Subsurface Layer Characterisation Research

Subsurface Layer Characterisation Research SUBSURFACE LAYER CHARACTERISATION FOR ENGINEERING PURPOSES USING VERTICAL ELECTRICAL SOUNDING IN BANANA ISLAND, LAGOS, NIGERIA OLADUNJOYE, H.T., 2OLALEYE, A.O. Unique Seven (7) vertical electrical sounding stations along two navigates were utilized to contemplate the profundity to equipped development for establishment of designing structures in Banana Island of Lagos. The examination is intended to disentangle the nearness of issue soils that can be antagonistic to structures.  The sounding information utilized in this investigation was obtained utilizing Schlumberger Electrode Configuration. The VES information were deciphered into a model bend utilizing forward demonstrating programming called Winglink. The translation parameters (layer resistivities and thicknesses) for each VES point were assembled into three geo-electric areas to speak to the 2-D models of the subsurface layers in the investigation territory. The last translation results (resistivities and profundities) demonstrated as geoelectrical areas and pseudosections were looked at inside the reviewed region and related with the borehole log-inferred profundities to decide a likely profundity for establishments of tall structure. The examination inferred that the geophysical examination is of extraordinary noteworthiness in incomplete bend coordinating understanding procedure gives the most reliably low rate deviation mistake edges among the three VES primer translation methods. There is steady increment inside and out expectation mistake, with profundity, in all the translation strategies. Presentation Geophysical resistivity methods depend on the reaction of the earth to the progression of electrical flow. In all techniques, an electrical flow is gone through the ground and the subsequent potential anodes permit us to record the resultant potential contrast between the flow terminal is estimated. The evident resistivity is then a component of the deliberate impedance (proportion of potential to current) and the geometry of the cathode cluster. Contingent on the review geometry, the obvious resistivity information are plotted as 1-D soundings, 1-D profiles, or in 2-D cross-segments so as to search for irregular locales. In the shallow subsurface, the nearness of water controls a great part of the conductivity variety. Estimation of resistivity (converse of conductivity) is, all in all, a proportion of water immersion and network of pore space. This is on the grounds that water has a low resistivity and electric ebb and flow will follow the easiest course of action. Expanding immersion, expanding saltiness of the underground water, expanding porosity of rock (water-filled voids) and expanding number of cracks (water-filled) all will in general abatement estimated resistivity. Resistivity estimations are related with fluctuating profundities relying upon the partition of the present and potential anodes in the overview, and can be deciphered as far as a lithologic as well as geohydrologic model of the subsurface. The decision for Direct Current (DC) resistivity study in this venture is because of its capacity to give a fast brief picture of lithological and groundwater qualities regarding its clear resistivity differentiate. In this work the electrical resistivity character gathered were model with Winresist and Winglink reversal programming. The layers parameters (resistivities and thicknesses) acquired from information handling were then altered with Winglink to give two-dimensional pictures of the subsurface clear resistivity variety (Pseudosection). Nonetheless, information on the topography of the territory from obtained borehole log has assumed a fundamental job in deciding the best fit model. MATERIALS AND METHOD OF STUDY The vertical electrical resistivity is utilized to decide the subsurface resistivity appropriation and profundity of the sub-surface layers with the guide of electrical resistivity meter. In this examination, Schlumberger cluster of electrical resistivity strategy was received due to the ease of the field activity, decreased coordinations labor and furthermore quicker. A sum of Six (6) vertical electrical sounding (VES) were estimated inside the zone. The resistivity estimation is taken by presenting little bit of power through two current cathodes (An and B) and estimating the resultant voltage drop between a couple of potential anodes (M and N). A basic determined estimation of evident resistivity is the parameter bearing the data for the qualities of the dirt (Tonkov, 2008). From the establishment of electrical resistivity hypothesis is the ohms law (Grant and West, 1965; Dobrin and Sarvit, 1988):    â â â â â â â â (1) Where Ï  is the resistivity, R is the opposition, L is the length of the directing chamber and An is the cross sectional region. For the strong earth, whose material is overwhelmingly comprised of silicates and essentially non conveyors, the nearness of water in the pore space of the dirt and in the stones cracks improves the conductivity of the earth when an electrical ebb and flow I is gone through it, subsequently making the stones a semi conductor.â Since the earth isn't care for a straight wire and it's anything but an anisotropic, at that point Equation 1 can be redone as:  â â â â â â â â â â â â â â â â â â â â (2) Where change in voltage and r is the span of current cathodes side of the equator. Since the earth isn't homogeneous, Equation 2 is utilized to characterize a clear resistivity which is processed as the result of the geometric factor and the obstruction recorded in the electrical resistivity meter and resistivity the earth would have on the off chance that it were homogeneous (Grant and West, 1965;):  â â â  â â  â â â â â â â â â (3) Where, 2ï€r is then characterize as the geometric factor (G) fixed for a given anode setup and is subject to the terminal game plan. The Schlumberger terminal setup was utilized for this examination. The geometric factor is consequently given as:  â â (4) Where AB is the separation between two current terminals, MN is the separation between two potential anodes. Area AND GEOLOGY OF THE STUDY AREA The investigation territory is situated in the south-eastern piece of Lagos State between Latitudes 6â ° 25 to 6â ° 28 North of the Equator and Longitudes 3â ° 24 to 3â ° 30 East of the Greenwich Meridian (Fig.1.1). The investigation region Lekki and its environ issituated inside the western Nigeria waterfront brooks and tidal pond created by boundary sea shores related with sand testimony (Pugh, 1954). It shapes some portion of the Lagos tidal pond framework known as the biggest of the four tidal pond framework in the bay of guinea coast (Adepelumi and Olorunfemi, 2000). Banana Island is a counterfeit island in Lagos State, Nigeria. It is situated in the Lagos Lagoon and appended toward the North-Eastern Ikoyi Island by a committed street strip which is connected to the current street organize. The banana-formed Island is around 1,630,000 square meters in size (not exactly a square mile) and is separated into 536 plots (of somewhere in the range of 1000 and 4000 square meters in size) basically masterminded along circular drives, so intended to upgrade the verifiably private nature of Ikoyi. Banana Island is a counterfeit island recovered from bits of the Lagos Lagoon, topographic rise acquired at the site found the middle value of 4 meters above mean ocean level. The investigation territory is situated inside the beach front area of Nigeria with an atmosphere of long wet season (April to October) and short dry season (November to March). Normal yearly precipitation is above 2000mm and fills in as a wellspring of groundwater energize. The zone is vulnerable to flooding after substantial precipitation during the wet season. Nigeria lies in a broad Pan-African versatile belt which isolates the West African and Congo Cratons. The belt is deciphered to have developed from the mainland impact between the West African craton and the Pan-African belt (Black et al., 1979; Caby et al., 1981). Geochronologically, the Precambrian rocks of Nigeria can be gathered into four significant classes (Liberian Orogeny 2800  ± 200 Ma, Eburnean Orogeny 2000  ± 200 Ma, Kibaran Orogeny 1100  ± 200 Ma and The Pan-African Orogeny 600  ± 150 Ma). These classes relate with the four orogenic occasions that have punctuated the Precambrian history of Africa (Odeyemi, 1979). Figure 2.1: Regional Geological Map of Nigeria (Modified after Ajibade et al., 1988) Beach front Nigeria involves two sedimentary bowls: The Benin-Dahomey bowl and the Niger Delta bowl isolated by the Okitipupa edge. The stones of the Benin bowl are mostly sands and shales with some limestone which thicken towards the west and the coast just as down plunges to the coast. Ongoing residue are underlain by the Coastal Plains Sands which is then underlain by a thick dirt layer the Ilaro Formation and other more seasoned Formations (Jones and Hockey, 1964). The Coastal Plains Sands spring is a multi-spring framework comprising of three spring skylines isolated by silty or clayey layers (Longe et al., 1987). It is the principle spring in Lagos Metropolis that is abused through boreholes for residential and modern water flexibly. The sedimentary rocks of Lagos State are made out of five (5) sedimentary arrangements (Jones and Hockey, 1964) viz: Abeokuta Formation, Ewekoro Formation, Ilaro Formation, the Coastal Plains Sands and the alluvial stores. The waterfront fields sand otherwise called the Benin Formation extends over the Dahomey bowl into the Niger Delta Basin. The topographical developments of the examination zone made out of silt set down under fluviate, lascustrine and marine situations. These dregs grade into each other and change broadly in horizontal degree and thickness (Adepelumi and Olorunfemi 2000). Longe et al., (1987) grouped the spring into three sorts. Quantitative understanding of the bends included halfway bend coordinating utilizing two-layer Schlumberger ace bends and the aux