ESTIMATION OF BOREHOLE YIELD USING VERTICAL ELECTRICAL SOUNDING DATA: A CASE STUDY FROM KWABRE DISTRICT

ABSTRACT
Groundwater has been identified as the world’s largest resource that can be easily accessed especially when confronted with water shortages as a result of low river discharges. Successful execution of groundwater exploration using geophysical methods requires knowledge on the relationships that exist between hydrogeological and geophysical parameters. This makes data interpretation and delineation of groundwater zones easier, thereby avoiding the drilling of marginal holes. This study was carried out in the Kwabre District and was aimed at establishing a relationship between airlift yields and electrical resistivity parameters. Three methods, viz. borehole log analysis, cumulative resistivity method and the drilling rate with respect to formation analysis, were employed to obtain resistivity (rho) and thickness of the saturated zones from borehole logging and vertical electrical sounding data. The obtained resistivity and saturated thickness were used to compute transverse resistance (Tr) and longitudinal conductance (Sc). Each of these parameters (i.e. Tr and Sc) including the resistivities of the saturated zones were further correlated with airlift yields of the boreholes to evaluate their relationships. Also, the airlift yields were used to create a yield map of the district. Correlation results obtained from the borehole log analysis and cumulative resistivity method suggest there is no relationship between the airlift yield and the resistivity parameters. On the other hand, results obtained from the drilling rate with respect to formation analysis showed that the yield versus resistivity, longitudinal conductance and transverse resistance are related by 60.85%, 57.16% and 50.25% respectively. However, validations of the associated models were very poor in predicting measured airlift yield values. Thus, further studies may be required to improve and validate the method. This study also provided useful information on the variation of airlift yields in the district.


CHAPTER ONE
INTRODUCTION 
1.1       Background
Groundwater is known to be the world’s largest accessible storage of fresh water. It is considered as the logical resource to turn to when faced with water shortage which may arise from low and variable river discharges (Ifabiyi et al., 2016). The convenient nature and occurrence of groundwater have rendered it less expensive for treatment when exploited and could be developed at a desired location and at a reasonable cost.

The demand for adequate, good-quality water has increased extensively due to awareness and technology. Therefore, many people rely on the exploration and exploitation of groundwater, which is one of the valuable natural resources for sustenance of life. Its quality is good for human consumption; hence, it is the most preferred choice in various households and communities.

In the Kwabre District of Ashanti Region, groundwater has proven to be the most reliable source of water for household, agricultural and some industrial purposes. It has over the years, conveniently, served the inhabitants of the district during the dry seasons when surface water runs out. Groundwater consumption in the district has saved the inhabitants from contracting some water-borne diseases that could be obtained from the consumption of surface water, which is usually exposed to the glare of potential pollution. As the occurrence of groundwater depends on the nature and type of aquifer, the shallow aquifer system in the district permits groundwater to be easily tapped by hand-dug wells and boreholes.

Due to the growing needs of groundwater in most areas including Kwabre District, many methods have been used for its exploration. These methods include water witching (dowsing), fracture mapping, direct drilling, borehole logging and the use of geophysical techniques. Today, one of the most efficient ways of identifying groundwater is the use of geophysical methods. These methods include but not limited to, electrical resistivity, seismic, electromagnetic, gravity and magnetic. Out of all the methods, the direct current resistivity method is a common tool used in groundwater survey. This method can be successively employed for groundwater exploration where a good electrical resistivity contrast exists between the saturated and unsaturated layers. The vertical electrical sounding with dipole-dipole array as a low-cost technique and as an authentic tool in groundwater exploration, is more suitable for hydrogeological surveys in both sedimentary and hard rock terrains. It has been used to successfully map vertical variations in resistivity with depth, determine the depth to the water table, thickness of the saturated zone, delineate structures that serve as hosts for groundwater accumulation, map overburden thickness, determine the depth to bedrock and provide an understanding of the geometry of aquifers (Ifabiyi et al., 2016). This technique employs collinear arrays of electrodes designed to input a 1-D vertical apparent versus depth model at a specific observation point. Using this technique, a series of potential differences are acquired at successful greater electrode spacing while maintaining a fixed central reference point. The potential difference measurements are propositional to the changes in the deeper subsurface (Cardimona, 2002). Vertical electrical sounding (VES) technique for groundwater exploration has proven reliable in many areas. For instance, it was used in delineating groundwater zones in an arid region in Iran (Nejad et al., 2011). It has proven very popular with groundwater prospecting and engineering investigation due to the simplicity of the technique and has been used to map groundwater bodies in many places in Nigeria (Jatau et al., 2013).

Although VES has proven useful and shown to be one of the most reliable techniques for groundwater siting, unsuccessful and marginal holes are still encountered; hence there has been studies trying to improve on the success rate. Like many others, this study is investigating the use of VES data to estimate the yield prior to drilling by creating a model that relates yield to VES data. This model is to show that, results obtained from VES survey can further be used to estimate the potential yield of boreholes and, therefore, enhance selection of VES points for drilling. In so doing, it would help improve the success rate in drilling boreholes and cut down the losses from drilling dry and marginal holes.

1.2       Purpose and Specific Objectives
The main purpose of this research is to establish a possible relationship between borehole yield and some electrical resistivity parameters. The specific objectives are to:

* Develop a yield map of the area,

* Determine resistivity of the saturated zone,

* Determine transverse resistance and longitudinal conductance of the saturated zone, and

* Establish correlation between the yield and electrical resistivity parameters

1.3       Scope of Study
The research is limited to Kwabre District of Ashanti Region and concentrated on creating models that relate yield to vertical electrical sounding data. Secondary data consisting of dipole-dipole siting data, borehole logs and borehole construction data were used. The ArcGIS software was used to produce the yield distribution map of the area. Comparing the VES data with the borehole logs, resistivity and thickness of the saturated zones were obtained and used to compute transverse resistance and longitudinal conductance. Also, airlift yields obtained from the borehole logs were correlated with each of the electrical resistivity parameters to determine the relationship between them.

This thesis comprises five chapters. The background, problem statement, justification and scope of work are presented in chapter 1. Chapter 2 includes a review of literature on previous studies done in relation to the study while Chapter 3 provides a brief description of the study area and explains the methods employed in the study. The study results and discussion are presented in Chapter 4. Lastly, conclusion and recommendations based on findings from the study are provided in Chapter 5.

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Item Type: Ghanaian Project Material  |  Attribute: 38 pages  |  Chapters: 1-5
Format: MS Word  |  Price: GH50  |  Delivery: Within 30Mins.
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