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To get the overall picture of the depth estimates of the area, the radial spectrum and its log
of the aeromagnetic field of the Middle Benue Trough, Nigeria were computed and the
latter plotted against frequency number (Fig. 5). Logarithmic spectra are preferred for the
analysis than linear spectra because of the additive property of the former, which gives
better performance of ensemble average parameters (Spector and Grant 1970) as influences
simply add. The plotted radial spectrum seems to support the possibility of five linear
segments (marked
A
,
B,
C, D
and
E
). The segment marked
E
represents the
spectrumdominated by deep-seated contribution, while the mid-layers marked
D, C
and
B
come from near-surface contribution and other plate sources of intermediate depths. The
low gradient portion (marked
A
) represents two contributions: from near-surface structures
and magnetic terrain effect. From the slopes of the segments at these points it may be
inferred that the depths to corresponding magnetic layers are 20.62 km (portion
E
), 3.54 km
(portion
D
), 1.43 km (portion
C
), 0.73 km (portion
B
) and 0.26 km (portion
A
). Flight height
above mean sea level (m s l) in the survey area is 0.275 km. We suggest that the deep band at
20.62 km is probabily the lowest boundary of static magnetic sources.
4
o
1.00E+007
(a)
1.00E+006
1.00E+005
(b)
1.00E+004
1.00E+003
1.00E+002
1.00E+001
1.00E+006
4
o
1.00E+005
(c)
169
o
1.00E+004
140
o
36
o
56
o
75
o
102
o
1.00E+003
1.00E+002
1.00E+001
0
40
80
120
160
200
Angle (deg)
(A)
Fig. 6. (continued)
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