Characterization of Major Subsurface Structures in Ada’a Plain Using Intergated Geophysical Survey
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Date
2007-02
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Addis Ababa Universty
Abstract
This manuscript presents the results of geophysical data analysis from
Ada'a plane with a primary objective of characterizing major
subsurface structures and their connection to the subsurface
hydraulic system. Towards this end, verifying a barrier, inferred
through preliminary interpretations and conceptualizations of the
ground water system in the plane by Water Works Design and
Supervision Enterprise (WWDSE), was the primary task of this work.
Ada'a ground water is located in the southern Awash Basin, Oromiya
Regional State, between geographical coordinates of 38050'E - 39015'E
and 8030'N - 8045'N.
The geophysical data set employed in this project includes: the
potential field methods namely Gravity and Magnetics and are used to
investigate major subsurface structures. The other method found
adequate for this study is the electrical resistivity method as it is used
to evaluate ground water potentials.
Regional and residual Bougeur anomaly contour maps revealed large
scale regional subsurface structures that align along the rift and small
scale, local subsurface structures showing different orientation. The
residual feature with a discordant structural alignment shows spatial
correlation with the expected subsurface barrier. The total field
magnetic contour map also confirms this indication and gave further
evidence for the existence of such anomalous feature in the area of
interest A depth-slice stack map of apparent resistivity of fifty VES data also
manifested a subsurface feature that coincide to the anomaly revealed
by the residual gravity map. Deep, low apparent resistivity responses,
which can safely be interpreted as water saturated, aquifer zones, are
separated by a high resistivity, elongated feature. This feature could
represent an impermeable dry rock units related to domes and cones
of intermediate magma that can possibly impede the ground water
movement.
Inversion of 2D electrical resistivity imaging data gave a clear
subsurface picture and depicted the geo-electric substratum, which is
a resistive dry bottom, to rise above the expected water table at the
speculated region. Though it falls short to cover the lateral extension
of the inferred barrier, it presented strong evidence that supports
findings from the other methods. A 1D apparent resistivity pseudosection
along VESs 54,55,56,58 and 59 reflect the geologic reality as
they traverse across a high resistive formation where the barrier is
expected lying and complement the results of the 2D analysis
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Keywords
Ada’a Plain