Evaluation of hydraulic conductivity of subsoil using electrical resistivity and ground penetrating radar data: example from Southwestern Nigeria
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Groundwater seepage has being linked to the cause of building failures in Erinle area, Ede metropolis, SW Nigeria. A comparative study of the hydraulic conductivity (K) of first two subsoil or subsurface layers underlying the area of investigation was carried out using data acquired from ground penetrating radar (GPR) and electrical resistivity (ER) surveys. Ground penetrating radar (GPR) and electrical survey (ER) were performed along eight traverses over the shallow subsurface. From these a total of eight radar sections and geoelectric sections were developed along an electrode separation of 80 m. The results from the radar section and geoelectric section show that the area under investigated is made up of three stratigraphic layers (laterite topsoil, weathered layer, bedrock). The radar sections show that the second subsurface layer (weathered layer) or subsoil is pervious as compared to the others. The weathered layer shows relatively smooth reflections suggesting high groundwater flow which accounts probably for the groundwater seepage observed. Hydraulic plots and Porosity confirms the pervious nature of the weathered layer as oppose to the other two layers as expressed by high hydraulic conductivity (K) values of 1.0 × 10−5–7.4 × 10−6 m/s and high porosity values of 2.16–5.06 which makes it liable to groundwater seepages. Both the ER and GPR surveys confirmed that the second subsurface layer (weathered layer) is more prone to groundwater seepage especially in traverses (1, 2, 3 and 5) where the relative thickness of the first layer (lateritic topsoil) is less than 5 m and in traverses (4, 6, 7 and 8) having a resistive layer (basement). This study shows the importance of geophysical investigation in detect of groundwater seepage and post-foundation investigations.
KeywordsRadar Resistivity Seepages Hydraulic conductivity Porosity
Groundwater seepages, is one of the basic problems associated with tropical environments especially in seasons of heavy rainfalls. Groundwater seepages are often found to be in close association with geological and tectonic factors for example, faults, fractures and cavities within the subsurface [6, 9]. The mechanical strength of any subsurface layer or subsoil is connected to its soil type, hydraulic conductivity (K), resistivity and porosity. These are basic properties considered by engineers when constructing roads or building foundations . These properties (hydraulic conductivity, porosity, permeability and water content distribution etc.) which could be extracted from the radar sections and hydraulic plots are important in the occurrence of groundwater seepage and the prediction of sustainable foundation in the study area.
The hydraulic conductivity (K) of subsoil is a primary factor considered in the analytical design of civil engineering structures and the tackling of environmental related issues like groundwater seepages, drainages, landfills and efficient sewage disposal units. The variation in hydraulic conductivities of subsoil is a function of its intrinsic permeability, degree of saturation, viscosity of fluid moving through the subsoil, the structure and lithology of the geological formation .
Several authors have posited varying approaches in the determination of hydraulic conductivity of saturated and unsaturated soils in the tropics and temperate regions of the world using varying techniques. Unlike other approaches adopted in the determination of hydraulic conductivity; the adoption of a geophysical technique (use of GPR and Electrical Resistivity surveys) provide low cost and faster technique for acquiring vast information on the physical properties of the soils [10, 27]. The use of geophysical techniques in the determination of hydraulic conductivity stems from the works of Matsui et al.  and Antonio Costa . Matsui et al.  showed that a direct relationship exist(s) between resistivity and hydraulic conductivity in granitic rocks while  showed a permeability-porosity relationship between pores spaces and fractures found within the subsoil. The relationship showed that though the subsoil might be porous, they might not necessarily be permeable (clay) except in cases where pores spaces within the subsoil are interconnected (weathered material).
The objective of this study is to investigate ground seepage occurrence through the evaluation of the hydraulic conductivity of subsoil (lateritic topsoil, weathered layer) and the basement viz a viz the integrity of foundations in the study area.
Geology of study area
Structural geology of study area
A study of the structural geology of the study area was essential as the study of geological structures provide clues to how groundwater seepages could weaken the foundation of buildings. Previous works show that rocks within the area under investigation has been subjected to some form of deformational episodes characterized by geological structures such as fractures, faults [2, 18, 19] and events of plasticity. The presence of these structures coupled with individual subsurface properties (porosity and permeability) has led to either an increase or a decrease in the occurrence of groundwater seepages viz a viz its negative effect on stability of buildings within the study area.
Ground penetrating radar (GPR)
The GPR is efficient in mapping out shallow targets (fractures or voids within subsoil etc.). This is carried out by the use of high-frequency electromagnetic pulses (usually 10–1000 MHz) which are generated and transferred into the ground. This way, dielectric discontinuities existing between subsoil are detectable electromagnetic pulse travels through it. It is the contrast in the dielectric permittivity between the subsoil that causes reflections: the greater the difference in dielectric permittivity (variation in textural, lithology and porosity of subsoil), the greater the coefficient of reflectivity  which translates GPR reflection pattern variations. Best results for GPR surveys are obtainable when topographic cover is smooth and when material penetrated is dry .
The study was carried out using the GSSI SIR–2000 System along eight traverses each about 80–100 m long with station-station separation of 10 m. The GPR System was coupled with a 200 MHz shielded mono-static antenna oriented in a broadside survey direction for the investigation and characterization of the shallow subsurface. The following parameters were then adopted for use in the processing of the subsurface data acquired; Data collection mode: continuous; range: 300 ns; samples per scan: 1024. From this survey, eight radargrams were generated and subjected to processes of dewowing, filtering by distance, AGC (automatic gain control) and NMO (normal moveout).
Electrical resistivity (ER)
The use of the ER method for geophysical exploration seems to be the most applied for geophysical technique in shallow subsurface investigation. This is applied through the use of the vertical electrical sounding (VES) technique which measures vertical changes of electrical resistivity. In terms of field logistic it’s economical, easy and straight forward to use.
Data acquisition for the study was carried out along the eight traverses established during the GPR survey. The Schlumberger configuration was adopted during the electrical resistivity survey with a half-current electrode spacing (AB/2) varying from 1 to 80 m. A total of 32 VES points along eight (8) traverses were occupied. The resistivity data obtained at the various stations were plotted against the half-current electrode spacing (AB/2) and processed using the WinRESIST software . Processed results were stacked together to develop 2D geoelectric section of the subsurface to better characterize the subsurface.
Results and discussions
GPR sections along the traverses
Geoelectric sections along the traverses
The lateritic topsoil has resistivity values ranging from 69 to 426 Ωm with a thickness layer range of 1.2–7.2 m. The lateritic topsoil is fairly compacted with moderate resistivity values. The weathered layer had resistivity range of 19–281 Ωm with a thickness range of 7.0–17.4 m with infinity thickness on some VES data stations. The bedrock had resistivity range of 459–5289 Ωm and thickness range of 4.9–8.6 m and an infinite thickness.
The weathered layer as compared to the other two subsurface layers or geological units exhibits the lowest range resistivity values (19–281 Ωm). This low resistivity values could be attributed to loosely bonded sediments and probably conductive fluids and clay minerals (with anomalously low resistivity values of less than 65 Ωm). Clay minerals exhibit a plasticity nature which allows it to swell when wet and contract when dry. A continuous motion of this within the weathered layer leads to the development of cracks and differential settlement. The presence of cracks and other inclined features (probably fractures) serve as conduit for water movement to the surface resulting to events of groundwater seepages in the weathered layer. This is seen to be most prominent among traverses in Figs. 10 and 11 with thicker columns of the weathered layer and anomalously low resistivity values (<65 Ωm) as compared to traverses in Figs. 12 and 13 with high resistivity values (>300 Ωm).
Depression zones observed in traverses 1 (between VES 3 and 25) and 5 (between VES 15 and 13) contains probably conductive fluid and elements of sandy clay minerals evident by their anomalously low resistivity values (<65 Ωm). The basement depression delineated in Figs. 6 and 7 possibly correspond to a fracture zone in the area. Forward modeling results generated by Adepelumi et al.  indicates that such basement depressions probably represent fractured bedrocks which could serve as groundwater accumulation centres.
Hydraulic conductivity, resistivity and porosity of subsoil
Correlation of results
Results obtained from the GPR survey and electrical resistivity (ER) Survey is presented in form of radargrams and geoelectrical sections respectively with the aim of correlation. That is to know how much similar results obtained from the two different surveys are in terms of evaluating the hydraulic conductivity of the subsurface layers.
From both surveys (GPR and ER) it is evident that the study area is made up three subsurface layers (lateritic topsoil, weathered layer and basement) as evident from their radargram reflection patterns and resistivity variations.
It is also evident from both surveys (GPR and ER) that the study area is made up of two distinct traverse set. One of which is made up generally of low hydraulic conductivity while the other is made up of high hydraulic conductivity. This is evident from the near smooth reflection pattern (weathered layer) and anomalous low resistivity values shown (Figs. 5, 6) whereas the other traverse set (Figs. 7, 8) displays subsurface layers with low hydraulic conductivity evident by planar reflections/moderate resistivity values (lateritic topsoil) and chaotic reflections/high resistivity values (basement).
Both surveys (GPR and ER) reflect that certain traverses (1, 2, 3 and 5) at depths of about 5 m reflect high hydraulic conductivity values while others (traverses 4, 6, 7 and 8) reflect moderate to low hydraulic conductivity values. The traverses with high hydraulic conductivities correspond to traverses with low resistivities (with pores, cracks being water-filled) which makes them liable to groundwater seepage events. The traverses with moderate and low hydraulic conductivities correspond to traverses with high resistivities and less liable to groundwater seepages.
Conclusions and recommendations
Integrated GPR and electrical resistivity data have been successfully used to relatively determine (varied reflection patterns) and quantify the hydraulic conductivities (K) of subsoil (lateritic topsoil and the weathered layer) found in the tropical region of southwestern Nigeria. The integrated data showed that the weathered layer with smoother reflections and K values of 2.0 × 10−5–6.7 × 10−6 m/s is more liable to groundwater seepage as compared to the lateritic topsoil (semi-planar reflection and K values of 2.4 × 10−5–9.0 × 10−5 m/s) and the basement (chaotic reflections and K values of 1.6 × 10−5–9.3 × 10−5 m/s). This been based on the variation in hydraulic conductivity values of the different subsurface layers (lateritic topsoil, weathered layer and the basement). Based on the K values houses situated along traverses 1, 2, 3 and 5 are mostly prone to groundwater seepage than houses along traverses 4, 6, 7 and 8. For houses along traverse 1, 2, 3 and 5; the adaptation of a dewatering system (construction of deep drainages) would be essential to direct away the flow of groundwater. Also deep trenches could be dug around foundations and then gravel packed to inhibit the effect of groundwater seepage. Based on the results obtained from hydraulic Vs. resistivity plots and the resistivity Vs. Porosity plots It could be suggested that the high values of the porosity, hydraulic conductivity observed probably enhanced the groundwater seepage experienced perennially in the area been investigated.
- 2.Akinniranye OA (1985) Structural analysis of the basement complex of the Obafemi Awolowo University campus. Unpublished B.Sc. Thesis Obafemi Awolowo University, Ile-Ife, NigeriaGoogle Scholar
- 3.Antonio C (2006) Permeability–porosity relationship: a re-examination of the Kozeny–Carman equation based on a fractal pore-space geometry assumption. Geophys Res Lett 33(2). AGU PublicationsGoogle Scholar
- 4.Archie GE (1942) The electrical resistivity log as an aid in determining some reservoir characteristics. Petrol Transact Am Inst Mineralog Metallurg Eng 146:54–62Google Scholar
- 5.Archie GE (1950) Introduction to petrophyshics of reservoir rocks. AAPG Bulletin 34:943–961Google Scholar
- 7.Conyers LB, Goodman D (1997) Ground Penetrating Radar for Archaeology. Walnut Creek, CaliforniaGoogle Scholar
- 10.Knight RJ (1997) The role of ground penetrating radar and geostatistics in reservoir description. The leading edge 1617, 1622, NovGoogle Scholar
- 12.Matsui T, Kamiide S, Park S (1977) An applicability of resistivity-based high density prospecting to ground survey of mountain tunnel. Tsuchi-to-kiso 45:20–22Google Scholar
- 13.Møller I, Jørgensen F (2006) Combined GPR and DC Resistivity imaging in hydrogeological mapping. In proceedings of 11th international conference on ground penetrating radar, June 19–22, 2006, Columbus Ohio. p 5Google Scholar
- 14.MuCurry P (1976) The geology of the precambrian to lower paleozoic rocks of Northern Nigeria. In: Kogbe CA (ed) Geology of Ile–Ife, pp 15–39Google Scholar
- 15.Odeyemi IB (1976) Preliminary Report on the field relationship of basement complex rocks around Igarra, Mid—Western State, Nigeria. In: Kogbe CA (ed) Geology of Nigeria, University of IfeGoogle Scholar
- 17.Olarewaju VA (1988) Petrology and Geochemistry of the Charnockitic and Associated Granitic Rocks of Ado-Ekiti-Akure area, Southwest Nigeria. In: Oluyide PO, Mbonu WC, Ogezi AE, Egbuniwe IG, Ajibade AC, Umeji AC (eds) Precambrian Geology of Nigeria, Geological Survey of Nigeria, pp 129–143Google Scholar
- 18.Olorunfemi MO, Olarewaju VO, Ajayi MA (1986) Geophysical investigation of a fault zone a case history from Ile-Ife, South western Nigeria. Geophysical Prospecting, pp 1277–1284Google Scholar
- 19.Olorunfemi MO, Okhue ET (1992) Hydrogeologic and Geologic Significance of a Geoelectric Survey at Ile-Ife, Nigeria. Nigeria J Min Geol 28(2):221–229Google Scholar
- 20.Oyawoye MO (1964) The geology of the Nigeria basement complex. J Nigeria Min Geol Metall Soc 1:87–482Google Scholar
- 21.Rahaman MA (1976) Review of the basement geology of South–Western Nigeria. In: Kogbe CA (ed) Geology of Nigeria, Elizabethan Publishing Company, Nigeria, pp 41–58Google Scholar
- 22.Rahaman MA (1988) Recent advances in the study of basement complex of Nigeria. In: Oluyide PO (ed), Precambian geology of Nigeria, Geological survey of Nigeria, pp 11–43Google Scholar
- 23.Reynolds JM (1998) An introduction to applied and environmental geophysics. Wiley, New YorkGoogle Scholar
- 25.Telford WM, Sheriff RE, Geldert LP (1990) Resistivity methods. In: Applied geophysics. Cambridge University Press, Cambridge, pp 523–524Google Scholar
- 26.Vander Velpen BPA (2004) WIN RESIST™. Electrical resistivity inversion programGoogle Scholar
- 27.Van Overmeeren RA (1994) Georadar for hydrogeologist. First Break 12:402–417Google Scholar
- 28.Welsh LA, Allen DM (2014) Hydraulic conductivity characteristics in mountains and implications for conceptualizing bedrock groundwater flow. Hydrogeology J; 2014. http://www.researchgate.net/journal/1431-2174_Hydrogeology_Journal 1007/s10040-014-1121-5. Accessed 10 Mar 2014
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