Geophysical survey involving electrical resistivity method has been carried out in Erunmu, Oyo state south-western Nigeria with a view to delineate the geoelectric characteristics of the basement complex and evaluate its ground water potential in the area. A total of 7 vertical electrical sounding stations were established within the site along 7 transverses. The Schlumberger configuration was used for the data acquisition. The half current electrode (AB/2) used range from 1.0 to 75m. The quantitative interpretation of the VES curve involved the use of partial curve matching and the I-D computer iteration technique. The depth sounding interpretation results were used to generate geo-electric sections from which the aquifer was delineated.
The geoelectric section drawn from the result of the interpretation reveals five (5) subsurface layer which comprises of the top soil, latentic sand, partially weathered, weathered and fractured basement. The weathered and fractured constituted the aquoferous zone in all stations.
Hence, from this project work it is recommended that boreholes can be sited in high conductivity zone in VES 1,2,3, and 4 as they contain probable equifers. The depth of any borehole should be located between 15m and 30m to take advantage of the basement fractures.
Keywords: Geo electric, overburden, isoresistivity, latentic, foundation.
TABLE OF CONTENT
Table of content
List of Table
List of Figure
1.1 General Statement
1.2 Aims and objective
1.3 Location and accessibility
1.4 Relief and drainage
1.5 Climate and vegetation
1.6 Review of previous work.
2.0 Geological settings
2.1 Regional geology
2.1.1 Basement complex
2.1.2 The younger granite
2.1.3 The sedimentary series
2.1.4 The tertiary recent volcanic rocks
2.1.5 The tertiary recent volcanic rocks
2.3 Local geology
2.4 Banded gneiss
2.5 Quartz mica schist
2.6 Granite gneiss
3.0 Research Methodology
3.1 Principle of Resistivity method and electrode arrangement
3.1.1 Wenner Array
3.1.2 Schlumberger array
3.2 Field Techniques
3.2.1 Power source
4.0 Result and interpretation
4.2.1 Qualitative Interpretation
4.2.2 Quantitative Interpretation
4.3 Partial Curve Matching
4.4 Curve Fitting
4.4.1 Interpretation of 4 layer earth by curve matching
4.5 Computer Interpretation
4.6 Result and interpretation
4.6.2 Overburden thickness
1.1 GENERAL STATEMENT
Ground water is a vital natural resources for providing a community’s water supply and play a fundamental role in human well being, as well as that of many aquatic ecosystem and found in basement complex of Nigeria (Ariyo 2003). Ground water occurs in the joint, fractures and fault with contracting permeability (Ako et all 1986).
Ground water is a preferred source in many instances because water quality is generally good as it has been filtered by the soil (Ariyo 2003) (Olorunfemi and Oloromiwo 1985). It is reality available due to its better spahai extension and distribution. This is constantly increasing demand for is resource which as resulted in the tremendous development of the ground water resourcesduring the recent years. Unfortunately, it is not readily available like surface water and it exploitation involves cost and demand a form of technical – know- how.
The early 1900’s brought about development of the electrical resistivity method of deciphering ground water potential (Loehnert 1918, Agbede and Dosumu 1989). Since men, improve technologies have led to a more refined approach to the exploration and exploitation of ground water resource. Appling the resistivity method with the availability of computers for processing and analysing data has greatly helped the matters Reynolds 1997). Many researchers such as Ako et al (1985) and Annoy (2003) have successfully located prolific boreholes using this method.
Different methods of geophysical techniques are used but the well known method that that can be successfully employed for the ground water investigation for rural/ urban area in Nigeria is the electrical resistivity method because of its cost.
1.2 AIMS AND OBJECTIVE.
The geophysical field investigation formed an important component of the ground water exploration program. The prime objective was to unravel the hidden sub-surface hydro geological conditions to save money and time, which would have otherwise been spent on drilling of test boreholes.
The major objectives are as follows;
- Delineation of sub-surface water bearing horizons
- Identification of fractured and faulted zones in hard rock areas
- Estimation of overburden thickness and delineation of bed rock topography.
1.3 LOCATION AND ACCESSIBILITY
Erummu, me study town is located Egbeda L G A of Oyo-state, Nigeria with the region font code of 222meters above the sea level and its population amounts to 138,367.
Its coordinates are 70 271 0 11N and 4041 011 E in DMS (Degree minute seconds) or 7.45 and 4.066679 (in decimal degree). This study area is characterized by motorable road and foot paths roads. Thus accessibility seems easy.
1.4 RELIEF AND DRAINAGE
The relief lies mostly on low-lands which are punctuated by rocks; there are scattered hills that are inter flues between mostly of schist and quartzite inselbergs. The location of the state is seen in all rivers that transverse the state which flows southward either as tributaries or main river into the coastal lagoon and the Atlantic Ocean. These include OGUN, OSUN, YEWA, YEMOSI, ONA, SASA, ONI, OHU, OHIA, GBAFON, OYAN, IJA, and other, it is well drained by these streams and rivers, much of which dry up during dry season.
1.5 CLIMATE AND VEGETATION
The distinct season namely the rainy season which last from march/April to October/November and the dry season which last for the rest of the year, October/November till march/April. The vegetation pattern is that of the rain forest. The climate is favourable to the cultivation of crops such as yam, maize, millet rice, plantain, cassava, palm tree, cocoa tree etc.
Most of the areas is covered by the fertilise loamy soils derived mainly from the Precambrian hornblende biotitic gneisses.
1.6 REVIEW OF PREVIOUS WORKS
A combined geological and geographical was made of the ground water potential of the ajana, Remo north area in south –western Nigeria, the geology and other structural features of the rock there strongly influence and correlated the aquifer storability and transmissivity. Geological mapping revealed that the area was made up of granite, quartzite and varieties of gneiss some of which have good secondary porosity and permeability. Ten vertical electric sounding (VES) status were established using a Schlumberger electrode array. Five geo electric layer were the 38.3m thick 2837m resistivity sand/sandy clay and 55-518m resistivity fractured/ weathered basement other geo-electric parameter used in evaluating the area’s hydro geological potential induced curve type anisotropy coefficient and reflection co-efficient. The QH curve type was predominant in the area. The anisotropy coefficient suggested VES station having ground water potential ranging from 1.4-156 while reflection co-efficient for the area ranged from 0.21-099. The overall result showed that yes station 8, 9 and 10 could be possible ground water sources having high expected yield.
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