Abstract
Groundwater is the water that exists in pore spaces and fractures in rocks and sediments
beneath the water table. The need for groundwater has increased tremendously due to the
unavailability and the contamination of surface water bodies by the intrusion of saline water and
human activities.
The geo-electric soundings were carried out in Eleme Port Harcourt, capital of Rivers
state in order to delineate the subsurface geo-electric layers and the depth to the aquifer unit for
groundwater development. Ten (10) vertical electrical sounding stations were occupied within the study area using
the schlumberger electrode configuration. The Pasi Earth Resistivity Meter (16EL Model) was
used. The sounding curves were classified into five (5) curve types: KH, HKH, QHK, HKQH
and QQ curves. The quantitative interpretation of the geo-sounding curves by partial curve
matching and computer iteration revealed 5 geo-electric layers based on characteristic resistivity
ranges. The layers are;
1.0 INTRODUCTION
1.1 GENERAL STATEMENT
Groundwater is water that exists in the pore spaces and fractures in rocks and sediments beneath
the water table. It originates as rainfall or snow, and then moves through the soil and rock into
the groundwater system, where it eventually makes its way back to the surface streams, lakes, or
oceans. Groundwater makes up about 1% of the water on the Earth (most water is in oceans.
Groundwater is found beneath the unsaturated zone where all the open spaces between
sedimentary materials or in fractured rocks is filled with water and the water has a pressure
greater than atmospheric pressure. To understand the ways in which groundwater occurs, it is
needed to think about the groundwater bearing formation properties such as porosity and
permeability. The water bearing formation is termed AQUIFER.
Porosity: this is the property of a rock possessing pores or voids.
Permeability: this is the ease with which water can flow through the rock.
Aquifer: which is a geologic formation sufficiently porous to store water and permeable
enough to allow water to flow through them in economic quantities.
Storage coefficient:this is the volume of water that an aquifer releases from or takes into
storage per unit surface area of aquifer per unit change in the component of area normal to
surface.
The occurrence of groundwater resources in a basement complex depends mainly on the
secondary porosity (after deposition of sediments) and also permeability arising from weathering
and fracturing of parent rocks and also the pattern of the fracture (Carruthers, 1984).
Fractures in rock are very important pathways for the flow of groundwater and the transportation
of contaminants. In fractured rock systems, groundwater occupies voids that are formed by
fractures, fissures, faults and joint planes which are constantly distributed inside the rock
formation. Due to their nature, they exhibit unique problems in their investigation, evaluation
and management largely because of their heterogeneous nature and the dependence of aquifer
properties on fracture distribution and connectivity.
A relatively inexpensive way to prospect for groundwater, both on a small and large scale is by
using electrical resistivity method of geophysical prospecting; this method is fast, repeatable,
relatively cheap and non-intrusive, thus making it a practical alternative to traditional approaches
(Skinner and Heinson, 2004). The electrical resistivity of rocks depend on several factors, some
of which include; the presence of conductive minerals such as base metal sulphides or oxides and
graphites in the rock. Most rocks without these minerals are usually poor conductors and their
resistivity is determined primarily by their porosity, degree of fracturing and the degree of
saturation of the pore spaces (Cook et al., 2001).
Electrical methods have been successfully employed to monitor groundwater occurrence and
have also provided information on fluid electrical conductivity, fracture orientation and overall
bulk porosity (Dailey et al., 1992, Slater et al., 1996, Skinner and Heinson 2004, Adepelumi et
al., 2006, Batayneh 2006., Weiss et al., 2006).
AIM AND OBJECTIVES
The aim of the research is to use the electrical resistivity method to prospect for groundwater
devolopment in the study area. The objectives of the study are;
? To determine the geo-electric parameters of the different geologic units.
? To delineate the geo-electric layers, their thicknesses and lateral extent. ? To determine the depth to the aquifer unit ? From the above objectives, to locate a borehole point.
1.4 PHYSIOGRAPHIC SETTING
1.4.1 LOCATION, ACCESSIBILITY AND DRAINAGE PATTERN
The study area lies within the sedimentary terrain of southern Nigeria between longitudes E:
007° 06.215? to E: 007° 06.564? and latitudes N: 04° 49.480? to N: 04° 49?793 in Port Harcourt,
capital of Rivers state. The study area is accessible as a result of the availability of roads; the
terrain is generally low-lying with elevation between 8 -17m above mean sea level and slopes
unperceptively towards the Atlantic Ocean (International Journal of Science and Technology
Volume 3 No. 2, February, 2014). The drainage pattern is largely controlled by the Bonny River
and its tributaries and creeks which together drain various outcrops of relatively higher land
which are largely surrounded by mangrove swamps, (Bell-Gam 2002).
1.4.2 CLIMATE AND VEGETATION
Rainfall is high in Port Harcourt with annual mean of 240cm. the rainfall exhibits double
maxima regime with peaks in July and September. The area falls within the humid tropics with
humidity of 63- 79%, (Korean Report, 1980).The physiography conforms to the geomorphic
features of the Niger Delta governed by several factors which influence transport and ultimate
deposition of the sediment load, shape and growth of the delta. The Niger Delta comprises five
geomorphic sub-environments(Osakumi and Abam 2004); the undulating lowlands of the coastal
plain sands, the flood plain of the lower Niger with extensive sand deposits, the meander belts
consisting of wooded freshwater swamps, the mangrove swamps and estuary complexes and the
beach ridges. These sub-environments are zones where a vast amount of sediments are deposited
by rivers in their search for lines of flow, (Osakumi and Abam 2004).
1.5 REVIEW OF PREVIOUS WORKS
The electrical resistivity method has been used by many researchers to detect bedrock fractures,
overburden thicknesses, geoelectric layers, groundwater potential and so on.
Ozegin et al., (2008) used combined electromagnetics method and vertical electrical sounding to
establish groundwater viability in Oke-Agbe high school field located in Akoko North-west local
government area of Ondo state based on a clear-cut relationship between electromagnetics
method and vertical electrical sounding. Both methods were jointly used for investigation to
determine the overburden thickness, geo-electric parameters and groundwater potential.
Selemo et al (1995) worked on an appraisal of the usefulness of Vertical Electrical Sounding
(VES) in groundwater exploration in Nigeria and concluded that the results of VES with the
schlumberger array in many parts of Nigeria have being very useful in identifying viable
locations for water boreholes.
Adiat et al., (2009) used integrated geophysical techniques involving the VLF-EM and the
electrical resistivity sounding methods to map Oda town, Southwestern Nigeria to determine the
groundwater potential of the town. The qualitative interpretation of the VLF-EM results
identified areas of hydro-geologic importance and formed basis for vertical electrical sounding
(VES) investigation. On the basis of the geo-electric parameters, the area was zoned into good,
intermediate and poor groundwater potential zones.
Olayanju G.M (2011) carried out a geophysical mapping involving VLF electromagnetic
profiling along seven profiles, ten offset wenner and two azimuthal soundings in the study of
perennial spring sites at Iloyin community in Akure metropolis, Southwest Nigeria.
Odoh and Onwuemesi (2009) use Azimuthal Resistivity Survey (ARS) to determine and
characterize the anisotropic properties of fractures in Presco campus of Ebonyi state University
Nigeria, for evaluation of groundwater development and flow within the area. The azimuthal
resistivity survey results show that there is significant anisotropy between depth and fractures.
Variation of the coefficient of anisotropy has been shown to have the same functional form as
permeability anisotropy. Thus, a higher co-efficient of anisotropy implies higher permeability
anisotropy. The results also indicate better permeability and porosity.
Isifile and Obasi (2012) used radial vertical electrical sounding (RVES) around Ifon, south
western Nigeria, to determine electrical anisotropy and map trend of concealed structure. The
results show that the concealed bedrock is anisotropic with causative features such as foliation,
joints and fault which could favour groundwater storage.
Ajibade et al, (2012) used Azimuthal Resistivity Survey (ARS) to investigate the groundwater
potentials and anisotropic properties of fractures for sustainable groundwater development within
Ibadan metropolis. Result of groundwater head contouring showed that groundwater flow is
dominantly in directions which are associated to fracture-controlled flow.
Olasehinde and Bayewu (2011) used evaluation of electrical resistivity anisotropy to report the
potency of combination of anisotropy polygon and isoresistivity map in reducing ambiguity
inherent in a single geophysical parameter in OdoAra, west central Nigeria.
Onabanjo (2001) carried out geophysical survey using resistivity method in Ago-iwoye
southwestern Nigeria and discovered two types of subsurface zones which are associated with
groundwater exploration namely the weathered basement and fractured basement in which both
are separated by barriers of unaltered rocks tending to reduce the possibility of groundwater
accumulation.