ORIGIN OF OHE POND IN NSUKKA, EVIDENCE FROM GEOPHYSICAL INVESTIGATION

ABSTRACT

The study area is located in Nsukka Local Government Area of Enugu State Nigeria. It lies between latitudes 6° 49′ N, and 6° 50′ N, and longitudes 7° 15′ E, and 7° 21′ E respectively. The area is underlain by Nsukka Formation, which is capped by the laterites, and underlain by heterolithic Sandstone / Siltstone layers with some clays or shale at some deeper horizons.  Geophysical investigation using Electrical Resistivity and IP methods with ABEM Terameter, SAS 1000, reveal the presence of fault in one section of the area, (Profiles 1 and 2, see Fig. 13, 14a and 14b of pages, 47 and 48 respectively) which potentially serves as the conduit through which water percolates to aquifer which eventually feeds the pond throughout the seasons.  This is indicated by the interpretation of anomalous low resistivity values within this zone, which shows a downward continuity within the geo-electric section in these profiles. A shallow possible source of the water in a perched aquifer following precipitation during the wet season, serves as recharge to the deep seated aquifer.  The result of the Profile 3 shows no continuity of the fault across that point, hence no anomalous drop in the resistivity in this area.

CHAPTER ONE

INTRODUCTION

1.1 Location and Accessibility

The study area is located within Nsukka town of Enugu State in Southeastern Nigeria. It lies within Latitudes 6° 49′ N and 6° 50′ N and Longitudes 7° 15′ E and 7°21′ E. The area is bounded by major communities which include Nsukka town, Obimo town, and Edem town. The study area is accessible through a network of tarred and untarred roads. The major roads include Obimo road, Ogurugu–Adani road, and Barracks Junction-Obimo road via St. Cyparin Special Science School. The minor roads include Umukashi road, Odoru-Nsukka road, and Edem road. Many foot paths connect the study area to the tarred and untarred roads. Fig.1. shows Location and Accessibility Map of the Study Area.

Fig.1. Location and Accessibility Map of the Study Area.

1.2 Statement of Problem

The Ohe pond is situated within the lateritic Sandstone cap of the Nsukka Formation. The Nsukka Formation is essentially made up of layers of Sandstone, Shales and Clays with some heterolithic of Siltsone and Clay, and capped by the lateritic overburden. The lateritic overburden is extensive and covers large area. Naturally a laterite serves as an unconfined aquifer for accumulation of water following precipitation. The Clay lenses serve as stratigraphic trap by terminating the downward percolation of the accumulated water.

The Ohe pond has been observed to be perennial having water throughout the year both wet and dry seasons. One striking feature about the area is that as much as an extensive area surrounding the pond is occupied by the same rock material which is the lateritic sandstone, yet the pond is only confined within a small radius of about 85m. This suggests a point source for the larger part of water that feeds the pond. That is to say, that the volume of the water and the perennial nature suggest that the major source of the pond water could be located somewhere beneath the Ohe pond surface. The only plausible explanation could be a connection between the pond area through a Fault / Fracture and a deeper source of groundwater beneath the area.

This research thus sets out to examine the area through the application of geophysical method to evaluate the structure, to determine the possible existence of fault to scientifically explain the reason for the existence of the Ohe pond in the area.     

1.3 Objectives of the Study / Expected Results

To find the source of water to the Ohe pond.

To determine if there is a linkage between the pond, and the underlain source through a connection by presence of fault/fracture.

1.4 Study Methodology

Two different approaches were employed; one is to know the detailed geology of the area. To identify possible aquifer system of the area through geologic mapping.

The second method is the geophysical survey to independently identify the geo electric layers and to identify any structure on the area, such as fault or fracture that could serve as conduit linking the pond to deep source of the groundwater. The geophysical method applied includes electrical resistivity method.   

1.4.1 Desktop Study

A desktop study was carried out in March 2012, and it involves the collection of the base map of the Nsukka and its environs. These include the base map, that is, its location, accessibility and drainage pattern. Planning which has to do with the Geology of the area and Geophysical investigation of the area.

1.4.2 General Field Work Approach

A reconnaissance survey was done in April 2012, and it comprises of location of out crop in the study area, seeking consent and permission from the community leaders before going into the detailed geologic mapping and geophysical survey, exercise was carried out on May 2012, and lasted for some days. On the surface geology, fault was identifying while on the subsurface geology, fault was also identify with the help of pseudo section from Electrical Resistivity Survey, (profiles 1 and 2). Some of the equipment used in the course of this study include, Global Positioning System (GPS) for measuring altitudes of the area, measuring tapes, geologic hammer, topographic map, field note book, cameras, and geophysical equipment (ABEM Terrameter SAS 1000,) which was used for the Wenner array configuration profiling.

1.4.3 Field Method at Ohe Pond

The profiling was done at a constant electrode spacing of ten meters (10m) each in all the three profiles. (Profiling 1 along St. Cyprian Special Science School Road; profiling 2 between St. Cyprian Special Science School Road, and Nsukka-Adani Express road; profiling 3 along Nsukka-Adani Express Road). The electrodes were connected to the central switching system (Terrameter). Employing theWenner array, four electrodes were chosen. Two electrodes each for the current and potential electrodes were at the centre. These electrodes were moved along at a specified interval. During the traversing, current were passed into the ground through the current electrodes. The potential differences between the two electrodes were measured and the resistivity of the ground measured. The general formula for Wenner resistivity measurement using four electrodes is simple for specific array of current and potential electrodes. The separation of the current electrodes is chosen so that the current flow is minimized in depths where lateral resistivity contrast is expected, (Lowrie, 1997). The equation  is a conversion factor used in converting the resistance to resistivity, where R = ^V/₁(i.e. resistance measured in the field and a = distance between the electrode). Fig. 2. shows Field equipments used at Ohe pond.

1.4.4 Field Data Acquisition and Processing

This acquisition and processing technique was used for collection and Processing of the data acquired in this study. The first imaging data was acquired using the ABEM Terrameter SAS 1000 which employed a multi electrode LUND image system using Wenner array. The study also utilized the latest iteration 3 RMS of the (Root Mean Square) method. The second imaging method used was the Electrical Resistivity Imaging (ERI), which is an indirect technique that records the resistivity of an area or volume within the subsurface (Ramirez et al., 1993). The RESIN2DIV Software was used for the processing of the data. The survey layouts were designed to provide the greatest depth penetration along with high resolution enough to image a fault’s fractured zone. Since this zone can be large, the largest electrode spacing possible was used. The terrain was also a major consideration in the electrode line layouts.

The surveys also had to cross the fault toward the center of the line for the best resolution at depth. This was the most critical part in designing the survey and choosing the site. The key was to find property that had enough open range on both side of the fault to allow the survey line to extend out for at least 100 meters on either side. The site that was chosen was open enough for each of the survey to fit the criteria. This limited data acquisition to the Ohe Pond study area.

1.5 Geomorphology and Topography

 The study area is characterized by the three main land forms. They are residual hills, low land and broad dry valley that bear vegetation. The residual hills are the remnants of Nsukka Formation and they form prominent physiographic features in the area. They form characteristic elongate ridges that generally trend in a NE-SW direction. These residual hills recapped by laterites which are resistant to erosion thus accounting for the present geomorphic features (ridges and dome-shaped outliers) of the area.

The topography of the map area is undulating, featuring the North-South trending escarpment, cone-shaped hills and broad dry valleys. This escarpment is steep in some places, noted that this escarpment in part of the North-South trending Nsukka–Okigwe cuesta of the Ajali sandstone whose dip is generally South-West ward.The cuesta is asymmetric in shape and is more or less parallel to one another. Isolated hills with laterite capped that occurred in the area are outliers of Nsukka Formation overlying the Ajali-Sandstone; the outliers are often sigmoid in symmetry surrounded by dry valley.

These hills are often separated by lowland and broad dry valley.

The lowland is made up of overburden red earth and sediment weathered and eroded from the upper lateritic cap. The lowland and valleys are underlain by Ajali Sandstone occupying lower elevations and they are predominantly sandy. Though the absence of stream in the area is pronounced due to geologic nature of the underlying rock, there are two ephemeral springs oozing out from the Nsukka Formation where thick bands of impervious rock underlies a pervious rock, it may be as a result of fault in the areas.