1.1 Background of the Study
This work involved the observation, description of various rock types and their field relationships. These rocks give information on the general Geology of the area. Generally, a number of geophysical exploration techniques are available which enable an insight to be obtained rapidly into the nature of subsurface layers. These include geo-electric, electromagnetic, seismic and geophysical borehole logging techniques. The choice of a particular method is governed by the nature of the terrain and cost consideration. These methods have been used extensively in groundwater investigation, geologic mapping and engineering site investigation Etu-Efeotor and Akpokodje, 1990; Obiakor and Chukwudebelu, 1992; Okwueze and Ezeanyim, 1985; Mbipom and Archibong, 1989.
In geophysical investigations for lithological boundary, ore prospecting, water exploration, depth to bedrock determinations, sand and gravel exploration. The electrical resistivity method can be used to obtain quickly and economically, details about the location, depth and resistivity of subsurface formations, (Udoinyang, 1999). The basis of the electrical resistivity method employs the measurement of electrical potential associated with subsurface electrical current flow generated by a Direct Current (DC) or slowly varying alternating current source (AC). Factors that affect the measured potential include the presence and quality of pore fluids and clays (Boyld, 1999).
Wiebeng, (1955) noted that the ratio of the current applied, with a geometric factor K, which depend on the electrode separation gives the quantity termed apparent resistivity. This study describes a direct current geo-electric investigation for subsurface lithological boundary impacts of Afikpo in Southern Benue Trough, Southeastern Nigeria. The aim of this investigation is to obtain from the geophysical characteristics of the study area, a meaningful delineation of the subsurface lithological boundary and other sedimentary structures across different rock type for better understanding of the subsurface geology of the study area.
1.2 Aim and Objectives of the Study
The main objective of this research is to study the lithologic characteristics of the Afikpo and it environ with a view to delineate it lithological boundary using resistivity profiling.
1.3 Scope of the Study
- Description of the exposed outcrop sections of the study area.
- Mapping and demarcation of the rocks encounter in to lithologic units.
- Determination of the geo-electric sections at each profiling point in order to detect the change in resistivity between different lithologies.
- Inferring of geo-electric and geologic layers from the inverses resistivity plots.
- To integrate the results above to construct and digitize a geological map of the study area.
1.4 Location of the Study Area
The study area is geographically located in southeastern Nigeria. The geographical limits are defined by Latitude 5051’N and 5056’N and Longitude 7051’E and 7056’E, and within part of the 1:25,000 Sheet 313 (Afikpo NE) topographic map. It covers an area of about 60 km2(sixty square kilometers). Major access roads into the area are through the Abakaliki – Afikpo Road, Okigwe – Afikpo Road, and Okposi – Amasiri – Afikpo Road, as well as Cross River – Afikpo Road.
Good network of roads link up such areas as Owutu Edda, Ibi, Amasiri, Amuro and Afikpo which are the major towns in the study area. Figs. 1.1a and Fig.1.1b are the location maps of the