report of geophysical investigation for groundwater

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Fig. 1 and 2 show the plots of apparent resistivity against its corresponding current electrode spacing (AB/2) of VES 1 and 2 respectively. The results of the ...
REPORT OF GEOPHYSICAL INVESTIGATION FOR BOREHOLE AT LIPEDE ESTATE, ONIKOKO, ABEOKUTA, OGUN STATE, SOUTHWESTERN NIGERIA

CLIENT:

ENGR. R. ADENUGA

CONSULTANTS: Bayewu O.O, Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Nigeria

Mosuro G.O Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Nigeria

Oloruntola M.O Department of Geosciences, University of Lagos, Lagos , Nigeria

MARCH, 2012

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Executive Summary 1. The geophysical survey was commissioned by Engr. Adenuga for the hydrogeophysical characteristic of the subsurface layers within his residence at Lipede Estate, Onikoko, Abeokuta, Ogun State with a view to identifying location that could be drilled for a productive borehole. 2. The Pre-drilling surveys aimed at delineating the several geologic units beneath the surface of the area and determining the suitability of drilling productive borehole within and outside the compound were carried out. 3. Two locations were surveyed based on the Constant Separation Traverse were

selected

for

detailed

investigations

of

the

hydrogeological

characteristics of the subsurface layers using the Schlumberger Array of Vertical Electrical Sounding (VES) study. 4. The area is characterized by the presence of sandy overburden and fresh basement rocks occurring at varying depth 5. Based on the interpretation of the VES data, the area around VES 2 is recommended for drilling to a depth of 40m. This borehole might not sustain more than 0.5HP submersible pump.

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INTRODUCTION Le BOM consult was commissioned to conduct pre-drilling geophysical survey within the premises of Engr. Adenuga at Lipede Estate, Onikoko, Abeokuta, Ogun State Aims of the Survey The Geophysical Survey was aimed at the following: a. Delineating the several geologic units beneath the surface of the area b. Ascertaining their hydro-geologic significance of these units. c. Determining the suitability of drilling a productive borehole at the points investigated d. Making appropriate recommendation, if any, to ensure the success of the boreholes.

The Geology and the Hydrogeology of the Area The study area lies within the Precambrian basement of South-Western Nigeria. The rocks of the South-Western basement complex include the older granites, amphibolites, gneisses and granites, as well as the schist belt rocks mainly made up of schist of various types, granites gneisses underline the study area. The rock type typifying the geology of Imomo area is described as follows:  Coarse porphyritic granite It is type of granite with coarse texture (grain size) containing feldspar phorphyry embedded in the Granite ground mass.  Pegmatite

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It is a very coarse-grained Igneous rock with phenocrysts over 250mm in length, usually of Granitinic composition and forming at a late stage of crystallization.  Biotite hornblende gneiss They are usually intercalated with amphibolite and variably migmatized and undifferentiated biotite are found in the area. The rock type is characterized by biotitehornblende banding.  Quartzite and Quartz schist  Amphibolite schist.  Undifferentiated Gneiss Complex probably mainly schist. The study area falls in the region of the metamorphosed region of the southwestern basement complex area. Methodology A detailed Vertical Electrical Sounding (VES) using the Schlumberger Array method was used for the study in the area. The VES was carried out on the three points within the vicinity of the compound; they were selected for the survey due to space availability and good coverage of the compound. Campus Ohmega resistivity meter was used for the survey using the Schlumberger array for the VES data acquisition. This array is capable of isolating successive hydrogeologic layers beneath the surface using their resistivity contrast. A current electrodes spread of 300m (AB) was used during the study due to the spread achievable in each location. The field parameter obtained from the resistivity meter was resistance (Ω) which was then multiplied by its corresponding Geometric Factor (G) to obtain the resistivity needed for the interpretation. i.e. ρ=GXR.

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The resistivity values obtained were then plotted on a log-log paper as points with the resistivity values being on the vertical axis and the electrode spacing (AB/2) on the horizontal axis. The points were joined and curve matched manually using precalculated master curves and their auxiliaries. The results obtained from the exercise were used as the input-model for the eventual computer aided iteration. WINRESIST software was used for the iteration. The locations of the VES points are shown in fig 1.

VES 1 Engr. Adenuga’s residence

VES 2

Untied Road

Fig.1: Location of the VES points within the study area.

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Results and Discussions Fig. 1 and 2 show the plots of apparent resistivity against its corresponding current electrode spacing (AB/2) of VES 1 and 2 respectively. The results of the iterated curves are presented in fig. 3 and 4. The interpreted results showing the inferred lithologies are presented in Table 1. Based on the geology of the area, lithologic inferences were made from the iterated data, the iterated (computed) resistivity values range from 191.0 Ωm to 6721.4 Ωm. At VES 1, there are three geoelectric layers with resistivities of 347.5, 191.0, and 6721.4 Ωm respectively. The respective depths of the layers are 0.7, 24.7 and infinity. These are inferred as topsoil, sandy weathered horizon, and fresh basement.

At VES 2, there are five geoelectric layers inferred, the computed resistivities values are 332.8, 195.4, 320.9, 188.6 and 3123.9 Ωm respectively. The corresponding depths of the layers are 0.9m, 5.0m, 7.2m, 33.3m and infinity. These are inferred as topsoil, Weathered

Horizon( Sandy), Weathered

Horizon( Sandy) and Fresh basement.

Fig. 1: The plotted VES 1 curve.

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Horizon( Sandy) Weathered

Fig. 2: The plotted VES 2 curve.

Fig. 3: The Iterated curve for VES1

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Fig. 4: The Iterated curve for VES2 Table 1: The Summary of VES and Inferred Lithologies. Layers VES 1

VES 2

1 2 3 1 2

Resistivity (Ωm) 347.5 191.0 6721.4 332.8 195.4

Thickness (m) 0.7 23.9 0.9 4.1

Depth (m) 0.7 24.7 0.9 5.0

3

320.9

2.2

7.2

4

188.6

26.1

33.3

3123.9

-

-

Inferred Lithology Top Soil Weathered Hor Fresh Basement Top Soil Weathered Horizon( Sandy) Weathered Horizon( Sandy) Weathered Horizon( Sandy) Fresh Basement

Conclusion The results of the Vertical Electrical Sounding of the area around the two VES points within the compound revealed the presence of three to five geoelectric layers, the top soil, Weathered Horizon( Sandy)underlain by the fresh basement rock. The overall result revealed a fairly thick overburden which can host groundwater. The overburden is essentially clayey sand with maximum thickness of 26.1m at VES 2. No fracture that was observed within the premises to serve as a secondary feature for groundwater accumulation.. Recommendation 8

On the basis of the above discussion and conclusion, the following recommendations are made: 1.

A borehole is of 40m is recommended around VES 2.

2.

Drilling should be done by competent driller. Care should be taken when the well completion is done so as to prevent the failure of the hole.

3.

A maximum of 0.5HP submersible pump should be used.

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