Evaluation of resistivity anisotropy of parts of Ijebu Igbo, southwestern, Nigeria using azimuthal resistivity survey (ARS) method
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Date
2014
Journal Title
Journal ISSN
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Publisher
Canadian Center of Science and Education
Abstract
This study was carried out to determine the anisotropic properties of parts of the Southwestern Basement Complex
of Nigeria in Ijebu Igbo, which is underlain by migmatite gneiss and older granites.
The surface water level (SWL) of 107 hand-dug wells were measured to determine the groundwater flow direction
of the area. Thirty Azimuthal Resistivity Survey (ARS) were investigated along four azimuths; 0°, 45°, 90°, and
135° to determine the electrical anisotropy, the trend of fractures and their behavioural pattern with depth. The
quantitative interpretation of the Radial Vertical Electrical Sounding (RVES) curves involved the use of partial
curve matching, and computer iteration using WINRESIST program.
The results of the wells measurement showed that the major directions of groundwater flow pattern of the study
area are N-S and E-W while the NE-SW or NW-SE are less prominent. Geo-electric interpretation of the RVES
data revealed the area to be underlain by three geo-electric layers; Top soil, (160Ωm - 1454Ωm), lateritic Horizon
clayey/sandy clay horizon, (22Ωm - 160Ωm), and weathered/fresh Basement (421Ωm - 3122Ωm) with depth to
fracture systems ranging from 6.3-54m. The reflection coefficient observed ranges from 0.6-0.98. The direction of
electrical anisotropy was derived from the polar plot and lies predominantly at NW-SE and E-W, N-S while the
NE-SW is less prominent. Coefficient of anisotropy ranges between 1.09 and 1.36 with a mean of 1.2. The
apparent anisotropic graphs show the behavioral pattern of the fracture with depth and reveal that the predominant
fractures in the area are closing with depth.
The orientation of geologic structures from the study area showed an E-W, and NW-SE direction and this
corresponds to the directions of the electrical anisotropic polygons. The fractures are known to be complex and in
turn interconnected, hence a fracture controlled flow.
Areas with low bedrock resistivity, thick overburden, presence of two interconnected anisotropy polygon
directions and high co-efficient of anisotropy indicate intense fracturing, thus, a potential sites for the drilling of
water borehole
Description
Keywords
resistivity anisotropy, groundwater flow direction, anisotropy polygon, coefficient of anisotropy, fracture direction