ABSTRACT
Electrical resistivity survey has proven to be an effective
tool for groundwater exploration and has been widely embraced to help reduce
the drilling of unsuccessful wells. Currently, information from electrical
resistivity survey is often used in locating points for drilling, but it does
not give indication of the yield of the borehole. The lack of this information
therefore sometimes results in the drilling of dry and marginal wells. This
study therefore looks at the possibility of using resistivity data, which is
readily available from electrical resistivity surveys for groundwater
exploration, for estimating the yield of yet to be drilled borehole. The study
was limited to the Garu Tempane and Bawku West districts. Secondary data on
vertical electrical sounding (VES) and drill logs for 49 boreholes in the
selected districts were used. The thicknesses, apparent resistivities,
longitudinal conductance and transverse resistance of the various subsurface
layers of the boreholes were determined from drill logs and VES data.
Correlations between borehole yields and the third layer apparent resistivity,
longitudinal conductance and transverse resistance were then investigated to
develop regression models for estimation of the borehole yields. The results
showed that the third layer is fractured and contributes significantly to
borehole yields in the area; hence the fractured subsurface layer is of primary
interest to be considered in groundwater exploration and estimating potential
borehole yield from VES data. The results obtained further indicated that
apparent resistivity, longitudinal conductance and transverse resistance had
good exponential and positive linear relationships with borehole yield.
CHAPTER 1: INTRODUCTION
1.1 Background
The high cost associated with
borehole drilling has necessitated for extensive detailed subsurface
investigation before selected sites are chosen for eventual drilling to reduce
the number of unsuccessful wells. Electrical resistivity survey has
demonstrated to be an effective means for groundwater exploration. Geophysical
methods application has commonly proved to be highly effective for water
content estimation and locating of the depth to water table and bedrock
(Hubbard and Rubin, 2002). Successes have also been attained using the
resistivity method for siting wells in areas underlain by crystalline rock
terrains (Ballukraya, 2001; Gupta et al., 2000; Patangay et al., 1977; Satpathy
and Kanungo, 1976). Mazac et al. (1985) assessed the correlation between
aquifer and geoelectrical parameters in both the saturated and unsaturated
zones of the aquifers. VES is ideal for ascertaining the depth, thickness, and
boundary of an aquifer (Zohdy, 1969; Young et al., 1998) and the water content
of aquifer (Kessels et al., 1985). But still dry and marginal wells are
recorded even after groundwater investigations are conducted. There is
therefore the need to augment the groundwater investigation to improve the
success of drilled holes and to cut down the cost of drilling unsuccessful
wells.
Garu Tempane and Bawku West
districts are two districts with serious water challenges in Upper East region.
The districts rely mostly on dugouts, dams, hand-dug wells and some boreholes
for their water supply needs. The preferred water supply alternative in the
area is groundwater because it is usually present even in drought situations
and of comparatively good quality. The groundwater is not only viable but also
the most cost effective means of potable water for these rural and scattered
settlements (Gyau-Boakye and Dapaah-Siakwan, 1999).
Although boreholes have been
drilled to complement surface water supplies in Garu Tempane and Bawku West
districts, some of these boreholes have been abandoned as a result of low
yields. Conventionally, the yield of a borehole is determined after drilling
and carrying out pumping test. The borehole is declared unsuccessful and
abandoned if the yield is below 13 liters per minute according to Community
Water and Sanitation Agency (CWSA, 2014) standard.
Hence it
would be beneficial for the yield of an intended borehole to be estimated from
electrical resistivity data obtained from geophysical exploration survey which
is conducted to aid in selecting drilling sites. This would help to reduce the
drilling of dry and marginal wells.
This study therefore explores the
possibility of estimating borehole yield from electrical resistivity data by
developing models relating borehole yield to electrical resistivity parameters.
The resultant models will, hopefully, help to improve on selection of potential
sites for drilling and reduce the drilling of unsuccessful wells. This will
enable the limited financial resources in developing countries to be channelled
into providing boreholes with substantial yields.
1.2 Research
Objective
The primary objective of the study
is to investigate the possibility of estimating borehole yield from sub surface
electrical resistivity data in Garu Tempane and Bawku West districts in Upper
East Region of Ghana. The specific objectives of the study are:
* To assess the relationship between
individual subsurface layer apparent resistivities and borehole yield within
the study area.
* To determine the relationship
between longitudinal conductance and borehole yield.
* To determine the relationship
between transverse resistance and borehole yield.
* To assess the reliability of
estimating borehole yield from individual layer apparent resistivities,
longitudinal conductance and transverse resistance.
1.3 Scope
of the thesis
The study is limited to Garu
Tempane and Bawku West districts. Basically secondary VES data, drill logs and
air lift yield of boreholes in these two districts were acquired for the study.
Thicknesses and apparent resistivities of various subsurface layers were
determined using drill logs alone first and secondly from the VES data.
Longitudinal conductance and transverse resistance were the main electrical properties
focused on. Correlation relationships between the borehole yield and the third
layer apparent resistivity, longitudinal conductance and transverse resistance
were investigated. Regression models were developed and used to estimate yield
in these two districts.
The thesis comprises of five
chapters. Chapter one is the background of the thesis, research objectives and
scope of the study. Chapter Two deals with review of literature and study area
focusing on: crystalline bedrock aquifers, groundwater storage and flow in
basement complex rocks and relations between electrical resistivity and yield.
Chapter Three describes data acquisition and the methods used in the study.
Chapter Four presents and discusses the results from the study. Finally,
Chapter Five presents the conclusions and recommendations on results of the
study.
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