Surficial Geophysical Deduction of the Geomaterial And

Home , Aboh Mbaise, Imo River, Imo State

Cent. Eur. J. Geosci. • 3(4) • 2011 • 368-374 DOI: 10.2478/s13533-011-0033-3

Central European Journal of Geosciences

Surﬁcial Geophysical Deduction of the Geomaterial and Aquifer distributions at Ngor-Okpala local Government Area of Imo State, South Eastern Nigeria

Research Article

Obianwu I. Victor1∗, Chimezie C. Innocent1, Akpan E. Anthony1, George N. Jimmy1

1 Department of Physics, University of Calabar, Calabar, 540242, Cross River State, Nigeria

Received 23 June 2011; accepted 14 September 2011

Abstract: Twenty seven vertical electrical sounding (VES) proﬁles surrounding four known traverses were obtained in Ngor- Okpala local government area of Imo state to examine the subsurface geomaterials and the associated groundwater potential. The VES data, constrained by borehole data, provided useful information about the subsurface hydrogeologic and lithologic conditions. From the validated interpretation, the area assessed has loamy soil, medium grained sands, well-sorted medium-grained/gravelly sands and river sand as the lithologic succession from top to the bottom of the depth penetrated. The aquifers in the area were found in the medium-grained sands and well-sorted medium-coarse-grained sands. The aquifer depth for all-season groundwater that would be de- void of draw-down can be found at a depth range of 42-50 m. The resistivity maps of selected depths exhibit sharp resistivity changes at depth due mainly to undulating subsurface topography. A map of the distribution of the kσ-values shows that good quality groundwater can be found in most parts of the area. Keywords: Aquifer • lithology • VES • Ngor- Okpala and Schlumberger conﬁguration © Versita Sp. z o.o.

1. Introduction The distribution of the primary and secondary physical properties of the subsurface is diagnostic of the rocks and minerals of the subsurface which can only be obtained from the interpretation of these physical proper- In any geographical setting, the geophysics and the ge- ties through the close application of real data obtained ology of the subsurface changes from place to place. The from borehole lithologic logs [2]. At Ngor–Okpala local local geology at times may be slightly or significantly dif- Government area of Imo State, this work was designed ferent from the regional or general geology. On the basis to specifically discern lateral and vertical distributions of of this slight or significant change, the study of the local sediments from the top to the limiting current penetra- geology becomes paramount as the geomaterial deduced tion depth which was determined by the maximum current may be used to determine the economic potential of the electrode separations of the vertical electrical sounding area. Specifically, masked geotechnical and hydrogeo- method, employing a Schlumberger configuration. The logic information can be unraveled [1]. aquifer units of the deduced sediment were earmarked and described for the area. The information becomes the ∗ E-mail: [email protected] baseline data about the hydrologic and hydrogeologic pa-

368 Obianwu I. Victor, Chimezie C. Innocent, Akpan E. Anthony, George N. Jimmy

mation which was laid down in paralic, brackish marine fluviatile, coastal and fluvio-marine environments and this formation consists of interbedded sands and shale. The Akata Formation is overlain by the Agbada Formation and this formation is shalier than the above two forma- tions [4–7]. For this study, only the Benin Formation was involved. Coastal plain sands are made up of an alter- nating sequence of gravels and sands of different sizes which combine to form multiple aquifers [8]. The Benin Formation of the study area is expected to be relatively shallow and consists of predominantly coastal sediments which are essentially sand, sandstone, siltstone and minor clay and lignite series [9]. The area has sediments and sedimentary rocks formed either as terrestrial or marine deposits which may be unconsolidated or lithified with no relation to age [10].

2. Material and method Figure 1. Location Map of Ngor-Okpala L.G.A. Imo State (Adopted from Ministry of Land and Survey, Owerri). The materials used in this work were Oyo McOHm resistivity meter (Model 2115 A), G.P.S. radar, borehole rameters within the location and its environs. The area is lithologic logs and geologic maps. The vertical electrical riddled with failed boreholes which according to informa- sounding (VES) technique employing the Schlumberger tion gathered are due to wild cat drilling of boreholes that electrode configuration was used to determine the appar- have no scientific basis. ent resistivity in the study area. For each of the 27 VES profiles surrounding four traverses, the distance between 0 1 The study area is located between longitudes 7 05 and the potential electrodes was gradually increased in steps 0 1 0 1 0 1 7 19 E and latitudes 5 10 and 5 25 N. It is bounded in starting from 0.25m to 14m to obtain a measurable po- the north by Aboh Mbaise local government area, south tential difference. The half current electrode separation by Rivers State, east by the Imo River and west by Owerri was increased in steps starting from 1.5m to 350m. In West Local Government Area of Imo State (Fig.1). Climat- this configuration, all the four electrodes were arranged ically, the area has two distinct seasons: rainy and dry collinearly and symmetrically with respect to the centre. seasons which are described in common terminology as an The potential electrode separation was made very small equatorial climate. There is no sharp boundary between compared to the current electrode separation [11]. The the two seasons. The rainy season starts from March distance between the potential electrodes was increased and ends in October while the dry season commences in only when the signal was too small to measure. Apparent November and ends in February each year. According resistivity ρ a was calculated at the field using equation 1; to the Meteorological Service Department in Owerri, the mean monthly rainfall during rainy season is about 325mm n o AB 2 − MN 2 and this drops to about 65mm during dry season. The to- 2 2 V ρa = π • (1) pography deduced from a global positioning system (GPS) MN I shows that the study area is a gently undulating low-land whose height varies from a minimum of 40m to a maximum where (V/I) is the earth’s resistance measured by the of 122m above sea level. The average height of the study equipment and the remaining parameters constitute the area is about 71m above sea level. geometric factor which is a function of the potential elec- Geologically, the study area lies within the deltaic de- trode separation, MN and current electrode separation, positional environment of Southern Nigeria [3]. The up- AB [12]. permost formationof this environment is the Coastal Plain The apparent resistivity was plotted against half current sands otherwise known as the Benin Formation which is electrode separations on double logarithmic graph paper the youngest in the Niger Delta Sedimentary Basin of and the sounding curves were obtained for each of the pro- Nigeria. The formation is underlain by the Agbada For- files. Smoothing was done on the resulting manual curves

369 Surﬁcial Geophysical Deduction of the Geomaterial and Aquifer distributions at Ngor-Okpala local Government Area of Imo State, South Eastern Nigeria

Representative curve from the K Group. Figure 2. Representative curve from the KHK Group. Figure 5.

Figure 3. Representative curve from the HK Group. Figure 6. Correlation between borehole lithology log (BH) and geoelectric section at VES 21.

due to the possibility of inherent or irreducible extraneous signals which the precautions taken cannot remove. The resistivity obtained at different AB/2 became the input data for electronic interpretation which was done using Resist Software. Resist Software employs the principle of inversion with the help of a well-known resistivity trans- form. The backward difference formula was specifically used for the analysis. The interpretation produced mod- els which fit the acquired field data with the least root- mean square error between the observed and calculated resistivities. The method of iteration was performed until the fitting error between the field data and the theoret- ical model curves, examples of which are shown in Figs 2-5, became less than 10%. The interpretation was made Figure 4. Representative curve from the AK Group. valid by constraining the result by the lithologic logs of boreholes in the area (see example in Fig. 6).

370 Obianwu I. Victor, Chimezie C. Innocent, Akpan E. Anthony, George N. Jimmy

Figure 9. E-W Geoelectric section constructed from VES points.

Figure 7. NE-SW Geoelectric section constructed from VES points.

Figure 10. N-S Geoelectric sectionconstructed from VES points.

Figure 8. NW-SE Geoelectric section constructed from VES points. study area are shown in Figs7-10. The sections were drawn from the interpreted field data in conjunction with 3. Results and discussion the geology and the lithologic log of a nearby borehole (Fig.6). The geoelectric sections define the electrical resistivities of the sediments, the depth to bottom of the The representative curves in Figs 2-5 indicate that the sediments, thickness of the sediments, their horizontal area is grossly characterized with KHK, HK, AK and K- spreads and the sounding points. The resistivity values curve types. This explains the alternate high and low that were of close range and close depth for each of the values of resistivities found in all the sounding points. sounding points were used to produce the geoelectric lay- The H and K-curve types translate into saturated beds ers of the study area. Generally, four geoelectric layers of water. This speaks volumes of the high groundwater were identified with few sounding points extending to the potential for the wells whose depths fall within the prolific fifth layer within the study area. Also few sounding points aquiferous geomaterials. Since electric resistivity or its identified only three layers. Some layers did not neces- inverse (which is conductivity of sediments) depends on sarily conform in totality with the geologic description geology, water content, and salinity [13–16], it was highly presented by the lithologic log of borehole used in the necessary to correlate the VES data with the lithological study. This minor variation with geology did not make information obtained from adjacent boreholes (see Fig. 6). the interpretation invalid because some geological infor- It was this correlation that removed all forms of conjectures mation may itself be an interpretation [17] and should not in the interpretation. be considered to be the only solution as geophysics may The geoelectric sections of some sounding points of the disprove the geological hypothesis. The variations may

371 Surﬁcial Geophysical Deduction of the Geomaterial and Aquifer distributions at Ngor-Okpala local Government Area of Imo State, South Eastern Nigeria

really be due to the principle of suppression [18] which is common in the interpretation of VES data and also may be caused by the distance between the borehole and the VES stations used for correlation. In general, the first layer is thin and has a moderate range of resistivity values of 112-2,198Ωm and its thickness is between 0.5-5.8m. This layer runs almost horizontal in Fig. 8 and thins out at VES 7 and 13 of Fig. 7. The thin- ning out of this layer in some VES points is attributed to erosion in the area and this layer is identified to be loamy top soil. The second layer is of fairly high resistivity, with values between 213-12,529Ωm, and a thickness of 2.5-9.5m. It is very thin at VES 2, 7, 13, 18 and 23. In Fig. 8, it runs through the sounding points with apprecia- ble thickness and thins out at VES 23. The sediment in this layer is classified as medium grained sands. The third layer is comparatively showing lower resistivity values of 154-29,106Ωm with few exceptions and between 5-50m thick. It is thicker near VES 9 and thinner at VES 23 in Fig. 8. This layer is very thick at VES 5 and 6 and thins out in VES 7 and 13. The sediment here is classified as medium-coarse grained sand with impregnation of clay lenses, which brings about a drop in resistivity values in Figure 11. Resistivity maps of the study area, A,B,C,D and E show- most parts of the study area and high resistivity in other ing the apparent resistivity variation with depth. parts where loose sands dominate. The fourth layer consists of sediments whose grain sizes are classified as well sorted medium–coarse grained the horizontal plane defined by the selected depth values sands. The resistivity values range between 132 and within the study area. Maps A and B have relatively low 42,292Ωm. In VES 5, 6, 9, 16, 21, 24 and 27, the bottom density of contour lines, indicating that layers of sedi- of this layer is not defined within the maximum current ments at those depths, exhibit low level of inhomogeneity. electrode separation used in this study. However, its bot- This is reflected by the nature of the first three layers tom is clearly shown in VES 4, 7 and 13 of Figs 7, as well in Figs 7-10. However, across the 60m, 80m and 100m as VES 13, 18 and 23 of Fig. 9. Generally, the thickness depths, the corresponding maps: C, D and E, show high of this layer is in excess of 40m. density of contour lines especially at the flanks, which The fifth layer is of relatively low resistivity values which is an indication of higher level of inhomogeneity in the range between 332 and 2,150Ωm. Since the depth of the composition of the sediments and irregular topography, boreholes used in this study did not get to this layer, one resulting from possible changes in the depositional envi- cannot really deduce the nature of the sediment in this ronment during the period of deposition of the sediments. layer from the lithology of the area. However, with the The uneven nature of the deeper layers in Figs 7-10 is inferred resistivity values, it is suspected that the sediment diagnostic of the depositional history of sediments within should be of river sand and gravelly sand. The layer runs the study area. Apart from natural activities (like erosion, at the bottom of VES 4, 7 and 13 of Fig 7 as well as in structural irregularities and changes in the composition VES 18 and 23 of Fig. 9 and its thickness is undefined and orientation of sediments within a layer), human activ- within the maximum current electrode separation of the ities (like discharge of dirty water from detergents, poor study. management of effluent, application of chemical fertilizers, insecticides, etc.) may constitute an additional input to As part of the deductions, apparent resistivity maps shown the sharp lateral changes in apparent resistivity observed in Fig. 11 were obtained by contouring apparent resistiv- in maps C, D and E. ities at selected depth values of 20m, 40m, 60m, 80m and 100m within the study area. The contours were respec- On the whole, the lithologic succession of sediments in the tively denoted by A, B, C, D and E. The maps give appar- area have been found from top to bottom to be loamy top ent resistivity variations with lateral displacement along soil, medium grained sand, medium coarse grained sand,

372 Obianwu I. Victor, Chimezie C. Innocent, Akpan E. Anthony, George N. Jimmy

Table 1. Ranges of the apparent resistivity of rocks (sediments) in the study area.

Rock types Resistivity (Ωm) Loamy top soil 112-2198 Medium grained sands 213-12529 Medium – coarse grained sands 154 – 27106 Well sorted medium coarse grand sand 132 – 42292 River send and gravelly sand 332 - 2150 well sorted medium coarse grained sand, and river and not likely to fail or draw-down at any of the seasons gravelly sands. Table 1 gives the summary of the sed- provided the screen is placed deep enough within the iments and their range of resistivity values deduced in aquifers. From a depth of 47m, any of the aquifers men- this study. From the analysis and interpretation which tioned above depending on the location can be reached are constrained by some local and regional geologic pre- and tapped for all-season groundwater. The study of this existing information, the aquifer is found to be in the fourth nature is considered to be of relevance in the manage- layer whose geologic unit is described as well sorted ment of groundwater schemes in the area and also it can medium–grained sand. This sediment is predominantly serve as a reconnaissance, as useful information within found in VES 1, 2, 4, 6, 7, 1 3, 21 and 23. It has a and outside the environs can be deduced. thickness in excess of 40m. The medium coarse grained As recommendation, wildcat drilling should be discour- sand which is the third layer in VES 5, 6, 9, 16, 21 and aged. Instead, additional information can be deduced 22 is also an aquifer with thickness in excess of 30m. At mostly in those areas within the study area that have no VES 3 and 24, the second layer may be an aquifer with lithologic logs. This can be achieved through reasonable geologic unit of medium grained sand and it is assumed generalization of the information obtained from the areas to be coarser with depth. This layer has its thickness in with geophysical data that are supported by lithologic excess of 60m. In all, the aquifers are unconfined (open) logs. Though the resistivity values of the aquifers encoun- types as predicated by the resistivity ranges and this im- tered in this study are high enough to suggest that the plies that recharge is by direct percolation of rainwater groundwater is safe and not brackish, water quality testing into the aquifer. is deemed necessary, because the aquifers are unconfined and susceptible to contamination from human activities at 4. Conclusion the ground surface. The knowledge of hydraulic conductivity, k, is one of the essential requirements for the determination of natural This study shows that Ngor-Okpala local government area water flow through an aquifer [? ]. The estimated av- is extensively characterized by sandy layer units from top erage permeability, k, of 8.64m/day for the study area to bottom within the allowable penetration depth of the was combined with the conductivity values, σ, to show the maximum current electrode separation used. The area lateral and vertical distribution of the product of perme- has a lithological succession of loamy top soil, medium- ability and electrical conductivity (kσ-values). The map grained sands, medium-coarse-grained sands, well sorted of the kσ-values shown in Fig. 12 indicates that the area medium-coarse sand, and river and gravelly sands from promises prolific and good quality water. This is because top to bottom. The medium grained sand is intercalated of the generally low kσ-values. Areas with relatively high with clay lenses. This shows that the study area is of kσ-values are indicated by the bright spots in the map. sandy units of various grain sizes. With geophysical methods therefore, areas that are prone The water-bearing unit (aquifer) is made up of medium– to poor quality water like the bright spot zones can be coarse-grained sands in some parts of the study area and detected and assessed with caution. The method is thus well sorted medium–coarse-grained sand in other parts of benign, noninvasive and friendly to the environment. Also the study area. aquifer layers, once delineated, can be targeted at cor- rect depth, thus avoiding failures associated with wildcat These layers are well developed in the parts where they drilling and the attendant economic losses. are identified as aquifers. This is suggestive of the fact that in any location within the study area, a borehole drilled to cut across the medium grained sand layer is

373 Surﬁcial Geophysical Deduction of the Geomaterial and Aquifer distributions at Ngor-Okpala local Government Area of Imo State, South Eastern Nigeria

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