Estimating Geothermal Energy Resource Potential within Jema’a Local Government Area of Kaduna State, Nigeria

This study used aeromagnetic data of Jema’a, sheet No.188 to estimate the geothermal energy resource potential in Jema’a Local Government Area of Kaduna State, Nigeria. The study area lies between latitude 9 � 11 � and 9 � 27 � N and longitude 8 � 00 � and 8 � 17 � E. Total magnetic intensity (TMI) map of the area was produced from the gridded data. Residual map was obtained by removing regional field from the TMI which was divided into 36 blocks for spectral analysis. The centroid depth (Z � ) was obtained by dividing the gradient of the lengthiest wavelength part of the spectrum by the wave number. The depth to top (Z � ) of the magnetic source was obtained from the gradient of high wave number portion of the power spectrum. The values of Z � and Z � were used to obtain the values for depth to bottom of the magnetic source ( Z � ). The geothermal gradient ( �� �� ) values were obtained by dividing 580℃ by Z � while the heat flow(q) values were obtained by multiplying �� �� by the constant � (2.51) .The �� �� values obtained range from 13.560 to 4000.000℃/km with an average value of 46.393℃/k and having the least ( 13.560 ℃/�� ) around Kafanchan axis and the highest ( 4000.000 ℃/km ) around Sanga. The q values range from 34.036 to 1004.00 ��m �� with an average value of 118.089 ��m �� . The �� �� values were observed to be greater than 100 ��m �� around Sanga and Andaha axes. The Curie depth (28 km) was deepest at the South-South, South-East and South-West regions. Since magnetic properties of minerals disappear at this temperature and thermal conductivity of rocks increases with depth, these areas are potential geothermal energy source.


I. INTRODUCTION
eromagnetic data of an area can be quantitatively and qualitatively analysed and interpreted with the aim of estimating the Curie point depth and heat flow within an area based on reconnaissance for geothermal energy.The Curie point which is the bottom of magnetic source is that point beneath the earth crust at which the magnetic properties of rocks disappear and the materials begin to exhibit paramagnetic properties.This temperature is taken to be about 580°C for magnetite under atmospheric pressure [1].The earth is not flat and the rocks are buried at different depths within the sub-surface.The temperature of these rocks is not the same as it varies from location to location.The magnetic contents in rocks is temperature dependent and the depth at which temperature reaches the Curie point is assumed to be the bottom of the magnetized bodies in the earth crust.It largely depends on the geology and magnetic contents of rocks and so, the Curie point temperature varies from place to place.The study of variation of temperature across various depths is essential as it will help provide appreciated information about the area temperature distribution and depths with potential geothermal energy within the sub-surface [2].Some scholars such as [3] stated that regions within the subsurface with potential geothermal energy are likely to have minimum Curie point depth (CPD).At these regions, there could be a young volcanism and a thin crust.A geophysical method known as spectral analysis can be applied to study geothermal energy at any location.The significant of spectral analysis in physics and science in general, cannot be overemphasized as it is applied in the interpretation of potential field data to determine the basement depths as well as to study crustal temperatures [4].Several researchers applied this method at different locations to study different things within the sub-surface.For instance, [5] applied 1D spectral analysis to aeromagnetic anomalies in the Anambra basin, to interpret aeromagnetic anomalies over Maiduguri-Dikwa depression of Chad basin.Reference [6] also applied spectral analysis to evaluate aeromagnetic anomalies over parts of upper Benue through Southern Chad basin, Nigeria.Spectral analysis method has a lot of advantages over other geophysical methods because it has the ability to filter all the noise away from the data, keeping the desired information intact and easy to use especially when transformation from space to frequency domain is performed [7].The application of spectral analysis to the interpretation of potential field data is a method that can be used to determine the basement depth [8].The procedures involved in this study include digitization of the aeromagnetic maps, separation of magnetic data and production of magnetic anomaly map, production of the first vertical derivative of the total field and analysis of magnetic anomaly data amongst others.To perform this analysis, the average radial energy spectrum will be calculated and graphs of the natural logarithm of energy against frequency will be plotted and the gradient of the linear segments will be computed in order to obtain the depths to the basement [9] and [8].Within Nigeria, there are places such as Ikogosi spring which is a warm spring, an evidence for a potential geothermal energy being generated within the sub-surface; this geothermal energy is a renewable energy source that can be used for generating electricity.Considering the unstable power supply bedeviling Nigeria today, generating electricity from this source of energy will go a long way in curtailing this challenge.Thus the study of the Curie-Temperature Depth and heat flow deduced from spectral analysis of aeromagnetic data within Jema'a Local Government Area of Kaduna State, Nigeria was conducted in order to estimate geothermal energy resource potential of the area.

II. GEOLOGY OF THE STUDY AREA
Jema'a Local Government area is one of the 23 local governments of Kaduna state in north-western Nigeria.The area has an average elevation of about 650 m above sea level; it is of low relief with slightly undulating topography and lowlying outcrops and a few prominent ridges of weathered granite scattered over the landscape.Jema'a local Government lies between latitude 9 11 ′ and 9 27 ′ N and longitude 8 00 ′ and 8 17 ′ E. The Local Government shares border with Kagoro to the East in Kaura Local Government, with Ungwan Rimi of Zangon Kataf Local Government to the North, with Jaba Local Government to the west and with Nasarawa state to the south.The population of the area has grown tremendously in the last 30 years to around 278, 202 according to 2006 Census [10].The local government has several villages among them are Kafanchan, Zikpak, Gidan Waya, Godo-godo, Ambe, Kagoma, Ungwan Baki, Ungwan Fari, Takau, etc.The geological map of Jema'a Local Government area is depicted in Fig. 1.Fig. 1 Geological map of the study Area [11]

A. Materials
The materials used for this research include Aeromagnetic data (Sheet No. 188) procured from Nigerian Geological Survey Agency (NGSA), Geological map of Jema'a Local Government; suffer 13 software, Oasis Montaj 8.4® software and Micro Excel®.The data used was acquired on a scale of 1:100,000 in half degree sheets collected at a flight altitude of 80m along North-East to South-West flight lines spacing of 500 and a tie line spacing 2 .

B. Method
The data was gridded and yielded the total magnetic intensity (TMI) map of the study area (Fig. 2).The first-order polynomial regression fit was applied on the total magnetic intensity (TMI) data of the study area to remove the regional field within (Fig. 3) which shows a North-East to South-West trend with the decrease in the magnetic intensity in the North-West direction.After removing the regional field within, what was left is the residual map of the study area (see Fig. 4).In an attempt to understand the subsurface architecture of the study area, spectral analysis was applied to the magnetic data procured from Nigeria Geological Survey Agency (NGSA).This spectral analysis uses Fast Fourier Transform (FFT) as a mathematical tool which is usually applied to a regular spaced data and it provides relationship between the spectrum of the magnetic anomalies and the depth to magnetic sources by transforming the spatial data into frequency domain.This study adopted spectral analysis used by [6] to determine depths to the top, centroid and bottom of the magnetic source of the study area.This work adopted the method and formulae used by researchers in estimating the Curie point depth.Equation ( 1) is used in estimating CPD [1].
Where is a constant, and denotes the depth to the top and bottom of the magnetic contacts respectively and K is the wave number of the field.
According to [12], the centroid depth (ZB) of the inmost magnetic contact can be evaluated from the gradient of the lengthiest wavelength part of the spectrum divided by the wave number using (2).The residual data was divided into 36 overlapping blocks of equally spaced area.Each of the blocks was subjected to zero padding and tapering to correct for edge effects.It was then subjected to fast Fourier transform to convert the data from a space domain to a frequency domain.The Fourier transformed data was then subjected to spectral analysis to yield its power spectrum and wave number, thus the depths to the top, and the centroid were determined based on the procedure of [14].Two out of the 36 spectral plots were shown in Fig. 5.The depth to a centroid ( )for each of the 36 blocks was obtained by the ratio of the gradient of the lengthiest wavelength part of the spectrum to the wave number.The depth to top ( ) of the magnetic source was obtained from the gradient of high wave number portion of the power spectrum.

spectral plots
The depth values obtained from Z and Z were used to obtain the values for depth to bottom of the magnetic source (Z ) which yielded the Table I.
The geothermal gradient and the heat flow of every location within the study area were given in the table above.These values were obtained by dividing 580 by the value of of every block while the values of the geothermal gradient were multiplied by the constant (2.51) to obtain the values of the heat flow.The values of the geothermal gradient range from 13.560 to 4000.000℃/ with the average value of 46.393℃/ .The least value was at block five (5) around Kafanchan axis while the highest value (400.000℃/ ) was at block seventeen (17) south of Sanga.The values of the heat flow across the study area as shown in table 1, range from 34.036 to 1004.00 mW with the average value of 118.089 mW .The Curie point depth varies according to the terrain and geological architecture as reported by [1].Reference [15] revealed that heat flow up to the magnitude 80-100 is a good pointer of geothermal anomalous conditions.Result of the findings of [16], revealed that heat loss anomalies usually occur at locations with thermal manifestation.
The maps of the depth to centroid (with depth range from 3 − 14.5 ), depth to top with depth range from 0.8 -4.4 and depth to bottom (with depth range from 3 -28 ) of the magnetic source are depicted in Fig. 6, 7 and 8 respectively. .The Curie depth is deepest at the South-South, South-East and South-West regions ( 28) ©DOP_KASU Publishing and the highest heat flow is around South-South (1004.000).Since magnetic properties of minerals disappear at this temperature and thermal conductivity of rocks increases with depth, those areas where these properties are exhibited are regions with potential geothermal energy source and there's a need to exploit this as it will serve as alternative source of energy to energy from fossil fuel and also curtail the energy challenges currently facing Nigeria.

Fig. 2 TMI
Fig. 2 TMI Map of the Study Area

Fig. 6 Fig. 7
Fig. 6 Map of depth to Centroid of the magnetic source of the study area

Table I
Depth value for the blocks in the study area VOLUME 02, ISSUE 01, 2022 95 ©DOP_KASU Publishing