The primary goal for upward continuation is to enhance the visibility of deeper, regional geological structures by attenuating the effects of shallow near surface features and cultural noise. Mathematically transforms magnetic field to a higher level. This process enhances long wavelength anomalies (associated with deeper structures). The high resolution aeromagnetic data was subjected to upward continuation to simplify the appearance of regional magnetic anomaly in the study area. The data was continued upward to 4 km. The regional features are often overly detailed, obscured by the proliferation of local magnetic anomalies. Thus, these disruptions were smoothed out without affecting the primary regional features via upward continuation. The main objective of upward continuation is to monitor the magnetic field intensity above the level of flight in order to minimize the occurrence of short wavelength anomalies and enhance longer wavelength anomalies that correspond to regional features [25] ( Figure 4 ).

The equation of upward continuation is given by [26] . The upward continuous F (magnetic anomaly) at a higher level (Z = −h) is given by:

$F\left(x,y,-h\right)=\frac{h}{2\pi }{\displaystyle \iint \frac{f\left(x,y,0\right)\partial x\partial y}{{\left({\left(x-x^{\prime }\right)}^2+{\left(y-y^{\prime }\right)}^2+h^2\right)}^{1/2}}}$ (1)

where $F\left(x,y,-h\right)$ = total field at a point $F\left(x^{\prime },y^{\prime },-h\right)$ above the surface on which $F\left(x^{\prime },y^{\prime },-0\right)$ is known. h = continuation height.

The total magnetic intensity data was continued upward to remove effects due to shallow magnetic sources and then divided into four overlapping blocks, each measuring 60 × 60 kilometers, totaling four data blocks. Each block was further exported into MATLAB programme to plot the spectral depths, the Top boundary (Z_t) and the centroid (Z_o).

Two steps are involved in estimating Curie point depth [27] . The first step in doing the analysis is to use the slope of the longest component of the wave length spectrum to estimate the depth to the centroid of the magnetic source (Z_o).

$\ln \left[\frac{p{\left(s\right)}^{1/2}}{\left|s\right|}\right]=\ln A-2\pi \left|s\right|Z_o$ (2)

where A is a constant, $\left|s\right|$ is the wave number, and p(s) is the anomaly’s radially average power spectrum.

The slope of the second longest wave length special segment is used to estimate the depth to the top boundary (Z_t) of that distribution in the second step.

$\ln \left[p{\left(s\right)}^{1/2}\right]=\ln B-2\pi \left|s\right|Z_t$ (3)

where B is the sum of the constant, the basal depth independent of $\left|s\right|$.

In calculating Curie point depth, the basal depth ( $Z_b$) of the magnetic source in the area is assumed to be the Curie point depth [27] . Therefore, the basal depth ( $Z_b$) of the magnetic source is calculated as follows:

$Z_b=2Z_o-Z_t$ (4)

The earth’s core cooling process and radioactive heat generation in the upper 20 to 40 kilometers of the crust are the main sources of heat on Earth. Fourier’s law is the fundamental formula for conductive heat transfer [28] . Using Fourier’s Law, Empirical formula was used to calculate the heat flow and thermal gradient as follow:

$q=\lambda \left[\frac{\partial T}{\partial Z}\right]$ (5)

In order to relate the Curie point depth ( $Z_b$) to Curie point temperature variation, the vertical direction of temperature variation and the constant thermal gradient was assumed. The geothermal gradient $\left(\frac{\partial T}{\partial Z}\right)$ between the earth and the Curie point depth ( $Z_b$) was defined by the equation:

$\frac{\partial T}{\partial Z}=\frac{580˚\text{C}}{Z_b}$ (6)

where 580˚C is the Curie temperature at which ferromagnetic minerals are converted to paramagnetic minerals. Furthermore, the geothermal gradient was related to heat flow ( $q$) using an empirical formula. However, by this calculation an average thermal conductivity value of 2.5 W·m⁻¹·˚C⁻¹ for the dominant rock in the region (Igneous rock) was also applied.

$q=\lambda \left(\frac{\partial T}{\partial Z}\right)=\lambda \left(\frac{580˚\text{C}}{Z_b}\right)$ (7)

where $\lambda$ is the coefficient of thermal conductivity.

The results obtained from this spectral analysis are presented on Figure 5 and the summaries of calculations using empirical formula are presented on Table 1 .