GEOPHYSICAL INVESTIGATION OF THE ORLE RIVER FRACTURE SYSTEM IN IGARRA TOWNSHIP, SOUTHWESTERN NIGERIA

MUSLIM B AMINU

Abstract


The subsurface structure and propagation geometry of the fracture system controlling the Orle River Channel in the
Igarra Township, Southwestern Nigeria, have been investigated via a multi-method geophysical survey. The goal was
to delineate the nature, distribution, and spatial propagation geometry of the fracture system and evaluate its potential
to serve as storage and distribution features for groundwater within adjoining areas. 2D electrical resistivity, total field
magnetic, and co-planar loop conductivity measurements were collected along four traverses using an ABEM1000
Terrameter unit, a GEMS Magnetometer, and an EM34 Co-planar loop electromagnetic system. Traverses were
established to run across and parallel to the river channel. The observed electrical resistivity field data were inverted
for subsurface 2D resistivity structure using a commercially available 2.5D finite element modelling inversion software.
Magnetic field intensity data and ground conductivity data were presented against station positions. Three subsurface
layers were delineated at the survey site; (1) surficial humus-rich and wet top-soil, (2) a thin poorly developed
weathering layer, and (3) the fresh bedrock which occurs as relatively shallow levels and often outcrops. The River
channel is controlled by multiple fractures usually located at or near the contracts between contrasting rock types.
Fracture dip is usually in the northerly direction but conjugates, dipping southwards also occur in the most northerly
extremes of the imaged fracture system. Upstream the fracture path is wider and along with the imaged overburden, is
juxtaposed northwards of the channel axis suggesting a much broader river channel in the geologic past. Low
aperture fractures imaged tangential to channel axis likely serve to funnel surface and groundwater from the channels
to the surrounding areas. Where such low aperture fractures can be delineated, they offer the best chances of
groundwater abstraction within adjoining areas, particularly during the dry season.


Keywords


Igarra Township, subsurface resistivity structure, fracture geometry, bedrock structure, ground electrical conductivity.

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