AERO-RADIOMETRIC DATA OF KANO, NIGERIA

This study focused on investigating the geothermal resource potential in Kano using high resolution aeromagnetic and radiometric data. A total of sixteen (16) sheets of aeromagnetic data with a block dimension of 55 km x 55 km were processed. Thus, the spectral centroid method which result from low wave number part of the wave number-scaled (depth to the centroid) and high-wave-number portion of the power spectrum (depth to the top) were estimated. Thus, the Curie point depth (CPD), geothermal gradient and heat flow were determined using appropriate parameters. The results showed that the CPD values range from 4.27 km to 19.59 km with an average of 11.83 km, and geothermal gradient ranges from 29.6 o C/km to 135.7 o C/m with an average of 82.7 o C/km while the heat flow ranges from 74 mW/m 2 to 339.2 mW/m 2 with the average of 206.6 mW/m 2 . Areas with high heat flow and shallow CPD could be prospective areas for geothermal energy. The values of the radiogenic heat obtained in the study area range from 0.15 to 2.77 μW/m 3 . It was observed that the highest radiogenic heat generation in the area is greater than the average radiogenic heat values in the continental crust. The information obtained from temperature to depth model reveals that, the Curie temperature could be reached at the depths of 6 km within the regions.


INTRODUCTION
Aeromagnetic and airborne radiometric data could be used in a variety of ways, to minimize noise, enhance particular aspects of the data and/or integrate with other geophysical data (Megwara et al., 2013). Recent works by Nwankwo and Shehu (2015), Ezekiel (2019), Nwankwo and Sunday (2017), among others have shown great potential for geothermal heat flow in the Sokoto and Bida Basins respectively. Thus, geothermal resource (energy) can be extracted within the Earth and could be used either directly for heating applications or transform into electricity. The locations at which these resources are stored can be identified by estimating the Curie point depth from magnetic data over an area of interest. The Curie-point depth (CPD) may be very deep or shallow depending on the heat flow in the region and composition of the rocks. The shallow CPD region is preferred as compared to the deep depth due to the cost of exploitation. A Curie temperature of 580 °C for magnetite is considered applicable to continental crust because magnetite is the most common magnetic mineral (Tanaka et al., 1999). The Gamma rays arising from the decaying of unstable nuclei in the earth's rocks are recorded in this survey. The major elements that are of interest in the radiometric investigation are uranium, 238 U, thorium, 232 Th and potassium, 40 K (Keary et al., 2002). Though, contributions from radio-elements in the subsurface to geothermal heat production would assist in the assessment of potential zones for geothermal resources and its possible exploitation. Nevertheless, to determine the CPD within a particular region of interest around the world several studies have been conducted: California, U.S.A. (Ross et al., 2006); Yellow stone National Park, U.S.A. (Bhattacharyya and Leu, 1975); Cerro Prieto geothermal area, Baja California, Mexico (Espinosa-Cardena and Campos-Enriquez, 2008); East and South east Asia (Tanaka et al., 1999); Kyushu Island, Japan (Okubo et al., 1985); north east Japan (Okubo and Matsunaga,1994); India (Rajaram et al., 2009); south-central Europe (Chiozzi et al., 2005); the African-Eurasian convergence zone in south-western Turkey (Dolmaz et al., 2005); and Bulgaria (Trifonova et al., 2009). However, several studies have been conducted to investigate the Curie point depths within Nigeria which include (Ezekiel, 2019;Nwankwo and Shehu, 2015;Kasidi and Nur., 2012;Bemsen et al., 2019;Abdulwahab et al., 2019;Ayuba and Lawal 2019;Ibe and Uche 2020;but, to the best of my knowledge, no previous work in the area that assess the variation of Curie temperature with depth. In this study, the CPD, geothermal gradient, heat flow, radiogenic heat generation and temperature to depth variation could be determine to assess the geothermal resource potential in the study area using high-resolution aeromagnetic and radiometric data.

Location of the Study Area
The area of study covers Kano State and parts of Jigawa State. The area is bounded by latitude 11° ˈ -13° N and longitude 8° ˈ -10° E (see figure 1a). The climate of Kano state is the tropical wet and dry type which is marked by two distinctive conditions: the rainy season which usually lasts from June to September, and the dry season which is hot with temperatures exceeding 39 o C. February, March, and April are the hottest months, with day time temperatures exceeded to 45°C (Balogun, 2003). The natural vegetation of the study area is that of the Sudan Savannah type. The area of study lies on the Basement Complex of northern Nigeria (Figure 1b). The Nigerian basement complex is made up of gneisses, migmatites, and metasediments of Proterozoic age which was intruded by series of granitic rocks of later Precambrian to Lower Palaeozoic age. These rocks were variably metamorphosed and granitized through the tectonicmetamorphic cycles converted to migmatites and granite-gneiss (McCurry, 1971). Younger Granites rocks series were also noticed to be present within the area (Falconer et al., 1911) and these are of the Jurassic in age, as well as volcanic, and occasional younger dikes.  (After, Britannica, 1914), (b) Geology of the study area (after NGSA, 2004)

Data Acquisition and Processing
The high-resolution aeromagnetic (HRAM) data was acquired by Fugro Airborne Survey from 2003 to 2010 using seven Cessna Caravan fixed-wing aircraft each carrying three Scintrex Cesium vapor magnetometers. The survey was done on row spacing of 500 m with average topographic gap of 80 m, resulting in a total of about 2 million row-km of data. The data logging interval is 0.1 seconds or less than 7 minutes. The projection methods used to process the data are Universal Transverse Mercator (UTM) and WGS 84 as a reference. The ellipsoid model of Clarke 1880 (modified), 33ºE central meridian, zoom factor of 0.9996, 500,000 m X-deviation, 0 m Y-deviation were adopted. Also, 50 m grid size for drawing specifications and IGRF 2005 model were used for magnetic declination and inclination calculations. The sixteen (16) Sheets of HRAM data of the study area were windowed to the dimension of 55×55 km. The Fugro stored the acquired data in the following format; Column X represents longitude/easting, column Y represents latitude/northing, and column Z represents total magnetic field intensity. Thus, for ease of handling of the data, the total magnetic field strength value (Z) of 33,000 nT was removed. Therefore, after obtaining the data from NGSA to derive Z total (TMI), these values most be compensated in each value for column Z. The X and Y columns were geo-referenced in the UTM projection system and then set as the preferred columns. Thereafter, the data were gridded to generate total magnetic field intensity (TMI) of the study area. (b) Regional field map of the study area.
Each of the 16 blocks of the gridded aeromagnetic data were subjected to various data processing techniques such as regional-residual separation ( figure 2a & b), zero-padding, tapering, and fast-Fourier transform. Thereafter; the spectral centroid analysis method was applied to estimate the Curie point depths in the study area using the Step-by-step filtering option on the MAGMAP menu of Oasis Montaj.

METHODOLOGY
The estimation of Curie point depths of each blocks was done by using the Spectral analysis of the processed magnetic data and hence the assessment of the geothermal resources potential of the study area. There exist different approaches for carrying out estimation of depth to the bottom of magnetic sources. More so, the centroid method is one of the most commonly used approaches because it gives better estimates to depths with fewer errors compared with others (Ravat et al., 2007).  The mathematical models of the centroid method are based on the assessment of the shape of isolated magnetic anomalies in crust introduced by Bhattacharyya and Leu (1975; and the study of the statistical properties of magnetic ensembles by Spector and Grant (1970). Consequently, the introduction o f power spectral density of total magnetic field by Blakely (1995), Tanaka et al. (1999) reveals that the depths to the top bound and centroid of magnetic sources could be calculated from the power spectrum of magnetic anomalies. Hence, the basal (bottom) depth of the magnetic sources will be obtained from it. The centroid depth is obtained from the low wave number part of the wave number-scaled power spectrum using the relation as shown in equation 1: Where, ( ) is power spectrum, k is wave number, is a constant and is the centroid depth. Similarly, the depth to the top of the magnetic sources is derived from the slope of the medium to high wave number portion of the power spectrum using the relation in equation 2: Where, is a constant and is depth to the top of magnetic sources. Fitting the slope to a higher wave number part of the spectrum as suggested by Tanaka et al. (1999) leads to deeper magnetic bottom estimation that, at times, appear to be desirable (Ravat et al, 2007). The depth to the bottom of the magnetic source ( ) which is considered as the CPD is obtained from the relation in equation 3, as (Okubo et al., 1985): The geothermal parameters were obtained using the basal depth (Zb) information; the geothermal gradient (dT/dZ) can be estimated from the relation shown in equation 4 (Tanaka et al, 1999;Ross et al, 2006): Where, θc is the Curie-temperature. The Curie-temperature depends on magnetic materials in the rock, but magnetite (Fe3O4) is the most common magnetic mineral in igneous rock and has an approximate Curie-temperature of 580 o C. The heat flow (qz) of the study area was estimated using equation 5: Where k is thermal conductivity ( Table 2) and (dT/dz) is the geothermal gradient, as Tanaka et al., (1999). According to Braun, (2009), the conductivity of the rock decreases as temperature increases, this means that the thermal conductivity in the lower crust is much lower than in the surface. The relationship between the thermal conductivity and the temperature changes was studied by Robertson, (1988). The negative sign indicates the direction of the heat flow which moves in the opposite direction to the depth. This result was also compared with the heat flow of the geothermal fields in the region and these defined zones with geothermal resource potential in the area of interest.

Interpretation of Radiometric Data
The radio-elements are mostly the elements of radioactive origin which include, Uranium (U), Thorium (Th) or Potassium (K). The contributions of these elements are found in abundance in rocks of earth's crust. This study is only concern with U, Th and K which are discussed as follow: In this study, a total of four (4) sheets of airborne radiometric dataset of the anomalous CPD zone (hotspot) within the study area were obtained from the Nigerian Geological Survey Agency.
The radiometric data was gridded to obtain the values of each radioelement in all the blocks within the study area and these values were used to determine the radiogenic heat generation, concentration of radio-elements and temperature to depth variation model maps in the study area. The purpose of gridding the data is to identify and interpret signatures related with the source rocks for potential geothermal sources within the area under study.

Determination of Radioactive Heat Production
The three most pronounce types of isotope decay series abundant in rocks are the uranium series (decay of 238 U and 235 U), the thorium series (decay of 232 Th), and the decay of the potassium isotope 40 K. The radioactive heat production (Hp) as a result of isotope decay is expressed after Rybach (1976) as shown in equation 6: Hp = 9.25 + 2.56 + 3.48 (6) Where, Hp is the radioactive heat production and C is concentration of each of the three mentioned radio-elements.

Determination of Radiogenic Heat Generation
The radiogenic heat generation (Rhg) in µW/m³ produced by rock radioactivity with concentrations of uranium (CU), thorium (CTh) and potassium (CK) was estimated using the values of radioactive heat production and the density of the  (Rybach, 1988). Rhg = 10¯⁵ Hp (7) Where Hp is radioactive heat production, is density of the rock (in kg/m 3 ) (Table 1), U and Th are concentration of uranium and thorium (in ppm), while K is the concentration of potassium (in %). The computed values of radiogenic heat generation of the within the study area were used to produce the map of radiogenic heat generation in the area.

Temperature to Depth Variation Model
The temperature to depth variation model was estimated using equation 8 which was obtained from the linear relation and exponential distribution from Ravat et al., 2016. The relation was drive by obtaining a solution of the 1-Dimensional heat flow equation. Thus, this relation was used to determine the temperature to depth variation within the study area. Therefore, the values of the temperature to depth variation within the study area were obtained using model (equation 8) by varying the depths (Z) from 0 km, 2 km, 4 km, 6 km, 8 km and 10 km in the study area. = where; qs is heat flow obtained from the magnetic data; As radiogenic heat generation; is temperature at Z-depth (temp-to-depth variation); surface temperature (from Schoenech and Askira 1987); b is the empirical heat production depth distribution parameter (assumed to be 10 km) as (Ravat et al., 2016) and k is thermal conductivity of the rock types (Table 1).  Table 2 presents the results of the estimated depths for the 16 blocks of the gridded aeromagnetic data of the study area. The depths to the top of magnetic sources range from 0.82 km to 1.24 km with an average of 1.03 km. Also, the depths to the centroid of the magnetic sources in the study area range from 2.66 km to 10.20 km with an average of 6.43 km. The values were used in calculating the CPD (equation 3) of the study area, which range from 4.27 km to 19.59 km with an average of 11.83 km.    (Table 2) and thermal conductivity (

Results from Radiometric Data
In addition to the heat coming from the upper mantle, one parameter needed, to estimate the geothermal resource, is radiogenic heat generation which is produced by decay of radioactive elements, majorly U, Th, and/or K, in the crust which is an important contributor to heat flow at the surface (Clauser, 2011). Therefore, in order to obtained a clear picture of the result from this investigation, the study also adopt the radiometric method to determine the radiogenic heat generation of the CPD anomalous region within the study area Radiometric data of the shallow CPD region which was considered as a geothermal hotspot in the area was obtained and used to investigate the radiogenic heat generation and temperature to depth variation of the study area. Aside heat source due to earth's crust (CPD), radioactivity is another source of heat contributor that established a geothermal resource potential in the continental crust. The result (figure 7) shows high concentration of potassium (K) at the central and southern boundary with values ranging from 0.87 % to 2.93 % , low K concentration dominate the northern boundary with values ranging from 0.12 % to 0.84 %. More so, figure 8 reveals high concentration of Thorium (Th) at the SW and southern boundary of the area with values ranging from 12.08 ppm to 22.02 ppm, low Th concentration was observed in the NE boundary with values from 3.81 ppm to 11.82 ppm. Also, figure 9 shows high concentration of Uranium (U) at the SW, central part through the southern boundary with values ranging from 2.01 ppm to 5.39 ppm, low U concentration was observed in the NE boundary with values ranging from 0.12 ppm 1.94 ppm.

Distribution of U, Th & K within the Study Area
The compositions of K, Th and U were shown in the Ternary map of the study area. Ternary presented triangular plots of the radio-elements (figure 10) which displayed images by combining the three radio-elements which are also useful in classifying some rock types within the study area. Moreover, ternary plots of the radio-elements usually give a superior image of the geology (Salem et al., 2005). Figure 10 show that Uranium occupied almost the entire central part through the NW boundary of the study.

Radiogenic Heat Generation
The radiogenic heat (Rhg) generation (figure 11) produced by rock radioactivity with concentrations of uranium (CU), thorium (CTh) and potassium (CK) was determined by using equation 7. Figure 11 shows high values of radiogenic heat generation ranging from 1.33 µW/m 3 to 2.77 µW/m 3 was observed in the NW, SW through the central parts and to the southern boundary. Low radiogenic heat generation values ranging from 1.29 µW/m 3 to 0.32 µW/m 3 was observed in the entire north and NE boundaries of the study area, including places like Bichi, Kano and Wudil.

Temperature to Depth Variation Model
The model presents how temperature changes from the surface to a given depth in the crust. It also provides information about the temperature range at a particular depth, thus reveals the depth at which Curie temperature could be attain. Table 3 shows the estimated values of temperature to depth variations of the hotspots in the study area. The values were determined using the solution of a 1 -dimensional heat equation as shown in equation 8. The temperature to depths variation values in tables 3 were used to plot the temperature against depth graph which is the pictorial representation of the model. The result shows that at depth of 6 km 8 km and10 km. The plot reveals that some points have temperature values greater than the Curie temperature as assumed for magnetite rocks (> 580 o C). Thus, at depths of 2 km and 4 km the temperature was shown to have values less than the Curie temperature (see figure  12).  (Kasidi and Nur, 2012). In other words, shallow Curie point depth indicates Crustal thinning/Updoming/ Intrusion which also lead to high temperature in the affected area (Nwankwo and Shehu, 2017). Therefore, the shallow CPD regions suggest crustal thinning while regions with deep CPD imply the crust thickens. The shallowest CPD regions (hotspot) was identified, and selected for further analysis of radiogenic heat generation, and temperature-depth variation.  (Jaupart and Mareschal, 2007). Heat flow values between 80 mW/m 2 to 100 mW/m 2 are an indication of viable good geothermal conditions (Ludvik, 2009). Meanwhile, a heat flow values greater than 100 mW/m 2 is suggests anomalous geothermal condition (Jessop et al., 1976). Therefore, it was shown that, almost the entire study area reveals high heat flow, >100 mW/m 2 , which suggest potential geothermal source of energy. The average heat flow in a thermally normal continental crust is 60mW/m 2 , but average oceanic heat flow is 101 mW/m 2 (Jaupart and Mareschal, 2007). Heat flow values between 80 mW/m 2 to 100 mW/m 2 are an indication of viable good geothermal conditions (Ludvik, 2009). Meanwhile, a heat flow values greater than 100 mW/m 2 is suggests anomalous geothermal condition (Jessop et al., 1976). Therefore, majority of the study area have anomalously high heat flow, >100 mW/m 2 , which suggest potential geothermal source of energy.

INVESTIGATION OF GEOTHERMAL ENERGY …
The study area shows high values of radiogenic heat generation ranging from 1.33 µW/m 3 to 2.77 µW/m 3 , observed in NW, SW through the central part and southern boundary. Similarly, low values ranging from 1.29 µW/m 3 to 0.32 µW/m 3 was observed in the north and NE boundaries which include places like Bichi, Kano and Wudil. This radiogenic heat value of 2.77 μW/m 3 is within the values reported for average continental crust radiogenic heat generation (Alistair et al., 2014;Wollenberg and Smith, 1987). The high concentration of uranium, potassium and thorium in the area also shows high potentiality for geothermal energy. The ternary map shows that Uranium dominates almost the entire central part and the NW boundary of the study area and this indicates the presence of granites and pegmatites rocks in 306 the area. Potassium was observed in large amount at the SW and NE boundaries and indicates high concentration of felsic igneous basalt and granite rocks in the area. Thorium was observed in the area dominated by uranium and this reveals the presence of mafic igneous rocks in the area. Thus, from the temperature to depth model in this study, it was found that, the Curie temperature could be attained at a minimum depth of 6 km within the area with shallow CPD regions. Therefore, based on the analysis from the CPD, geothermal gradient, heat flow, radiogenic heat and temperature to depth variation the study area could be consider geothermal energy potential region.

CONCLUSION
The Curie point isotherm deduced from the study, indicate that the crust is thinning, shallow Curie point depth, in the Central and Eastern parts of the study area. The geothermal gradient shows that the area is classified into low and high temperature zones. High temperature zones could result into thermal maturation of sediments and hence probable oil generation. This study provides thermal information which could guide in geothermal energy exploitation within the study area. The areas with high heat flow were delineated and could be prospective for geothermal energy in generating electricity. The high concentration zones of Uranium and Thorium observed in the area is found to a trend extending to the western boundary which trend to Niger Republic across the mid-northern border. This concentration also follows the pattern of radiogenic heat generation in this area. The Curie temperature within the study area could be attained at the depth of 6 km, and sedimentary maturation could occur at a minimum depth of 2 km. Thus, the study area could be considered for geothermal resource exploitation in order to boost power (electricity) generation for domestic and industrial usage in the region.