Appraisal of geophysical methods used for groundwater exploration in South Africa
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Date
2023
Authors
Kolobe, Libuseng T.
Journal Title
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Publisher
University of the Free State
Abstract
Groundwater is a crucial resource that has been a means of survival for humans for many years, as seen in the tendency to settle near springs (Fitts, 2013). Interestingly, groundwater was extracted from dry mountain basins by Persians in what is now modern Iran (Fitts, 2013). Indeed, many methods have been developed over time to explore groundwater, as has been done with other resources of the Earth that are not readily visible.
According to Arefayne 𝘦𝘵 𝘢𝘭. (2016), groundwater exploration is “the investigation of underground formations to understand the hydrologic cycle, know the groundwater quality, and identify the nature, number, and type of aquifers”. Numerous exploration methods are employed and are often used in conjunction with one another to ensure the most accurate estimation of groundwater potentiality in a given area.
These exploration methods may be direct or indirect (Balasubramanian, 2017), with surface methods often indirect. These include esoteric techniques, which are the most ancient, also known as water witching or dowsing. Here, operators using a forked stick or an egg (Balasubramanian, 2017) claim to be able to detect water underground, the accuracy of which is fairly unproven (Arefayne and Abdi, 2016). Geomorphological methods study the landforms in the area of interest to show where the groundwater is likely to occur (Balasubramanian, 2017). Features such as pediplains or valley flats often show good groundwater potential (Sedhuraman et al., 2014). The slope and drainage density are also particularly important features as there is more infiltration in gently sloping areas, and a higher drainage density indicates more runoff and, hence, less potential for recharge (Sedhuraman et al., 2014).
Other surface exploration methods may be geological in nature (Balasubramanian, 2017). Such methods first collect and analyse topographic maps, aerial photographs, geological maps, and core logs. This is then supplemented by geological field reconnaissance and evaluation of hydrological data on stream flows, springs, well yields, groundwater recharge levels, and water quality (Balasubramanian, 2017). Faults, joints, and lineaments are also noted, as these may form preferential flow paths for groundwater (Van Tonder 𝘦𝘵 𝘢𝘭., 2001).
The structure of the water-bearing strata can also give indications for groundwater occurrence (Balasubramanian, 2017). Contacts between permeable rock overlying impermeable strata along the sides of valleys are suitable locations for groundwater (Balasubramanian, 2017). Quartz veins and dykes can be linear and form ridges (Nel, 2017). There, erosion-prone rock types may form depressions and are connected with structures ideal for water storage (Nel, 2017).
Geobotany, which is the study of how plants are related to the subsurface, can also be used in the exploration of groundwater (Odhiambo, 2016). This means observing any anomaly in the growth of vegetation or trees growing in a straight line can be an indicator of subsurface moisture changes (Nel, 2017). The presence of phreatophytes can also allude to the presence of groundwater (Nel, 2017). Halophytes and white efflorescence of salt on the ground may also be an indication of shallow brackish or saline groundwater (Balasubramanian, 2017). Playas and precipitation of salt are also good indicators of possible groundwater occurrence (Balasubramanian, 2017). Photogeology, which is the use of aerial photography to study geology, can also provide valuable information for targets of a groundwater exploration exercise (Mekel, 1988).
Another manner through which groundwater exploration can be conducted is through the use of geophysical methods. There are a variety of geophysical methods used in groundwater exploration employed for several reasons. However, the main objective of geophysical surveys in hydrogeology is to provide parameters for the creation of models essential for decision-making in groundwater exploration and protection (Mpofu 𝘦𝘵 𝘢𝘭., 2020).
Geophysical survey methods can be airborne, conducted on the surface, or even conducted down boreholes (Balasubramanian, 2017). These methods operate primarily by detecting anomalies or contrasts in the physical properties of the Earth (Balasubramanian, 2017). These properties include but are not limited to density, elasticity, magnetism, and electrical resistivity. Such anomalies indicate the potential zones which can be exploited for groundwater. The use of geophysics in groundwater exploration is very important as it removes the need to drill many boreholes before the target is reached (Mpofu 𝘦𝘵 𝘢𝘭., 2020). Geophysical tools are invaluable in groundwater exploration; therefore, this study will be primarily based on the use of geophysics for groundwater exploration in South Africa.
Description
Dissertation (M.Sc.(Geohydrology))--University of the Free State, 2023