The results of the data obtained using 2D ERT were analysed with the EarthImager 2D Software and they vary from profile to profile outlined in the sections below.
4.1 Ward 1
Profile 01 is located at coordinate N 6°58’60.723, E 3°41’13.7405 to N 6°59’00.104, E 3°41’52.8400 with a profile length of \(840m\) (Fig. 3). From the inverted resistivity section, it was evident from the section that the geological formations for this profile were characterised with measured resistivity values that ranged from \(20.4\varOmega m\) to \(1832\varOmega m\) within a depth of \(174m\). The result from the ERT suggested a geological environment with lateral differences along the survey lines. The resistivity model indicated the presence of sand in the topsoil and this occupied a depth of \(25m\), except a small zone of aquifer between lateral distance \(610m\) and \(680m\). This portion is actually close to a stream around that location. Below this layer comprise a body of lateritic soil with resistivity ranging from \(500\varOmega m\) to \(1800\varOmega m\) extending eastwards from a depth of \(10m\) downwards between \(440m\) to \(780m\). A low resistivity anomaly occurred below lateral distance of \(340m\) at a depth of \(26m\) extending to \(150m\) indicating huge potentiality of groundwater and it is a huge aquifer as depicted in the resistivity section. The resistivity of this aquifer zone varies from \(27.8\varOmega m\) to \(100.0\varOmega m\) representing the deposition of clay saturated with water. This is a potential site for exploitation of the groundwater resource at deeper depth for long term sustainability of the aquifer system in the area. On the eastern side, the high resistivity body (\(500\varOmega m\) to \(1800\varOmega m\)) is completely massive and devoid of groundwater between lateral distance of \(440m\) to \(790m\).
Profile \(02\) is situated at coordinate N 6°59’05.571, E 3°40’54.3604 to N 6°58’50.893, E 3°40’48.7011 with a length of \(498m\) using \(6m\) electrode spacing and a total of 84 electrodes. A depth of \(78m\) was investigated and the resistivity section ranged from \(11.6{\Omega }m\) to \(327{\Omega }m\) as shown in Fig. 3. The resistivity model indicated the presence of topsoil and sand with resistivity values that ranged from \(53.7{\Omega }m\) to \(287.6{\Omega }m\) within a depth of \(12m\). Below this section, was a low resistivity zone which ranged from \(11.6{\Omega }m\) to \(35{\Omega }m\) and occured between \(15m\) and \(40m\) depth. This lateral layer extended from the north to the south of this profile. This zone is inferred as the water bearing horizon at a shallow depth which is suspected to be of low yield. Profile \(02\) depicts three horizontal layers with topsoil layer of \(10m\) thickness and sand formation (\(53.7{\Omega }m\) – \(287.6{\Omega }m\)). This layer is underlain by a thick layer of weathered/ moderately weathered formation (\(11.6{\Omega }m\) to \(35{\Omega }m\)) laterally and is extended up to \(40m\) depth. The second layer occupied the depth between \(8m\) and \(40m\) and had resistivity that ranged from \(\tilde11{\Omega }m\) to \(35{\Omega }m\). It consisted of saturated clay and was regarded as the most favourable zone for groundwater exploration within the site. The third resistivity zone represents a body of saturated sandy clay and shows higher resistivity values ranging from \(50{\Omega }m\) to \(82{\Omega }m\) when compared to the second layer.
Profile \(05\) is situated at coordinate N 6°58’54.424, E 3°41’24.0664 and N 6°59’10.295, E 3°41’15.1925 having a profile length of \(664m\) and electrode spacing of \(8m\) (Fig. 3). A total depth of \(105m\) was investigated and the inverted resistivity section ranged from \(25.7{\Omega }m\) to \(1607{\Omega }m\). The model shows a surface layer of low to moderately high resistivity with the northern region dominated by high resistivity values that ranged from \(615{\Omega }m\) to \(1600{\Omega }m\) and this was interpreted as lateritic topsoil. The mid to southern region shows a distribution of similar materials at the surface with lower resistivity values that ranged from \(400{\Omega }m\) to \(600{\Omega }m\) sparsely distributed. Other materials with lower range of resistivity values \((80{\Omega }m-200{\Omega }m)\) representing dry sandy clay occupied a large portion of the profile (\(0-450m\)) down to an average vertical distance of \(6m\). Saturated clay with resistivity values that ranged from \(26{\Omega }m\) to \(105{\Omega }m\) occupied a greater portion beneath the surface layer and was interpreted to be a zone with high groundwater potential. This zone has a thickness of \(10m\) which increased vertically to \(65m\) between lateral distance of \(213m\) and \(600m\). Other than this region, the remaining part of the profile comprised of moderate resistivity layer with resistivity value that ranged from \(180{\Omega }m\) to \(500\varOmega m\) and was classified as clayey sand occupying a greater portion of the section.
4.2 Ward 2
Profile \(03\) is situated at coordinate N 6°59’14.701, E 3°40’46.0956 and N 6°59’04.122, E 3°40’20.1059 with profile length of \(840m\) using \(10m\) electrode spacing. A total depth of \(197m\) was investigated and the resistivity ranged from \(6.4{\Omega }m\) to \(2954{\Omega }m\) as depicted in Fig. 4. The model indicated a contrasting resistivity body at the top layer. The top layer had resistivity that ranged from \(47.0{\Omega }m\) to \(3361{\Omega }m\) and extended from the surface to a depth of \(5m\) within the profile. Between lateral distances of \(590m\) to \(840m\), the resistivity was high and greater than \(500{\Omega }m\) and this zone was considered to be devoid of groundwater. At lateral distance between \(370m\) and \(380m\), the resistivity was low \(44{\Omega }m\); an indication of groundwater potential. The other resistivity values \(>200{\Omega }m\) suggested the composition of sandy clay within that zone. Beneath the topsoil, there was evidence of water saturation in small patches occurring between lateral distance \(100m\) and \(570m\) at a depth between \(5m\) and \(47m\) and had resistivity values that ranged from \(6.4{\Omega }m\) to \(70{\Omega }m\). This zone is suspected to be of low yield aquiferous zone. Below the second zone, there was no potential groundwater prospect as this region seems to be composed of dry clayey sand.
Profile \(04\) is situated at coordinates N 6°59’23.344, E 3°40’36.6894 to N 6°59’20.027, E 3°40’10.4492; parallel to profile \(03\). The profile length was \(840m\) with \(10m\) electrode spacing (Fig. 4). The depth of investigation was \(174m\) and the values of the resistivity ranged from \(1.9{\Omega }m\) to \(2767{\Omega }m\). The inverted model revealed the topsoil layer which comprised of laterite mostly and resistivity that ranged from \(570{\Omega }m\) to \(2315{\Omega }m\) between a distance of \(350m\) to \(840m\).
There was a progressive increase in the depth of the lateritic soil from a depth of about \(5m\) at a lateral distance of \(330m\) to a depth of \(87m\) at a lateral distance of \(840m\). There was a saturated clay region from lateral distance of \(330m\) to \(570m\) at a depth between \(20m\) and \(60m\) with resistivity values that ranged from \(17{\Omega }m\) to \(40{\Omega }m\). This region depicts a low yield aquiferous zone. Also, on the eastern part, there was a presence of clayey sand, indicated by low resistivity that ranged from \(200{\Omega }m\) to \(400{\Omega }m\) and dry sandy clay comprised the region below the saturated clay region.
4.3 Ward 3
Profile \(06\) is situated at coordinates with a profile length of \(840m\) using \(10m\) electrode spacing (Fig. 5). A depth of \(174m\) was investigated and the resistivity values ranged from \(51.6{\Omega }m\) to \(929{\Omega }m\). From the inverted resistivity model, a surface layer of moderately high resistivity body \((230{\Omega }m\) to \(920{\Omega }m)\) was revealed from the surface to a depth of \(15m\). Between lateral distances of \(310m\) to \(380m\), a low resistivity zone \((110{\Omega }m-150\varOmega m)\) was present, and this suggested a water body close to this region. This zone appeared to be the recharge site of an aquifer body which occupied the west side of the profile. This aquifer is favourable for groundwater exploration as it occupies to a depth of \(13m\) to \(126m\) and could be a prospective borehole location. Other than this region, smaller prospective groundwater zones appear in patches centrally between lateral distance of \(441m\) and \(469m\) with a thickness of \(16m\), at a distance of \(553m\) to \(575m\) with a thickness of \(20m\) and towards the eastern part of the profile between \(730m\) to \(774m\) at a depth of \(22m\) to \(51m\). Other regions in this profile comprise moderate of high resistivity value \((>200{\Omega }m)\) interpreted as dry sandy clay and compacted sandstone.
Profile \(07\) is situated at coordinate N 6°59’29.381, E 3°41’12.8413 to N 6°59’54.973, E 3°41’14.2475 perpendicular to profile \(06\). The profile length was \(840m\) using an electrode spacing of \(10m\) and a depth of \(170m\) was investigated while the resistivity ranged from \(35.7{\Omega }m\) to \(2659{\Omega }m\) (Fig. 5). The resistivity layer indicates the presence of a three-layer model represented by the 2D section. The first layer comprises of the lateritic topsoil and sand formation with resistivity that ranged from \(200{\Omega }m\) to \(2659{\Omega }m\) from the surface to a depth of \(25m\). This layer is immediately underlain by a low to high saturated layer with resistivity that ranged from \(37{\Omega }m\) to \(190{\Omega }m\) and a thickness of about \(50m\) average. This layer is favourable for groundwater exploration within the area. The third layer represented a body of sandstone, clayey sand and sandy clay having moderate resistivity values that ranged from \(240{\Omega }m\) to \(650{\Omega }m\).
Profile \(08\) is situated at coordinates N 6°59’52.164, E 3°40’58.0130 to N 6°59’49.440, E 3°41’12.6751 parallel to profile \(06\) and the profile length was \(498m\). A depth of \(105m\) was investigated and the resistivity ranged from \(42.5{\Omega }m\) to \(1562{\Omega }m\) (Fig. 5).
The resistivity model revealed a contrasting resistivity body at the top layer. It ranges from \(82{\Omega }m\) to \(127{\Omega }m\) at a distance of \(214m\) to \(252m\), \(450{\Omega }m\) to \(1200{\Omega }m\) at lateral distance of \(374m\) to \(495m\) and \(170{\Omega }m\) to \(300{\Omega }m\) interspersed between the first two resistivity zones. Beneath the top layer, from a depth of \(5m\) to \(38m\) on the western part, a low resistivity body (\(42.5{\Omega }m\) to \(91{\Omega }m\)) which suggested the presence of groundwater table within the area was observed. It suggested the possibility of high yielding hand dug wells extending from lateral distance of \(0m\) to \(200m\). Also, between lateral distance of \(330m\) to \(418m\), a saturated body was also identified from a depth of \(18m\) downwards. This suggested huge potentiality of groundwater and this site appears to be favourable for borehole drilling within the region. The other regions within the model seem to be comprised of sandy clay.
Ward 4
Profile 09 is situated at coordinates N 7°00’00.625, E 3°41’01.9037 to N 7°00’12.174, E 3°41’08.9277. and a profile length of \(415m\) was investigated using \(5m\) electrode spacing. The depth of investigation was \(99m\) and the resistivity section ranged from \(49.4{\Omega }m\) to \(6113{\Omega }m\) (Fig. 6). From the inverted resistivity model, a surface layer of moderately high resistivity body (\(100{\Omega }m\) to \(1000{\Omega }m\)) was revealed except at lateral distance of \(348m\) to \(360m\) where the resistivity value was very high (\(2998.2{\Omega }m\) to \(6113.2{\Omega }m\)) and indicated the presence of basement rock and coarse sand to a depth of \(10m\). Between lateral distance of \(304m\) and \(345m\), the resistivity was lowest (\(49.4{\Omega }m\) to \(82{\Omega }m\)) and occurred at a depth of \(11m\) to \(31m\). This zone appeared to be the only favorable zone for groundwater exploration within the area. At lateral distances from \(38m\) to \(208m\), the resistivity was relatively high \((1000{\Omega }m\) to \(4105{\Omega }m\)) and revealed the occurrence of sand stone and quartzite rock within the depth of \(8m\) to \(41m\). This zone also appeared between lateral distance of \(374m\) to \(400m\) at a depth of \(5m\). Other regions had moderately resistivity values \((300{\Omega }m-620{\Omega }m)\) which can be interpreted as shale/dry sandy clay.
Profile \(10\) is situated at coordinate N 7°00’03.079, E 3°40’53.5964 and N 7°00’05.049, E 3°41’05.0389 perpendicular to profile \(09\). The profile length was \(415m\) and electrode spacing of \(5m\) was used. A depth of \(99m\) was investigated and the resistivity section ranged from \(82{\Omega }m\) to \(6783{\Omega }m\) (Fig. 6). The model revealed the presence of topsoil and sand with resistivity that ranged from \(82.3{\Omega }m\) to \(1150{\Omega }m\) within a depth of \(6m\). Below this layer, between lateral distance of \(18m\) and \(317m\), a large high resistivity body occurred (\(1000{\Omega }m\) to \(6782{\Omega }m\)), and this suggested the presence of massive quartzite rock from a depth of \(9m\) to the end of the profile. Other region within the profile revealed moderate resistivity (\(600{\Omega }m\) to \(1000{\Omega }m\)) which indicated the presence of compacted sandstone. From the inverted resistivity section, there was no groundwater prospective zone within the profile.
DISCUSSION
The study revealed the lithological composition of the subsurface within of the study area. It revealed resistivity values ranging from \(20.4{\Omega }m\) to \(1832{\Omega }m\); \(11.6{\Omega }m\) to \(327{\Omega }m\) and \(25.7{\Omega }m\) to \(1607{\Omega }m\) for profile \(01\), profile \(02\) and profile \(05\) at depths of \(174m\), \(78m\) and \(105m\) respectively in Ward 1. The geoelectric model composed of laterite, sand, saturated clay, sandy clay, and clayey sand. Profile 02 had the lowest resistivity values recorded within the town. Due to the low resistivities observed, this location has the highest groundwater prospect. The results obtained in this region were similar to the results obtained at Ode-Remo in the study by Ariyo and Adeyemi in 2012 and this region can be depicted as a sedimentary zone.
Ward 2 revealed the resistivity values to range from \(6.4{\Omega }m\) to \(2954{\Omega }m\) and \(1.9{\Omega }m\) to \(2767{\Omega }m\) for profiles \(03\) and \(04\) to a depth of \(197m\) and \(174m\) respectively. The lithology within the region compose of laterite, sandy clay, clayey sand and dry sandy clay and saturated clay deposits within the area. Profiles 03 and 04 are parallel to each other and revealed similar model composition. The region had weak/ poor groundwater potentials where available and only to a depth of about \(50m\). The groundwater potential in this region is low due to the high resistivities. The lithology of this region was inconclusive from the resistivity result and aquifer potential obtained in this study.
For Ward 3 profiles \(06\), \(07\) and \(08\) have the resistivity value to range from \(51.6{\Omega }m\) to \(929{\Omega }m\), \(35.7{\Omega }m\) to \(2659{\Omega }m\) and \(42.5{\Omega }m\) to \(1562{\Omega }m\) and the geoelectric model revealed the composition of laterite, sandy clay, clayey sand, sandstones, compacted sandstones and saturated clay up to depths of \(174m\), \(197m\) and \(105m\). Profile 08 revealed the best groundwater potential in the ward. The results obtained in the region and the moderately low resistivity values recorded depicted the region as a sedimentary terrain.
For Ward 4, the resistivity model ranges from \(49.4{\Omega }m\) to \(6113{\Omega }m\) and \(82{\Omega }m\) to \(6783{\Omega }m\) for profiles \(09\) and \(10\) respectively and their depths of investigation were \(99m\). The geological model layer of the region revealed the presence of shale, dry clay, clayey sand, sandy clay, coarse sand, sandstone, compacted sandstone, quartzite rock, basement rock and fractured basement. Profile 09 had the lowest groundwater prospective zone while at Profile 10, there was no noticeable confined aquifer structure. The results obtained in this ward were similar to the results obtained in the adjourning town as reported in the study by Ariyo and Adeyemi (2009) conducted in Fidiwo and Ajebo. Due to the composition of the studied region and the geology of the adjourning towns, it could be concluded that the ward was a basement complex region.