The data obtained from ground magnetic survey was qualitatively analyzed. The Quantitative interpretation revealed anomalous sources with an average depth of 5.0-25.0 meters, beneficial for engineering. This also involved magnetic profiles, contour maps, and surface maps.
Qualitative interpretation:
Magnetic Contour maps:
Grid no 1 (Spina Bandai)
This grid covers a total of 1800 square meter area and trends in North South direction while 1717 magnetic observations were recorded across the grid. The 2D contour map and 3D surface map of Spina Bandai area was produced using the residual magnetic intensity values as shown in Fig.2. The variation in the total magnetic intensity across the grid ranges from -190 to 40 nT. is a function of variation in lithology, change in depth, degree of strike, difference in magnetic susceptibility, dip and plunge.
The legend is the rectangular shape attach to the map with different color, it gives information about the map. The area is marked by the high (Red Color) and low (Deep blue) magnetic signatures. In Fig.2. high magnetic susceptibility values with closely packed contour lines are present towards the Southern East side of the study area. This shows the presence of thin overburden or near-surface iron bearing sedimentary rocks and can serve as a competent zone for engineering/building purposes. Low magnetic intensity anomalies forming elliptical closures are also present in the Eastern and South East corner of the map and are interpreted as non-magnetic source rocks near the surface or local fractures/cavities. While dispatched contour lines are experienced towards the Northern, Western and Central region of the study area. This suspect the presence of diamagnetic minerals like rock salt, gypsum or deeper cavity/fracture formed due to dissolution of salt in the region and should be avoided for construction purposes and perhaps explore for groundwater prospects especially for the industries that uses water for the production of their products.
Grid no 2 (Bahadur-Khel 01)
The Bahadur-Khel grid 01 covers a total of 600 square meter area and trends in North-South direction while 1094 magnetic measurements were recorded across the grid. The total magnetic intensity across the grid ranges from -38 t0 -34 nT. The North South trending low magnetic anomalies dominated the entire grid area (Fig. 3). and it depicts areas of thicker sediments or deeper lying magnetic bodies. The low magnetic anomaly contours are characterized by the presence of elliptical closure and nosing which are closely spaced, linear and sub-parallel in orientation and are suspected to be due to the presence of diamagnetic minerals e.g. halite, anhydrite or fault, local fracture/subsurface cavities formed due to the dissolution of salt so the area should be avoided for construction of buildings. It further shows that the investigated area is a promising area for hydrogeological prospect because of low magnetic intensities.
Grid no 3 (Nari Panos)
The Nari Panos grid covers a total of 4000 square meter area and trends in North South direction. A total of 2907 magnetic observations were recorded across the grid. The said grid generally shows high and low magnetic values from -100 to 76 nT depicting magnetic heterogeneity of the area. In Fig. 4, towards the Eastern side where the contour lines forms elliptical closure are closely spaced and represent steep gradient or change in value which is the zone with higher magnetic susceptibility and can host high building as thin overburden and presence of magnetic sedimentary rocks could serve as a strong foundation for them. Low magnetic intensity anomalies forming elliptical closures are also present in the North East and South East side of the map are interpreted as non-magnetic source rocks near the surface while widely spaced contour lines experienced in the Western, Central and Southern region of the study area are indication of relatively thick overburden or deep lying source bodies and are interpreted as non-magnetic minerals or fault, deeper cavity/fracture formed due to dissolution of evaporite in the region. This zone will be dangerous for the construction of high-rise building and therefore should be avoided for construction purposes. Further, the area may be of hydro-geological importance due to low magnetic intensities.
Grid no 4 (Bahadur-Khel 02)
The Bahadur-Khel 02 grid covers a total of 1200 square meter area and trends in North-South direction and a total of 898 magnetic measurements were recorded on the grid. The total magnetic intensity values ranges from -42 to -6 gamma. The 2D contour map and 3D surface map of the area was produced using the residual magnetic intensity values as shown in fig. 5. The East West trending negative magnetic anomalies dominated the entire grid area and it depict areas of thicker sediments or deep source bodies. Some localized low magnetic lenticular anomalies of limited extension in the Western, Central and Eastern part of the map are suspected to be due to near surface nonmagnetic source rocks or local fractures/cavities while the dispatched contour lines present in the Northern and Southern region of the map are indication of relatively thick overburden or deep source bodies and are suspected to be due to the presence of diamagnetic minerals e.g. rock salt or faults, subsurface cavities/fractures and are considered to be weak zone for construction purposes. Furthermore, the area should be investigated for hydrogeological purpose because of low magnetic intensities.
Reduction to Pole
Magnetic anomalies should be reduced to the pole to remove the asymmetry due to inclination of magnetic field. The procedure is nothing more than a correction factor applied across the study area to remove the non-vertical magnetic component (causative body) in its correct spatial position. Reduction to the pole (RTP) transforms an observed TMI anomaly into an anomaly that would be measured at the north magnetic pole (David and Oliver, 1998). The RTP maps for Spina Banda, Bahadur-Khel 01, Nari Panos, and Bahadur-Khel 02 (Fig. 6a-d) were produced with angle of inclination of 51. They indicate a small shift of the anomalies and tends to align themselves vertical towards North relative to the Residual magnetic anomaly maps. The vertical alignment enhanced overview and interpretation of the anomalous magnetic anomalies.
Vertical Gradient (Derivative)
The Vertical gradient filtering process were also applied which is physically equivalent to measuring the magnetic field simultaneously at two points vertically above each other, subtracting the data and dividing the result by the vertical spatial separation of the measurement points (Labbo and Ugodulunwa, 2007). The vertical gradient (derivative) usually enhance the effects of shallow/local anomaly at the expense of deep ones and sharpens the edges/boundaries of anomalies. Vertical magnetic gradient maps were produced from RTP maps for Spina Banda, Bahaur-Khel 01, Nari Panos, and Bahadur-Khel 02 (Fig. 7a-d) which indicate that the anomaly sources are shallow.
Upward Continuation
Upward Continuation is a technique that project data taken at an elevation to a higher elevation. The effect is that short wavelength features are smoothed out because one is moving away from the anomaly. This function enhances deep and low frequency features while diminishing the shallow and high frequency sources (Gunn., et al 1997). RTP maps of Spina Banda, Nari Panos Bahadur-Khel 01 and Bahadur-Khel 02 continued up to 25 meter (Fig. 8a-d). The Shallow features disappeared on these maps while deep sources are evident. Both the vertical gradient and upward continuation maps shows that all anomaly sources are shallow and extends from near surface to 25 meters.
In comparison to the global magnetic model, the study region investigated for the aim of engineering site characterization yielded favorable results with the international geomagnetic reference model, as shown in the table below (Table no 1). The area displays negative anomalies (readings), which are dominated by the salt. These locations include BahadurKhel, Banda Spina, the Flank regions of Nari Panos, and the Karak region. While the synclinal part of the Nari Panos and other extremities had greater values reported in the field, this was owing to the movement of the salt component of the Eocene evaporites to the anticlinal portions of the examined area due to diaperic effect.
Table 1 Values Recorded in Field vs Observed by IGFRF Model
Sno
|
Coordinates
|
Recorded Value
|
IGRF Model
|
1.
|
33 15.814N 70 97.238E
|
48735nT
|
48534.1nT
|
2.
|
33 11.981N 70 08.056E
|
48660nT
|
48628.7nT
|
3.
|
33 11.984N 71 08.390E
|
49845nT
|
49914.3nT
|
4.
|
33 12.716N 71 05.140E
|
49989nT
|
50217.2nT
|