried out whereby volcanic sediments of the
quaternary-tertiary age were found to dominantly cover most of the areas. The
study area is within the Phanerozoic basin in the Kenyan rift which is characterized
by highly tectonic events that lead to rise of fault zones. These faults cause aquifer
discontinuity hence a need for a keen study on the subsurface zones before
borehole drilling, dam construction or even settlement.
The resistivity results acquisition and processing was reviewed and Schlumberger
array was found to be more effective when dealing with zones of high resistivity
and shallow in nature. Twenty eight (28) vertical electrical soundings (VES) were
conducted within the area of study using the Schlumberger array and were
visualized in a three-dimensional (3D) format which represented the geoelectric
structure beneath Olbanita well field to a depth of 350 M. The 3D presentation is
of more significant value since it allows one to appreciate how the resistivity varies
in all the three directions of the Cartesian coordinate system. This was attained by
use of a commercial interpretation package, Visual Data 3D visualization software
from GraphNow Inc. This study focuses mainly on application of VES method
aimed in mapping out the subsurface resistivity, which is then interpreted so as to
provide information such as fluid filled fractures within the targeted site.
Resistivity results obtained from Olobanita area revealed a shallow high resistivity
zone close to the surface which is a result of unsaturated rocks and a low resistivity
zone to depths of up to 350M which was inferred to show the presence of
aquiferious zones where alteration by water has lowered the resistivity of the rocks.
The aquiferous zones constitute the weathered and/or the fractured volcanic rocks.
Genrally the lithology of the area can be described by the presence of unsaturated
terrestrial deposits and the Menengai volcanic at the top hence the reason for high
ground resistance to the electrical sounding at these stratas. Further down the rocks
can be associated with fracturing hence having the capability to store groundwater.
Basaltic stata from the laikipian can be described by the systematic decrease of the
resistivity downward. Basaltic sediments have resulted from erosion of the
Simbara series and have been deposition on the Laikipian plain. These basalts
overlies the Bahati and Kinagop tuffs (Nyeri tuffs) which in turn overlies basaltic
agglomerates and autobreccias. The exact lithology can’t be driven from the
interpretation of the resistivity results due to the complexity nature of geology, but
this can only be attained after drilling and examination of samples.