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.

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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Item Type: Ghanaian Topic  |  Size: 60 pages  |  Chapters: 1-5
Format: MS Word  |  Delivery: Within 30Mins.


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