Investigation of soil moisture content over a cultivated farmland in Abeokuta Nigeria using electrical resistivity methods and soil analysis

1.1 Description of the study area

The study area is an agricultural farm managed by Directorate of University Farms (DUFARMS), Federal University of Agriculture, Abeokuta (FUNAAB). The study area is located at Latitudes 7^°14′4″N − 7^°14′7″N with Longitudes 3^°26′7″E − 3^°26′10″E and lies within a forest-savanna transition zone in southwestern part of Nigeria (Fig. 1). Abeokuta experiences two local climates (rainy and dry seasons). The wet season starts from March-October while the dry season occurs from November-March when the area is under the influence of North-easternly winds (Badmus and Olatinsu, 2010). The amount of rainfall varies between 750 and 1000 mm in the rainy season and 250–500 mm during the dry season (Akanni, 1992). Abeokuta is characterized by an undulating topography with elevation value ranging from 100 to 400 m above sea level (Akanni, 1992; Oloruntola and Adeyemi, 2014). The mean monthly temperature ranges between 25.7 °C in July to 30.2 °C in February with the mean annual temperature of 26.6 °C.

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Fig. 1

Map showing location of the study area.

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1.1.1

1.1.1 Geology of the study area

The study area falls within the Basement Complex formation of southwestern Nigeria. Abeokuta is located on crystalline basement complex of igneous and metamorphic origin (Gbadebo et al., 2010). The basement rocks are of Precambrian age to early Paleozoic age and extend from the north-eastern part of Ogun state, of which Abeokuta belongs and dipping towards the coast (Ako, 1979). The basement complex rock comprises of coarse-porphyritic hornblende-biotite-granodiorite, biotite granite gneiss, pegmatite, porphyoblastic granite gneiss, quartz-schist and amphibolite/mica-schist (Jones and Hockey, 1964; Kehinde-Phillips, 1992). The foliation and joints on these rocks control the course of the two major rivers of Ogun and Oyan causing them to form trellis pattern. The prolonged weathering of the crystalline rock has led to the development of regoliths of varying thicknesses which ultimately reduced the conductivity of the parent rocks (Badmus and Olatinsu, 2012). The degree of weathering also depends on the depth of the rock beneath the earth surface. Abeokuta belongs to the stable plate which was not subjected to intense tectonics in the past, thus the underground faulting system is minimal (Ufoegbune et al., 2009). Abeokuta is also said to be part of the transition zones of the southwestern Nigeria. The northern side of Abeokuta is characterized by pegmatitic veins underlain by granite while the southern part enters the transition zone with the sedimentary formation of the eastern Dahomey Basin. The western part of Abeokuta is characterized by granitic gneiss of less porous nature together with various quartzite intrusions (Key, 1992). At the southeastern and southwestern parts of the area is the outlier of the ISE formation of Abeokuta group which consist of conglomerates and grits at base and in turn overlain by a coarse medium grained loosed sand (Aladejana and Talabi, 2013). Outcrops of cross-bedded, ferruginous sandstones and pebbly sandstones have been noted near the base of the succession among the outliers south-east of Abeokuta (Jones and Hockey, 1964). The terrain of Abeokuta is characterized by two types of landforms, these are sparsely distributed low hills and knolls of granite, other rocks of basement complex and nearly flat topography (Adekunle et al., 2013). The soil of the study area is gravelly loamy sand underlain by undifferentiated basement complex of an alluvio-colluvial parent material (Busari and Salako, 2015). Taxonomically, the soil was classified as Arenic Plinthic Kandiudalf and Arenic Lixisol based on United State Department of Agriculture (USDA) and Food and Agricultural Organization (FAO)/United Nations Educational, Scientific and Cultural Organization (UNESCO) classification system respectively (Soil Survey Staff, 2006; FAO, 2015). The dominant rock type in the study area is migmatite gneiss. The geological map of the study area is shown in Fig. 2.

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Fig. 2

Geological map showing the rock type that underlies the study area (adapted from Jones and Hockey, 1964).

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2.1 Vertical electrical sounding (VES) survey

A total of six VES were carried out along the 2D profile lines laid within and outside the farmland using Campus Tigre Resistivity Meter in March 2016. Four lines were laid within the farmland in the E -W direction with a separation of 7 m between the image lines while the remaining two (2) profile lines with a separation distance of 48 m were measured along the entrances to the farm as shown in Fig. 3. The electrode arrangement employed for VES was Schlumberger with a maximum current electrode separation of 110 m. Each VES location was chosen at the middle of 2D profile line with a current electrode spread (AB/2) of 55 m. The VES resistivity data obtained on the field were partially curve matched before being computer iterated with WINRESIST software with a R.M.S error of less than 5.0 so as to obtain the true resistivity and layer parameters. The iterated geoelectric parameters obtained were used to generate geo-electric sections.

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Fig. 3

Field layout for the 2D Traverse Lines and VES Points.

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2.2 2D electrical resistivity survey

2D Electrical Resistivity survey was also carried out within the seasonally cultivated farmland. The profiles layouts were as described for VES survey. Each 2D profile line was 72 m in length. The electrode arrangement employed for 2D ERT was Wenner array configuration. Fig. 3 shows the data acquisition map for resistivity survey. The electrode separation distance for each traverse ranged from a = 3 to 18 m with a station interval of 3 m. The 2D ERT raw data obtained on the field were inverted through the use of RES2DINV software (Loke and Barker, 1996). RES2DINV program automatically determines the 2D resistivity model of the subsurface from the input 2D resistivity data using the smoothness constrained least square method (Sasaki, 1992). The iterative optimization method attempted to reduce the differences existed between the measured resistivity values and calculated resistivity values with the inversion model. The accuracy of fit is expressed in terms of RMS error (Loke and Barker, 1996).

2.3 Soil samples Collection

Twenty four (24) soil samples were collected along the profiles at depths of 0.5, 1.0, 1.5 and 2.0 m respectively on each profile for laboratory analysis of particle size distribution and soil moisture content. Collection of soil samples at these depths should offer information on the estimated depth to the shallow water table within the farmland (Olayinka and Oladunjoye, 2005) and also assist in evaluating the suitability of the soil for cultivation of tree crops with effective depths greater than 30 cm of most arable crops. Soil samples were collected with the aid of soil auger and core samplers. The collected soil samples were analyzed in soil laboratory of the Department of Soil Science and Land Management of FUNAAB, Nigeria. The particle size analysis of the collected air dried soil samples was done according to the procedures outlined by Gee and Bauder (1986) with the help of hydrometer method while the textural classification was done using the USDA textural soil classification system. Soil moisture content was determined using the weight loss method based on ASTM D4959-07 standard.

3.1 VES result

The results obtained from the VES are shown in Fig. 4. The study area is underlain by three or four layers of varying lithologies. Two resistivity sounding curve types were obtained and are mostly H-types $({\rho }_1>{\rho }_2<{\rho }_3)$ except at VES 1 which have KH type $({\rho }_1<{\rho }_2>{\rho }_3<{\rho }_4)$ . The range of the values of thicknesses of each layer for all the six VES points are given as: Topsoil ranges between 0.5 and 1.3 m, clayey sand layer between 1.1 and 11.9 m and weathered basement between 1.1 and 14.0 m. This is shown in Table 1.

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Fig. 4

Layer model interpretations for VES 4, VES 5 and VES 6.

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The topsoil resistivity values ranged from 78.0 to 1094.0 Ωm while the layer thickness ranged from 0.5 to 1.9 m. The range of resistivity values obtained for the topsoil for VES 1, 2, 5 and 6 were between 78.0 and 349.0 Ωm which is within that of sandy-loam soil class (Loke et al., 2004) while VES 3 has topsoil resistivity values of 1094.0 Ωm. The variations in topsoil resistivity could be as a result of different degree of compaction due to reworking activities at the farmland. The clayey sand layer resistivity values range from 110.0 to 275.0 Ωm with thickness values from 1.1 to 11.9 m. The weathered basement resistivity values lie between 19.0 and 274.0 Ωm while the layer thickness varies between 1.1 and 14.0 m. The fractured basement has resistivity values ranging between 160.0 and 893.0 Ωm. fractured basement columns were delineated beneath VES 1, 2, 3 and 4 while partially fractured basement were delineated beneath VES 5 and 6.

From the VES results, relatively thin sandy loam/sandy clay loam constitutes the topsoil of the study area.

3.2 Geo-electric section

The lateral and vertical variation in depth and thickness of the subsurface layers was revealed with the aid of geo-electric sections. The sections showed that the study area is underlain by geologic sequence consisting of the topsoil, thin clayey sand, sandy clay and fractured basement.

3.2.1

3.2.1 Cross section between VES 1 and 2

The cross section between VES 1 and 2 that serves as entrances to the farm is presented in Fig. 5a. It showed that the study area is underlain by four layers representing the topsoil, clayey sand, sandy clay and fractured basement. The first two units in the section is the overburden with resistivity and thickness values ranging from 78.0 to 349.0 Ωm/0.5 to 1.1 m and 26.0 to 98.0 Ωm/1.1 to 5.7 m respectively. The lowermost fractured basement resistivity values ranged from 160.0 to 475.0 Ωm.

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Fig. 5a

Cross section between VES 1 and 2.

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The topsoil layer has a near surface horizontal geometry while the weathered layer (predominantly clayey) has its thickness increasing towards the southern end of the profile.

3.2.2

3.2.2 Geo-electric section across VES 3, 4, 5 and 6

Fig. 5b represents the geoelectric section across the profiles within the farmland. The resistivity of the topsoil ranges from 112.0 to 1094.0 Ωm while that of the weathered basement ranges from 18.0 to 274.0 Ωm. The weathered layer (clay) with resistivity value less than 30.0 Ωm in both VES 5 and 6 are similar while that of VES 3 and 4 (clayey sand) are also identical in nature. The fractured basement resistivity value varies from 181.0 to 893.0 Ωm. The topography of this section is uneven with thickness range of 3.9–14.0 m and depth range of 4.8–15.3 m. The basement is much closer to the surface with a depth of 4.8 m occurring at offset 7.0 m towards the east axis. The fractured basement model resistivity is less than 500 Ωm in VES 5 and 6 justifying the incompetent and fractured nature (Aina et al., 1996). The resistivity values of fractured layer beneath VES 3 and 4 is >500 Ωm. The highest resistivity value of fractured layer with 893.0 Ωm occurs at VES 3.

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Fig. 5b

Geo-electric section across VES 3, 4, 5 and 6.

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3.3 Interpretation of 2D resistivity models

Fig. 6a−f show the inverse model sections of the subsurface and resistivity distribution derived from the 2D inversion of resistivity data. As shown in Fig. 6a, the resistivity-depth model shows relatively high resistivity variation on the near surface almost along the entire traverse. This may be due to high evaporation rate, high stiffness value and untilled state of the farm during the time of surveying. The resistivity of the top soil layer from the 2D resistivity model ranged from 176.0 to 600.0 Ωm up to a depth of about 3 m. This is an indication of unsaturated topsoil (loamy sand) with low soil moisture content. The first layer, which was 0.5 m thick with resistivity of 78.1 Ωm was the topsoil from 1D model obtained from VES 1 (Fig. 4a). An isolated conductive zone of resistivity values less than 20.0 Ωm (clay rich layer/loam) was noticed at lateral distance of 12.0–21.0 m while another relatively low resistive region within the bedrock were noticed at lateral distance of 38.0–62.0 m along the profile. This corresponds with a layer below the topsoil of VES1 with a resistivity of 26.2 Ωm of clay (moist soil) nature. A relatively high resistivity value (174.0–516.0 Ωm) appears as a transition interval between two conducting zones at lateral distance of 19.0–38.0 m along the traverse. Fig. 6b shows model section for traverse 2 with the topsoil (loamy sand) characterized by resistivity values of 176.0–561.0 Ωm with thickness ranging from 0 to 2.0 m. The 1D model for VES 2 indicates topsoil (loamy sand) layer with a resistivity of 349.0 Ωm at a depth of 1.1 m. The second layer with resistivity values of 56.0–99.0 Ωm and thickness 2.2–3.6 m represent highly weathered layer. The third resistivity anomaly zone represent a continuous spread of conductive region with low resistivity values <25.0 Ωm almost along the entire traverse from about 4.0–8.0 m depth. This region serves as the clay rich zone. This corresponds to the clay layer below the topsoil of VES 2 with resistivity of 98.0 Ωm at a depth of 6.0 m. Fig. 6c shows the inverted section of traverse 3, at horizontal distance of 5.0–68.0 m, the near surface has resistivity values of 98.0–561.0 Ωm suggestive of sandy loam topsoil. This is an indication of unsaturated top soil and localized lateral resistivity inhomogeneities. At a depth of about 3.8–7.5 m, there is a continuous low resistive region along the entire length of the traverse. This represents the presence of clay rich region in saturated condition up to a depth of about 8.0 m. Below 8 m depth is the presence of a weathered/fractured layer with resistivity values 31.0–99.0 Ωm at a lateral distance 20.0–52.0 m.

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Fig. 6

2D inverse resistivity model for traverses 1, 2 and 3.

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Fig. 6

2D inverse resistivity model for traverses 1, 2 and 3.

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The inversion model of traverse 4 (Fig. 6d) depicts near surface resistivity anomaly (176.0–314.0 Ωm) suggestive of sandy loam/loamy sand topsoil at lateral distances 7.0–15.0 m and 42.0–65.0 m along the traverse at a depth of about 1.5 m. The 1D model of VES 4 showed topsoil with resistivity value of 393.0 Ωm at a depth of 0.9 m (Fig. 4d). This may be due to high evaporation rate and unsaturated near surface soil or untilled status of the soil during survey time. This was followed by clay/humid soil at a depth of about 3.8 m up to a depth of 8.0 m. The weathered/fractured layers have resistivity values ranging between 31.0 and 176.0 Ωm at lateral distance 20.0–54.0 m below a depth of 8.0 m. The maximum depth of investigation for the profile model was about 10 m. Fig. 6e shows inverse model section of traverse 5 in which resistivity values of 176.0–314.0 Ωm was noticed at lateral distance 25.0–36.0 m and 62.0–65.0 m towards the end of the traverse up to a depth of about 1.5 m. This is suggestive of topsoil/sandy clay/sandy loam nature. The 1D model for VES 5 indicates that the resistivity of topsoil is 122.0 Ωm at a depth of 1.3 m followed by a sandy clay zone with a resistivity of 26.0 Ωm at a depth of 15.0 m. Weathered layer was noticed in the 2D model at lateral distances 5.0–24.0 m and 37.0–44.0 m with resistivity values 56.0–99.0 Ωm. Low resistive region with resistivity value <20.0 Ωm protruded to the surface at position 45.0–55.0 m along the traverse. This was followed by clay/sandy clay material with resistivity values ranging between 56.0 and 99.0 Ωm. In Fig. 6f, the 2D resistivity model showed relatively high resistivity anomaly close to the surface with values of 176.0–314.0 Ωm, indicating sandy loam/loamy sand topsoil at lateral distances of 15.0–25.0 m and 34.0–52.0 m along the traverse. The 1D model of VES 6 (Fig. 4f) indicates that the resistivity of the topsoil layer is 112.0 Ωm followed by clay (18.0 Ωm) and fractured basement zones respectively. The 2D resistivity model showed clay/moist soil region of low resistivity anomaly values <20.0 Ωm which occurs throughout the entire traverse. This was noticed from a depth of about 3.8–7.5 m. Below the clay rich zone is the presence of fractured/weathered bedrock layer with resistivity values ranging between 56.0 and 99.0 Ωm at a depth of about 9 m.

It should be noted that the 2D resistivity models of traverses 1 and 2 showed continuous occurrence of high resistivity anomaly on topsoil. This may be due to the fact that the two profiles serve as entrances to the farm. The VES and 2D imaging revealed soil type and topsoil effective depths suitable for a well developed root systems of cash crops such as Oil palm (Elaeis guineensis) and Cocoa (Theobroma cacao) cultivation (Goh and Chew, 1994; Mutert, 1999; Ibiremo et al., 2011; Agunbiade and Ojoawo, 2014). The 2D models also show that the clay rich/moist soil zones were noticed at approximate depth of about 3.8–7.5 m for traverses 2, 3 and 4 while it extends from the near surface up to 10 m depth for traverse 5 at lateral distance 45.0–55.0 m along the traverse. Comparison between results of VES and 2D imaging showed that there is similarity in thickness and resistivity values for some electrical layers/horizons.

3.4 Soil samples analysis

Soil classification test using particle size distribution result showed that soils at cultivated farmland belong to Sandy loam (Table 2). This result agrees with earlier work by Elias and Gbadegesin (2012) who reported that dominant soil type of the basement complex rocks are loamy sand or sandy loam underlain by a variety of alluvial or colluvial materials over coarse-grained granitic gneiss and pegmatite.

The values of soil moisture content at different soil depths of 0.5–2.0 m ranged from 45 to 74% (see Table 3). It was observed that the soil moisture content increases with depths for most of the traverses. This is in agreement with work of Halfmann (2005) and Olorunfemi and Fasinminri (2011). It was further observed that there is significant increase in soil moisture at 1.5–2.0 m depth. This corresponds with the transition from high resistivity near surface layer to relatively low resistivity weathered layer.