Empirical Relation Between Magnetic and Radiometric Survey in Bitumen Area, Ogun State, Nigeria

Imeri, the study area, is well recognised as a bitumen-rich area in the eastern part of Ogun state. It was found on geographical coordinates between the latitude 6 °45′N to 6 °48′N and longitude 3 °58′E to 3 °59′E along the Benin–Sagamu expressway, in the tropical rainforest region, Southwestern Nigeria. The study area is bounded by Idono in the north, Igbaga in the east and Imagbon in the south, and lies on the Abeokuta formation (Figure 1). The lithostratigraphical classification of the study area reported that the rock is basically made up of ferruginous sandstone, limestone, and shale [46]. The bitumen, from geochemical perspective, was detected to be rich in mineral-bearing elements such as iron, nickel, and lead [50].

Figure 1:

With the aid of a global positioning satellite (GPS) along the magnetic traverses, soil samples were collected. A total number of 40 soil samples were taken and prepared for radiometric analysis. The prepared samples were then taken to the radiometric laboratory of Federal University of Agriculture, Abeokuta, for gamma ray spectrometric analysis.

The radiometric measurement was carried out using a gamma ray spectrometer with 2″ by 2″ sodium iodide, NaI(Tl) detector coupled to the digibase multichannel analyser. The system calibration was performed using International Atomic Energy Agency [51] reference materials: RGK-1, RGU-1, and RGTh-1 for K, U, and Th activity measurements, respectively. The procedure for calibration and measurements were adopted from Grasty et al. [52].

The laboratory radioelement concentrations were obtained in Becquerels per kilogram (Bqkg⁻¹). The concentration in Bqkg⁻¹ was then converted to ppm and % using a conversion factor given by the Polish Central Laboratory for Radiological Protection [53], as shown in equations (1–3) for uranium in ppm, thorium in ppm, and potassium in %, respectively. (1) CUppm =CUBqkg−1×0.08045 {C_U}\left( {ppm} \right)\; = {C_U}\left( {{{Bqkg}^{ - 1}}} \right) \times 0.08045 (2) CThppm =CThBqkg−1×0.24331 {C_{Th}}\left( {ppm} \right)\; = {C_{Th}}\left( {{{Bqkg}^{ - 1}}} \right) \times 0.24331 (3) CK% =CKBqkg−1×0.003296 {C_K}\left( \% \right)\; = {C_K}\left( {{{Bqkg}^{ - 1}}} \right) \times 0.003296 where C_u, C_Th, and C_k are the concentrations of²³⁸ U, ²³²Th, and ⁴⁰K, respectively.

In this study, three empirical relations (models) are to be deduced as presented in equations (4–6). The models show the possible relationship existing between the magnetic anomalous response and the three radiometric parameters: uranium, thorium, and potassium, respectively. (4) M=pUr+q M = {{pU}_r} + q (5) M=pThr+q M = {{pTh}_r} + q (6) M=pKr+q M = {{pK}_r} + q

M is the measured magnetic anomalous data set.

In order to validate the models generated, a statistical inference test must be carried out on the measured survey data for the two geophysical methods used. In this regard, the t-test was found suitable and appropriate.

This test was carried out for the testing hypothesis concerning the mean difference of pairs of the data set, provided the variance of the two samples are equal [19].

The t-test was given by Dixon and Massey [54] as presented in equation (7): (7) tn1+n2−2=x1−x2sp21n1+1n2 {t_{{n_1} + {n_2} - 2}} = {{{x_1} - {x_2}} \over {s_p^2\left( {{1 \over {{n_1}}} + {1 \over {{n_2}}}} \right)}} where Sp2=n1−1S12+n2−1S22n1+n2−2 S_p^2 = {{\left( {{n_1} - 1} \right)S_1^2 + \left( {{n_2} - 1} \right)S_2^2} \over {{n_1} + {n_2} - 2}}

x₁ and x₂ are the mean of the two samples;

Two hypotheses are as follows:

H0: There is significant relationship between the anomalous values obtained for the two geological surveys.

When t calculated > t critical, the H1 hypothesis is rejected, while H0 is adopted.

If t calculated < t critical, then H1 hypothesis will be accepted.

In order to visualise the pattern of magnetic anomalies and radiometric responses in the bitumen deposit, the 2D and 3D pictorial representations were adopted. Figures 4 and 5 showed the 2D distribution of magnetic anomalies, and the three radioelement concentrations in T1 and T2 respectively. In similar context are the 3D representation of the magnetic anomaly and the three radioelement concentrations in Figures 6 and 7 for the two traverses.

Figure 4:

Figure 5:

In our study, the anomalous responses due to both the magnetic intensity and the radioelement concentrations were classified into three distinct levels: low, moderate, and high. The region with pink colour signifies the area of intense or high response of both magnetic and radioelements, while the yellowish-green represents the moderate response to the bitumen deposition. The blue colours denote the low responses in both the magnetic and radioelements.

The magnetic anomaly (Figure 4a) behaves in a similar way to uranium concentration (Figure 4b) response. Thorium concentration responses (Figure 4c) were also found to be closer to the magnetic anomalous response but not as much as that of uranium response. In Figure 4a, low and moderate magnetic anomalous levels were observed at the eastern and western part while the high level of magnetic response was found associated with the central part, trending southward along the traverse. Figure (4b) accounted for low and moderate uranium concentrations in the western part trending south, while high uranium concentration was found in the SW corner and the central part, trending east along traverse 1.

In the central part of Figure 4 (a, b, and c) exists a distinct circular shape with intense response due to magnetic, uranium, and thorium to bitumen deposition. The circular portion with intense response was found to be the exact location of the bitumen deposit. The SS part of the magnetic anomaly corresponds with that of thorium concentration, having high responses and inversely varying with potassium, having low response in the identified area. In Figure 4d, a distinct very high potassium concentration was observed in eastern and western sections, while moderate followed by low concentrations were accounted for in the SS section of traverse 1. At the upper part of the high level lies the moderate level of potassium concentration.

The magnetic anomalous response (Figure 5a) to the bitumen deposition in traverse 2 was not as high as in traverse 1. Moderate and low magnetic responses dominate the area with some spots of intense responses. The high uranium concentrations are found in line with those spots identified in the magnetic intensity. The high level of uranium concentrations is found to be more dispersed, compared to that of magnetic intensity (Figure 5b). Four zones of low thorium concentration level were identified along the traverse, while high levels of thorium concentrations were found in the SE and SW, trending SS along traverse 2 (Figure 5c). Similar anomalous features were observed in the SW corner of Figures 5a–c) for magnetic, uranium, and thorium responses, respectively.

Three distinct levels were experienced in traverse 2 for potassium distribution (Figure 5d). Low potassium concentration was observed at the extreme western section, while high concentrations were found associated with the central and NE part. The moderate concentration was observed at the central part of this section encompassing the high concentration region. Both traverses 1 and 2 accounted for high potassium concentration, which may be due to the presence of sandstone, the predominant bedrock composition of the area.

The clearer views of the magnetic and radiometric anomalous responses are shown in Figures 6 and 7. The region with a sharp peak indicates the zone of high levels of anomaly for both magnetic and radiometric surveys. The steep region is also an indicator of anomalous response, but with low magnetic or radiometric intensity, while the flat area implies uniformly distributed magnetic and radiometric intensity. The magnetic map gives an account of magnetic anomalies due to local variations of the magnetic field produced by magnetic material in the subsurface layers. The 3D surface plot (Figures 6 and 7) provides a better presentation of the anomalous responses accounted for by magnetic and radiometric survey over the bitumen deposition when compared to that of the 2D contour plot. The potassium surface plot accounted for more anomalous points compared to that of uranium and thorium. The region where the magnetic anomaly was situated was similar to that of uranium anomalous response around the NE section of traverse 1, (Figures 6a and b). The anomalous point was not only situated in the NE region of traverse 1 for thorium but also visible in the SW region of the traverse.

Figure 6:

Figure 7:

The 3D surface plot presented for magnetic and uranium responses (Figures 7a, b) also show a similar distinct feature of anomalous peak and steep regions along traverse 2, different from that accounted for in traverse 1. At the central part of traverse 2, a very steep anomalous point was accounted for by magnetism and uranium, while for potassium two sharp high anomalous points were observed.

Generally, potassium is one of the most abundant radioelements in the earth's crust. In view of this, the points of anomalous response are more associated with potassium distribution, as presented in the 3D surface plot. Conversely, the regions with high peaks found in the potassium response are found to be the region with steep or uniform anomalous magnetic and uranium responses. This implies that the potassium anomalous responses vary inversely to that of magnetism and the uranium. The results obtained by both the contour and surface plots showed that the magnetic anomaly associated with the bituminous area has a very strong direct relationship with the uranium and thorium anomalous responses, while potassium has inverse relation.

In order to establish a significant relationship between the magnetic and the radiometric survey, there is need for a linear and simple mathematical model. The model was obtained with the aid of a regression approach. The magnetic survey data set and the radioelements (U, Th, and K) were subjected to regression analysis, and a linear relation was established (Figure 8). The result obtained for the two traverses were merged for both the magnetic and radiometric evidence, such that a generalised model for the area can be deduced. Figure 6 shows a positive linear slope for uranium and thorium with magnetic anomaly while negative gradient was obtained for potassium responses to the magnetic anomaly.

Figure 8:

Figures 8a, b fall in line with the claim that the bituminous soils on which the survey was carried out are ferruginous in nature, which is rich in both uranium and thorium. Being a ferruginous area, it consists of minerals such as iron (Fe), which has a strong radiometric response to magnetic intensity, with a correlation coefficient (r-value) of 0.84 and 0.67 for uranium and thorium, respectively. On this account, a good agreement is therefore expected between the magnetic and uranium models, likewise that of magnetic and thorium. According to Bassey and Ishaku [39], potassium was reported to respond well in a feldspar area, unlike that of ferruginous material. This was revealed in Figure 8c, where an inverse relationship was obtained between magnetic and potassium response to bedrock composition of the bituminous area, with a correlation coefficient of −0.38.

The regression model obtained for magnetic to uranium response is as presented in Equation (8): (8) M=0.558 U−0.847 M = 0.558\;U - 0.847

The regression model obtained for magnetic to thorium response is as presented in Equation (9): (9) M=1.348 Th−2.024 M = 1.348\;Th - 2.024

The regression model obtained for magnetic to potassium response is as presented in Equation (10): (10) M=−31.230 K+7.984 M = - 31.230\;K + 7.984

Inferential statistics are basically used to validate the result obtained between two or more variables. In this study, a t-test was used to validate the response of magnetic flux and that of radiometric flux over the study area. This is necessary in order to validate the established relations between the magnetic intensity and the radioelements. The result obtained for the three radioelements and magnetic intensity for the two traverses shows that there is no significant difference between them. The critical value is greater than the calculated value (Table 1). This is also in agreement with the result obtained for the p-values. The p-values results are greater than 0.05, which implies that the hypothesis H1 was adopted while the hypothesis H0 was hereby rejected.

The t-calculated tests indicate negative value at traverse 1 and 2, which is in accordance with the r-value, with a low negative correlation coefficient. The result obtained opines that the model generated by uranium and thorium with magnetic intensity due to the two traverses are reliable. The inference result was concluded, as the correlation result depicts a very strong relation of uranium and thorium with magnetic intensity along the two traverses. The models are found to vary from place to place depending on geological bedrock composition. The model generated in this study fit better for uranium and thorium compared to potassium.