_{1}

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Analysis of high resolution of aeromagnetic data was carried out over Lamurde, Adamawa state north-eastern Nigeria to determine the Curie point depth (CPD), heat flow and geothermal gradient. The aeromagnetic data used for this work was obtained at Nigerian geological survey agency, the total magnetic intensity was processed to produce the residual magnetic map which was divided into 4 overlapping blocks, each block was subjected to spectral analyses to obtain depths to the top boundary and centroid, while depth to bottom of the magnetic sources was calculated using empirical formula. The depths values obtained were then used to assess the CPD, heat flow and geothermal gradient in the area. The result shows that the CPD varies between 9.62 and 10.92 km with an average of 10.45 k, the heat flow varies between 150.73 and 132.78 mWm
^{−20}
⋅
°C
^{−1} with an average of 139.12 mWm
^{−20}
⋅
°C
^{−1} and the geothermal gradient in the study area varies between 12.16 and 15.67
°C/km with an average of 13.39
°C/km. In view of the above results, the high heat flow may be responsible for maturation of hydrocarbon in Benue Trough as well as responsible for the lead Zinc Mineralization. Again by implication, Lamurde area can be a good area for geothermal reservoir exploration for an alternative source for power generation.

The study area lies in the upper Benue Trough between latitude 9˚00N and 9˚3'N and longitude 11˚00E and 12˚00'E (

This research work aimed at estimation of Curie Point Depth (CPD), heat flow and geothermal gradient in Lamurde and Environs, North-Eastern Nigeria, by analysis of high resolution aeromagnetic data. One of the tools used in investigating thermal structure of crust via aeromagnetic studies is spectral analysis. Several studies have shown that magnetic data can be used to determine the thermal structure of the earth’s crust in various geologic environments [_{4}) is the most common magnetic materials in igneous rocks and has an approximate CPT of 580˚C [

The surface lineaments around the study area and their influence on the hot springs manifestations have not been previously investigated. The depth to the

heat sources which could provide information on the thermal structure has not been investigated either. In this research, the thermal structure around Lamurde and Environs, North-Eastern Nigeria was investigated in order to explore the geothermal potential using aeromagnetic data.

Thermal structure of the crust involving Curie depth isotherm estimations has been published for various tectonic settings (e.g. [

Studies of Curie Point Depth (CPD) in some other parts of the world especially African-Eurasian convergence zone, SW Turkey by [

The geology of the area is made up of Precambrian basement complex rocks which are believed to be mainly of gneiss-migmatites complex remnants of meta-sediments and older granite. Deformation described as Augen gneisses due to their eye shaped structure.

The older granites of Nigeria are calc-alkaline in chemical composition. These are also granite rock seen to have intrude the basement complex. Analysis have shown that, older granite are muscovite, granite, biotite granite aplite, quartz diorite etc. they show weak alignment of constituent platy minerals which a times result to their weak foliation.

The Cretaceous sedimentary rocks, which include rocks of Dukul, Yolde and Bima sandstone formation [

The Tertiary-Recent volcanic rocks in the study area consist of the basalts, trachyte, rhyolite, and newer basalts of eastern arm of Cameroon volcanic line. [

Stratigraphically, the basement complex rocks are the oldest and the Quaternary alluvium deposit being the youngest which formed mainly from the weathered rocks dominating the study area (

The data for this study is a high resolution aeromagnetic data obtained from Nigeria geological survey Agency [

The analysis was done using computer software (Oasis Montaj) and Mat lab designed for analysis of potential field data. In this research, the spectral analysis was made using interactive Oasis Montaj, Version 8.2 which enables two dimensional frequency domains processing of potential field data. The results of the analysis are plotted on a logarithmic scale against the radial wave number using a simple programme in Mat lab. On such a plot, if a group of sources has the same depth, they will fall onto a line of constant slope (tangent of the line fitted to the power spectra). Thus, if there are sources at different depths, such as a shallow plutonic formation over a deep basement, the plot will be separate into two or more sections with different slope. The reciprocal of the angle of slope is a measure of the depth to the source. This process was carried out to obtain the depth to the top boundary (Z_{t}) and depth to the centroid (Z_{0}) for the four overlapping blocks (

To carry out the analysis, the initial step according to [_{0}) of the magnetic sources from the slope of

the longest wave length of the spectrum as given:

log ( P ( K ) 1 2 / K / ) = log ( A ) − 2 π Z 0 / K / (1)

where P(K) is the radially averaged power spectrum of the anomaly / K / is the spatial wave number and A is a constant.

The second step was to estimation of the depth to the top boundary (Z_{t}) of the magnetic source from the slope of the line with the second wave length.

log ( P ( K ) 1 2 ) = log ( B ) − 2 π Z t / K / (2)

where, B is a sum of constants independent of / K / . The basal depth (Z_{b}) of the magnetic source is then computed from the equation:

Z b = 2 Z 0 − Z t (3)

The obtained basal depth (Z_{b}) of a magnetic source is assumed to be equal to the CPD.

Analysis of CPD is one of the methods used to estimate the temperature gradient and the heat flow in the crust. Heat flow is defined by Fourier’s Law as:

Q = d [ d T d Z ] (4)

where Q is the heat flow and d is the coefficient of thermal conductivity. In this equation, it is assumed that the direction of the temperature variation is vertical and the temperature gradient dT/dZ is constant. Accordingly, the Curie temperature; can be obtained from the Curie Point depth (Z_{b}) and the thermal gradient dT/dZ using the following equation:

ϕ = Z b [ d T d Z ] (5)

Equations (4) and (5) give a relationship between the CPD (Z_{b}) and the heat flow (Q) as:

Q = d [ Q Z b ] (6)

In Equation (6), the Curie point depth is inversely proportional to the heat flow. In this research, the Curie Point Temperature of 580˚C and thermal conductivity of 2.5 mWm^{−1}∙˚C^{−1} as average for igneous rocks was used as Standard. In order to calculate the thermal gradient and heat flow in the study area, Equations (5) and (6) were used.

Having examined both the TMI and the residual map of the study area (

are of magnitude 7.0 to 136.45 nT, and magnetic low observed are of magnitude −2.0 to −3 4.51 nT After regional residual separation the magnetic high observed on the residual data (

The average power and wave number spectra for each of the four overlapping blocks were calculated and used to estimate the CDPs (_{t}), while the left-hand side indicated the slope of the lower-wave-number-scaled spectra, which was also used to estimate the depth to centroid (Z_{0}) of the magnetic sources.

The results from the analysis and the one obtain from empirical formula are shown on

Block | Depth to Top Boundary (Z_{E}) in km | Depth to Centroid (Z_{0}) in km | Curie Point Depth (Z_{b}) in km | Heat Flow in mWm^{−20}∙˚C^{−1} | Geothermal Gradient ˚C/KM |
---|---|---|---|---|---|

1 | 2.38 | 6.46 | 10.54 | 137.57 | 13.10 |

2 | 2.58 | 6.10 | 9.62 | 150.73 | 15.67 |

3 | 2.80 | 6.86 | 10.92 | 132.78 | 12.16 |

4 | 2.73 | 6.72 | 10.71 | 135.38 | 12.64 |

compared with what is obtain across the world, it can be opined that volcanic, tectonic and associated geodynamic environments have CPD shallower than 10 km, while CPDs ranging between 15 to 25 km are as a result of island arcs and ridges and deeper than 25 km in plateaus and tranches.

Similarly, the results on ^{−20}∙˚C^{−1} with an average of 139.12 mWm^{−20}∙˚C^{−1}, while the geothermal gradient varies between 12.16 and 15.67 ˚C/km with an average of 13.39 ˚C/km. Though, the region that comprises of high heat flow values correspond to volcanic and metamorphic regions since they have high thermal conductivity. With the results obtained it is evident that the study area is potential site for geothermal exploration for alternative source for power generation and in addition to that the shallow CPD and high heat flow is responsible for some hot springs in Lamurde and environs.

The high resolution aeromagnetic anomaly data over Lamurde and Environs, North-Eastern Nigeria have been analyzed to estimate Curie Point Depth, heat flow and geothermal gradient. The result shows that CPD varies between 9.62 and 10.92 km with an average of 10.45 km, the heat flow varies between 150.73 and 132.78 mWm^{−20}∙˚C^{−1} with an average of 139.12 mWm^{−20}∙˚C^{−1} and geothermal gradient varies between 12.16 and 15.67 ˚C/km with an average of 13.39 ˚C/km.

The area is observed to have shallow Curie point depth and corresponding high heat flow (above 150 mWm^{−20}∙˚C^{−1}), thus, suggesting anomalous geothermal condition. Hence, further detail studies are recommended in the study area. It is known that geodynamics processes are mainly controlled by the thermal structure of the earth crust; therefore, this study is anticipated to contribute significantly to the quantitative appraisal of the CPD, heat flow and geothermal gradient in Lamurde and Environs.

The authors are grateful to the Nigerian Geological Survey Agency (NGSA) for releasing the aeromagnetic data at subsidize rate and to the Geosoft for providing Oasis Montaj software which was used for processing the data.

The author declares no conflicts of interest regarding the publication of this paper.

Kasidi, S. (2019) Determination of Curie Point Depth, Heat Flow and Geothermal Gradient from High Resolution Aeromagnetic Data around Lamurde Area, Adamawa State, North-Eastern Nigeria. Open Journal of Geology, 9, 829-838. https://doi.org/10.4236/ojg.2019.911093