Geophysical investigation of remotely sensed suspected fault Zones around iwaraja-ijebu ijesa area of osun state, nigeria.
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Date
2015
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Abstract
Magnetic profiling and electrical resistivity 2-D Dipole Dipole imaging and 1-D Vertical
Electrical Sounding (VES) were carried out over two suspected remotely sensed delineated mega
faults in the area around Iwaraja – Ijebu Ijesa in Osun State, Nigeria. This was with a view to
establishing the existence of the mega structures.
Four geophysical traverses varying in length from 1 to 4.55 km, were established across the
suspected fault zones. Total field magnetic measurements were made at 10 m interval along each
of the traverses and the corresponding base stations. The measurements were corrected for
diurnal variation and offset using standard methods. The residual magnetic profiles were
qualitatively interpreted and semi-quantitatively inverted into 2-D subsurface images. Dipole-
Dipole resistivity profiling was carried out along profiles with lengths varying from 520 – 600 m,
constrained by the magnetic model. A dipole length of 20 m and inter dipole expansion factor (n)
varying from 1 to 5 were adopted. The Dipole Dipole data were subsequently inverted into 2-D
resistivity structures. The VES involving the Schlumberger array were carried out along the
Dipole Dipole profiles. The VES sounding curves were quantitatively interpreted using the
partial curve matching technique and computer assisted 1-D forward modeling. The
interpretation results were used to generate geoelectric sections. The magnetic profile and model;
2-D resistivity structure and geoelectric section were synthesized along each investigated
traverse and used to establish the existence of the suspected fault zones.
Five VES type curves comprising HA, KH, AKH, HKH and HKHKH were obtained. Five
geologic layers comprising the topsoil, laterite, weathered layer, weathered/fractured basement
and fresh basement were delineated. Synthesized images along Traverse 1 (TR 1b) showed an undulating magnetic basement relief, overburden thicknesses varying from 3.0 to 50.0 m and
signatures of steeply dipping suspected fault zone, centered around distance 460 m and about
100 m wide. The synthesized images along a segment of Traverse 2 (TR 2a) showed gently
undulating basement relief; overburden thicknesses varying from 1 to 20 m and characteristic
feature of a steeply dipping suspected fault zone with center at around 1335 m and width of
about 60 – 200 m. Over another segment of Traverse 2 (TR 2b), the synthesized images
delineated overburden thicknesses varying from 20 – 90 m and a suspected steeply dipping fault
centered at 4460 m and about 200 m wide. Along a segment of Traverse 3 (TR 3), the magnetic
model delineated overburden thicknesses varying from 25 – 30 m and a diagnostic signature of a
typical fault zone centered at around distance 2160 and about 90 m wide. The correlated centers
of the geophysically delineated fault zones A' and B' along Traverses TR 3 and 2b and TR 1b
and 2a respectively were shifted northward away from the corresponding centers of the remote
sensing delineated fault zones A and B.
The study concluded that the remotely sensed delineated suspected mega structures A and B
existed but were spatially shifted north of their present locations.
Description
136p
Keywords
Iwaraja, Osun State, Nigeria, Magnetic, geophysical