Correlation and mapping of geothermal and radioactive heat production from the Anambra Basin , Nigeria

Twelve sheets of aeromagnetic and aeroradiometric data covered the study area. The data was used to investigate heat sources. The aeromagnetic data were combined to form a composite maptotal magnetic intensity (TMI) anomaly map and aeroradiometric data of each radio-element were combined to produce the radioelements maps. Regional-residual separation of the total magnetic intensity data was performed using polynomial fitting method on the aeromagnetic data. The filtered residual data was Fourier transformed after dividing the whole area into thirty-five overlapping sections for spectral analysis, to determine Curie point depth, geothermal heat flow and magnetic trends. Calculation of ratios was used for the radio-elements to estimate the radioactive heat values in the study area, and the surface geology of the study area was delineated to outline each rock unit to match their density and corresponding radio-elements. The results of the analysis of aeromagnetic data showed that the shallow magnetic source ranges from 0.59 to 3.86 km, deeper source ranges from 8.03 to 19.85 km, Curie point depth values ranges from 14.64 to 38.62 km and geothermal heat flow values ranges between 37.54 and 99.02 mWm -2 . The results of the analysis of the radioactive heat production of the study area range between 0.01 and 5.43 μWm -3 . The highest heat produced was from the Shale with radioactive heat production as high as 5.43 μWm -3 . There are high geothermal heat flow and radioactive heat values in Aimeke and Ogobia.


INTRODUCTION
Aeromagnetic and aeroradiometric data were used to correlate the geothermal heat flow and radioactive heat production of the Anambra basin to ascertain if area with high geothermal heat flow values corresponds with that of the radioactive heat production.In this research work, Curie point depth was used to calculate the geothermal heat flow because of its importance in earth science (Artemieva et al., 2001;Megwara et al., 2012).The radioactive heat map was interpreted to know productive area based on the geographic projection of important towns.The digitized and georeferenced geological map of the study area outlined the rocks' boundaries.Apart *Corresponding author.E-mail: kuforijimi@gmail.com.

Attribution Creative Commons
Author(s) agree that this article remain permanently open access under the terms of the License 4.0 International License from geothermal exploration, radioactive heat can also be applied to detect uranium exploration (Killen et al., 2009;Grasty, 1979), identify sedimentary facies for oil and gas exploration (Myers et al., 1979;Bristow et al., 1989;Davies et al., 1996), detect radioactive contamination (Rybach et al., 1995;Sanderson et al., 1989) and mineral exploration (Mero, 1960).Radioactive decay of rocks is probably the greatest overall source of heat in the Earth's crust by a substantial factor, although, there are other sources that may be peculiar to specific area (Jessop, 1990).In some studies, radioactive heat production was calculated from concentrations of radio-elements measured in the laboratory by Fernández et al. (1998) and directly from gamma-ray log by Bücker and Rybach (1996) in order to get the accurate radioactive heat values.Also, radioactive heat production was assessed from airborne gamma-ray data (Salem et al., 2005;Richardson and Killen, 1980;Thompson et al., 1996).

The geological setting
The geology of the study area is presented in Figure 1.The area of study is bounded by latitudes 6°00'N and 7°30'N and longitudes 6°30'E and 9°00'E of the Anambra basin.The Anambra basin is an elongated NE -SW trends as marked out in Figure 2 and is located at the south-western fringe of the Anambra basin bordered on the west by the Precambrian basement complex rocks of western Nigeria and on the east by the Abakaliki Anticlinorium.The Asu River Group in the Abambra basin consists of shales, limestones and sandstone lenses of the Abakaliki Formation in the Abakaliki area and the Mfamosing Limestone in the Calabar Flank (Petters, 1982).The sedimentation in the Anambra basin started with the marine Albian Asu River Group, even though some pyroclastics of Aptian -Early Albian ages have been scarcely reported (Ojoh, 1992).The general stratigraphic cross-sections of this region are presented in Figure 2.

Data acquisition and processing
Twelve sheets of 268 -271, 287 -290 and 301 -304 covered the study area.They were measured and acquired on a scale of 1: 100,000 series by Fugro Airborne Surveys for Nigerian Geological Survey Agency (NGSA).The sheets were used as basic data for determining the nature of magnetic anomalies over the area.The survey was carried out along a series of North-South lines with a spacing of 3 km and an average flight elevation of 80 m above the sea level.The magnetic data was obtained from digitization of the total magnetic intensity contour maps at an interval of 0.0271 units at a flight line spacing of about 3 km.Also, the radiometric data for this study was obtained by windowing out the data of the same twelve (12) radiometric sheets.The radiometric data were acquired at a flight elevation of 80 m, line spacing and tie-line spacing were 500 and 5000 m, respectively.

Airborne magnetic data processing
A super (composite) map (Figure 3a and b) was produced after merging the data of each smaller map that covered the study area with "Oasis Montaj version 8.3"-a geospatial software and "Golden Surfer 11"a contouring software with a grid cell size of 55 m (Dentith, 2011).The results are presented in three columns: longitudes, latitudes and magnetic values of the given data point, respectively, and each magnetic data was placed with their corresponding longitudes and latitudes.Magnetic field reduction to the equator (RTE) filter was performed since the study area is located within the low magnetic latitudes (that is, areas with geomagnetic inclination less than 15°) where a reasonable reduction to the pole (RTP) of magnetic data is not achievable (Anudu et al., 2014;Sheriff, 2002;Rajagopalan, 2003;Wijins et al., 2005;Fairhead and Williams, 2006;GETECH, 2007;Geosoft Inc., 2011a).A Butterworth low-pass filter was applied during the RTE transformation to eliminate high-wavenumber connected with noise in the data.The parameters used during RTE filtering are: geomagnetic inclination of -8.571°, geomagnetic declination of -1.779° and amplitude correction of -20, while the Butterworth lowpass filter parameters include: cut-off wavelength of 500 m and filter order of 8.The -8.571° and -1.779° are mean values of the geomagnetic inclination and declination, computed for the area based on the IGRF-11 model for year 2006 to 2007 as adopted by the International Association of Geomagnetism and Aeronomy (IAGA) (NOAA/NGDC, 2010; Anudu et al., 2014).This process converts the TMI anomaly map of the area into one with better directions of magnetization field and the reduction to the equator total magnetic intensity (RTE-TMI) anomaly map was produced.Two noticeable disturbances were observed: residual and regional, they were separated using polynomial filtering method.The filtered residual data was Fourier transformed and the area was divided into 35 overlapping sections (Figure 4) for spectral analysis (Udensi et al., 2004).Figure 5a and b show the plots of spectral energies against their corresponding wave-numbers of one of the 35 plots to calculate the values of the shallow magnetic source and deeper magnetic source depths for each sections.The Curie point depth was calculated using Equation 1: The value of the geothermal heat flow is expressed by Fourier's law with Equation 2: (2) Tanaka et al. (1999Tanaka et al. ( , 2005) ) used the same equation to calculate geothermal heat flow, where (q) is the geothermal heat flow, (λ) is the coefficient of thermal conductivity, the Curie temperature (θ) can be obtained from the Curie point depth (Zb) and the thermal gradient ) using the following equation: (3) The combination of Equations 2 and 3 gave: ion of Equations 2 and 3 gave:   Geothermal gradient is calculated as the ratio of Curie point temperature to Curie point depth: In this study, 2.5 Wm -1 °C-1 (Reiter et al., 1985) was taking as an average of thermal conductivity value, because the predominant lithology in this area is Shale.

Radiometric data processing
The radioactive heat values were calculated from the energy released from the Alpha, Beta and Gammay decay of rocks (Salem and Fairhead, 2011) using an empirical equation by Rybach (1976) expressed as: A(µW/m 3 ) = ρ(0.0952Cu+ 0.0256 CTh + 0.0348 Ck) (5) Where, A = radioactive heat, ρ = density of rock adapted from Telford et al. (1990), Cu, CTh and Ck are the concentrations of uranium (ppm), thorium (ppm) and potassium (%), respectively.The method applied in this research is similar to that of Salem et al. (2005) where the rock unit boundaries were outlined to avoid mix-up while assigning densities to each rock unit with their corresponding radio-elements.

RESULTS
Figure 3a and b show the image of the total magnetic intensity and the contour map of the study area.The anomalies present in the study area were analyzed quantitatively from the combined data of 12.The total magnetic intensity (TMI) in the area is in the range of -300 to 225 nT, and is characterized by short-wavelength (high wave number), medium-wavelength (moderate wave number) and long-wavelength (low wavenumber) anomalies.The range of the TMI quite agrees with the work of Anudu et al. (2014) stating that the range of the TMI in the middle Benue Trough is in the range of -370 to 270 nT.Most of the anomalies in the TMI anomaly map have predominantly NE-SW and E-W trends.The residual magnetic anomaly map of the study area is  presented in Figure 6 after the composite map has been reduced to the equator.Table 1 shows the values of the shallow magnetic sources, deeper magnetic sources, Curie point depths and geothermal heat flow derived from the power-density spectra energy of the study area.The results of the analysis showed that the shallow sources have depths varying from 0.59 to 3.86 km, while the deeper sources have depths ranging from 8.03 to 19.85 km and Curie point depth values ranging from 14.64 to 38.62 km.The geothermal heat flow values obtained from the Curie point depth and geothermal gradient values vary from 99.02 mWm -2 in the upper part around Enugu and Agwu and also very high around Aimeke to 37.54 mWm -2 in the lower part around Iboko in the study area.Figure 7 shows the geothermal heat flow map of the study area.

Correlation of thorium abundance, total magnetic intensity and geological maps
The thorium abundance values range from 4.3 to 24.6 ppm.Correlation of Figure 8a and b shows that high thorium concentration corresponds with high total magnetic intensity anomalies around Nsukka, Mbasere, Ekuaro, Ogobia, Aimeke, Abakaliki and Iboko while low thorium concentration corresponds with low magnetic intensity anomalies around Emandak, Igumale, Amagunze, Ankpa, and Akpanya.The Ogobia and Aimeke area shown in the geological map (Figure 8c) have occurrences of shales and basement complex; which are associated with thorium mineralization and basement granitic rocks which are associated with magnetic mineralization.

Correlation of uranium abundance, total magnetic intensity and geological maps
The uranium abundance values range from 0.6 to 6.8 ppm.Correlation of Figure 9a and b shows that high uranium concentration corresponds with high total magnetic intensity anomalies around Ogobia and Aimeke while low uranium concentration corresponds with low magnetic intensity anomalies around Ankpa and Akpanya.Ogobia and Aimeke area shown in the geological map (Figure 9c) has occurrence of shales and basement complex which are associated with uranium mineralization and basement granitic rocks which are associated with magnetic mineralization.

Correlation of potassium abundance, total magnetic intensity and geological maps
The potassium abundance values range from 0.0 to 1.3%.Correlation of Figure 10a and b shows that high potassium concentration corresponds with high total magnetic intensity anomalies around Ogobia, Iboko, Ishieke, Aimeke and Nde, while low thorium concentration corresponds with low magnetic intensity anomalies around Ankpa, Ejule, Akpanya, Nsukka and Ngwo.The Ogobia and Aimeke area shown in the geological map (Figure 10c) have occurrences of shales and basement complex; which are associated with potassium mineralization and basement granitic rocks which are associated with magnetic mineralization.

Radiometric anomalies hotspots
Based on the radiometric heat flow map (Figure 11) of the study area, Ogobia and Aimeke show high concentration values of potassium, uranium and thorium.These particular areas are suitable for geothermal resource because of the high values of the three radioelements.

Qualitative and quantitative analysis of the radioactive heat analysis
The rock units with their average densities identified and assigned were used for the estimation of the radioactive heat production based on the concentrations of potassium, thorium and uranium within each rock unit.The average of the density of each rock unit was used in this research work as presented in Table 2.The boundary of each rock unit was outlined with the concentrations of radio-elements (K (%), eU and eTh) (Figure 10).Summary of the results of the analysis of radioactive heat value for each rock unit is presented in Table 3 and illustrated as map in Figure 11.High radioactive heat concentrations were observed around Aimeke, Iboko, Mbashere, Ogobia and Nde.Radioactive heat production values for each rock unit were calculated based on Equation 5.

DISCUSSION
Figure 12 shows that the structural trends are in E-W direction and this agrees with the magnetic trend results.
It is also an indication of the fault lines in the Anambra basin.The green arrows indicate that the radioactive heat flows in E-W direction.Majority of the anomalies in the total magnetic map are mostly NE-SW and E-W trends.
The radioactive results showed high concentration of uranium, thorium and potassium in Aimeke and Ogobia areas.Also, this research work provides new insights on the geothermal setting of the Anambra basin using airborne radiometric data.Radioactive heat production was mapped from the aero-radiometric data.The investigated basin has a range of radioactive heat production values which is above 4.0 μWm -3 for a high radioactive heat production (Alistair et al., 2014).Most of the radioactive heat is produced from uranium as its constant is more than twice of the constants of the other two radio-elements (Equation 5).The high concentration of uranium, potassium and thorium in Aimeke and Ogobia shows potentiality for geothermal energy production.
The radio-elements' maps showed that there is high concentration of thorium in Nsukka, Mbasere, Ekuaro, Ogobia, Aimeke, Abakaliki and Iboko.These areas have abundance of shale and sandstones.Potassium existence is correspondingly abundant in Ogobia, Iboko and Aimeke because of abundance of shale while high concentration of uranium was found around Ogobia and Aimeke.
Analysis of the radioactive heat map showed that Ogobia, Aimeke and Nde have high concentration of radioactive heat.The high concentration of uranium, potassium and thorium in Aimeke and Ogobia shows potentiality for geothermal energy production.
Ankpa has a high geothermal heat flow due to the occurrence of granitic rocks underlain the area.This particular area has a low radioactive heat value; this shows that the geothermal heat in Ankpa is not from radioactive heat values found in this study.This may be due to the heat from the mantle or the ambient heat in the hot granitic rocks in this area.The same thing is applicable to Enugu with high geothermal heat flow but low radioactive heat values; this may be due to the occurrence of coal underlain the area.The geothermal heat flow is at its highest value around Ogobia, Aimeke and Enugu.Based on the results from the analysis, there is likelihood that the study area may be prospective for geothermal energy utilization because heat flow values fall between 60 and 100 mWm       Comprehensive ground radiometric and magnetic surveys with soil test should be carried out at Ogobia and Aimeke which are considered to be hotspot.Also, detailed ground radiometric and magnetic surveys should be carried out at Ankpa because of the low radioactive heat values but high heat flow from the magnetic data.

Figure 1 .
Figure 1.Geological map showing the study area in red (Source: MacDonald et al., 2014).

Figure 2 .
Figure 2. Idealized N-S stratigraphic cross-section across the Chad Basin-Benue Trough-Niger Delta showing Anambra basin underlain by basement complex depicting connected Trans-Atlantic seaway between the South Atlantic and the Tethys Sea during the Coniacian-Turonian (Source: Obaje, 2009).

Figure 3a .
Figure 3a.Reduction to the Equator of total magnetic intensity (RTE-TMI) anomaly map of the study area.

Figure 3b .
Figure 3b.Contour of total magnetic intensity (TMI) anomaly map of the study area (the axes are in degrees and contour interval is 25 nT, respectively).

Figure 4 .
Figure 4. Residual magnetic anomaly map of the study area showing overlapping sections using the coordinates at the centre for each location.

Figure 5a .
Figure 5a.Typical plot of spectral energy against wave-number showing deeper magnetic source depths.

Figure 5b .
Figure 5b.Typical plot of spectral energy against wave-number showing shallow magnetic source depths.

Figure 6 .
Figure 6.Residual magnetic field map of the study area.The x-y-axes are longitude and latitude in decimal degrees, respectively.

Figure 7 .
Figure 7. Geothermal Heat flow map of the study area (Contour interval is 5.0 mWm -2 ).

Figure 8 .
Figure 8. Correlation of (a) thorium abundance (b) total magnetic intensity and (c) geological maps of the study area.

Figure 9 .
Figure 9. Correlation of (a) uranium abundance (b) total magnetic intensity and (c) geological maps of the study area.

Figure 10 .
Figure 10.Correlation of (a) potassium abundance (b) total magnetic intensity and (c) geological maps of the study area.

Figure 11 .
Figure 11.Airborne radioactive anomalies heat flow map showing hotspots in black rectangles.

Figure 12 .
Figure 12.Radioactive heat map showing structural trends and fault lines in the green arrows.

Table 1 .
Summary of the results of the shallow depth, deeper depth, Curie point depth and geothermal heat flow for the 35 over-lapping cells and their corresponding longitudes and latitudes.

Table 2 .
Average density for each rock unit.

Table 3 .
Radioactive heat production corresponding to each rock unit (in μWm -3 ).