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 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.
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.
Aim Of The Study
The general aim of this study is to rely on the application of resistivity method to determine and model the direction of underground water as well as the hydrogeological pattern around Obubra area of Cross River State, Nigeria.
Literature Review Of The Area
In basement provinces groundwater occurrence depend exclusively on discontinuities like fractures, joints, fissures, and weathered litho - zones. The fissures of crystalline rocks are limited to shallow depths, and water movement is lateral in the direction of the gradient downwards to the drainage area. Fracturing and fissuring is a common phenomenon in basalts because of the tectonic chilling effects on them, which develops fractures. About60% of ground water is habited in weathered - fresh bedrock transition with aquifer yields of 0.2 - 3.5 l/sec.(CRBDA, 1982). According to Petters (1989) recharge to the weathered zones and joints system is greatly retarding significantly lateritic cover areas. This is attributed to the high content of the impermeable clay in the laterite. CRBDA (1982) put the yield for this province (weathered zones) at 84.4 - 345.6 m3/day. Static water level (SWL)is between 4.6 - 19.8 m in Obubra and 12.2 - 21.4 m for part of Ikom in the study area. Boreholes depths range between 25 - 47m in the study area. Shale - sandstone or shale/siltstone province is the largest hydro- geological province in the study area, occupying about 70% of the study area. This area cuts across locations like Obubra,Apiapum, Nko, Ekori, Ugep, Ochom, and Agara Ekureku. It constitutes the geologic Asu River Groupand Eze - Aku formation. These sediments are slightly folded, tilted and at times broken by faults. Fractures, fissures and joints commonly occur in sandstones and sandstone affiliated sediments, but are commonly restricted to shallow depths of 20 - 50 m. Permeability of the study area is influenced by the nature and texture of the sediment type, constituting the study area. For example permeability is moderate in porous, fissured and fractured sandstone/Shale but very low in impervious shale and siltstones. (www.ccse. net.org/ Journal of Geography and Geology Vol. 4, No. 3; 2012). Shale/siltstones province record very low aquifer yield of 0.05 - 0.5 l/sec, while some sub area like siltstone/limestone record up to 2.02 l/sec (CRBDA, 1982).
Chapter Two: Ground Water Movements
Ground water in its natural state is invariably moving. Groundwater moves from areas of higher elevation or higher pressure/hydraulic head (recharge areas) to areas of lower elevation or pressure/hydraulic head. This is where the groundwater is released into streams, lakes, wetlands, or springs (discharge areas). The base flow of streams and rivers, which is the sustained flow between storm events, is provided by groundwater. The direction of groundwater flow normally follows the general topography of the land surface. Groundwater moves extremely slowly usually inches per day, whereas rivers move more swiftly feet per second (ft/sec). However, in the sandy soils, groundwater moves more quickly, between 1-5 feet per day.5 Even at this rate, groundwater and substances dissolved in it may take 5 years to travel about 1 mile. In comparison, a small twig moving downstream in a river at about 1-2 ft/sec would only take about 1 hour to travel 1 mil This movement is governed by established hydraulic principles. The flows through porous and permeable rocks can be expressed by what is known as Darcy’s law. Hydraulic conductivity which is a measure of the permeability of the media, is an important constant in the flow equation. Determination of hydraulic conductivity can be made by several laboratory or field techniques. Application of Darcy’s law enables ground water flow rates and direction to be evaluated. The dispersion, or mixing resulting from flows through porous media produces irregularities of flows that can be studied by tracers and in the aeration, the presence of air add a complicating factor to the flow of water.
Darcy’s experiments demonstrate that the volume of water which passes through a bed of sand of a giving nature is proportional to the pressure and inversely proportional to the thickness of the bed traversed. Darcy’s law states that the flow rate through a porous media is proportional to the head loss and inversely proportional to the length of the flow path. The law, more than any other contribution, serves as the basis for present- day knowledge of ground water flow. Therefore the resulting head loss is defined as the potential loss within a sand cylinder, this energy been loss by frictional resistance is dessipated as heat energy. It follows therefore that the head loss is independent of the inclination of the cylinder.
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