AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

© 2015,Science Huβ, http://www.scihub.org/AJSIR

ISSN: 2153-649X, doi:10.5251/ajsir.2015.6.3.58.68

Evaluation of effective soil depth and heterogeneous sub-surface mapping using electrical resistivity geophysical methods to aid oil palm plantation

planning at Ile-oluji, Ondo State, Nigeria.

O.J Agunbiade and A.I Ojoawo

Department of Physics, University of Ibadan, Ibadan, Nigeria.

ABSTRACT

Considering the general decline in the production rate of oil palm in Ondo State in recent time, it is expedient to embark on the development of site specific principles of crop management based on the variability of soil and hydrological properties for proposed oil palm plantation project in the state so as to obtain optimum productivity. Electrical resistivity method of geophysical survey was used to investigate a proposed oil palm plantation farm land with the presence of rocky outcrops at Ile-oluji, Ondo State which is confined within latitudes 7

o12.128

1 N and 7

o12.187

1 N and

longitudes 4o 50.303

1 E and 4

o 50.419

1 E for the purpose of mapping the heterogeneity of the

sub-surface, delineating the textural class of the soil and the evaluation of the effective soil depth so as to make a proper recommendation for oil palm plantation usage. Using a Geopulsetera meter for data acquisition, twenty (20) Vertical Electrical Sounding (VES) data were acquired on the proposed land adopting the schlumberger electrode configuration. The acquired VES data were presented as depth sounding curves and further interpreted based on access to the iterative computer modeling software using a finite difference algorithm.The result was used to estimate the geoelectric parameters of the soil layers at each of the sounding points and presented as geoelectric sections.T he site was found to have resistivity and overburden thickness/depth thresholds of the top soil and weathered layers between 72 to 428.1Ωm and 0.8 to 14.5 m respectively. Points around VES 3, 6, 10 and 19 were delineated to have a very thin overburden thickness (between 0.8 m and 2.4 m) and therefore considered as non-supportive for effective root system growth because of the shallow basement around the stated VES points. About 50% of the resistivity values of the topmost geoelectric layer delineated fall within 101 and 200 Ωm range which typified a clayey/sandy loamy soil and a geological formation with interconnected pore, therefore, can store nutrient and transmit water for effective root system growth thereby facilitating optimum productivity.

Key words: Electrical Resistivity, Soil and Hydrological properties, Schlumberger electrode, configuration, Vertical Electrical Sounding, Overburden thickness, Ile-oluji. Nigeria.

INTRODUCTION

The production of oil palm crop for export in Nigeria has been inhibited by a limited rate of expansion. With the exploding population, increasing industrialization and urbanization, the available areas with fertile soils for agricultural production are on the reducing side hence precision agriculture and careful soil management are needed for optimum agriculture production. One of the challenges facing the adoption of precision agriculture technology is the identification of productivity related variability of soil properties accurately and cost effectively (Lund et al., 1998). Accessing spatial variability of soil properties generally often require high density and repetitious sampling which is costly, time consuming and labour intensive.

Oil palm generally classified as Elaeis guineensis is a tropical tree crop which is widely grown in Nigeria and some other countries in West Africa and is considered a highly effective crop in terms of oil production and energy balance. Oil palm grows in almost all types of soil but for optimal growth and production condition, a well-drained, deep, fertile loamy to clayey loam/sandy loam soil are the most suitable for cultivation (Goh and Chew, 1994). Establishment of Oil palm plantation is a long-term and money intensive, but highly pay-off project. In many regions, the future development of new areas for agricultural production is limited by shortage of productive lands because many Nigerian farmers involved in the production of Oil palm are faced with the lack of fertile and deep soil for their cultivation especially in basement complex zones and rocky

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outcrop regions. Some Oil palm stands in an observed plantation with rocky outcrops in Ile-Oluji have been noticed to have stunted growth even after 10 months to a year of healthy after the field planting process. Also, some palm trees were found uprooted during a rainfall accompanied by wind due to lack of adequate deep soil that can hold/support the fibrous root system. The inability to access water and nutrients which eventually led to the increase in inflorescence abortion rate has been found to be a major factor contributing to lower productivity of fruit bunches in these areas. Accessing information about variation of soil electrical properties and effective soil depth with conventional methods of digging and excavation is extremely slow and rigorous (Parasnis, 1997). Therefore, it is justifiable to carry out a relatively fast and less rigorous electrical method of geophysical survey which makes it possible to define areas of electrically contrasting soil layers, provides information about spatial and vertical variation of soil electrical and hySsmouse@aesop.rutgers.edudrological properties and depth of rocky layer/basement within the sub-surface as there is no record of such investigation on the proposed farmland.

The aim of this work is to measure the vertical variation in the resistivity of the subsurface materials of a proposed oil palm plantation at Ile-oluji, Ondo

State for the purpose of detecting the depth of subsurface rocky materials that poses threat to the growth and well establishment of oil palm root system and to develop recommendations on the farmland usage for oil palm plantation.

Physiography and geology of the study area: The study area is located at Ile-oluji, Ile-oluji/Oke-igbo L.G.A., Ondo State. It is confined within latitudes 7

o12.128

1 N and 7

o12.187

1 N and longitudes 4

o

50.3031 E and 4

o 50.419

1 E. The farm site is

underlain by the Precambrian basement complex rocks of South-Western Nigeria. The basement rock exposures are however found as lowland outcrop in few places within the survey area particularly where basement is shallow and erosion activities are prominent and surrounded by mountains on the eastern part of the area. However, according to Rahaman (1976 & 1988a), the area is underlain by migmatite-gneiss-quartzite complex with the banded gneiss and biotite granite being the major units while the minor units include quartz schist, granodiorite, granite gneiss and quartzite (Figure 1). The basement is covered by the superficial overburden derived from the weathering of the basement rocks and the farmland is characterized by relatively gentle undulating terrain with elevations between 269 m and 282 m.

Fig. 1: Geological map of study area.

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THEORY

Electrical resistivity method is one of the oldest geophysical survey methods. It employs the introduction of a known quantity of artificial source of current into the ground via point electrodes called current electrodes. The potential difference due to the injected current can be measured in the vicinity of the current flow. Information on the form and electrical properties of the subsurface inhomogeneity can then be derived from the effective or apparent resistivity of the subsurface when the current and potential differences are known.

For a conducting material of length l, resistance R and cross-sectional area A, the resistance R offered by this conducting body is directly proportional to the length l and inversely proportional to the cross-sectional area A

That is

1

2

Where is the resistivity of the material.

Recall from Ohm’s law which relates the potential difference, electric current I and resistance R

That is V = IR 3

R = V/I 4

Equating (2) and (4)

=

*

5

But

is the electric field E and

is the current

density J. Therefore,

J =

6

since conductivity is the inverse of resistivity,

that is;

therefore equation (6) becomes

J = 7

To understand the concept of current flow in the ground, we need to consider a homogeneous conducting half space with constant resistivity under an insulating medium. Let us consider a single point source of current at the surface of the homogeneous medium. Current flows radially from the electrode so that the current distribution is uniform over hemispherical shells centered on the source. At a distance r from the electrode, the shell has surface area of 2πr

2 so that the current density J is given by

J =

=

8

Equating (6) and (8) and rearranging, we have

E =

9

The potential at a distance r is obtained on integrating (9) from infinity to r

Vr = - ∫ = ∫

=

10

Equation (10) is used to calculate the potential at any point on or below the surface of a homogeneous half space.

For a general four electrode array as shown in figure 2 below,

Fig. 2: The general four electrode array for resistivity method.

The potential at M due to the current at A is

While potential at M due to the current at B is

Therefore the potential at M due to current at A and B

is the sum of the contributions

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Also, the resultant potential at N due to current at A and B can be derived similarly as

The potential difference (∆v) between M and N due to

A and B, is ∆v =

=

∴

11

From (4), we can see that R =

, so that (11)

becomes

= KR where K is the

geometric factor which depends only upon the relative position of the four electrodes. Equation (11) defines the general resistivity equation for four electrodes. Since the Earth medium is heterogeneous and anisotropic, the resistivity becomes the apparent resistivity aand is defined as the resistivity of an electrically homogeneous and isotropic half space that would yield the measured relationship between the applied current and the potential difference for a particular arrangement and spacing of electrodes.

The schlumberger electrode array was used in this study to determine overburden thickness. The array is best used in carrying out vertical electrical sounding VES with ultimate goal of observing variation of resistivity with depth.

In schlumberger electrode array, the current and potential pairs of electrode have a common midpoint but the distance between potential electrodes is very smaller than the current electrode spacing. The midpoint of the array is kept fixed while the distance between the current electrodes is progressively increased. This causes the current line to penetrate to ever greater depth, depending on the vertical distribution of resistivity.

With reference to figure 3 below

Fig. 3: Special geometries of current and potential electrodes for Schlumberger configuration

Where r1

, r2

, r3

and r4

Apparent resistivity a

=

12

Hence for schlumberger array, the apparent

resistivity a is given by a ( )

13

MATERIALS AND METHODS

In this study, vertical electrical sounding VES approach of electrical resistivity method was adopted to determine the electrical resistivity and depths of the subsurface layers with a highly sensitive Geoplulseterrameter using schlumberger electrode configuration. This array was applied due to its relative fast speed, low cost of field operation and reduced man power.

For the purpose of investigation, three geophysical horizontal profiles were established, each along 100 m distance within the proposed oil palm plantation. A total of twenty VES stations (5 points at a distance of 25 m apart per profile and 5 randomly selected points) were marked out for the VES survey. The overburden in the basement area is not as thick as to warrant large current electrode spacing for deeper penetration, therefore, the current electrode spacing AB used was 84 m, that is AB/2 = 42m. The locations of each VES point were recorded with the aid of Global Positioning System (GPS) meter. The equipment arrangement consist of a pair of current and potential electrodes which are driven into the subsurface to make a good contact with the Earth at each VES point and connected to the terrameter by means of cables. After the setup has been completed

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and the current sent, the Earth medium resistance reading at each VES point was automatically displayed on the digital readout screen and then recorded on the data sheet. The apparent resistivity

of the subsurface was then derived using a= K where K is the geometric factor.

The VES data were presented as depth sounding

curves by plotting the apparent resistivity a in (Ωm) against half of current electrode spacing AB/2 in (m) on a bi-logarithmic coordinate transparent paper. Matching the field curve with theoretical model curves made the quantitative interpretation possible and this involves partial curve matching using master curve in conjunction with auxiliary curves. The preliminary interpretation and initial estimates of the resistivity and depth of the various geoelectric layers at each VES station which was obtained from the partial curve matching was later used as input into computer software (WINRESTIST) for a fast computer-assisted interpretation. Repeated iterations were made on the initial geoelectric model until a satisfactory fit to the field data was obtained and the error is minimized to an accepted level.

RESULTS AND DISCUSSIONS

The geographic coordinate information of the survey area was presented as contour maps. A 3-dimensional surface modeling as shown in figure 4a was used to model the presence of rocky outcrops in the survey area. It can be deduced that the elevation is minimum around the north-eastern part of the farm land, highest at the center and gradually reduces towards the south-western part of the area. Figure 4b shows the superposed base map of the VES points and the contour map.

The computed geoelectric parameter corresponding to VES data obtained from the survey area and the lithological interpretation were presented in table 1. Three resistivity sounding curve types were obtained from the survey area and these are; A – Type (3

layers in which 1< 2< 3), H – Type (3 layers in which

1> 2< 3) and K – Type (3 layers in which 1< 2> 3) curves. However, there are some sounding points which show two geologic layer cases. The percentage frequency of occurrence of the curve types are 60% for A-type, 25% for the two layer type, 10% for H type and K type has 5% occurrence. Typical computer iterated curve types derived from the survey are shown in figure 5a – 5d. The geoelectric parameters which were modified appropriately with the iteration procedure repeated for satisfactory fit to be obtained were used to make

geoelectric sections which are as shown in figure 6a - 6e. The geoelectric section of the VES analyses was used to give a detailed pictorial lithology interpretation of the subsurface layers and constituents relating the several layers associated with the VES which include the topsoil, partially weathered basement layer and fresh basement.

Generally, topsoil resistivity of the medium in the range of 20 to 100Ωm suggest clayey and porous layer while resistivity between 100 to 350Ωm suggest porous clayey sand/sandy loam layer and resistivity values above 350Ωm are diagnostic of laterite/lateritic materials or partially weathered near-surface crystalline outcropping rocks in the area. The resistivity and thickness parameter of the overburden geoelectric layer in the study area was used to assess the vulnerability of oil palm growth since an effective oil palm rooting system, presence of adequate soil water and nutrient requires a deep, fertile, well drained loam clayey/clayey sandy loam and a gentle slope to prevent waterlog (Opeke, 1992). Also, Mogaji et al, 2011 stated that rooting network is greatly supported by sufficient soil overburden thickness and low hydraulic conductivity.

Fig 4a: A 3D surface modeling

Fig 4b: Superposed base map of the VES sampling points and the contour map

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In the present study, the northern, extreme southern, extreme south western and central parts of the study area are considered to have sufficient overburden thickness that will support optimal root growth of oil palm hence maximum productivity can be achieved considering every other meteorological factors favorable.

The second order thickness parameter of all the materials above the fresh basement at each of the VES locations obtained from the computer software curve interpretation were also utilized in generating the overburden thickness map of the survey area as shown in figure 7a. The overburden thickness map shows areas with relatively thick overburden suitable for oil palm cultivation (3 m and above) and an averagely thin overburden which is not suitable for such cultivation (less than 3 m). However, the depth to basement from Earth surface varies between 0.8 and 14.5 m shallowest at VES 6 and deepest at VES 20. Figure 7b was used to show a statistical distribution of the overburden thickness of the survey area.

The iso-resistivity map shown in figure 8a gives the variation in the resistivity values of the topsoil within the study area. The variation of the soil resistivity may be explained by the wide textural variation in the topsoil (Olorunfemi and Fasuyi, 1993; Mohammed et al., 2012). The resistivity of the survey area vary from 72 Ωm to 2.1 Ωm which is relatively high suggesting soil textural classification ranging from clayey sand to stony/lateritic loamy sand soil from weathered bedrock. The resistivity is relatively high (between 300 to 450Ωm) at the northern part of the area (VES13), eastern part at VES19 and around VES1, VES2 and VES 7 towards the southern part of the largest outcrop in the study area. The resistivity of the topsoil is generally low with resistivity that is less than 250Ωm in the extreme north-west, north-east, south-west, south-east with the minimum resistivity at VES6 and VES 20. Figure 8b shows the statistical distribution of the resistivity of topsoil of the survey area.

Based on the geophysical investigation carried out on the study area while keeping other optimal meteorological factors constant, the evaluation of soil potential for optimum major tree crops especially oil palm growth and production is based on two

thresholds; resistivity and thickness of the upper two layers and the geology. However, the two strata/columns are considered most important zones as far as oil palm plant root penetration is concerned. The identification of areas/strata with relatively thick columns (not less than3m) of the sandy loam/clayey sand and resistivity range between 72 and 428.1 ohm-m may have suggested a desirable site for oil palm soils.

Fig. 5a: A typical computer iterated 2 layers curve Fig. 5b: A typical computer iterated A - type curve

in which 1< 2

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Fig. 5c: A typical computer iterated H – type curve Fig. 5d: A typical computer iterated K-type curve

Table 1:Geoelectric parameters andtheir lithological interpretation corresponding to VES data obtained from the survey area

VES NUMBER

LAYER LAYER RESISTIVITY (Ω-m)

THICKNESS (m)

DEPTH (m)

LITHOLOGIC DESCRIPTION CURVE TYPES

1

1 2

428.1 1062.7

4.3 -.-

4.3 -.-

Stony topsoil (loamy sand) Fresh basement

A 2 layer curve

2 1 2

318.7 931.9

2.9 -.-

2.9 -.-

Stony topsoil (loamy sand) Fresh basement

A 2 layer curve

3

1 2

337.0 1101.9

2.2 -.-

2.2 -.-

Stony topsoil (loamy sand) Fresh basement

A 2 layer curve

4

1 2 3

147.0 441.0 14,040.0

0.9 4.5 -.-

0.90 5.4 -.-

Topsoil (loamy sand) Partially Weathered basement Fresh basement

A

5

1 2 3

185.5 519.8 12,206.3

1.6 4.1 -.-

1.6 5.7 -.-

Topsoil (loamy sand) Partially Weathered basement Fresh basement

A

6

1 2 3

72.0 1412.5 978.0

0.8 3.4 -.-

0.8 4.2 -.-

Topsoil(sandy clay) Fresh boulder Fresh basement

K

7

1 2

405.3 857.1

3.0 -.-

3.0 -.-

Stony topsoil(loamy sand) Partially weathered basement

A 2 layer curve

8

1 2 3

259.5 762.9 2092.9

1.1 8.9 -.-

1.1 10 -.-

Topsoil(loamy sand). Partially Weathered basement. Fresh basement

A

9 1 2 3

187.6 574.4 1396.2

1.0 6.1 -.-

1.0 7.1 -.-

Topsoil(loamy sand) Partially Weathered basement Fresh basement

A

10

1 2 3

231.4 1572.9 6663.7

2.4 11.1 -.-

2.4 13.5 -.-

Topsoil (loamy sand) Fresh basement Fresh basement

A

11

1 2 3

177.7 323.0 670.0

1.2 6.4 -.-

1.2 7.6 -.-

Topsoil(sandy loam) Weathered layer (sand) Partially weathered basement

A

12 1 2

173.5 346.0

1.1 3.5

1.1 4.6

Topsoil(sandy loam) Weathered layer(sand)

A

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3 518.6 -.- -.- Partially weathered basement

13

1 2 3

426.9 270.3 1090.5

0.9 3.3 -.-

0.9 4.2 -.-

Stony topsoil(loamy sand) Highly weathered layer (sand) Fresh basement

H

14

1 2

174.2 4094.4

5.6 -.-

5.6 -.-

Topsoil(sandy loam) Fresh basement

A 2 layer curve

15

1 2 3

141.1 265.2 8997.0

1.0 7.5 -.-

1.0 8.5 -.-

Topsoil (sandy loam) Highly weathered layer (sand) Fresh basement

A

16 1 2 3

171.4 643.3 1001.6

1.0 5.1 -.-

1.0 6.2 -.-

Topsoil(sandy loam) Partially weathered basement Fresh basement

A

17 1 2 3

229.5 571.5 8622.9

1.0 7.9 -.-

1.0 8.9 -.-

Topsoil(loamy sand) Partially weathered basement Fresh basement

A

18 1 2 3

150.0 300.0 810.0

1.5 3.5 -.-

1.5 5.0 -.-

Topsoil(sandy loam) Sandy weathered layer Partially weathered basement

A

19 1 2 3

341.9 193.4 743.4

1.2 1.4 -.-

1.2 2.6 -.-

Stony topsoil(loamy sand) Highly weathered layer Partially weathered basement

H

20

1 2 3

106.9 360.7 5040.4

1.6 12.9 -.-

1.6 14.5 -.-

Topsoil (sandy loam) Weathered basement Fresh basement

A

Figure 6a: Geo-electric section of profile 1 Figure 6b: Geo-electric section of profile 2

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0

2

4

6

8

Fre

qu

en

cy

Overburden thickness

Figure 6c: Geo-electric section of profile 3 Figure 6d: Geoelectric section relating VES 6, 16 and 18

Figure 6e: Geoelectric section relating VES 1, 19and 20 Figure 7a: Overburden thickness map of the survey area

Figure 7b: Statistical distribution of the overburden Figure 8a: Iso-resistivity map for the top soil thickness of the survey area

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Figure 8b: Statistical distribution of resistivity

of topmost soil layer of survey area

CONCLUSION AND RECOMMENDATIONS

Basement often occur at shallow depths in the crystalline basement terrain, thus posing threat to agricultural activities such as tree crop cultivation. The geoelectric survey for the evaluation of subsoil for optimal oil palm production was undertaken on a proposed oil palm plantation in Ile-oluji, Ondo State. This is considered expedient considering a general decline in the production rate of the crop in Ondo State in recent times. From the above findings, relatively thick sandy loam/clayey sand and/or thin loamy sand covers constitute the most probable sites for optimum oil palm yield. The site was found to have resistivity and thickness/depth thresholds of the topsoil and weathered layers between 72 – 428.1 ohm-m and 0.8 - 14.5m respectively. Points around VES3, VES6, VES10, and VES19 were delineated to have a very thin overburden thickness (between 0.8m and 2.4m) and are considered as non-supportive to effective root system growth because of the shallow basement around the stated points. About 50% of the resistivity values of the topmost geoelectric layer delineated fall within 101 to 200Ωm range which typified a geological formation with interconnected pores and hence can store and transmit water for root system growth (Ololade et al., 2010). However, about 30% of the resistivity values of the same geoelectric layer (topsoil) estimated fall above 300Ωm bracket, thus suggesting considerable stony loamy sand.

While other environmental/meteorological factors play essential roles in choosing a site for oil palm production, geology and sub-soil appraisal remain limiting factors for optimum oil palm production in the state and other areas with similar geologic settings.

The integration of a 2-dimensional geoelectric survey and vertical electrical sounding should be conducted on the farmland so as to delineate the lateral and vertical variation of resistivity with a better resolution of the heterogeneity of the survey area. Furthermore, grain size analysis should be carried out in the area to evaluate the stone content of the subsoil which is also another factor that affects the productivity of oil palm.

REFERENCES

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Lund E., C.D. Christy, and P. Drummond., (1998). Applying soil electrical conductivity technology to precision agriculture. In: Abstracts of 4

th

International Conference on Precision Agriculture. July 19-22. St Paul, MN.

Mogaji K.A., Omosuyi G.O., and Olayanju G.M., (2011). Groundwater system evaluation and protective capacity of overburden material at Ile-oluji, Southwestern Nigeria. Journal of Geology and Mining Research Vol. 3(11). 294-304.

0

2

4

6

8

10

1-100 101-200 201-300 301-400 401-500

Fre

qu

en

cy

Resistivity (ohm-m)

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Mohammed M. Z., Olorunfemi, M. O. and Idornigie, A. I. (2012). Geoelectric sounding for evaluating soil corrosivity and the vulnerability of porous media aquifers in parts of the Chad Basin Fadama Floodplain, Northeastern Nigeria. Journal of S. Dev., Vol. 5, No. 7; Canadian Center of Sc.& Ed., doi:10.5539/jsd.v5n7. 111

Ololade, I. A. , Ajayi, I. R., Gbadamosi, A. E., Mohammed, M. Z., Sunday, A. G. (2010). A Study on Effects of Soil Physico-Chemical Properties on Cocoa Production in Ondo State. Modern Applied Science Vol. 4, No. 5; 35 – 43.

Olorunfemi M. O. and Fasuyi, S. A. (1993). Aquifer types and the geoelectric/hydrogeologic characteristics of part of the central basement terrain of Nigeria (Niger State). Jour. of African Earth Sciences. 16, 309-317.

Opeke, L. K. (1992), Tropical Tree Crops, Published by Spectrum Books Limited, Ibd. Reprinted, 1 – 322.

Parasnis D.S. (1997). Principles of applied geophysics. Chapman & Hall, 2-6 Boundary Row, London SE1 8HN, UK.

Rahaman M.A., (1976). Review of the Basement Geology of Southwestern Nigeria. In: Geology of Nigeria. Kogbe, C.A. (ed). Elizabethan Publishing: Lagos, Nigeria.

Rahaman M.A., (1988a). Recent advances in the study of the basement complex of Nigeria. In: P.O. Oluyide (Co-ordinate), Precambrian Geology of Nigeria. Geol. Surv. Nigeria. Publ., 11-43