British Journal of Applied Science & Technology 4(): 978-988, 204 SCIENCEDOMAIN international www.sciencedomain.org Vertical Electrical Sounding Survey for Groundwater Exploration in Parts of and its Environs, in the Anambra Basin of Nigeria M. Onimisi*, M. S. Kolawole, A. Daniel and A. Ebeh Department of Earth sciences, Kogi State University,, Nigeria. Authors contributions This work was carried out in collaboration between all authors. Author MO designed the study interpretation of the analysis results, author AD performed data analysis using IP2WIN software and writing of final manuscript, author AE wrote the first draft of the manuscript and managed literature searches. Author MSK managed literature searches and redesigned the manuscript. All authors read and approved the final manuscript. nd Short Research Article Received 2 September 20 th Accepted November 20 th Published March 204 ABSTRACT Aim: The aim of the study is to delineate the subsurface geologic layers and evaluate the aquifer potential in and its environs. Study Design: The resistivity sounding surveys were carried out randomly within the study area with half of the total surveys carried out within town. Lithologic logging of boreholes close to the survey lo\ations were also carried out. Place and Duration of Study: The study area is and its environs in Kogi state, central Nigeria. Field geophysical resistivity survey and borehole lithologic logging of the study area were carried out between March 200 and December 202. Methodology: The field resistivity sounding surveys were carried out with an ABEM SAS (000) Terrameter using the Schlumberger electrode array. Drill cuttings from boreholes sampled at meters interval were used for the identification of the subsurface lithology. Results: The results of the study reveal the presence of four to six geologic layers within the study area. The geologic layers and the mean thicknesses of the layers are: i. Lateritic top soil (.022m), ii. Lateritic sandy layer (.28m), iii unsaturated loosely compacted sandstone layer (9.84m), iv. unsaturated compact sandstone layer (4.8m), *Corresponding author: E-mail: monomisi@yahoo.com;
v. partly saturated sandstone layer (26.9) and vi. A fully saturated sandstone layer. Conclusion: Four to six geologic layers have been delineated within the study area. The fifth (partly saturated sandstone) layer and the sixth (fully saturated sandstone) layer constitute the main aquifer units in the study area. Keywords: Aquifer; sandstone; geoelectric; layer; resistivity.. INTRODUCTION and its environs in Kogi state, central Nigeria, lie within the Anambra sedimentary basin. The area has witnessed a large influx of people following the creation of Kogi state and this has led to an increase in demand for water supply. The principal stores of groundwater in Africa are concentrated in the earth to a depth of 400metres []. Groundwater is widely known to be more hygienic than surface water, thus the possibility of utilizing it as a source of water supply for public use is always attractive [2]. The Vertical Electrical Sounding survey method of geophysical exploration is very useful in groundwater exploration []. Olorunfemi and Fasuyi [4] identified aquifer types/hydrogeologic characteristics of part of the central basement terrain of Nigeria using the vertical electrical sounding method. Olayinka [] used the resistivity sounding method in groundwater investigation in the basement complex terrain of Egbeda- Kabba in Kogi state, Nigeria. This study is aimed at delineating the geological sequence and identifying the aquifer units in and environs using the vertical electrical sounding method of geophysical surveying.. Geologic and Hydrogeologic Setting The study area is part of the Anambra sedimentary basin and lies within latitude 6º 2 0 N to 7º2 0 N and longitudes 6º4 to 7º2 E The Anambra basin is a nearly triangular shaped embayment covering an area of about 00, 0000 sq. km. It stretches from the area just south of the confluence of the rivers Niger and Benue to as far as Awka, east of the river Niger (Fig.). The lithostratigraphic units underlying the study area are: (i) The basal Nkporo Group which is of Campanian age and consists of dark shales and mudstones with occasional thin beds of sandy shale and sandstone. (ii) The Mamu Formation, which overlies the Nkporo Formation is Lower Maestritichtian in age and consists of distinct assemblages of mudstones, sandstone, siltstone, sandy - silty shale, grey shales, carbonaceous shales and coal seams [6]. The Mamu Formation serves as a confining Formation for the overlying Ajali Formation. (iii) The Ajali Formation (Upper maestrichtian) is the main aquiferrous Formation within the Anambra basin. The Formation has an average thickness of about 00 meters. The Ajali Formation consists essentially of stratified sandstone units with planar cross bedding and coarse to fine grained texture. The Formation is capped by a considerable thickness of red earth at the top resulting probably from weathering and ferrugenisation. The Formation is readily recharged in its outcrop belt around the Idah-Nsukka-Enugu escarpement. (iv) The marine shales of the Imo and Nsukka Formation which were deposited in the Paleocene overlie the Ajali Formation. 979
(v) The continental Nanka and Ameki sandstones of Eocene age overlie the Imo and Nsukka Formation (Fig. 2) Alluvium Ogwashi Asaba Ameki Formation Imo Clay Shale Ajali Formation Mamu Formation Nkporo Shale Older granite Undifferentiated Metasediment Undifferentiated Basement Study Area. Fig.. Geologic Map of the Anambra basin showing the study area Fig. 2. Stratigraphic successions in the Anambra basin (After Obaje, 2009) 980
2. METHODOLOGY A total of 0 VES sounding surveys were carried out in the study area as shown in Fig.. The VES survey was carried out using the Schlumberger array. Current Electrode spacing (AB/2) was varied from to 600m with a maximum spread length of 200m. The VES data were acquired using an ABEM (SAS 000) terrameter. The Apparent resistance value, (Ra), as displayed by the terrameter was multiplied by the geometric factor for the Schlumberger array to obtain the apparent resisitivity (ρa) of the spread concerned. The apparent resistivity values for the corresponding current and potential electrode separation was plotted against half of the current electrode separation (AB/2) on a log-log graph paper using the IP2WIN software. The software iteratively adjusts the field curve until it matches the theoretical curve within a tolerance of.0% Root Mean Square (RMS). 8 6 0 2 0 4 9 7 Fig.. Map of And its Environs showing the VES points. RESULTS AND DISCUSSIONS The Apparent Resistivity values for the different electrode spacing for the VES locations are presented below (Table ). Plots of Apparent Resistivity Versus Current Electrode Separation (AB/2) were generated. The VES curves showing the field resistivity, calculated resistivity and thickness of the subsurface layers in the area are presented below (Figs. 4-). 98
Table. Apparent resistivity values for different electrode spacing obtained in the survey area Electrode Spacing AB/2(m). 2 7 0 20 2 0 40 0 60 70 80 00 20 0 200 00 400 00 600 VES 6 49 78. 260 70 98 70 420. 4. 400 6. 22. 00.2 270. 27. 22 278 270 4 66 82 46 00 20. Ra (Ohm m) VES 2 00.2 26. 40. 90. 2 200 20 207 240.6 282.6 20. 8 47. 470 24 89. 70 69. 740.6 67.7 60.7 0. 00 VES 80.4 28.6 70. 079.9 700 220.8 2288 282.9 97.6 90.2 999 4022 4809 0. 077. 4288. 27. 8.6 88.4 4.2 4 0 288.2 Electrode Spacing AB/ 2(m)..8 2.4.2 4.2.6 7. 0. 8 24 2 42 6 7 00 80 240 20 VES 4 60 76 884 980 27 000 800 60 707 800 000 200 00 800 0 00 200 900 920 000 200 Ra (Ohm m) VES VES 6 20 20 620 00 700 470 760 442 80 429 70 6 700 20 600 260 4 28 700 60 800 900 640 4 7 00 79 00 970 90 78 80 80 800 276 900 2849 000 200 00 280 VES 7 940 872 7 607 28 6 600.8 72.7 84 870 989 00 289 400 92 76 224 200 Electrode Spacing AB/2 (m)..6 2..4 7. 0.7.8 2. 4. 0 7. 08 8.7 2. 4 04. VES 8 4.6.4 0.. 267. 6.9 289.72 729.2 896.44 020.4 824.96 848.7 74. 8. 04.6 474.96 772.44 Ra (Ohm m) VES 9 VES 0 904.64 49.49 8.4 68.0 4.79 770.0 962. 904.64 4. 8.4 2068.46 4.79 2802. 962. 779. 4. 26. 2068.46 72.64 2802. 807.79 779. 7247. 26. 7697.42 72.64 78.64 807.79 926.7 7247. 6664.98 7697.42 78.64 982
Table 2. Geoelectric layer Resistivities and Thicknesses obtained in the VES locations in the survey area Community Mean VES Location 2 4 6 7 8 9 0 RMS Errors.9.2 4.4 4..27.24 2.. 2.98 4.26.904 69.2 7 227 409 6 08 04 69 04 248.92 Geoelectric Layer Resistivity 2 4 2 0 240 9.9 77 82 97.99 2992 6076 4 7666 6 20 6 67 24 8 40 48088 67 066 26 280 4966 409 07 262269 02 904 699 90402 2792 2792 20 802 9. 8.4 08.9 4.82 6 448 828 68 0. 0.66 0. 0. 0. 2. 0.9.4.06 0.7.022 Geoelectric Layer Thickness 2 4.9 2.6. 2.8 4.62 90 0.4 4.9 0.9.7 0..06 2.0 7.22 26.7 27.8 0.8.9.4.49 0.8 29.4 2.9 20.8 47. 0. 0.08.8.28 9.84 4.796667 2. 0. 26.9 98
The Root mean Square errors for the analysis were found to be very low with an average of.90 (Table 2). This underscores the reliability of the analysis tool for this type of work. Figs. 4- also show the various number of geoelectric layers expected in the subsurface as outlined in Table 2. The table shows the geoelectric layers penetrated at each VES location. Fig. 4. Typical depth sounding curve showing synthetic curve (Red) VES Fig.. Typical depth sounding curve showing synthetic curve (Red) VES 2 984
Fig. 6. Typical depth sounding curve showing synthetic curve (Red) VES Fig. 7. Typical depth sounding curve showing synthetic curve (Red) VES 4 Fig. 8. Typical depth sounding curve showing synthetic curve (Red) VES 98
Fig. 9. Typical depth sounding curve showing synthetic curve (Red) VES 6 Fig. 0. Typical depth sounding curve showing synthetic curve (Red) VES 7 Fig.. Typical depth sounding curve showing synthetic curve (Red) VES 8 986
Fig. 2. Typical depth sounding curve showing synthetic curve (Red) at Post office VES 9 Fig.. Typical depth sounding curve showing synthetic curve (Red) VES 0 The mean thickness and resistivity of the geoelectric layers for all the sounding locations have been calculated and are presented in Table. The study reveals the presence of six geoelectric layers within the study area. These geoelectric layers have been identified with lithologic type using lithology logs obtained from boreholes drilled in the study area and standard rock resistivity charts. Table. Geoelectric layer Parameters and the interpreted lithologic types Number of Geoelectric Layers 2 4 6 Mean thickness of Geoelectric Layers (m).022.28 9.84 4.8 26.9 Mean depth of Mean Geoelectric resistivity Layers (m) of Geoelectric Layers (Ωm).022.92 6.07 9. 6.20 8.4 62.0 08.9 88.9 4.82 68 Lithologic type Lateritic top soil Lateritic sand Unsaturated sandstone Unsaturated sandstone Partly saturated sandstone Saturated sandstone unit 987
The fifth and sixth geoelectric layers constitute the main aquifer units within the study area. 4. CONCLUSION The aquifer units in and environs of the Anambra basin have been delineated using the Vertical Electrical Sounding survey technique. The aquifer units in the study area consist of the partly saturated sandstone and fully saturated sandstone units corresponding to the fifth and sixth geoelectric layers of the VES curves respectively. These layers occur at mean depths of 88.9m and beyond. It is therefore recommended that the minimum depth for successful borehole yields in the study area should be about 00m. COMPETING INTERESTS Authors declare that there are no competing interests. REFERENCES. Rybkina AJ. World water balance and water resources of the earth. UNESCO Publishers, USSR; 978. 2. Abdullahi AS, Musa SM, Illiya AG. Aquifer depletion and groundwater situation in Damaturu, Northeastern Nigeria. Water Resources. 200;6:9-64.. William L. Fundamentals of Geophysics. Cambridge University Press, Cambridge, U.K. 997;208. 4. Olorunfemi MO, Fasuyi SA. Aquifer types and the geoelectric/hydrogeologic characteristics of parts of the central basement terrain of Nigeria (Niger state). Journal of African Earthscinces. 99;6():09-7.. Olayinka AI. Electromagnetic profiling and resistivity soundings in groundwater investigation near Egbeda-Kabba, Kwara state. Jounal of Mining and Geology. 990;26:24-20. 6. Obaje NG. Geology and mineral resources of Nigeria. Lecture Notes in Earth Sciences. Springer- Verlag Berlin Heidelberg. 2009;22. 204 Onimisi et al.; This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Peer-review history: The peer review history for this paper can be accessed here: http://www.sciencedomain.org/review-history.php?iid=49&id=&aid=980 988