Pre-foundation geophysical investigation of a site for structural development in Oka, Nigeria

ABSTRACT Frequency of structural failure globally has necessitated geophysical investigation of subsurface geology of a site for engineering construction works. Combined very low frequency electromagnetic (VLF-EM) and electrical resistivity methods were used to provide detailed information on subsoil profile for documentation and references for durable and sustainable construction works. Thirteen traverses were established from which geophysical data were acquired. Major conductive geological interfaces suspected to be faults/fractured zones were identified from the plots of VLF-EM data. These points serve as 50 sounding stations further investigated using Schlumberger electrode array with vertical electrical sounding technique and electrical resistivity imaging on selected four traverses of the site. The acquired data were processed, inverted and interpreted. VLF-EM 2-D inverted models revealed conductive zones at some locations suggesting incompetent zones, responsible for structural instability. Saturated clayey subsoil and uneven bedrock topography with depressions at some points could cause differential settling which has negative impact on engineering structures. Structural failure may arise from existence of concealed geological structures, deep weathering/fractured bedrock, heterogeneous and structurally deformed (F1–F16) subsurface geological setting. Thus, classified unstable sections are considered priority in structural design and construction to mitigate unforeseen challenges. Deep foundations in form of piers and piles are encouraged to avert structural failure.


Introduction
Geophysical methods are widely applied in engineeringgeological survey to provide quality information on the nature of subsoil, geologic sequence and structures. These methods offer valuable information concerning the early discovery of possible risky subsurface settings. The threat to civil engineering works are basically from concealed near surface; fractures, cavities, buried stream channels, sheared zones, voids, sink holes and inhomogeneities in the subsurface materials. Also, information regarding the geologic condition of the area is very important for safety of building foundations. Geophysical methods have been effectively used in near-surface engineering and foundation assessment (Sharma 1997;Ademila 2015). Electromagnetic and electrical methods among others are valuable tools for geo-engineering investigations to confirm the competence of foundation subsurface structures. Subsurface exploration is a vital step from which foundation stability response is acquired for the design and construction of sustainable civil engineering works (Oluwafemi and Oladunjoye 2013;Ademila 2015). Failure of civil engineering works makes no news as this happens on a daily basis in different geologic settings throughout the nation. The understanding of the properties and parameters of subsurface materials (rocks or soils) from measurements made at the surface of the earth provide subsurface information in solving engineering infrastructure problems. Civil engineering structures in Nigeria are constructed with no detailed information of the subsoil which acts as basic geomaterial upon which the foundations of these structures would be placed for support and stability. Structural failure is usually linked to substandard structural materials and poor plan with no consideration of the subsoil (Ademila 2018). Combined geophysical investigations provide the basic information of the subsurface sequence and structural disposition necessary for foundation design. This is to offer the assurance of the suitability of the subsoil and the type of foundation for building construction. The intrinsic reason for structural failure lies on inadequate understanding of geophysical and hydrogeological information as regards the nature/type of soils, geologic sequence and structures, which are the water bearing units detrimental to stability of structures. These are paramount to effectively characterise the heterogeneous subsoil properties beneath the engineering site and to enhance successful structural development. Site response studies involving geophysical approach serve as an efficient step of characterising subsurface geology of site for the design of civil engineering works from which detailed structural nature of the subsurface are generated (Khalil et al. 2010). This also makes provision for early discovery of potentially unstable subsurface conditions and accounts for the effects of spatial variability of the subsurface which can hamper soil-structure interaction and cause structural failure (Capilleri et al. 2018;Chandran and Anbazhagan 2020). Electromagnetic (VLF-EM) and electrical resistivity methods being nondestructive from which measurements are made at the surface of the earth have been effectively used for groundwater exploration, mapping and detecting groundwater contamination, mineral exploration, engineering site investigations and related geological structures, landfill and contaminant characterisation, archaeological prospecting and foundation instability analysis (Bayrak 2002;Cardarelli et al. 2007;Oluwafemi 2012;Oluwafemi and Oladunjoye 2013;Ademila 2015Ademila , 2021Pazzi et al. 2016;Ademila and Ololade 2018;Ademila et al. 2020). This has been supported with computing power and image processing algorithms for inversion of geophysical data sets for subsurface images and evaluation of geoelectric parameters (Loke 2000). The quest for stable and sustainable structural development prompted the need for subsurface characterisation of engineering site. Oka-Akoko, the study area being in Basement Complex geological setting requires a detailed knowledge of the geological and geo-engineering materials, nature of subsurface conditions of the construction site due to the heterogeneity and variations in the subsurface environments. This becomes imperative for development as the subsoil would interact with the structure on site. Hence, this research used combined very low frequency electromagnetic (VLF-EM) and electrical resistivity techniques to characterise subsurface geological disposition of the engineering site with the aim of identifying subsurface geological structures with possible engineering threat to proposed building projects. This will help in decision making on the suitability of the site for construction and to design effective structural foundations for sustainable building constructions. The sufficient subsurface data acquired would also give detailed information on the foundation type and dimensions of the proposed buildings. This serves as a preconstruction investigation of a site for structures in order to prevent tilting and sudden collapse of buildings with resultant needless loss of lives and valuables. These data would also guide the suitable location of borehole site in the area to support its potential groundwater development.

Area of study (location and geology)
The study area in Akoko Southwest of Ondo State, Nigeria, is between latitudes 7° 25ʹ N and 7° 30ʹ N and longitudes 5° 45ʹ E and 5° 50ʹ E (Figure 1) with approximately area of 42 square kilometres. It has boundary with Ugbe/Isua towards north, Ikun towards south west, Akungba towards southeast and Ayegunle/Oba-Akoko to the south. The topography of the area is distinguished by highlands in the form of hills and inselbergs and lowlands in the form of plains and valleys. It has highest elevation approximately 600 m (Figure 1). It is one of the most populous towns in Ondo Northern Senatorial district. The area is accessible through different footpaths and good road network particularly the Owo-Akungba-Iwaro and Isua-Ibillo-Okene roads. It is characterised by tropical rainforest belt of Nigeria having two distinct (rainy and dry) seasons. The mean annual rainfall is about 1650 mm with mean annual temperature, 27.5°C and relative humidity of over 70%. The vegetation is evergreen rainforest type characterised by some trees. The study area is drained mainly by Rivers Ogbehli, Agbagbara, Asawa and Ojomirin with a dendritic drainage network pattern. The survey site (National Open University of Nigeria) in Oka-Akoko is approximately 2.64 km 2 . Lithologically, the area is characterised by migmatite-gneiss-quartzite complex of crystalline rocks of Nigeria (Rahaman 1989). Field mapping exercise showed granite gneiss, grey gneiss and older granite rocks ( Figure 2) and minor occurrences of pegmatite, aplite and charnockitic rocks as the main lithologic units in the area. The grey and granite gneisses dominant the area (Figure 2). Rocks in the area are subjected to several phases of deformation with resultant hydrogeological structures, which aid groundwater accumulation and serve as groundwater prospective zones in the area. Aquiferous zones in basement complex constitute the weathered and fissured/fractured bedrock from which residents derived water for use through hand dug wells having depth in the range 2.87-12.43 m. This shallow depth of water in the area could contribute to structural failure as a result of interaction of foundation subsoils with water (Ademila, 2019).

Materials and methods
This study includes thorough geological mapping exercise to establish the lithologic rock units of the area. The coordinates and elevations of each location were measured with a global positioning system (GPS). This is done in an attempt to produce the geological map and geophysical field layout map of the survey area. A total of 13 traverses were established with seven traverses orienting approximately in NW-SE and six traverses in NE-SW directions along which VLF-EM and electrical resistivity data were acquired ( Figure 3). The VLF-EM measurements were conducted on the 13 traverses with 10 m station interval with ABEM WADI VLF system and the length in the range 250 to 600 m ( Figure 3). These traverses were set to guarantee adequate conductor coupling and the VLF instrument measures the horizontal (H p ) and vertical (H s ) components of magnetic field in the form of real and imaginary parts of vertical magnetic field component. These data are displayed as profiles by plotting raw real and filtered real components against distance (Figures 4a-16a). The qualitative interpretation of the profile presents the conductive zones in the subsurface at intersection of the raw real and positive peaks of the filtered real (Nabighian 1988). These data which are indicative of near surface geologic structures were thereafter processed also for qualitative interpretation. They were presented as inverted pseudosections with the aid of KH Filt software (Pirttijarvi 2004). The electrical resistivity method employed Schlumberger vertical electrical sounding (VES) technique and ABEM 1000 terrameter system was engaged to acquire VES data with electrode spread (AB/2) in the range 1-150 m. Fifty (50) VES acquired across the site were the major conductive points delineated from VLF survey along the traverses (Figure 3), to investigate the vertical changes in the resistivity distribution. The acquired data were processed manually (plot of apparent resistivity versus AB/2 on bi-log graph). The data were partially curve matched employing Zohdy (1965) technique and Orellana and Mooney (1966) using two-layer master curves and appropriate auxiliary charts as the first round investigation. This gives the resistivity and thickness of different subsurface layers at every VES station. The concluding phase of the interpretation was done with WinResist, an iteration modelling technique, where the model derived from the preliminary interpretation was inputted into the inversion algorithm (Vander Velper 2004). The values of resistivity and thickness obtained from the final model derived from the software were used to construct geoelectric sections (Figures 18b,19b,20b,21b,22,23,24,25 and 26) along the various stations. The 2-D electrical resistivity imaging employed dipoledipole array on selected four traverses to investigate lateral and vertical variations in conditions of the subsurface (Figure 3). Dipole spacing on the traverses was 10 m, and greater depth of penetration was attained by increasing the current dipole and potential dipole. The apparent resistivity values acquired were processed and inverted with the aid of DIPROfWIN 4.01 (Dipro for Windows 2001). This program computes an initial model and reduces the difference between the model and resistivity fields observed till a satisfactory fit is obtained. 2-D subsurface resistivity structures were interpreted with respect to subsurface lithology using the resistivity distribution, depending on the lateral and vertical continuity and geometry of the image responses as low continuous resistive subsurface substratum with resistivities ≤120 Ωm characterised by clay-rich material/water absorbing clayey layer/weathered basement, partially weathered/conductive fractured basement with resistivities between 130 and 950 Ωm and high laterally and/ or vertically continuous resistivities (>1000 Ωm) interpreted as fresh bedrock.

VLF-EM profiles and VLF-EM 2-D inverted models
The composite plots of the raw real and filtered real components against station positions as presented in Figures 4a-16a, enable qualitative identification of conductive zones in the subsurface. These positive peak anomalies of the VLF-EM profiles signify conductive zones and constitute the 50 vertical electrical sounding (VES) stations obtained along the 13 traverses across the study area (Figures 4a-16a). These conductive zones are indicative of faults/fractures, lithologic contacts, sheared zones, basement depression, weathered basement and other weak zones that enhance accumulation of water and form pathways for groundwater, and are disastrous to structural foundation of civil earth works. The Karous-Hjelt 2D inverted models give the pictorial distribution of conductive geologic features relative to depth (Figures 4b-16b). Varying conductive subsurface materials (green to red) were recognised trending differently across the sections.

Traverse 1
Conductive subsurface features were identified at distance of 75, 170, 255, 365 and 410 m along traverse 1 (Figure 4a). These features indicate conductive fractured zones. These zones although are significant in groundwater development but act as weak zones that could pose significant threat to

Traverse 3
Conductive bodies at distance 170, 240, 367 and 480 m along traverse 3 ( Figure 6a) indicate faults, fractures, depressions, clay and geologic contacts which are unsuitable for building construction. The 2-D inverted model identified several oval shapes of high conductive bodies across the area which suggest pocket of clay materials between station distance 100-250 and 300-520 m (Figure 6b). Identified conductive features on the profile relate to that on 2-D inverted model. The observed linear conductive features between distance 180-250 (F5-F5ʹ) and 300-370 m (F6-F6ʹ) signify incompetent geologic unit with potential subsurface structural foundation problem ( Figure 6b).

Traverse 4
Conductive structures at distance 60,145,200,355 and 410 m along traverse 4 ( Figure 7a) signify basement depression or fractured zones. These conductive zones accumulate groundwater and serve as pathway for groundwater, thus constitute weak

Traverses 5 and 6
The VLF-EM profiles revealed conductive features at distance 25 and 190 along traverse 5 ( Figure 8a It also shows resistive features, which may be due to closeness of outcrop to the location. The inverted sections in the survey area depict an uneven subsurface topography reflecting different degree of conductivities. The observed conductive linear features at these locations suggest the likelihood of conductive materials at depth, indicative of the presence of fractures and basement depressions constituting water collection zones and saturated clay which are unsuitable to support massive and multistorey building projects. The Identified conductive features on the profiles relate to conductive zones established in 2-D inverted models. The fractured zones (F8-F8ʹ and F9-F9ʹ) are engineering incompetent zones that could pose significant threat to structural foundation of the proposed construction works in the study area (Figures 8b and 9b), thus needed to be considered during foundation design and adequately addressed during construction for safe and sustainable structures.

Traverses 7, 8 and 9
The points of inflection of the raw real coincide with the positive peak of the filtered real at distance 125, 250, 310 and 490 m along traverse 7 (Figure 10a   as subsurface conduits for migration and accumulation of groundwater, unfavourable to construction projects as they permit interaction of subsoil with water resulting in reduction of its load bearing capacity.

Electrical resistivity results
The 50 VES stations investigated from the VLF-EM survey are displayed as sounding curves, tables, geoelectric sections and maps. The quantitative interpretation of the sounding curves provides geoelectric parameters used to generate the geoelectric sections (Figures 18b,19b,20b,21b,22,23,24,25 and 26). The geoelectric sections represent the geologic sequence mapped with respect to depth. The 2-D subsurface resistivity structures were interpreted with respect to subsurface lithology using the resistivity distribution, depending on lateral and vertical continuity and  geometry of the image responses and interpreted in terms of subsurface lithology. Results from the electrical resistivity method revealed the pattern of resistivity variations within the study area with insight into the geoelectric characteristics of the geologic units giving the engineering competency/suitability of each  layer with depth. Two curve types are identified from the site: HA and AA curve types (Table 1 and Figure 17). The curve types represent four distinctive lithologic layers: topsoil (top layer), weathered basement, partially weathered/fractured bedrock and fresh basement ( Table 1). The HA-curve type dominates the  study area with a percentage frequency of 82%, while AA-curve type has 18% of occurrence. The inverted subsurface 2-D resistivity structures along the four traverses are characterised by three different resistivity responses: low resistivity response (generally <120 Ohm-m), intermediate resistivity    responses (resistivity in the range 130-575 Ohm-m) and high resistivity responses having resistivity above 1000 Ohm-m.

Traverse 1
The 2-D electrical resistivity subsurface structure along traverse 1 is characterised by three different resistivity responses; low resistivity response (generally < 120 Ohm-m), intermediate resistivity responses   composition and water saturation of the subsoil are potential threat to the proposed civil engineering works. The low resistivity clay weathering materials along the traverse coincide with the high conductivity zones in the bedrock. The basement topography is irregular and forms depression at VES station 2 with depth to bedrock from 13.6 (VES 5) -18.5 m (VES 2) ( Figure 18b). Bedrock depression is a typical groundwater collecting zone in form of trough, receiving water discharging from the crest. The basement depression is overlain by highly conductive layers (clay), thus unsuitable to sustain substantial civil engineering structures. The partially weathered/fractured basement (261-467 Ohm-m) with thickness range 7.6-9.9 m cut across all the VES stations of this location. Resistivity values of this layer beneath the weathered layer indicate water-saturated fractured bedrock. Its existence with the basement depression constitutes potential weak zones that could pose a threat to the stability of the engineering projects. The fresh bedrock possesses resistivity 1052-1618 Ohm-m ( Figure 18b). Classified incompetent zones may expose the various proposed civil works to future failure, if necessary precautionary measures are not taken to mitigate future challenges. From the geologic setting of this location, soils at shallow depth are unsuitable as foundation geo-materials. Thus, shallow foundation type is not encouraged as the overburden thickness increases significantly with water saturated subsoil, which could result to subsidence/differential settling that poses risk to foundation of buildings resulting to failure even shortly after construction. Deep foundations in form of piers and piles are encouraged to transmit structural load to competent and reliable fresh bedrock for safe and stable construction except in cases of soil stabilisation.

Traverse 2
Subsurface image along traverse 2 is also characterised by the three different resistivity responses (Figure 19a)  Ohm-m of this unit is an indication of water saturation of the clay materials. Saturated clayey nature of the subsoil poses potential threat to the proposed building foundation. The water saturated clay along this traverse coincident with the high conductivity zones trending in different directions across the section. The topography is irregular which forms depression at VES station 7 with depth to bedrock from 15.1 (VES 8) -19.2 m (VES 7) (Figure 19b). The basement depression is overlain by highly conductive layers of saturated clay, thus unsuitable to sustain the proposed civil engineering structures. The partially weathered/ fractured basement has resistivity and thickness range 346-402 Ohm-m and 9.1-12.6 m respectively, which cut across all the VES stations at this location. Resistivity values of this layer indicative water-bearing fractured basement which form a viable site for groundwater development with maximum yield. This layer therefore forms an incompetent layer for building construction which could result to subsurface subsidence and differential settling of the subsoil with resultant building failure. The existence of this layer and the basement depression constitute potential weak zones that would threaten stability of civil engineering structures constructed on it. The fresh bedrock is with resistivity 1123-2195 Ωm (Figure 19b). Those categorised incompetent layers would expose the proposed earthworks to failure, if necessary precautionary measures are not put in place to mitigate future challenges. Soils at shallow depth at this location are unsuitable layer for foundation. Hence, shallow foundation is not encouraged because there is likelihood of subsurface subsidence and differential settlement for structures constructed on the soils. Deep foundations in form of piers and piles are encouraged for safe and sustainable construction except in cases of soil stabilisation.

Traverse 3
The 2-D resistivity image of the subsurface along traverse 3 displayed three resistivity responses (Figure 20a) (Figure 20b). The partially weathered/fractured basement  and range of thickness 8.0-11.5 m, cut across all the VES stations at this location. Resistivity values of this layer indicate water-bearing fractured basement which form groundwater potential zone within the location. Thus, the weathered basement and fractured/ partially weathered bedrock form two main aquiferous units in the area. Taking into consideration the resistivity value of aquifer units and its overburden thickness, VES 12 signifies a viable groundwater potential zone in the area for sustainable groundwater development. These layers therefore are incompetent for building construction because they could result to settling of parts of the building constructed on it. Also, interaction of the subsoil which form foundation base with water would reduce its load bearing capacity. The existence of these layers and the basement depression constitute potential weak zones that would threaten stability of civil engineering structures at the site. The fresh bedrock has resistivity values varying from 947-2217 Ohm-m (Figure 20b). The classified incompetent layers would contribute to the failure of proposed earthworks if not properly considered during the foundation design and construction to mitigate future challenges. The identified incompetent layers may pose risk to building. From the study of this location, future failure of earthworks may arise from the existence of concealed geologic structures and inhomogeneous subsurface settings as a result of abrupt changes in resistivity values.

Traverse 4
The 2-D resistivity image of the subsurface along traverse 4 displayed three resistivity responses (Figure 21a) the section of the traverse and between the low resistivity and high resistivity responses at 250-364 m, where it is observed below the trough filled with low resistive materials but intrudes to the surface at some other points of the location. The high resistivity response extends beyond 20 m depth forming dome-like basal high resistivity bodies beneath the intermediate resistivity zones at approximately 90-135 m and 240-290 (Figure 21a). The low resistive portions on the 2-D subsurface structure coincide with weathered bedrock, indicative of saturated clayey zones. The intermediate resistivity responses are coincident with the partially weathered/fractured basement on the geoelectric section. Also, the subsurface features on the inverted 2-D resistivity structure are coincident with the features on VLF-EM 2-D section. Traverse 4 shows four geologic units (Figure 21b), resistivity of the topmost layer varies from 34-461 Ohm-m with thickness range 1.3-6.8 m corresponds to clay, clayey sand and lateritic sand. The weathered bedrock is majorly clay (50-153 Ωm) and thickness range of 4.2-12.6 m ( Figure 21b). The resistivity range of < 80 Ohmm except beneath VES stations 18 and 19 is an indication of water saturated clayey subsoil. The water saturated clay along this traverse coincident with the high conductivity zones trending in different directions across the section. The topsoil and weathered layer (subsoil) are largely composed of clay and clay composition of the weathered layer indicates water saturation of the zone. Saturated clayey nature of the subsoil is an incompetent layer that cannot support the proposed civil works, thus, poses potential threat to building foundation due to exposure to excessive water leading to strength reduction of the soils. The basement relief is irregular and forms depression at VES station 18 with depth to bedrock from 13.6-24.8 m (Figure 21b). The resistivity values of the third unit in the range 301-467 Ohm-m with thickness range of 6.8-11.0 m lies beneath the weathered layer in all the VES stations of the location. Resistivity values of this layer indicate waterbearing fractured basement which form groundwater potential zone within the location. Thus, the weathered basement and partially weathered /fractured basement form the two main aquiferous units in the area. Relative to the resistivity value of aquifer units and its overburden thickness, VES 18 serves as a suitable groundwater potential zone in the area for sustainable groundwater development. These layers form incompetent zones for building construction which may result to subsurface subsidence and differential settlement. Building constructed on these classified aquiferous unit would definitely collapse as the interaction of the subsoil with water would result to reduction of the load bearing capacity of the soil, thus stability of the civil engineering structures constructed on it is not guaranteed. Failure of the buildings would be influenced by the near surface incompetent clayey subsoil with existence of subsurface structures; fracture, basement depressions and lithologic contact (Figure 21b). The classified incompetent layers would contribute to the failure of proposed earthworks if not properly addressed during the foundation design and construction, to prevent future failure. From this study, future failure of engineering structures may arise from the existence of concealed geologic structures like faults, fractures and inhomogeneous subsurface units as a result of abrupt variations in resistivity values of the location.

Traverses 5 and 6
Four different lithologic units are determined on traverses 5 and 6 ( Figure 22). The resistivity of the uppermost layer varies from 77 to 307 Ohmm with thickness in the range 0.8-9.5 m corresponds to clay, sandy clay and clayey sand/lateritic sand. The weathered layer is majorly clay with resistivity range of 51-175 Ohm-m and thickness range of 3.4-12.3 m (Figure 22). This clayey geomaterial in the weathered layer suggests low resistivity water saturated zone. The topsoil and water saturated unit (weathered layer) are largely clayey with resistivity values <176 Ohm-m except beneath VES station 24 only with resistivity value of 307 Ohm-m, indicative of water absorbing clayey subsoil. The soils at shallow depth of these locations are poor geo-engineering materials for foundation. The low resistivity clay weathering materials along the traverses coincide with the high conductivity zones in the bedrock. Bedrock depression is groundwater storage zone in the form of trough, receiving water flowing down freely from the crest and it is overlain by high conductive units/low resistive materials (clay). The basement relief is irregular and forms depression at VES station 21 with the thickest overburden at the location. The depth to bedrock varies from 11.8 (VES 24) to 31.7 m (VES 22) (Figure 22). The low resistive saturated layer and basement depression in the locations are subsurface structures favourable for groundwater development, but they are incompetent zones unsuitable to sustain and support substantial civil engineering earthworks, as they pose threat to structural stability. The partially weathered/fractured basement with resistivity varying from 360 to 510 Ohm-m and thickness range of 7.6 to 10.0 m cut across all the VES stations of the locations. The established incompetent regions identified as conductive zones, linear conductive features; fractures (F8-F8ʹ and F9-F9ʹ) from the VLF-EM 2-D inverted models, basement depression and the low resistive saturated units from the geoelectric section are inimical to structural earthworks. Possible failure of the proposed building may arise from concealed subsurface structures and abrupt lateral inhomogeneous geologic units as a result of large resistivity variations. The classified incompetent zones may expose the proposed construction works to future failure if necessary preventive measures are not put in place especially during structural design and construction to mitigate the future challenges. Soils at shallow depth <12.5 m are not suitable for building foundation; thus, shallow foundation is not encouraged because there is a possibility of differential settlement to structures constructed on soils at such depth. Deep foundations, except in cases of soil stabilisation in the form of piers and piles, are encouraged.

Traverses 7, 8 and 9
Four lithologic units are present in this area (Figures 23 and 24). The resistivities of the top layer vary from 183-507 Ohm-m along traverse 7 (Figure 23), 34-326 Ohm-m along traverses 8 and 9 ( Figure 24) with thicknesses, 0.8-1.6 m and 0.6-4.7 m, respectively. The resistivities signify clay/ sandy clay/clayey sand/lateritic sand. The weathered layer composed majorly of clay with resistivity range of 54-70 Ohm-m along traverse 7, 62-143 Ohm-m along traverses 8 and 9 with thicknesses in the range of 2.6-6.4 m and 2.6-7.2 m, respectively (Figures 23 and 24). The resistivity range of <105 Ohm-m of this unit with the exception of VES 34 is an indication of water saturation in the clay materials. The resistivity value of VES 34 in the weathered layer is 143 Ohm-m, characterised by sandy clay. Clayey and water saturated subsoil are incompetent layer that cannot support  civil works. Saturated clayey nature of the subsoil could negatively impact the integrity and stability of the building foundation. The water saturated clay along these traverses coincident with the high conductivity zones trending in different directions across the sections. The bedrock topography is irregular and forms depressions at VES stations 26, 29, 33 and 35 with depth to bedrock from 10.4 to 18.0 m and 11.7 to 18.1 (Figures 23  and 24). Bedrock depressions identified at some points of the study area could also threaten the foundation stability of the construction works causing differential settlement of the earthworks. The depressions are good groundwater potential zones but inimical to engineering earthworks. Thus, to avert the threat of ground variations, foundation should be designed to rest on competent rock layer. The third layer of the locations has resistivity variations of 400-629 Ohm-m and 305-524 Ohm-m and thickness range of 6.4-11.4 m and 4.6-14.9 m, which cut across all the VES stations at the locations. The weathered and partially weathered/fractured bedrock form two main aquiferous units of the area. Thus, taking into consideration the resistivity values of aquifer units and its overburden thickness, VES 26, 29, 33 and 35 are viable groundwater potential zones for maximum productivity and sustainability to enhance groundwater development in the area. These layers are incompetent subsoil layers that would permit interaction of foundation soil with water with reduction of load bearing capacity of the soil, posing risk of subsidence to construction earthworks within the survey area. The fresh bedrock is characterised by resistivity 1052-1610 Ohm-m (traverse 7) and 1068-4105 Ohm-m (traverses 8 and 9) (Figures 23 and 24). The classified incompetent layers would contribute to the failure of proposed earthworks if not properly considered during the foundation design and construction to mitigate future challenges. Resistivity values of subsoil strata dictate its engineering suitability; thus, the fresh basement is considered as sound and competent layer for the building constructions within the area. Future failure of engineering structures may arise from the existence of concealed geologic structures like faults, fractures, bedrock depressions and heterogeneous subsurface units. Thus, soils at shallow depth at these locations are unsuitable for foundation. Shallow foundation is not encouraged because there is likelihood of differential settlement and to avert the risk of structural failure after construction, deep foundations except in cases of soil stabilisation in the form of piers and piles are encouraged for sustainable construction.

Traverses 10, 11, 12 and 13
Four major lithologic units are distinguished in this area (Figures 25 and 26). The resistivities of the topsoil vary from 27-466 Ohm-m along traverses 10 and 11 (Figure 25), 219-456 Ohm-m along traverses 12 and 13 ( Figure 26) with thicknesses, 1.4-6.0 m and 1.2-2.5 m, respectively. The resistivities indicate clay, sandy clay and clayey sand and lateritic sand. The weathered basement corresponds to clay with resistivity range of 30-130 Ohm-m along traverses 10 and 11 and 21-73 Ohm-m along traverses 12 and 13 with thicknesses in the range of 4.0-9.5 m and 4.0-9.8 m, respectively (Figures 25 and 26). The resistivity range of subsoil ≤130 Ohm-m of this unit is indicative of water saturation in the clay materials. Clayey and water saturated subsoil are incompetent layer that cannot support civil works. Saturated clayey nature of the subsoil could negatively impact the integrity and stability of the building foundation. The water saturated clay along these traverses coincident with the high conductivity zones trending in different directions across the sections. The basement topography is irregular and forms depressions at VES stations 37,39,40,44,48 and 50 with depth to bedrock from 14.0 to 24.3 m and 12.1 to 22.9 m (Figures 25 and 26). Bedrock depressions identified at some points could threaten earthworks foundation stability causing its differential settlement. These depressions are good groundwater potential zones but inimical to engineering construction works. Thus, the threat of ground variations can be prevented, if structural foundations are designed to rest on competent rock layer. The partially weathered/ fractured basement of the locations has resistivity variations of 269-559 Ohm-m ( Figure 25) and 330-495 Ohm-m and thickness range of 8.0-11.4 m and 6.2-10.6 m, which transpire across all the VES stations of the locations. Resistivity values of this layer indicate water saturated fractured basement which constitute groundwater potential zone within the locations. Thus, taking into consideration the resistivity values of aquiferous units (weathered and fractured/partially weathered bedrock) and its overburden thickness, VES 37, 39, 40, 44, 48 and 50 are viable groundwater potential zones for maximum yield and sustainable groundwater development in the area. These layers are incompetent subsoil layers that would permit interaction of foundation subsoil with water with potential to pose threats to construction earthworks within the survey area. The fresh bedrock has resistivity values varying from 1246 to 3229 Ohm-m (traverses 10 and 11) and 1272 to 3923 Ohm-m (traverses 12 and 13) (Figures 25 and 26). The categorised incompetent layers would contribute to failure of the proposed civil-works if necessary preventive measures are  not put in place during foundation design and construction to mitigate future challenges. Resistivity values of subsoil strata dictate its engineering suitability; thus, the fresh basement is considered as sound and competent layer for the building constructions within the area. Future failure of engineering structures may arise from the existence of concealed geologic structures like faults, fractures, bedrock depressions and heterogeneous subsurface layers. Soils at shallow depth at these locations are unsuitable for foundation building. Shallow foundation is not encouraged for buildings within the area because there is likelihood of differential settling which can also pose considerable threat to the engineering structures after construction. Thus, for sustainable structural development in order to prevent the risk of structural failure in the area, deep foundation works in the form of piers and piles are recommended except in cases of soil stabilisation.
The direction of groundwater flow identified aid to establish the discharge zones or groundwater highyield zones which are detrimental to foundation works (Ademila and Saloko 2018). The surface elevation map (Figure 27a) demonstrates that groundwater moves from north to the south and northeastern regions to southwestern parts of the study area. The 3-D elevation map (Figure 27b) shows the topographic characteristics of the study area with the depressions serving as the groundwater storage centres within the area. The topographical high zones are the recharge points, while the topographical lows are the discharge areas. This suggests that structural foundations should be erected towards the northern and northeastern flank to prevent water ingress into foundation works and avert future structural failure.

Subsurface characterisation
Resistivity values of subsurface units dictate its engineering suitability and soil corrosivity classified based on resistivity of soil (Agunloye 1984;Baeckmann and Schwenk 1997), showed that low soil resistivity has impact on corrosivity of buried metallic pipes (part of building materials). The soil layer accommodating buried pipes must be noncorrosive otherwise the buried metallic pipes would rust, thus, failure of engineering structures in the area. High conductivity/low resistivity geologic unit is a good electrical conductor resulting to reduction in aeration. The depth of buried pipes usually do not exceeds 3 m, which is within subsoil layers of the study area with resistivity in the range 21-507 Ohm-m (Figures 28 and 29) and thickness variation of 0.6-14.6 m, indicating corrosive to practically non-corrosive. The buried metallic engineering construction works at some locations within the site would be at risk of being corroded. Thus, the fresh bedrock with higher resistivity is considered as sound and competent layer for the building constructions within the area shown on the fresh bedrock resistivity map of the site (Figure 30a). Foundation of various civil-works in the site should be designed to rest on competent bedrock (Figure 30a) for sustainable infrastructural development. 3-D map of the fresh bedrock resistivity of the site (Figure 30b) shows the variations of high layer resistivity which are good site for safe and sustainable construction, while that of low resistivity values are good  groundwater potential zones, disastrous to foundation of engineering structures. Deep foundation works are encouraged in form of piers and piles except in cases of soil stabilisation for stable engineering structures. Documentation of the results of this study is recommended for foundation  design and construction on the site and future development of other engineering sites. Structural characterisation map is usually useful in engineering, environmental geophysics and groundwater studies, fracture zones in this area are distributed over the observed geologic units, cross cutting in different directions and show their lithologic boundaries ( Figure 31). These features resulting from deformation of underlying parent rock are indicators of subsurface geological structures incompetent for civil engineering construction works due to its negative impact on foundation stability. Considering the results from the VLF-EM and electrical resistivity given in this study, it can be concluded that the two techniques are efficient in subsurface characterisation of a site for sustainable engineering construction works, revealing information on nature of the subsurface geological setting. The major problem discovered in this geophysical study is the existence of geological structures, uneven bedrock topography with depressions filled with low resistive geo-materials, deep weathering and fracturing of bedrock and saturated clayey subsoil in some locations. The engineering site with undulating bedrock topography is classified into stable and unstable sections ( Figure 32) following the results of the study. The stable section is considered as sound and competent layer for the building constructions within the area and recommended for substantial engineering structures coupled with the use of standard quality construction materials. The unstable sections constitute the deep weathering sections of the survey area with potentially unstable geologic materials that tend to conduit groundwater through the engineering site. They are considered as unsuitable/incompetent zones that cannot sustain civil works and should be avoided by the building developers/building contractors to avert the risk of structural failure. The presence of concealed geologic structures beneath the unstable section could lead to differential settling which poses threat to civil-works at the site. The established unstable section would contribute to failure of the proposed civil-works if necessary preventive measures are not put in place during foundation design and construction to mitigate future challenges. It is recommended that foundation of buildings should not be constructed on classified unstable zones. This bedrock topography should be taken into consideration and used as basis for design or construction planning of engineering structures in the site and to provide preventive measures against the risk of building failure.

Conclusion
VLF-EM electromagnetic and electrical resistivity techniques involving vertical electrical resistivity sounding and 2-D electrical resistivity imaging have been successfully utilised in the characterisation of subsurface geological setting of a site for sustainable structural development. This study was to give detailed information on the existence of subsurface geological structures with possible engineering threat to proposed buildings. This is viewed to help in decision-making on the suitability of the area for construction and design effective structural foundations for sustainable building constructions. The VLF-EM mapped conductive zones, fractures (F-F', F1-F1ʹ to F16-F16ʹ and F16") which are unfavourable to engineering construction works. These characteristic weak zones constitute structural instability in complex geological terrain if   precautionary measures are not put in place to mitigate possible challenges. Four distinct subsurface layers are delineated. Water saturated clayey subsoil at some points within the site is incompetent layer that cannot sustain civil engineering works. The saturated clayey nature of the subsoil could negatively impact the integrity and stability of building foundation which may extend to the region. Uneven bedrock topography with depressions at some locations could also threaten the foundation stability of the construction works causing differential settlement. This bedrock topography should be taken into consideration in providing appropriate precautionary measures against building failure. The engineering site is categorised into stable and unstable sections; the unstable sections being the incompetent zones would contribute to the failure of proposed construction works hence should be avoided, except proper preventive measures are put in place during foundation design and construction to mitigate future challenges. Failure of engineering structures in the site may arise from the existence of concealed geologic structures: lithologic contact, bedrock depressions, water saturated clayey subsoil, heterogeneous and structurally deformed (F1-F16) subsurface geological setting. Groundwater moves from north to south and northeastern regions to southwestern parts. Thus, structural foundations should be erected towards northeastern and northern sides to prevent water accumulation at base of the foundation. The subsoil at shallow depth of the location is incompetent and unsuitable for foundation; thus, shallow foundation is not encouraged. To avert the risk of structural failure after construction, deep foundations in the form of piers and piles are encouraged except in cases of soil stabilisation for sustainable structural development. Documentation of findings of results of this study is recommended for foundation design, construction of the site and future development of other engineering sites. This study would proffer significant solutions against building failure, guide in engineering investigation for planning, design and construction of new buildings, highway, bridges, dams and other engineering structures. It is also useful for groundwater exploration, environmental studies and forms baseline information for further geophysical investigation in a complex geological terrain. Thus, the techniques serve as invaluable tools efficient for subsurface characterisation for sustainable structural development.

Disclosure statement
No potential conflict of interest was reported by the author(s).