1.1 Introduction
The resistivity method is used in the study of the horizontal and vertical discontinuities in the electrical properties of the ground and also in the detection of three dimensional bodies of anomalous electrical conductivity. In the study of ground water movement in obubra area, the resistivity method commonly employed are the electrical resistivity method. Electrical resistivity method is one of the most useful techniques in groundwater geophysical exploration, because the resistivity of rocks is sensitive to its ionic content (Alile, et al., 2011).
The method allows a quantitative result to be obtained by using a controlled source of specific dimensions. Records show that the depths of aquifers differ from place to place because of variation in geo-thermal and geo-structural occurrence (Okwueze, 1996). Therefore, the need to study the area for groundwater potential especially in terms of determining the flow direction is a prerequisite for portable ground water exploration and exploitation in this area.
1.2 Location and Geology of the Area
The study area lies between latitudes 50 15′ and 60 15′N and longitudes 70 45′ and 80 45′E. It is located within the sub-equatorial climatic region of Nigeria with a total annual rainfall of more than 300 to 400cm. Temperature ranged from 250C to 280C. The area experiences two seasons, these are the wet season which lasts from April to September with a peak in June and July while the dry seasons lasts from October to March (Iloeje,1991).
The study area is underlain by two major lithologic units: Crystalline basement and Cretaceous sediments.
The crystalline basement rocks occupy the extreme south of the study area. Also, there are intermediate rocks scatteredin patches around Obubra, Iyamayong, Iyamitet, Ikom, Nkpani and Usumutong.
The Cretaceous sediments cover about 90% of the study area. Asu River Group is the basal and oldest recorded sediment in the study area. It is dominated by bluish gray/black to olivine brown shale and sandy shale, fine - grained micaceouscalcareous sandstone and siltstone with limestone lenses.
The shale is often carbonaceous and pyritic which indicates that the sediments were deposited under a poorly oxygenated shallow water environment of restricted circulation, an indication of low energy environment (Petters et al., 1987). In general, Southern Obubra lies within the Cross River plain and the clastic beds in the study area can be ascribed to the Ezillo Formation. The Ezillo Formation comprises mostly dark gray shales with fine sandstone and siltstone intercalations in the lower part, and an upper unit that is highly bioturbated, fine medium sandstone, similar to the sandstone of the Amaseri Formation. The Ezillo Formation between Appiapum and Ikom was deposited in a deltaic coastal plain, in brackish marshes and inter-distributary bays (Barth, et al., 1995). A major river (Cross River) exists in the study area into which minor streams empty their loads. The elevation of the study area ranged from 14 to 170m above sea level.
The relief is characterized by undulations running at undefined direction and variably demarcating the very lowland areas from moderate relief landmarks. The occurrence of the low plains is occasionally broken by inselbergs of granite and basalts in the southern portion of the study area. In the sediment filled portions, the low plains are occasionally broken by flat -topped hills of sandstone ridges and igneous intrusive with highly ferroginized sandstones with gravels resulting from uplifts. The area is drained by the Cross River with major tributaries like, Udip, Ukong, Lakpoi, Okwo, and Okpon rivers. These rivers form a network of dendritic drainage system.
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