Groundwater potential zone map

CHAPTER 2: LITERATURE REVIEW

3.10 Groundwater potential zone map

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Step 3: The consistency ratio (CR) is thus computed using equation 3.2 as follows:

𝐶𝑅 = ^𝐶𝐼

𝑅𝐶𝐼 3.2 Where (𝐶𝑅) is the consistency ratio, and 𝑅𝐶𝐼 is the random consistency ratio.

The value of the random consistency ratio is derived from Saaty’s table of random index values.

The Consistency Ratio should be equal to 0.1 (i.e. 10%) or less for all consistent weights, otherwise the corresponding weights should be reevaluated for inconsistency (Saaty, 1980). Consistency Ratios greater that exceed 0.1 are too inconsistent and hence unreliable.

The features of each theme were further assigned a knowledge-based hierarchy ranking of 5 classes based on their degree of influence to groundwater occurrence. The most influential attribute in the theme was placed in class 5, and the least influential attribute placed in class 1. The ranking of parameters in all four themes were conducted whilst considering the works carried out by researchers such as (Tessema et al., 2014; Palaka and Sankar, 2015).

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Figure 3.2: Schematic representation of the overlaying process (Modified from Geo-world, 2003)

3.11 Validating the Groundwater Potential Zone Model 3.11.1 GRIP Borehole dataset

Validation of the groundwater potential zone map was done by comparing the resultant map with GRIP borehole drilling information as well as the results of follow-up resistivity surveys. GRIP borehole database contain significant information on boreholes drilled by the municipality as well as private boreholes. The dataset come in tabular form as a spreadsheet file showing parameters such as the geographic position, discharge rate, water level, borehole depth and operational status of the boreholes. Therefore, the X, Y data was used to make a plot in ArcMap to show the spatial distribution of low and high yielding boreholes across the study area. Subsequently, a shapefile was created and superimposed on the potential zone map to assess the relationship between the borehole yields and groundwater potential zones.

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Validation of the final map will involve a comparative analysis of the frequency of occurrence of successful and unsuccessful boreholes in various groundwater potential zones. In this case, the map will be considered reasonable if a high frequency of high yielding boreholes (> 0.5 l/s) occur in moderate-high groundwater potential zones. In addition, the frequency of occurrence of low yielding boreholes in poor groundwater potential zones will be analyzed and used for validation.

3.11.2 Follow-up Geophysical Surveys

From the detailed structural maps that were produced from the combined interpretation of satellite imagery, aeromagnetic data and published geological maps, target areas were delineated for follow up using ground electrical resistivity surveys (figure B-4.4 in appendix B). In general, all the surveys were conducted in close proximity to settlements to ensure that the results benefit the surrounding water-stressed communities.

3.11.2.1 Electrical Resistivity survey

Electrical resistivity survey profiles were selected based on the interpretation of the integrated lineament map and groundwater potential zone map. Areas with major and minor lineaments were chosen in order to infer the mode of groundwater occurrence in areas with cross-cutting structures. The survey profiles were mainly designed to cut across the geological structures.

3.11.2.2 Survey Instrumentation

The resistivity survey was carried out using the Ares-G v5.0 Resistivity and IP imaging instrument. This resistivity-measuring instrument is equipped with 12 volts onboard rechargeable battery used as a current source, four separate cables, and four stainless steel metal stakes used as electrodes. A 12-channel GPS receiver was used to determine the location in terms of coordinates and elevations of the sounding points. A hammer was used to implant the electrodes into the ground, and the lateral extent of the survey as well as the spacing between the electrodes were determined using a measuring tape. Vertical Electrical Sounding method with Schlumberger electrode configuration was used in this study.

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Measurements were carried out by introducing current into the ground through the current electrodes and measuring the resulting potential difference through a pair of potential electrodes (Anomohanran, 2013). At each measurement, the resistivity meter displayed all the necessary subsurface parameters, which included the apparent resistivity, induced polarization, potential difference, and the amount of transmitted current. All these parameters were recorded automatically and saved in the device. The parameter of interest in this case was the resistivity of the ground, which is controlled primarily by the porosity, and water content of the underlying media (Bernard, 2003). These recorded resistivity values were used to compute the apparent resistivity of the ground, using a geometric factor for schlumberger array used in this study.

3.11.2.4 Data processing and presentation.

The recorded VES data was downloaded from the resistivity meter using surfer, a computer software that automatically converts the data into a readable format. The converted data was then imported into excel format and later into IP2WIN where it was used to plot the curve of apparent resistivity versus current electrode spacing (the VES curve) and resistivity sections.

3.11.2.5 VES data interpretation

The interpretation of the resulting resistivity curves was based on the assumption that the subsurface consist of horizontal layers. In this case, the subsurface resistivity only changes with depth, but does not change in the lateral direction. The curve will show the number of underlying geoelectric layers as well as their corresponding resistivity values, depth and thicknesses. These parameters allow for the identification of conductive layers that may be saturated with groundwater. A simplified pictorial form of the subsurface resistivity distribution is often provided by a resistivity-section constructed using resistivity data acquired from two nearly parallel profiles. The resistivity curves and resistivity sections were then used for interpretation.

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CHAPTER FOUR: DATA PRESENTATION, INTERPRETATION AND DISCUSSION OF RESULTS.

This chapter presents the analysis, results, interpretations and discussions of the collected data. In addition, a brief summary of the preparation and processing of the datasets is outlined prior to presentation of the results; refer to chapter three for detailed description of data preparation techniques.