## 4 Materials and Method
### Dataset
#### 4.1.2 Aeromagnetic Data  
Aeromagnetic data over the survey area was also captured alongside Air-FTG(r) and the data is utilized to screen FTG anomaly patterns and quickly detect igneous, volcanic or metamorphic source from the responses in the subsurface. Magnetic data usually helps to image the main aspects of the geology across much of the Earth's crust (including the surface) and provide a cornerstone for geological model's construction [46]. An important caution over application of magnetic data is the strength of the Earth's magnetic field varies across the globe, with strongest at the poles and weakest in the equatorial regions (Figure 3B). A DAARC500 magnetometer system comprises the airborne magnetometer and a base station were mounted in a boom (stinger) of the Basler turbo BT-67 aircraft and collects compensation parameters while flying a figure of merit (FOM). Aeromagnetic data were recorded at the same time of the airborne gravity data at 10 Hz. Since the study area is close to the equator, the magnetic field lines are roughly parallel to the Earth's surface, thus the Geometrics 882 caesium vapor sensor is only sensitive in some headings [41]. The aeromagnetic data then underwent (i) Removal of Earth's regional magnetic field, (ii) Removal of magnetic diurnal drift, (iii) Final line levelling, and finally (iv) De-meaning [41]. The magnetic response from the Air-FTG(r) was analyzed based on the Total Magnetic Intensity (TMI) that had been Reduced to Equator (RTE) (Table 1) to eliminate the magnetic effect of the latitude. The TMI_RTE data must be interpreted with caution because the magnetic body will be stretched in east-west direction relative to the horizontal dimension of the body [47]. Ideally, both Reduction to Pole (RTP) and Reduction to Equator (RTE) should be interpreted together in area of low latitude. However, RTP data is not available for this survey and thus, the interpretation is supported by TMI_RTE that has been filtered with the first vertical derivative (1 vd) high-pass filter (TMI_RTE_1 vd) to provide a more reliable outcome that is readily interpretable in the context of magnetic rock bodies [46]. The first derivative filtering is effective in enhancing the anomaly due to shallow sources by improving the spatial and structural resolution. On the first vertical derivative, the width of magnetic bodies appears narrower, thus it is very effective to locate the source bodies accurately. Any structures' appearance on the first vertical derivative of the aeromagnetic data was interpreted and compared with the structures seen on the TMI_RTE data to delineate high frequency features where they are shadowed by large amplitude, low frequency anomalies. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No** & **Data Type** & **Unit** & **Purpose** & **Short Form** \\ \hline \multirow{2}{*}{**1**} & Total Magnetic & & & \\  & Intensity (Reduced to & nT & Highlight magnetic zonation & TMI\_RTE \\  & Equator) & & & \\ \hline \multirow{2}{*}{**2**} & 1 st vertical derivative & & & \\  & of TMI-RTE & nT/m & & \\  & & & magnetic zonation & TMI\_RTE\_1 vd \\ \hline \multirow{2}{*}{**3**} & Analytical Signal of the & & & \\  & first derivatives & nT/m & & \\  & & & & \\  & & & & \\
4 & Computed Free Air & & & \\  & Gravity (levelled and & & & \\  & noise reduce) for depth & & & \\  & (Z) tensor & & & \\ \hline \multirow{2}{*}{**5**} & Contact Linear & & & \\  & Processed (variable & & & \\  & wavelengths) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data components derived from the Air-FTG® acquisition and their purposes in geological interpretation. The local amplitude of the TMI_RTE data was further analyzed for residual and regional field Analytical Signal (AS). Analytical signal of TMI_RTE (Table 1) data helps in reducing the sensitivity for the inclination of the geomagnetic field than the original TMI data and thus, provides means to analyze low latitude magnetic fields [47] such that in Central Luconia Province.