EUNICE PRESENTATION.pptx
- 1. GEOELECTRICAL IMAGING SURVEY FOR ENVIRONMENTAL AND ENGINEERING STUDIES
- 2. ABSTRACT INTRODUCTION AIM AND OBJECTIVES LOCATION OF STUDY GEOLOGY OF STUDY AREA LITERATURE REVIEW METHODOLOGY RESULTS AND INTERPRETATION
- 3. A geo-electrical imaging survey was conducted at Uhunmode Local Government of Edo State Nigeria for site investigation in order to determine the applicability of 2-D resistivity imaging for Environmental and Engineering studies The Wenner array was employed. Field data were obtained for four electrical imaging lines Using PASI Earth resistivity Meter. The field data was subjected to inversion Using RES2DINV in order to remove geometrical effects from the pseudosection and produce an image of true depth and true formation resistivity. Each of the profiles indicates that resistivity increases with depth. The first 3 layers across the 4 transverse indicates the presence of alluvium deposits with scattered deposit of clay in small quantities from 0- 20m in depth underlain by laterite varying about 15m to 20m in thickness, across the four transverses with resistivity values ranging from 600ohm.m to about 1000ohm.m. The areas of intermediate resistivity zone 1000 to 3500ohm.m indicate a natural material like sandstone that have been underlain by a thin layer of shale and The High resistivity zone 3500ohm.m and above has been interpreted as bedrock which comprises of gravels and granite. It has been shown that all the areas are competent for the construction and also suitable for agricultural purpose.
- 4. The knowledge of the subsurface geology and the characterization of the spatial distribution of subsurface physical properties are necessary for effective environmental monitoring, protection and remediation in polluted areas, as well as for infrastructure development purposes. These will, in addition, assist policy makers and environmental managers to take quality decisions required to preserve and sustain a healthy environment for mankind and the ecosystems in general, and to effectively and safely manage our natural resources. ( Ahzegbobor P.A 2010 ) The applications of geophysical methods are able to investigate and determine soil properties, bedrock depth, topography of the bedrock surface below unconsolidated material, rock type, layer boundaries, water table, weak zones and expansive clays, inhomogeneities of the subsurface, cavities, ancient relics and generally underground structures or bodies that have different physical properties from their geological surroundings (Aubert, 1984; Carrara et al., 2001). Land that is contaminated contains substances at the surface or in the subsurface that are actually or potentially hazardous to health or the environment. In Nigeria there are numerous sites where land has become contaminated by human activities such as mining, industry, chemical and hydrocarbon spills and unregulated waste disposal. Contamination can also occur naturally as a result of the natural geology of the area, or through intensive agricultural use.
- 5. AIM OF STUDY The aim of this Research is to determine if the Lithology of the survey area is suitable for environmental and engineering studies. OBJECTIVES OF STUDY 1. obtain a 2D model of the subsurface by inverting the data set 2. Infer from the 2D model, the rock formation in the subsurface from which one can tell if the terrain is suitable for environmental and engineering studies
- 6. GEOLOGY OF THE STUDY LOCATION Geology of the study area is described in the geology of Niger delta. Niger delta is one of the ten major sedimentary basins of Nigeria. The others are Abakaliki basin, Anambra Basin, Benue trough, Bida basin, Bornu-Chad basin, Dahomey basin, Gongola basin, Sokoto basin and Yola basement complex. These are Western end of the Cameroun volcanic zone, Northern Nigeria massif and the eastern end of West African massif. The niger delta is divided into three formations AKATA FORMATION It is characterized by a uniform shale development. The formation is a marine sedimentary sequence laid down in front of an advancing delta. These prodeltaic shales are medium to dark grey, fairly hard or at places soft, gumbo-like and sandy or silty in several places, the shales of this formation were found to be undercompacted, and therefore mobile, and may contain lenses of abnormally high-pressured siltstone or fine-grained sandstone (Allen, 1965; Reyment, 1965; Short & Stauble, 1967 and Oomkens 1974).
- 7. AGBADA FORMATION: This sequence of strata forms the hydrocarbon prospective sequence in the Niger delta. The formation is characterized by alternating sandstones and shales of the delta front, distributary channel, and deltaic plain origin. Weber (1971) showed that the alternating sequence of sandstones and shales of the Agbada Formation is of cyclic sequences of marine and fluvial deposits. The sand content ranges from 50 to 75 %. The sandstones are medium to fine grained, fairly clean locally calcareous, and shelly. They consist dominantly of quartz and potash feldspar with subordinate and illite. BENIN FORMATION: This is the uppermost unit of the Niger delta complex. The formation can be easily distinguished based on its high sand percentage (70 – 1000 %). The sand is dominantly massive highly porous and freshwater bearing with locally interbedded shale beds, which are considered to be of braided stream origin. The sands are poorly sorted, ranging from fine to coarse – grained and occasionally pebbly and they contain abundant wood, fragments, which become lignitic with depth. Composition, structure and grain size show deposition in a probably upper deltaic environment.
- 8. BASIC RESISTIVITY THEORY Electrical resistivity is a material property which indicates how well a material retards electrical conduction. Resistivity relates electrical potential and current to the geometrical dimension of the specified region. It is the reciprocal of conductivity. Electrical conduction takes place due to the movement of charges. Charges are displaced from the original equilibrium condition under the application of electric potential. However, charge density depends on the applied electric field and resistivity of the material. Resistivity can be defined by considering current flow through a cylindrical section. To define resistivity, assuming a cylindrical section with cross sectional area and length of A and L, if current flow is I through section resistance R and potential drop across the section is V, then resistivity can be expressed by the following equation ρ=RA/L……………………………..(1) where, ρ=Electrical Resistivity, R= Resistance of the material, V= Potential, I= Current , A= Cross sectional Area and L= Length. The schematics of cylindrical section and flow of current are presented Figure1 : Diagram showing the flow of current through a cylindrical section
- 9. To obtain a good 2-D picture of the subsurface, the coverage of the measurements must be 2-D as well. The basic resistivity measuring technique is just similar to the conventional principles. Basically, four electrodes are required; two for current injection and two electrodes for potential measurement. Each array has their own advantages and drawbacks in data acquisition and processing works. But one might has better sensitivity and resolution power for vertical as well as lateral structural variations than the other. For instance, Wenner array in multi-electrode mode has good resolution power for horizontal structure having vertical variations but weak for horizontally variable geological structures. Figure 2: Arrangement of electrodes for a 2D survey
- 10. METHODOLOGY In this research work, the Wenner array in electrical resistivity survey was adopted. The basic field equipment for this study is the PASI Earth resistivity meter which displays apparent resistivity values digitally as computed from ohm’s law. It is powered by a 12.5 V DC power source. Other accessories to the Earth Resistivity meter includes the booster, four metal electrodes, cables for current and potential electrodes, harmers (3), measuring tapes, walking, phones for very long spread. In this configuration, the four electrodes are positioned symmetrically along a straight line, the current electrodes on the outside and the potential electrodes on the inside. The spacing between adjacent electrodes is “a”. With Wenner array, the first step is to make all the possible measurement with electrode spacing of “1a”. Electrode 1 is used as the first current electrode C1, electrode 2 as the first potential electrode P1, electrode 3 as the second potential electrode P2 and electrode 4 as the second current electrode C2. For the second measurement, electrodes 2, 3, 4 and 5 are used for C1, P1, P2, and C2 respectively. This is repeated down the line of electrodes until electrodes 17, 18, 19 and 20 are used for the last measurements with “1a” spacing. The spacing is then doubled without moving on, the first active electrodes with “2a” electrode spacing being 1, 3, 5 and 7. For the second measurement, electrodes 2, 4, 6 and 8 are used. This process is repeated down the line until electrodes 14, 16, 18 and 20 are used for the last measurement with spacing “2a”. The same process is repeated for measurements with “3a”, “4a”, “5a” and “6a” spacing.
- 11. To get the best results, the measurements in a field survey should be carried out in a systematic manner so that, as far as possible, all the possible measurements are made. This will affect the quality of the interpretation model obtained from the inversion of the apparent resistivity measurements (Dahlin and Loke, 1998). As the electrode spacing increases, the number of measurements decreases. The number of measurements that can be obtained for each electrode spacing, for a given number of electrodes along the survey line, depends on the type of array used. The Wenner array gives the smallest number of possible measurements compared to the other common arrays that are used in 2-D surveys. METHODOLOGY CONT’D EQUIPMENT USED FOR THE FIELD WORK INCLUDE Measuring Tape, Electrodes (Current And Potential Electrode), Cables, Terrameter, Hammer , Battery, Cutlass, Gps (Global Positioning System
- 12. RESULTS Figure 3: Inversion result from Location 1 Ekiugbo Community
- 13. Figure 4 Inversion result from Location 2 Ekiugbo Community
- 14. Figure 5: Inversion result from Location 3 Ekiugbo Community
- 15. Figure 6: Inversion result from Location 4 Ekiugbo Community
- 16. DISCUSSION OF RESULTS The obtained electrical resistivity images of the subsurface of the four transverses are presented as models in figures 4.1, 4.2, 4.3 and 4.4. The root mean square errors obtained in the inverted models were between a minimum of 4.6 to a maximum of 11.7% after 3 iterations for each of the transverse. The maximum depth of penetration for transverse 1 is 53.6m while that of transverse 2, 3 and 4 are 54.3m, 53.6 and 54.3 respectively. There is a very good correlation between the subsurface images of the four transverses which shows that the area of investigation has very well compacted soil with even deposition across the area. Each of the profiles indicates that resistivity increases with depth. The first 3 layers across the 4 transverse indicates the presence of alluvium deposits with scattered deposit of clay in small quantities from 0- 20m in depth underlain by laterite varying about 15m to 20m in thickness, across the four transverses with resistivity values ranging from 600ohm.m to about 1000ohm.m. The areas of intermediate resistivity zone 1000 to 3500ohm.m indicate a natural material like sandstone that have been underlain by a thin layer of shale and The High resistivity zone 3500ohm.m and above has been interpreted as bedrock which comprises of gravels and granite. From the interpretation it can be shown that the investigated area is competent for engineering purpose as revealed by the geoelectric imaging of all the transverses. It has been shown that all the areas are competent for the construction of factories that make use of heavy machines However; the area might be difficult to access aquifer because of its low conductivity. The four study areas also shows no form of contamination.
- 17. CONCLUSION The electrical resistivity imaging has been successfully used to determine the subsurface in the study area from which it was shown that the study area has compacted soil and an underlying layer of bedrock which comprises of gravels and granite the area also shows that there are no faults which could lead to collapse that are suitable for engineering purpose. There is also no sign of contamination which means the soil is not polluted and can be used from agricultural purposes. However low conductivity of the study area reveals that it is not aquiferous and thus might not be suitable for water supply purposes.
- 18. REFERENCES Ahzegbobor P.A 2010 Acquisition geometry and inversion of 3d geoelectrical resistivity imaging data for environmental and engineering investigations Allen, J.R.L., 1965. Late Quaternary Niger Delta and adjacent areas: sedimentary environments and lithofacies. Am. Assoc. Petroleum Geol. Bulletin, Vol. 49, pp. 547-600. Aubert, M. (1984). Resistivity and magnetic surveys in ground-water prospecting in volcanic areas. Case history Maar of Beanit, Puy de Dome, France Geophys. Prospect. 32: 63–554. Carrara, E. (2001). Resistivity and radar surverys at the archaeological site of Ercolano. J. Environ. Eng. Geophys. 6: 32–123. Short, K.L. and stauble A.J. 1967. Outline geology of Niger delta A.A. PG-V.5. I P. 761 -799. Oomkens, E., 1974. Lithofacies relations in Late Quaternary Niger delta complex: - Sedimentology. Vol. 21, pp. 195-222.