INTEGRATION OF ELECTRICAL RESISTIVITY AND INDUCED POLARIZATION FOR SUBSURFACE IMAGING AROUND IHE POND, NSUKKA, ANAMBRA BASIN

posted on August 13, 2019

ABSTRACT

Integration of 2D geophysical methods, Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) were used to study the subsurface geology and structures around Ihe pond, Nsukka in Anambra basin. The research work has delineated the geologic structures and strata responsible for the water in the pond, and determined the origin of the Pond. Three 2D resisitivity profiles of maximum spread lengths of 500, 400 and 500m were run around the pond. Horizontal profiling, using Wenner array configuration was employed in the data acquisition for both methods. Four faults designated Apo 1, 2, 3 and 4 with colour codes, blue, red, green and black and their fault zones were mapped. The faults were located at points 224, 265 and 325m on ER model line one, and at points 170, 205, 275 and 296m on ER model line two. Three lithologic units of consolidated coarse, medium grained, fine-grained sandstones and saturated zones were identified on the ER and IP Psuedosections. The ER and IP values from the three profiles range from 136 – 21559Wm and -81.0 to 240Ms respectively. The faults zone acts as water pathway to the pond. Inverse chargeability models established the faults zones, as large gradients of chargeability. Correlation of strata to known formation depicts the presence of consolidated coarse to medium grained sandstones, while known exposed fault was correlated to Apo 3 fault using coordinates readings. Analysed sample of water from the pond shows low Salinity and sulfides. Soil sample oxide content was also analysed to compliment the geology, and the result shows that Aluminum oxide (Al₂0₃), Silicon oxide (Si0₂), Ferrous oxide (Fe₂0₃), Zinc oxide (Zn0) and Sodium oxide (Na₂0₃) are relatively low and are in conformity with characteristics of laterite.

CHAPTER ONE

1.1 INTRODUCTION

Electrical Resistivity (ERI) and Induced Polarization (IP) surveys were conducted simultaneously around Ihe pond Nsukka, in Nsukka Local Government Area of Enugu State (Fig 1.1), to study the subsurface imaging of the prospective area. Electrical resistivity imaging survey was used to determine the subsurface resistivity distribution, by introducing artificial generated current DC through the rock layers mainly by the passage of ions in pore waters of the rocks (electrolytic process) (Kenrey and Brooks., 1991), the potential voltage is read through the potential electrodes at the surface. A good result is obtained if the rock is porous, as porosity is the major control of the resistivity of rocks, resistivity increases as porosity decreases.

Induced polarization (IP) is a second-order resistivity measurement that quantifies the charge storage capacity of earth materials. The technique over the past 30 years has proven to be one of the most successful geophysical methods in providing direct information in subsurface imaging about rock mineralogy especially in search for disseminated sulfides and massive sulfide mineralization. It has been demonstrated in field applications that IP has the potential to distinguish between sediments of different lithological composition (Slater and Lesmes, 2002a; Kemna et al., 2004) and of different groundwater salinity (Seara and Granda, 1987).

The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity survey has been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, it has been used for environmental surveys and mapping of geological faults.

The distribution of potential can be related theoretically to ground resistivities and their distribution for some simple cases, notably, the case of a horizontally stratified ground and the case of homogeneous masses separated by vertical planes (e.g., a vertical fault with a large throw or a vertical dike). Mineral grains comprised of soils and rocks are essentially nonconductive, except in some exotic materials such as metallic ores. Resistivity surveys can be useful in detecting bodies of anomalous materials or in estimating the depths of bedrock surfaces. In coarse, granular soils, the groundwater surface is generally marked by an abrupt change in water saturation and thus by a change of resistivity. In fine-grained soils, however, there may be no such resistivity change coinciding with a piezometric surface. IP also finds application in the study of clay minerals. It has been used, in the fields of hydrogeology (Vacquier et al., 1957), (Marshall and Madden, 1959), oil and gas field exploration (Sternberg and Oehler, 1990) and in environmental studies, such as mapping of polluted land areas (Towel et al., 1985).

The dependency of polarizability of rocks/soils upon their lithological composition and hydrogeological properties favours the application of the IP method for hydrogeological (groundwater) and engineering geologic investigations.

The usefulness of ERI and IP has necessitated integration of both mehods to delineate the geologic structures and strata responsible for the water in the pond and to determine the origin of the Pond. The study also analysed water and soil samples from the Pond and its environs for salinity, sulfide and oxides concentrations for the safety of the consumers and to compliment the goelogy of the study area. The result shows that ERI and IP is an effective tool for mapping, faults, fault zones, lithologic units and chargeability increases as salinity of the groundwater increases up to 500mg/L (Barker, 1990).

1.2 STUDY LOCATION

The study area around Ihe pond, Nsukka Town, in Nsukka Local Government Area of Enugu State. It is bounded by latitudes 6⁰ 49^I 50.1^II N – , 6⁰ 51^I 20.4^II N and longitutdes 7⁰ 21^I 55^II E, – 70 22^I 33.9^II E, it has an area coverage of about 1.4km². It is accessible by motorable roads and foot-path, especially those created by the villagers to fetch water from the pond. The base map of the study area, showing the accessibility to the study location is shown in fig 1.2.

1.3 PHYSIOGRAPHY

Igbozurike (1975) recognizes four vegetation zones in the eastern Nigeria; among these, the study area falls within rainforest savanna zone. The vegetaion is made up of grasses and trees, and occasinally shrubs. Grasses and shrubs covers the high land areas, while tall trees tend to be more concentrated in valleys and low lands where they form forest. The study area lies within the humid tropical rainfall belt of Nigeria. The rainfall regime in the area lasts between the months of April and October, while dry season ensues between November and March. The dry periods are characterized by very high temperatures and lower relative humidity. Temperature is high in most part of the year except during the hamattan (December- January). The month of December has low temperatures (20⁰C) at nights and mornings, and warmer days (up to 34⁰C). The hottest period is between February and early April. Rainfall often occurs as violent down pours accompanied by thunderstorms, heavy flooding, groundwater infiltration and percolation. The study area also shows two major types of landforms, which consist of a high relief zone with undulating residual hills, valleys and the lowland areas.