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

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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 49I 50.1II N – , 60 51I 20.4II N and longitutdes 70 21I 55II E, – 70 22I 33.9II E, it has an area coverage of about 1.4km2. It isaccessible 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. The lowlands are most profound in the northwestern part of the study area and serve as the collecting centre of run offs during the rainy season, (Chukwudi et al,

FIGURE 1.1:Map of Nigeria Showing the Location of Study Area

FIGURE 1.2: Base Map of the Study Area, showing Accessibility and the Three Profiles

1.4     AIMS AND OBJECTIVES

The aims and objectives of this study is for

  • Identification of lithology on the basis of resistivity/chargeability
  • Mapping of fault
  • Correlation of strata to known Lithology
  • Correlation of fault to known fault
  • Ascertain the degree of salinity & sulfide concentration
  • Determination of the pond origin

1.5     LITERATURE REVIEW

Many facts have been established due to many works done on the Southeastern Nigeria sedimentary basin of which the study area is a part. The study area lies within the Campanian – Eocene Anambra basin in Southeastern Nigeria. According to Petters (1978), the basin is an indication of local tectonism that activated source areas of the Benue trough region. The basin corresponds to the western complimentary syncline to the emergent Abakaliki anticlinorium in the lower Benue trough. The Benue trough has received consideration interest since 1903, when exploration for coal started.

A systematic study of the geology of the Enugu area was carried out by Wilson (1925) when he mapped the eastern railway line. However, further extensive mapping of southern Nigeria was carried out by Shell D’ Arcy, presently known as Shell Petroleum Development Company (SPDC),1938 – 1957, in their exploration for oil and gas.

Tattam (1944), during his review on the stratigraphy of Nigeria established that the coal measures recognized by Wilson consist of two coal bearing formations. He named them as the lower and upper coal measures. The two coal measures were separated by a thick series of sandstones, and this became important contribution to the stratigraphy of the Southeastern Nigeria giving rise to what we know as Mamu formation (Lower coal Measures) and Ajali sandstone (false bedded sandstone) and also Nsukka formation(upper coal measures).                                                                       

The Nsukka Formation, which overlies the Ajali Sandstone, begins with coarse to medium-grained sandstones and passes upward into well-bedded blue clays, fine-grained sandstones, and carbonaceous shales with thin bands of limestone (Reyment, 1965; Obi et al., 2001). Obi et al. (2001) used sedimentological evidence to suggest that the Nsukka Formation represented a phase of fluvio-deltaic sedimentation that began close to the end of the Maastrichtian and continued during the Paleocene (Figure. 1.3).

Murat (1972) recognized three tectonic phases, within the southern Nigeria sedimentary basin. This began during the lower cretaceous. The resultant sedimentation in the basin was controlled by these tectonic phases, during the second tectonic phase (Upper Santonian – lower Campanaian), the rift like Benue – Abakaliki uplift, the anambra basin where the study area lies within and Afikpo Syncline was formed along with anambra basin. The view of Benerjee (1979) was supported by Amajor (1984) and Ladipo (1988) that Ajali sandstone was likely deposited in an environmental spectrum probably ranging from marine through transitional to continental.

Reyment (1964) reviewed the Nigeria Cretaceous – Cenozoic stratigraphy and stated that the Mamu Formation in Southeastern Nigeria overlies the Nkporo Shale and is of Lower Maastrichtain age. He formerly refered to this Formation as the Lower Coal Measures. Overlying the Mamu Formation is the Ajali Sandstone, which lies conformably on it. He stated the age to be Upper Maastrichtian and noted that the Formation was deposited during the regressive phase of the sea and the sequence is entirely continental. Conformably on the Ajali Sanstone is the Nsukka Formation (Upper maastrichtian – Paleocene) which is parallic.

Murat (1972), Kogbe (1976), and Reyment (1965), had work on the Paleographic history of Nigeria from Albian time to recent time reviewing the marine sedimentation of the lower Benue trough which began with deposition of sediments of the Asu River Group, Ezeaku Shale Formation, length wise correlation of lithostratigraphic units shows a general increase in the marine character of the deposits from the Northeast to the Southwest of Benue trough of Nigeria.

Aquifer transmissivity of the Ajali Sandstone was determined by Onuoha and Mbazi (1988) using electrical sounding. They noted the hydrologic condition favouring the aquifers formation is provided by the lateritic red-earth and weathered top of the sandstone, and by clay/shale members of the Mamu formation that is immediately and conformably underlies the permeable and porous formation. The permeable and porous lithology are instrumental to saturation of the deep layers of the Ajali Formation and water storage. The stratigraphic history of the region is characterized by three sedimentary phases (Short and Stauble, 1967; Obi et al, 2001) during which the axis of the sedimentary basin shifted. These three phases were:

a,  Abakaliki – Benue Phase (Aptian – Santonian),

b, Anambra – Basin phase (Campanian – Mid Eocene), and

c, Niger – Delta phase (Late Eocene – Pliocene)

The second sedimentary phase resulted from the Satonian folding and uplift of the Abakalike region and dislocation of the depocenter into the Anambra platforms and Afikpo region. The correlation chart of the early Cretaceous to Tertiary strata in Southeastern Nigeria is shown in Figure 2.4

1.6     METHODOLOGY

The methodology employed in this research work as regards the acquisition, processing and interpretation of data obtained during subsurface imaging around Ihe Pond are presented in details in Chapter Four under materials and methods

1.7     DATA GATHERING

The data used for this project were acquired around Ihe pond opposite St. Cyprain Girls High School Nsukka, Enugu State. A total of 1,400m was covered with three profiles namely prifile 1, 2 and 3. Their maximum spread lengths were respectively 500, 400 and 500m  see figure 2.4 above. The data were used to generate six inverse models, three each for Electrical resistivity and Induced polarization imaging for further interpretation. Several visitations were also made to the study location for geological field mapping. Water and geological samples were also collected from the Ihe pond and its environs for laboratory analysis.

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