TRENDS OF STRUCTURES IN THE SOKOTO BASIN OF NORTH-WESTERN NIGERIA FROM THE INTERPRETATION OF AEROMAGNETIC ANOMALIES.
TRENDS OF STRUCTURES IN THE SOKOTO BASIN OF NORTH-WESTERN NIGERIA FROM THE INTERPRETATION OF AEROMAGNETIC ANOMALIES.
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Date
2000-03
Authors
ISHAKU, ROGER KENYUY
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Abstract
The total magnetic field data for the Sokoto Basin was
obtained from aeromagnetic maps each of size 55 km x 55 km
within the Basin. A qualitative study of the structural
trends in the total field map reveals an alignment of
anomalous zones in approximately linear patterns oriented in
the N-S, NNE-SSW and NW-SE directions. The residual field map
and the regional field were obtained by the method of
polynomial fitting using the least square technique. The
residual anomalies in the study area change little from the
anomalies of the observed total field The residual field was further analyzed by computing the
zero contours of the second vertical derivative. These zero
contours delineate the magnetic boundaries in the subsurface
structures. The elongation of these contours are mostly in
the NE-SW and E-W directions, though broken off at shorter
distances in some sections.
The method of crosscorrelatidn was used to determine the
structural trends and alignments for the twenty maps covering
the area of study. The main trends for most of the maps are
in the ENE-WSW and E-W directions with a moderately high
degree of correlation over data values in the northern
section of the area as opposed to the southwestern section
where a low degree of correlation is common. Spectral analysis of the residual magnetic field over
forty eight overlapping blocks of size 55 km x 55 km was
carried out. Two prominent magnetic layers were detected. The
results reveal that the depths to the first layer vary from
V 0 . 9 km to 2 .2 km with an average depth of 1.4 km. The second
layer depths vary from 2.2 km to 6.9 km with an average depth
of 4.3 km. The third layer magnetic sources are not uniformly
distributed. The first magnetic layer is attributed to
magnetic rocks intruded on the basement surface. The second
magnetic sources are considered to arise from magnetic rocks
within the basement which could include fractures and faults.
The depths to the basement map show that the magnetic basement beneath the sediments in the basin is generally
-V shallow with the highest depth of 2.2 km recorded in the
northwest corner of the study area. There are a series of
ridges and troughs on the basement surface suggesting that
the subsurface rocks must have undergone gentle folding.
All the analyses carried out in the study confirm the
fact that major structures are oriented in the NE-SW
directions. However minor features could be trending in
different directions as indicated in a few sections in the
crosscorrelat.ion analyses.
Description
A thesis submitted to the Ahmadu Bello University,Zaria , in partial fulfilment of the
requirements for the award of the Degree of Master of
Science in Applied Geophysics.Department of Physics
Faculty of Science
Ahmadu Bello University
Zaria, Nigeria
Keywords
TRENDS,, STRUCTURES,, SOKOTO BASIN,, NORTH-WESTERN NIGERIA,, INTERPRETATION,, AEROMAGNETIC ANOMALIES,