Hydrogeophysical Study of Parts of Charnockite Terrain of Akure Southwestern Nigeria

DOI: https://dx.doi.org/10.4314/gjpas.v23i1.11

GLOBAL JOURNAL OF PURE AND APPLIED SCIENCES VOL. 23, 2017: 107-121 COPYRIGHT© BACHUDO SCIENCE CO. LTD PRINTED IN NIGERIA ISSN 1118-0579 107 www.globaljournalseries.com, Email: [email protected] HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE SOUTHWESTERN NIGERIA

MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA (Received 18 October 2016; Revision Accepted 9 December 2016)

ABSTRACT

Hydrogeophysical study of parts of Charnockite terrain of Akure, southwestern Nigeria was carried out using the electrical resistivity method. The study involved one hundred and fifty two (152) Schlumberger geoelectric soundings (with seven (7) of them parametric)) alongside static water level measurements in 102 hand-dug wells. The Precambrian Crystalline Basement Complex rocks underlying the area consist largely of Charnockite with some granite and migmatite-gneiss bodies. The interpreted results enabled the determination of overburden thickness, longitudinal unit conductance (S) and coefficient of anisotropy (λ). The static water level measurements were used to determine the vadose zone thickness and groundwater flow direction. The overburden thickness in the study area ranges between 2.0 and 66.9 m; bedrock resistivity range between 748 and ∞ Ω-m while the vadose zone thickness ranges between 0.5 and 11.3 m. The groundwater potential of the area was classified based on the overburden thickness (h) and resistivity values (ρ). Thus, areas characterized by 100 ≤ ρ ≤ 800 Ω-m with h ≥ 45 m are classified as high, while areas presenting 15 ≤ h ≤ 45 m are classified as medium and areas of h ≤ 15 m are classified as low. A modified geologic classification of the study area using λ values was undertaken with λ ≤ 1.2 areas classified as Charnockite while areas with λ > 1.2 classified as granite and migmatite-gneiss complex. A correlation of the field geological map with the λ map presents a good correlation especially within areas underlain by Charnockite.

KEYWORDS: Charnockite, resistivity, groundwater, overburden thickness, longitudinal unit conductance and coefficient of anisotropy.

INTRODUCTION The Charnockite series include rocks of pyroxene and olivine (Obaje, 2009), Very often the The northwestern flank of Akure metropolis is different rock types occur in close association as one set underlain principally by Charnockite rocks (Owoyemi, forms bands alternating with another set, or vein 1996). In Akure, rocky outcrops are barely visible in the traversing it, and where one facies appears the other areas underlain by the Charnockite rocks as they are also usually are found. The Charnockite series include presumably easily weathered and consequently rocks of many different types, some being felsic and rich constitute the gently undulating terrains of the town. By in quartz and microcline, others mafic and full of careful inspection of the settlement and growth pattern pyroxene and olivine, while there are also intermediate within Akure town, it is observed that the gently varieties corresponding mineralogically to norites, undulating terrains are attracting rapid urbanization as it quartz-norites and diorites (Obaje, 2009). is easier and cheaper for settlers to undertake The term ‘Charnockite’ consequently is not the constructions. Thus, the areas play host to the city name of rock, but of an assemblage of rock types, square, many educational institutions, commercial connected in their origin arising by differentiation of the centers/offices, industrial layouts and residential estates. same parent magma. Olarewaju (1981) described three Charnockite is a series of foliated igneous rocks modes of occurrence. The first occurrence is within what of wide distribution in Africa and thus constitutes one of seems to be “core” of the granite rock. The second is the important petrological units within the Precambrian their occurrence along the “margins” of the granite Basement Complex rocks of Nigeria. According to Dada bodies. These two modes of occurrences are of the et al., (1989), it was at Toro that Charnockite was first coarse grained origin variety. The third form of described within the Nigerian Basement by Falconer occurrence is represented by the discrete bodies of (1911) where it was then referred to as a “quartz diorite gneissic fine-grained charnockitic rock within the country porphyrite”. Wright (1970) described it as undeformed gneisses. Of these three modes of occurrence, only the acid and basic dykes. They are generally characterized second mode of occurrence is found within the Akure by their dark greenish to greenish grey appearance area. Charnockites are usually of Proteozoic age (late which makes them easily recognizable in hand Precambrian). Tubosun et al., (1984) use the U-Pb specimen. They usually contain quartz + plagioclase + method on zircons, attributed precise Pan-Africa age of alkali feldspar + orthopyroxene + clinopyroxene + 634±21 Ma to the Charnockite of Akure. hornblende ± biotite ± fayalite. Accessory minerals are As Akure town expands most especially across usually zircon, apatite, and iron ores (Olarewaju, 2006). the Charnockite terrain, access to good quality water Michael Ilesanmi Oladapo, Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. Franklin Isimhenmhen Afuda, Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. 108 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA supply is becoming increasingly difficult due to The electrical resistivity method which is a fairly inadequate pipe borne water supply. The insufficient rapid cost effective means of determining the gross pipe borne water supply is thus augmented by configuration of an aquifer has been adopted for this groundwater abstracted through hand-dug wells and study. The method is useful in locating areas of some boreholes in the metropolis. Unlike other natural maximum aquifer thickness and has been used resources, groundwater is dynamic in nature and its extensively both in the basement and sedimentary occurrence and movement varies with time and space. environments. The movement, accumulation and yielding capacity of Basement Complex rocks are known for their groundwater depend upon the geological environment. characteristic low porosity and negligible permeability Charnockite terrain has unsure groundwater yielding except in cases of heavy weathering (Mogaji et al., potential because of the abundance of clay materials 2011). As such, good permeability development in the and iron oxides associated with its area of occurrence. form of interconnected network of joints and fractures However, for a successful groundwater development usually presents a favourable hydrogeological setting. programme in such unsure terrain, it is essential that These fractures/joint zones are usually delineated with painstaking exploration project be conducted in a their characteristic resistivity lows within host resistivity manner consisting of near accurate delineation of the highs as usually observed on geophysical field data and water bearing units and the probable depths to resistive hence the relevance of the survey method in this study. basement rocks. This research was therefore borne out of the LOCATION DESCRIPTION, CLIMATE AND need to evaluate the groundwater potential of parts of GEOMORPHOLOGY Charnockite terrain of Akure as the area is the fastest The study area which constitutes the developing part of the town. The result will enhance the northwestern flank of Akure metropolis occupies land description of underlying hydrogeological features in the area of about 42.74 square kilometres and lies between lowland Charnockite terrain of the town and evolve latitude N07° 14’ 50” and N07° 18’ 30” and longitudes spatial groundwater distribution pattern within the area. E05° 08’ 40” and E05° 12’ 05” (Figure 1).

Nigeria Political Map

SOKOTO

KATSINA JIGAWA BORONO ZAMFARA KEBBI YOBE KANO

KADUNA BAUCHI GOMBE

NIGER

ADAMAWA

PLATEAU Fed. Capital KWARA Territory NASSARAWA OYO TARABA KOGI EKITI

OSUN BENUE Akure OGUN ONDO EDO ENUGU LAGOS EBONYI

CROSS RIVER IMO DELTA ABIA

0 50 100 km RIVER AKWA IBOM 0 50 mi BAYELSA

5 8' 40" E 5 12' 5" E 7 18' 30" N OSRC ORITA OBELE

O. S.R. C GEN PET STATION FEDERAL HOUSING GATEWAY ARCON INN PETROL ESTATE SHAGARI COOP INDUSTRY CUDA PROPOSE IDEAL MARKET PAINT L.G. SEC O OWO ADE T SUPER NEW HOTEL STADIUM AULE RD I G L A E AKURE R S G HIGH H A G A SCH E WOMEN EDU SACRED CENTRE HEART CATHEDRAL AYEDUN & PRIM SCH SEMINARY

R T RCM PRIM S T SCH OYEMEKUN K M LEGEND GRAM SCH PRIMARY RD SECONDARY RD WOSEM RIVER CAC GRAM SCH SCS GROUND ISIKANMRKT

SCALE 7 14' 50" N 0 1 2KM

Figure 1: Layout map of northwestern flank of Akure (Adapted from Google Earth (2015) while inset is the political map of Nigeria) HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 109 Akure is situated within the rain forest belt of barracks) and 399 m in the northeastern flank (Shagari Nigeria with a climate of wet season spanning from April Estate). to October and dry season between November and March. The town experiences the hottest weather from GEOLOGY OF THE STUDY AREA February to March and also experiences the dry wind The rocks encountered in the area of study are (harmattan) from December to early February. The area mainly Charnockite and some bodies of granite and lies within the zone of moderate annual rainfall of about migmatite gneiss complex (Figure 2). Even though the 1,300 to 1,700 mm while humidity is relatively high Charnockite rocks outcrops were not encountered, during the wet season and low during the dry season evidences of their occurrences were observed from with temperature varying between 22°C and 31°C charnockitic boulders encountered in some places. It is throughout the year (Iloeje, 1981). thus presumed that areas in which Charnockite were The topography of the Basement Complex encountered cover a large extent of about 60% of the terrain of Akure is generally undulating with a virtually area from northwest to southeast and east. The rugged terrain consisting of hills and valleys with field Charnockite rocks occur as smooth rounded boulders in recorded elevation varying between 330 m above mean some areas in form of oval to sub-circular and elongated sea level in the southwestern flank (Nigeria Army bodies.

5 8' 40" E 5 12' 5" E 76 7 18' 30" N 77 15 78 148 79 OSRC 147 ORITA OBELE 12 13 145 151 108 11 42 O.107 S.R. C GEN PETA 43 B STATION 44 56 5857 104105 1 FEDERAL HOUSING GATEWAY ARCON 31 INN 12087 PETROL 121 ESTATE62 SHAGARI 16 86 E 109 64 102 17 1228485 106 61 100 COOP 91 30101 F 92 INDUSTRY 18 21 27 93 5654 20 90 146 49 73 1953 CUDA 9998 C 52 69 72 89 51 74 71 D 134 PROPOSE 88 29 96 95 75 70 IDEAL 135 MARKET 28 97 PAINT 32 L.G. SEC 26 6 1 7 532 ADE 8 4 SUPER NEW33 HOTEL STADIUM 37 22 AULE RD 1110 9 89 12 8 AKURE HIGH 90 SCH 51 49 WOMEN EDU SACRED 92 CENTRE HEART 38 CATHEDRAL AYEDUN 41 & 89 PRIM SCH SEMINARY 39 RCM PRIM 48 31 34 61 SCH40 35 OYEMEKUN 36 4645 33 32 36 GRAM SCH 70 143 62 34 47 60 35 69 138 67 133 WOSEM68 149 139 65 CAC GRAM 66 81 SCH 100 125 G 17 140127 71 20 19 SCS 126 18 GROUND 142ISIKANMRKT141 77 110 112 99 111 7 14' 50" N 78 113 114 116115 H SCALE LEGEND 0 1 2KM Charnockite Geological Boundary Approximate Granite Road Migmatite Gneiss River Hand-dug Well Line of Geosection VES Point Parametric Sounding Point

Figure 2: Geological Map of the Study Area (adapted from Geological Survey of Nigeria Sheet 61) showing locations of VES points, Hand-dug wells and geoelectric section profile lines. 110 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA The emplaced fine-grained granites were The interpretation of VES curves involved curve encountered in the northeast and southwest flanks while matching (Keller and Frischknecht, 1966) and computer the migmatite-gneiss rocks with limited occurrence were iteration using WinResist Version 1.0 (Vander Velpen, encountered in the northeastern flank around Federal 2004) to refine the initial geoelectric parameters Housing Estate (Shagari Village), southwestern and obtained from curve matching. central parts around Ondo State Industrial Park. The rocks occur as variant banded gneiss and granite RESULTS AND DISCUSSION gneiss. They are fine-to-medium grained strongly foliated biotite gneisses with varying dips. The migmatite The VES data interpretation (Figure 3) shows a diffuse contact with the Charnockite around the delineated three to four major geoelectric units Federal Housing Estate (Shagari Village). comprising the topsoil, lateritic/weathered layer, The geology and boundaries of the lithologic fractured basement and the resistive bedrock. The units in Figure 2 were inferred in places where they are topsoil resistivity values present 13 - 960 Ω-m range concealed by superficial residual soils. with thickness varying from zero (bare outcrop) around Isikan in the south to 2.0 m in the central area around MATERIALS AND METHODS OF STUDY Oyemekun Grammar School. The lateritic/weathered layer has resistivity in the range of 10 – 3671 Ω-m with ABEM SAS 1000 digital resistivity meter was most of the values less than 200 Ω-m. The resistivity used for field data acquisition. The electrical resistivity values indicate lithology in the suite of sandy clay and method was adopted using the vertical electrical clayey sand. Thickness values of the weathered layer sounding (VES) technique. The Schlumberger array was range from 0.4 m in Ondo Road area in the southwest to utilized for field data acquisition. One hundred and fifty 66.9 m at Stateline Road in the west. Fractured bedrock two (152) VES locations were occupied including seven columns delineated are defined by resistivity values in (7) parametric soundings. the range of 13 to 1189 Ω-m while the thickness values Hydrologic field study involving static water level range from 0.8 to 33.0 m. High resistivity in the area of measurements in one hundred and two (102) hand-dug study suggests possibility of dry fractured column that wells was also undertaken. Static water level may lack interconnectivity while low resistivity values in estimations were made using Buddermeier and Schloss this zone indicate high fracturing and saturation (2000) method. A total of seven (7) parametric conditions thus constituting areas of some favourable soundings were carried out near existing hand dug wells hydrogeologic setting. The geoelectric basement is to constrain the data utilized for the study. The assumed to be infinitely resistive in the entire area. parametric soundings were conducted with the rms- errors ranging between 2.4 and 5.5%. HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 111

Figure 3: Typical sounding curves obtained from the Study Area

Parametric Sounding layers above the static water level. A fair correlation The lithology log of each hand-dug well was correlated exists between the static water level in the hand-dug with the geoelectric sounding data (Figure 4). The wells and the presumed water saturation points in the lithology exposures recorded comprise the topsoil, VES. The results obtained for this study are presumed laterite/clay unit and the static water surface. However, to be of high quality and consequently reliable. the VES interpretations present up to four geoelectric 112 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA

Figure 4: Typical parametric sounding obtained from the study area.

Geoelectric Sections The Orita-Obele geoelectric sequence (line AB) The electrical resistivity contrasts existing N-W generated from results of VES 42, VES 43 and between lithological sequences in the subsurface are VES 44 and comprising four geoelectric layers (Figure often adequate to enable the delineation of geoelectric 5) consists of the overburden materials with resistivity layers and identification of aquiferous or non-aquiferous values of 30 - 355 Ω-m and thickness ranging from 21.0 layers (Lashkaripour et al., 2005; Dodds and Ivic, 1998). to 35.6 m. The characteristic low resistivity (< 200 Ω-m) Four interpretive geoelectric sections were generated and fairly thick overburden thickness within this vicinity along AB, CD, EF and GH for Orita-Obele, Akure South makes the zone a fairly satisfactory hydrogeological LGA Secretariat, Federal Housing Estate (Shagari setting environment. The fresh basement is Estate) and Ondo Road respectively for the correlation characterized by high resistivity values. of geoelectric sequences obtained from interpreted VES curves (Figures 5 – 8). HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 113

Figure 5: Geoelectric Section along Line AB (Orita-Obele Area).

Four geoelectric layers were delineated on the < 200 Ω-m) and thick column (> 26 m) within the vicinity West-East geoelectric section of line CD around Akure of the bedrock depressions make these zones to be of South L.G.A. Secretariat (Figure 6). The overburden is favourable hydrogeological setting. characterized by resistivity values varying from 52 Ω-m The N-E trending line EF geoelectric section of in the saturated zone to 2909 Ω-m in the upper lateritic Figure 7 was obtained around the Federal Housing column and thickness values ranging from 7.7 to 28.7 m. Estate. The overburden is typified by resistivity values A bedrock ridge was delineated beneath VES 54, VES ranging from 32 to 1324 Ω-m with thickness ranging 73 and VES 69 while bedrock depression underlies the from 4.1 to 58.7 m. The section shows existence of western and eastern flanks beneath VES 51, VES 52 bedrock depressions beneath the eastern and northern and VES 70. Bedrock depressions in a typical Basement flanks with bedrock ridge underlying the center of the Complex area are groundwater collection centers traverse. The bedrock depressions are areas of (Ayolabi et al., 2003). The characteristic low resistivity (ρ groundwater accumulation. 114 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA

Figure 6: Geoelectric Section along Line CD (Akure South LGA Secretariat Area)

Figure 7: Geoelectric Section along Line EF (Federal Housing Estate Area)

Three geoelectric layers were delineated on the The weathered layer is defined by resistivity within the geoelectric section of line GH around Ondo road (Figure range of 22 and 70 Ω-m with thickness values ranging 8). The topsoil is defined by resistivity in the range of 20 from 1.8 to 12.9 m. The third layer is the fresh basement to 403 Ω-m and thickness ranging from 0.5 to 1.3 m. and is resistive. HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 115

Figure 8: Geoelectric Section along Line GH (Ondo Road Area)

Overburden Thickness 9). Maximum overburden thickness of 66.9 m was The overburden is considered in this study to be obtained in the area thus constituting about 84.47% of the materials above the fresh bedrock at each VES 79.2 m maximum depth to bedrock obtained by location. The interpreted depths to the bedrock beneath Olorunfemi and Okhue (1992) in the basement complex all the VES stations were plotted and contoured as terrain at Ile-Ife southwestern Nigeria. overburden thickness map with classifications (Figure

5 8' 40" E 5 12' 5" E 7 18' 30" N OSRC ORITA OBELE Meter (m) O. S.R. C GEN PET 70 STATION 65 FEDERAL HOUSING GATEWAY ARCON 60 INN PETROL ESTATE SHAGARI 55 COOP INDUSTRY 50 CUDA PROPOSE IDEAL MARKET 45 PAINT L.G. SEC 40 ADE SUPER NEW 35 HOTEL STADIUM 30 AULE RD AKURE 25 HIGH SCH WOMEN EDU SACRED 20 CENTRE HEART CATHEDRAL AYEDUN & PRIM SCH SEMINARY 15 RCM PRIM OYEMEKUN SCH 10 GRAM SCH 5 0 WOSEM -5 CAC GRAM SCH SCS GROUND ISIKANMRKT

SCALE 7 14' 50" N LEGEND 0 1 2KM > 45 m Thick Overburden Zone 15 m - 45 m Fairly Thick Overburden Zone < 15 m Thin Overburden Zone

Figure 9: Overburden Thickness Map of the Charnockite Terrain of Akure 116 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA The classifications on the Figure 9 map are thin Groundwater Potential Evaluation of the Study Area ranking (h ≤ 15 m), medium ranking (15 ≤ h ≤ 45 m) and The groundwater potential map of the study thick ranking (h ≥ 45 m). Studies in similar Basement area (Figure 10) was generated from the synthesis of terrain (Omosuyi et. al., 2003; Bala and Ike, 2001) have the overburden thickness map and the water-level identified areas with thick overburden cover as high elevation contours. Studies in similar Basement terrain groundwater potential zones. Areas underlain by (Omosuyi et al, 2003; Bala and Ike, 2001) have medium ranking overburden thickness (15 ≤ h ≤ 45 m) identified areas with thick overburden cover and are the western flank (Akure South LGA Secretariat and resistivity values between 100 and 800 ohm-m as high Aule Road); central part (Ideal Paint Warehouse); groundwater potential zones. Correspondingly, areas northwest (Orita-Obele, AKAD); southern flank (CAC with thin (h ≤ 15 m), moderately thick (15 ≤ h ≤ 45 m) Grammar School and Isikan market); northeastern flank and thick (≥ 45 m) overburden covers are classified as (Federal Housing Estate (Shagari Village)). Areas of thin low, moderate and high groundwater potential zones ranking (h ≤ 15 m) are in the central (Araromi, Central respectively. Market) and eastern (new Stadium) flanks of the area.

5 8' 40" E 5 12' 5" E 7 18' 30" N OSRC ORITA OBELE Meter (m) O. S.R. C GEN PET 70 STATION 65 FEDERAL HOUSING GATEWAY ARCON INN PETROL ESTATE SHAGARI 60 COOP 55 INDUSTRY CUDA 50 PROPOSE IDEAL MARKET PAINT 45 L.G. SEC ADE 40 SUPER HOTEL NEW STADIUM 35 AULE RD 30 AKURE HIGH SCH WOMEN 25 EDU SACRED CENTRE HEART CATHEDRAL AYEDUN & 20 PRIM SCH SEMINARY RCM PRIM 15 OYEMEKUN SCH GRAM SCH 10 5 WOSEM 0 CAC GRAM SCH SCS GROUND ISIKANMRKT

SCALE 7 14' 50" N LEGEND 0 1 2KM > 45 m High Groundwater Potential Zone 15 m - 45 m Moderate Groundwater Potential Zone < 15 m Low Groundwater Potentiial Zone Water Elevation Contour in Meter above Sea Level

Figure 10: Groundwater Potential Map of the Charnockite Area of Akure HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 117 Areas around Akure South Local Government Susceptibility of Aquifer to Contamination Secretariat, Aule Road; Ideal Paint Warehouse; Orita- Geoelectric parameters derived from resistivity Obele; AKAD in the northwest; CAC Grammar School and thickness (Dar Zarouk parameters) include ground; Isikan market in the southern flank; the Federal longitudinal unit conductance, (Si = hi/ρi) and Coefficient Housing Estate (Shagari Village) in the northeastern of Anisotropy, (λ = √ρt/ρL). These secondary geoelectric flank are classified as moderate groundwater potential parameters are particularly important when they are zones. Apart from the fairly thick overburden cover of used to describe a geoelectric section consisting of this zone, the water-level contour depicts this area as several layers (Zhody et al 1974). groundwater collection zone (low hydraulic heads). The The total longitudinal unit conductance values zone also shows aquifer connectivity from the northeast have been utilized severally in the evaluation of (Orita-Obele) to the west and central part (Akure Local overburden protective capacities over hydrogeologic Government Secretariat and Ideal Paint Warehouse). structures (Oladapo and Omotola 2015; Okiongbo et al., The parametric soundings in this zone (Orita-Obele) 2011; Atakpo and Ayolabi, 2008; Braga et al., 2006). indicate water column of 22 m and 9 m at VES 147 and Longitudinal unit conductance values within the study 151 respectively. This zone presents some favourable area range from 0.01 to 2.7 mhos. The longitudinal unit hydrogeologic settings for groundwater development. conductance map of Figure 11 shows that the Araromi, Central Market and New Stadium, in hydrogeologic structures underneath Orita-Obele, the eastern and southeastern part of the study areas are Oyemekun, Isikan, Araromi (northwest through to classified as areas with low groundwater potential. The southeastern parts) have overlying materials consisting water-level contour in this zone describes discontinuous of moderate to good protective capacity rating (0.1 - 2.7 aquifers (cyclic contours) in the east (New Stadium) and mhos). The northeastern and southwestern flanks of the in the southeast (Oyemekun Grammar School and area are weakly protected (0.01-0.1 mhos) from Isolo). The parametric sounding within Oyemekun contaminant infiltration. Grammar School indicates water column of 2 m at VES 143.

5 8' 50" E 5 12' 5" E 7 18' 30" N

OSRC ORITA OBELE mhos O. S.R. C 2.7 GEN PET 2.6 STATION 2.5 FEDERAL HOUSING 2.4 GATEWAY ARCON 2.3 INN PETROL ESTATE SHAGARI 2.2 2.1 COOP INDUSTRY 2 CUDA 1.9 PROPOSE 1.8 IDEAL MARKET PAINT 1.7 L.G. SEC 1.6 ADE 1.5 SUPER HOTEL NEW 1.4 STADIUM 1.3 AULE RD 1.2 AKURE 1.1 HIGH 1 SCH WOMEN EDU SACRED 0.9 CENTRE HEART CATHEDRAL 0.8 AYEDUN & PRIM SCH SEMINARY 0.7 RCM PRIM 0.6 OYEMEKUN SCH 0.5 GRAM SCH 0.4 0.3 0.2 0.1 WOSEM 0 CAC GRAM SCH SCS GROUND ISIKANMRKT

7 14' 50" N LEGEND SCALE Good Protective Capacity Layer 0 1 2KM Moderate Protective Capacity Layer Weak Protective Capacity Layer

Figure 11: Longitudinal Unit Conductance Map of the Charnockite Area of Akure 118 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA Anisotropy (λ) and Geologic Boundary Modification low specific surface area and are thus more likely to be Anisotropy (λ) is related to preferential intensely fractured and permeable. permeability directions channelling groundwater flow Coefficient of anisotropy has also been used to within the aquifer (Steinich and Marin, 1996). The delineate lithology contacts in typical Basement terrain coefficient of anisotropy has been shown to have the (Oladapo et al., 2004; Olorunfemi and Okhue, 1992). A same functional form as permeability anisotropy to a first modified geologic classification of the study area using order (Bespalov et al., 2002). Thus a higher coefficient coefficient of anisotropy was undertaken (Figure 12). of anisotropy implies higher permeability anisotropy. The results of the geologic modification reveals that Higher-permeability anisotropy implies higher rocks of low anisotropy (1.0≤ λ≤1.2) and presumably groundwater flow. Thus areas of high anisotropy may be charnockitic underlie the Federal Housing Estate target areas for possible groundwater abstraction. The (Shagari Village) in the northeast, New Stadium in the coefficients of anisotropy (λ) values in the area vary east, Isikan in the south, Ilesha Motor Park and Femi between 1.0 and 1.98. Fractures at localities with Road in the central areas while the other parts of the relatively high values of coefficient of anisotropy area with higher anisotropy (1.2 ≤ λ ≤ 1.97) are possess relatively high fracture porosity and relatively underlain by granites and migmatites.

5 8' 40" E 5 12' 5" E 7 18' 30" N OSRC ORITA OBELE

O. S.R. C 1.9 GEN PET STATION 1.85 FEDERAL HOUSING 1.8 GATEWAY ARCON INN PETROL ESTATE SHAGARI 1.75 COOP 1.7 INDUSTRY CUDA 1.65 PROPOSE IDEAL MARKET 1.6 PAINT L.G. SEC 1.55 ADE SUPER 1.5 HOTEL NEW STADIUM 1.45 AULE RD 1.4 AKURE HIGH 1.35 SCH WOMEN EDU SACRED 1.3 CENTRE HEART CATHEDRAL AYEDUN & PRIM SCH SEMINARY 1.25 RCM PRIM 1.2 OYEMEKUN SCH GRAM SCH 1.15 1.1 1.05 WOSEM 1 CAC GRAM SCH SCS GROUND ISIKANMRKT

7 14' 50" N SCALE LEGEND Migmatite / Granite 0 1 2KM Charnockite

Figure 12: Modified Geological Map of Study Area using Coefficient of Anisotropy (λ)

Correlation of Results South. However, there is a slight departure around north A correlation of the field geological map with the of Federal Housing Estate in the northeast flank (Figure coefficient of anisotropy map shows a good correlation 13). The low level of correlation observed in this area is especially within areas underlain by Charnockite (λ ≤ possibly due to inferred geological contact determination 1.2). Charnockite underlay is confirmed in areas around during the geological fieldwork. Orita-Obele, in the northeast; Akure South LGA From the overburden thickness map, zones of Secretariat, in the west; C.A.C Grammar School in the thick to fairly thick overburden cover show good HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 119 correlation with areas presumably underlain with Orita-Obele, Akure South LGA Secretariat, Isikan and Charnockite. This suggests that zones underlain by the Federal Housing Estate (Shagari Village). Charnockite are more weathered. These areas are:

Zones of Correlation Zone of Departure

5 8' 50" E 5 12' 5" E 5 8' 40" E 5 12' 5" E 7 18' 30" N 7 18' 30" N OSRC OSRC ORITA ORITA OBELE OBELE

O. S.R. C 1.9 GEN PET GEN PET STATION STATION 1.85 FEDERAL HOUSING 1.8 GATEWAY ARCON FEDERAL HOUSING GATEWAY ARCON INN INN 1.75 PETROL ESTATE SHAGARI PETROL ESTATE SHAGARI 1.7 COOP COOP INDUSTRY INDUSTRY 1.65 CUDA CUDA PROPOSE PROPOSE 1.6 MARKET IDEAL IDEAL MARKET PAINT PAINT 1.55 L.G. SEC L.G. SEC 1.5 ADE ADE SUPER SUPER HOTEL NEW NEW 1.45 HOTEL STADIUM STADIUM 1.4

AULE RD AKURE AULE RD 1.35 HIGH AKURE SCH HIGH WOMEN SCH 1.3 EDU SACRED WOMEN CENTRE HEART EDU SACRED CATHEDRAL CENTRE HEART 1.25 AYEDUN & CATHEDRAL PRIM SCH SEMINARY AYEDUN & PRIM SCH SEMINARY 1.2 RCM PRIM RCM PRIM SCH 1.15 OYEMEKUN OYEMEKUN SCH GRAM SCH GRAM SCH 1.1 1.05 1 WOSEM WOSEM CAC 0.95 GRAM CAC SCH GRAM SCH SCS ISIKAN SCS GROUNDMRKT GROUND ISIKANMRKT 7 14' 50" N SCALE 7 14' 50" N LEGEND LEGEND 0 1 2KM SCALE Geological Boundary Approximate Migmatite / Granite Charnockite 0 1 2KM Granite Major Road Charnockite River Migmatite Gneiss Modified Geological Map of Study Area Using Coefficient of Anisotropy Simplified Geological Map of the Study Area Zones of Correlation

Figure 13: Correlation of the Field Geological Map with the Modified Geological Map Using Coefficient of Anisotropy (λ).

CONCLUSIONS Areas identified as moderate groundwater potential zones are: Orita Obele in the northeast, Akure South Hydrogeophysical study of parts of Charnockite LGA Secretariat and Aule Road in the west, Femi Road terrain of Akure, southwestern Nigeria has been carried and Isikan market in the south, and Gbeleaje Estate out using vertical electrical sounding involving the west of Shagari village in the north. These areas are Schlumberger array, and static water level underlain by Charnockite, granite and migmatite gneiss. measurements in hand-dug wells. The interpreted VES This shows groundwater occurrence in all the lithology data enabled the determination of the depth and units. subsurface configuration of the crystalline bedrock. High groundwater potential areas are quite Three geoelectric units were delineated viz: the topsoil, limited in size and comparatively insignificant. The the weathered/fractured layer and the highly resistive isolated areas are situated west of Oyemekun Grammar bedrock. The static water level data enabled the School in the city center area and northern fringes of determination of the unsaturated zone thickness with Federal Housing Estate on the northern flank of the values ranging from 1.5 in the flood plains to 11 m in the town. elevated areas. The study has also shown the The longitudinal unit conductance map reveals overburden thickness values to range from 2.0 to 66.9 m that the areas of medium and high groundwater potential with bedrock resistivity values varying between 248 and ratings are overlain by moderate protective capacity ∞ Ω-m. The study enabled the classification of the area layering units. Hence, aquifers in these areas are less into groundwater potential zones based on the vulnerable to contamination. delineated overburden thickness, second order geoelectric parameters and water-level elevation. REFERENCES Low groundwater potential zones are areas situated on the northern flank of the city (Arcon Petrol Atakpo E. A and Ayolabi, E. A., 2008. Evaluation of station, Ondo State Industrial Park and Federal Housing aquifer vulnerability and the protective capacity Estate (Shagari village)), some areas on the in some oil producing communities of western southwestern flank (Ondo Road) and central area of the Niger Delta. Environmentalist DOI town (Araromi, Oja-Oba (Central Market) and 10.1007/s10669-008-9191-3 Oyemekun Grammar School). 120 MICHAEL ILESANMI OLADAPO AND FRANKLIN ISIMHENMHEN AFUDA Ayolabi, E. A., Adedeji, J. K and Oladapo, M. I., 2003. A Okiongbo, K. S., Akpofure, E and Odubo, E., 2011. Geoelectric Mapping of Ijapo, Akure Southwest Determination of Aquifer Protective Capacity Nigeria and its Hydrogeological Implications. and Corrosivity of Near Surface Materials in Global Journal of Pure and Applied Sciences, Yenagoa City, Nigeria Research Journal of 10, (3): 441-446. Applied Sciences, Engineering and Technology 3(8): 785-791 Bala, A. E and Ike, E. C., 2001. The Aquifer of the Crystalline Basement Rocks in Gusau Area, Oladapo, M. I., Mohammed, M. Z., Adeoye, O. O and North-western Nigeria. Journal of Mining and Adetola, B. A., 2004. Geoelectrical Investigation of the Geology, 37, (2): 177-184. Ondo State Housing Corporation Estate Ijapo Akure, Southwestern Nigeria. Jour. of Mining Braga, A. C., Filho, W. M and Dourado, J. C., 2006. and Geol. 40(1), pp.41-48. Resistivity (DC) Method Applied to Aquifer Protection Studies. Revista Brasileira de Oladapo, M. I and Omotola, O. O., 2015. Geoelectric Geof´ısica (2006) 24(4): 573-581 Appraisal of Aquifer Contamination Risks within the Campus of Federal University of Bespalov, G. D., Tabarovsky, L and Schoen, J., 2002. Technology, Akure Southwestern Nigeria. On the Relationship between Resistivity and Journal of Emerging Trends in Engineering and Permeability Anisotropy: Annual Technical Applied Sciences (JETEAS) 6(6): 387- 398 Conference, Society of Petroleum Engineers, Proceedings, SPE 77715. Olarewaju, V. O., 2006. The Charnockitic Intrusives of Nigeria. In: Oshi O (ed) The Basement Complex Buddermeier, R. W and Schloss, J. A., 2000. of Nigeria and its mineral resources (A Tribute Groundwater Storage and Flow. http://www.kgs. to Prof. M. A. O. Rahaman). Akin Jinad & Co. Ibadan, pp 45–70 Dada S. S., Lancelot, J. R and Briqueu, L., 1989. Age and origin of the annular charnockitic complex at Olarewaju, V. O., 1981. Geochemistry of the Toro, Northern Nigeria: U–Pb and Rb–Sr charnockitic and granitic rocks of the basement evidence. J. Afr. Earth Sci., 9:227–234. complex around Ado-Ekiti, southwestern Nigeria. Ph. D. Thesis, University of London, U. K Dodds, A. R and Ivic, D., 1998. “Integrated Geophysical Methods Used for Groundwater Studies in the Olorunfemi, M. O and Okhue, E. T., 1992. Murray Basin, South Australia in Geotechnical Hydrogeological and Geologic Significance of a and Environmental Geophysics,” Society of Geoelectric Survey; Ile-Ife, Nigeria. Journal of Exploration Geophysicists. Mining and Geology. 28, 221-229.

Falconer, J. D., 1911. Geology and geography of Omosuyi, G.O., Ojo, J. S and Enikanselu, P. A., 2003. northern Nigeria. Public Macmillan, London. Geophysical Investigation for Groundwater around Obanla-Obakekere in Akure Area Within Iloeje, N. P., 1981. A New Geography of Nigeria (New the Basement Complex of Southwestern Revised Edition) Publish by Longman Group, Nigeria. Jour. of Mining and Geology 39, (2): London, pp.32-45. 109-116.

Keller, G. V and Frischnect, F. C., 1966. Electrical Owoyemi, F. B., 1996. Updated Geologic map of Akure, Methods in Geophysical Prospecting. Ondo State Pengamon Press, New York, pp.96 Steinich, B and Marin, L. E., 1996. Hydrogeological Lashkaripour, G. R., Sadeghi, H and Qushaeei, M., Investigations in Northwestern Yucatan, Mexico, 2005. Vertical electrical soundings for groundwater Using Resistivity Surveys. Ground Water, 34: assessment in Southeastern Iran: A case study. 640-646. doi:10.1111/j.1745-6584.1996.tb02051 J. Appl. Sci., 5: 973-977. .x

Mogaji, K. A., Olanyanju, G. M and Oladapo, M. I., 2011. Tubosun, I. A., Lancelot, J. R., Rahaman, M. A and Geophysical evaluation of rock type impact on Ocan, O., 1984. U-Pb Pan-African ages of two aquifer characterization in the basement Charnockite-granite associations from South- complex areas of Ondo State, Southwestern Western Nigeria. Contrib Mineral Petrol 88:188- Nigeria: Geo-electric assessment and 195. Geographic Information Systems (GIS) approach., International Journal of Water Vander Velpen, B. P. A., 2004. RESSIST Version 1.0. Resources and Environmental Engineering Vol. M.Sc. Research Project, ITC. Delft, 3(4), pp. 77-86, May 2011 Netherlands.

Obaje, N. G., 2009. Geology and Mineral Resources of Wright, J. B., 1970. Controls of mineralization in the Nigeria, Lecture Notes in Earth Science pp.9-32 Older and Younger tin fields of Nigeria. Econ Geol 65:945–951 HYDROGEOPHYSICAL STUDY OF PARTS OF CHARNOCKITE TERRAIN OF AKURE 121 Zohdy, A. A. R., Eaton, G. P and Mabey, D. R., 1974. Application of surface geophysics to groundwater investigations. Tech. of water Resources Investigation of the United State 0geological Survey Book, D1, pp. 42-55