Geoscience Reference
In-Depth Information
The angular and radial spectra of the residual total field magnetic map (Fig. 4) were
computed. First, for detailed delineation of angular features, the map was divided into two
sub-areas: north/south or upper/lower. The upper part (enclosing nearly 90% of the
sediments) has a size of 128x128 and the lower portion has a size of 57x120. It is
computationally efficient to use the Fast Fourier Transform (FFT) algorithm to accomplish
the analysis of the angular and radial spectra. The upper part of the map (128x128) is
already amenable for FFT application since the matrix size is already 2
N
, where N is an
integer. The lower part however, is cosine tapered and then padded with sufficient zeros to
make data matrix amenable for FFT application. This resulted into another data of 128x128.
Next, the averaging was carried out over three frequency bands (1-20, 20-40 and 40-60
frequency numbers) for each data matrix. This division represents the low frequency band
(1-20), mid frequency band (20-40) and high frequency band (40-60), noting that the highest
frequency number in the data is 64. The angular spectra for the three sub-bands in each sub-
area of the map are shown (Figs. 6a and b). Figure 6a shows correspondences between the
three spectra indicating peaks at 4
o
, 36
o
, 56
o
, 75
o
, 102
o
, 140
o
and 169
o
; where the first peak (4
o
)
appears in the three sub-bands, while the other six peaks appear in the mid and high
frequency bands. A similar pattern is obtained in the lower part of the map area where also
4
o
peak appears in the three sub-bands (Fig. 6b) and prominent peaks at 12
o
and 157
o
at the
mid and high frequency sub-bands amid a subdued peak at 149
o
.
24
20
E (Depth = 20.62 km)
16
D (Depth = 3.54 km)
12
C (Depth = 1.43 km)
8
B
(Depth = 0.73 km)
A (Depth = 0.26 km)
4
0
20
40
60
80
100
Fig. 5. Radial spectrum plotted against frequency number of the total field magnetic
anomaly over the Middle Benue Trough, Nigeria. Five linear segments are marked (A, B, C,
D, E). Estimates of depths from slopes are indicated.
The 4
o
peak appearing in the three sub-bands corresponds to the value of the inclination of
the Earth's magnetic field in the area. The peak at 56
o
corresponds to the general trend of the
Benue Trough (N56E). Other angular values have been fully mapped in the area (Benkhelil,
1982, 1989; Likkason, 2005).
Radial Frequency Number
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