ABRACT

This study is on the demonstration of an application of two-dimensional resistivity surveys to groundwater exploration in a problematic sedimentary terrain where thick clay layers impedes groundwater aquifer recharge. Six profiles of 830 m length each were surveyed to probe the subsurface lithologies and their groundwater potentials. Data acquisition system comprised a Super Sting Resistivity Meter, 84 metallic electrodes, and their accessories. Data acquired from the survey were forward modeled and tomographically inverted, using finite difference techniques. Results of the study revealed the different rock layers beneath the survey lines,   their spatial distribution and their resistivities. Two new boreholes, BH3 and BH4, drilled based on the inverted 2-D resistivity images are very productive and have respective yields of 46 l/s and 48 l/s in the dry season. The choice of the locations for the new boreholes was informed by the presence of low resistivity structures interpreted as saturated (wet) sand, good vertical and lateral extents of the saturated sands, the depth of the aquifer in relation to the water table, and the absence of impermeable (sandy clay) cover that could retard groundwater recharge and discharge. The resistivity images of the 2-D survey also show that the failed boreholes – BH1 and BH2 were located on low resistivity structures interpreted as aquifers. But the aquifers have limited vertical and lateral extents, and are adjacent to thick impervious sandy-clay layer. Overall, the study demonstrates the suitability and the superiority of 2D resistivity survey to the traditional 1D – four electrodes survey.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

1.0     INTRODUCTION

1.1     BACKGROUND OF THE STUDY

• • OBJECTIVE OF THE STUDY

• • GEOLOGY AND HYDROGEOLOGY

CHAPTER TWO

• • LITERATURE REVIEW

• • OVERVIEW OF GROUNDWATER AND AQUIFER

• • SOURCE OF GROUNDWATER

• • DIVISIONS OF SUB-SURFACE WATER

• • HYDROGEOLOGIC CLASSIFICATION OF ROCKS

• • TYPES OF AQUIFERS

• • HYDRAULIC PROPERTIES OF AQUIFER

• • TYPES OF WELLS

CHAPTER THREE

3.0     METHODOLOGY

3.1     DATA ACQUISITION, MODELING AND RESISTIVITY IMAGING

CHAPTER FOUR

• • DISCUSSION OF RESULTS

CHAPTER FIVE

• • CONCLUSION

REFERENCES 

CHAPTER ONE

1.0                                              INTRODUCTION

1.1                                 BACKGROUND OF THE STUDY

The increased global drought condition has negatively impacted water availability for human, agricultural, and industrial uses. There is increased pressure on groundwater to meet the daily demand for water supply. The use of geophysical survey for water well and borehole pre-drill investigation in order to mitigate cases of ‘dry wells’ and failed boreholes is on the increase. The purpose of pre-drill survey is to understand the hydrologic properties and stratigraphic relationship of the different subsurface rock layers that can potentially yield water to the borehole or well. The use of one-dimensional (1D) Electrical Resistivity sounding (e.g., Vertical Electrical Sounding) employing four metallic electrodes – two potentials and two current electrodes – is gradually phasing-out. 1D resistivity survey is laborious, time-expensive, and does not account for the lateral variation in the hydrologic properties of the aquifer units (Merrick, 1997; O’Neill et al., 1984; Roy and Apparao, 1971). Having recognized the superiority of 2-deimesional to 1-dimensional resistivity survey for subsurface investigations, regularly spaced 1D VES surveys are mapped into 2D during data processing using interpolation techniques (Corriols et al., 2014; Adepelumi et al., 2008; Osinowo and Olayinka, 2012; Kaya et al., 2015). However, rocks properties and hydrogeologic parameters are non-linear properties. Often, interpolated data do not represent the true subsurface condition (McGillivray and Oldenburg, 1990). This often leads to incorrect hydrogeological interpretation of geological layers, especially in areas of complex geology where pinch-out structure and porosity heterogeneities are common.

Direct 2D Multi-electrodes resistivity survey is gaining more attention in groundwater exploration. 2D survey provides excellent view of resistivity distribution of the rock in the depth and lateral axes, thus allowing reliable evaluation of the hydrogeological units in the subsurface beneath the profile line. The method is time inexpensive, has higher coverage, and gives deeper penetration compared to 1D survey. Because the data acquired are actual measurements of subsurface resistivity distribution, the uncertainties due to interpolation are removed. 2D resistivity survey have been successfully applied to groundwater exploration, dam seepage investigation, and other environmental studies in the different parts of the world (Benson and Mustoe, 1998). Coriols et al. (2014) applied 2D electrical resistivity surveys to evaluate the groundwater aquifer dynamics. The study revealed the presence of unconfined alluvial and consolidated volcanic aquifers, and concluded that 70% of the water used for irrigation in Leon Chinandega plain is sourced from the unconfined alluvial aquifer. 2-D resistivity survey method is operationally similar to multi-channel seismic survey where the arrival of seismic pulses generated at a shot point can be recorded at many receivers’ points to construct the velocity image of the subsurface. In 2D resistivity surveying and imaging, the resistivities values recorded at multiple electrodes points are used to map the subsurface layers around and beneath the electrode points. The inversion process uses Finite element or finite difference techniques and the resulting resistivity image is called resistivity tomogram (Loke and Baker, 1996; Loke, 2001).

Groundwater exploration in the sedimentary terrain is straightforward when sedimentary layers are deposited in horizontal or near-horizontal pattern. Geophysical exploration for groundwater aquifer in sedimentary terrain can be challenging when clay sediments are deposited around the aquifer in dispersed or irregular pattern (Revil et al., 1998; Reynolds, 1997; Shevin et al., 2007). The presence of clay layers around aquifers impedes groundwater recharge and discharge (Longe et al., 1987) thereby reducing water volume accumulating in the boreholes or wells. Delta sate, the study area, has zones of thick clay sediments that are deposited in irregular pattern (Adeoti et al., 2012; Aluko, 2014). Lenses of clay in porous sand units distort the expected resistivity pattern and makes interpretation of 1D resistivity data unreliable (Reynold, 1997) because 1D data does not account for lateral variation in lithology and resistivity. In this case, 1D resistivity survey cannot reveal the detailed geology needed to take informed decision on where to cite a borehole. A more detailed resistivity survey and a priori geological modelling are required to cite productive boreholes in such areas.

Delta state (asaba), the study area, is located on longitude 6°00′E and latitude 5°30′N with an altitude ranging from 15 m to 72 m above sea level. Two boreholes drilled in the area failed to yield sufficient water in dry season. The locations of the failed boreholes were cited by 1D vertical electrical sounding survey. This necessitated the use of 2D Electrical Resistivity Survey in this study. The goals of this study are to locate suitable places to drill new boreholes to supply portable water to the residents and investigate the failure of the previous boreholes. The current study took place in February 2013 in the peak of dry season.

1.2                                  OBJECTIVES OF THE STUDY

The objectives of the study are to:

• • select locations for citing new boreholes

• • demonstrate an application of two-dimensional resistivity surveys to groundwater exploration in a problematic sedimentary terrain where thick clay layers impedes groundwater aquifer recharge and

• • Investigate causes of the failure of the two boreholes previously drilled in the area.

1.3                             GEOLOGY AND HYDROGEOLOGY

Delta State is an oil and agricultural producing state in Nigeria. It is situated in the region known as the South-South geo-political zone with a population of 4,112,445 (males: 2,069,309; females: 2,043,136). The capital city is Asaba, located at the northern end of the state, with an estimated area of 762 square kilometres (294 sq mi), while Warri is the economic nerve center of the state and also the most populated. It is located in the southern end of the state. The state has a total land area of 16,842 square kilometres (6,503 sq mi). Seismic and drilling data show that the basement fracturing largely controlled the sedimentation and subsidence associated with rifting. The basin is filled with sequence of argillaceous sediments underlain by basement complex rocks. The sedimentary rocks are soft and friable, but in some places are cemented by ferruginous and siliceous materials (Omatsola and Adegoke, 1981). The sedimentary sequence comprised sandand shale with some limestone intercalations. These sediments have been described and reviewed by many authors including Adegoke, 1977; Ako et al., 1980; Omatsola and Adegoke, 1981; Okosun, 1990; Nton, 2001. The five lithostratigraphic formations, from the oldest to the most recent are: Abeokuta Group (Cretaceous), Ewekoro Formation (Paleocene), Akinbo Formation (Late Paleocene – Early Eocene), Oshosun Formation (Eocene) and llaro Formation (Eocene). The Abeokuta Group un-comformably overlies the basement complex rocks.

Delta recent sediments constitute different aquifers. The aquifer can be classified into confined, semi-confined, and unconfined depending on the nature of the aquifer units and the adjoining rock layers (Longe et al. 1987). The coastal plain sands and recent sediment aquifers are unconfined aquifer found at shallow depth. They are prone to pollution from ground sources and run-off water, and their depths vary with topography and seasons. The aquifers are confined where the sand unit is bounded at the top and bottom by the low- permeable clay layers. This aquifer is part of the aquifer system in Delta. At shallow depth, the marine sand aquifers are not confined but contain brackish water.