CASE STUDY: GROUNDWATER EXPLORATION IN THE BASEMENT GRANITES OF THE KRAAIPAN GROUP USING AEROMAGNETIC

SURVEYS

M Terblanche 1 and L Stroebel 2

1Water Unit, Aurecon, Pretoria, Gauteng, South Africa; email: Marius.Terblanche@aurecongroup.com

2Water Unit, Aurecon,

Pretoria, Gauteng, South Africa; email: Louis.Stroebel@aurecongroup.com

Abstract Historically groundwater exploration consisted of reconnaissance geophysical surveys followed by detail ground surveys. Where no potentially water bearing geological structures are shown on geological maps and aerial photos, the project area would be divided into a grid on which the ground geophysical survey would be done. This type of exploration is time consuming and expensive. In some cases the terrain or cultural noise prohibits the use of conventional geophysical methods, with only more expensive and time consuming methods being left as an option. This is where the high resolution airborne magnetic survey excels. The results obtained from this type of survey are of such a nature that ground geophysical surveys are only performed where potential drilling targets was identified from the aerial survey. Not only can there be cost and time savings on ground geophysical surveys, but drilling of dry boreholes can be limited which makes up the largest cost component of a groundwater exploration project. This paper will discuss successes achieved using high resolution aeromagnetic surveys as the basis for groundwater exploration in traditionally low-yielding igneous geology. 1. INTRODUCTION Aurecon was appointed to conduct a groundwater exploration program for the villages of Setlagole and Madibogo, located in the North West Province of South Africa. The villages of Madibogo and Setlagole are located approximately 75km south-west of Mafikeng and are accessible from the R27 road, linking Mafikeng and Vryburg (Figure 1). These towns are located in an arid part of South Africa on the edge of the Kalahari where the mean annual rainfall is approximately 443 mm/a (SA Weather Service, 2011). The topography can generally be described as flat and is covered by aeolian sand underlain by basement granite of the Kraaipan Group. These villages are dependent on groundwater, but due to a lack of management and maintenance of infrastructure, together with an increasing demand of potable water, a critical water shortage occurs within the area. During a previous study in 2002, the former Africon Engineering commissioned an aeromagnetic survey over the study area as part of the Setla-Kgobi and Environs Bulk Water Supply Investigation (Africon, 2002). This data was however never verified by a consequent groundwater exploration program. Having the data to our disposal, it was then interpreted and used to locate target areas for groundwater exploration.

Figure 1: Map showing project area (Bing Maps, 2012)

2. GEOLOGY AND GEOHYDROLOGY According to the 1:250 000 geological map (2624 Vryburg) (Figure 2), the project area is underlain by several different lithological units. The majority of the area is however underlain by Aeolian Sand of the Gordonia Formation of the Kalahari Group. Small igneous intrusions from the Kraaipan Group, consisting of undifferentiated pink, fine to medium grained granite, gneiss, migmaitite, schist, and amphibolite outcrop to the south of Setlagole. To the immediate north of Setlagole a small outcrop of igneous and metamorphic rocks from the Gold Ridge Formation of the Kraaipan Group, consisting of amphibolite, rhyolite, mica schist, pyrophyllite schist and quartz chlorite schist occur. Banded Iron Formation of the Kraaipan Group outcrops in the ridges to the east of Madibogo. Some lineaments also occur within the study area with the majority having an east-west strike. These lineaments can also clearly be seen on the aeromagnetic data. What is not shown on the geology map, are the structures caused by the postulated Setlagole meteorite impact as described in detail by Anhaeusser et al., (2010). Apart from the circular structure visible in the aeromagnetic data (Figure 5), possible impact breccias have been mapped in the field. Some of the existing boreholes have been sited on magnetic anomalies associated with this impact structure, but drilling results show them not to be ideal water bearing targets (Stettler EH, 2012). Based on the published 1:500 000 geohydrological map (2522 Vryburg), Setlagole and Madibogo falls partly on a fractured and intergranular aquifer type. A borehole drilled in this fractured/intergranular aquifer can be regarded as successful when it has a yield in excess of 360 litres/hour (l/h). Furthermore, the majority of boreholes on record within the study area have a potential yield of between 360 and 1800 l/h with nitrate concentrations generally exceeding the SANS 241 Class 1 Drinking Water Standards of 10 mg/l nitrate as nitrogen. The mean annual rainfall in the region is 443mm/a (SA Weather Service, 2011) and according to Vegter et al., (1995) the groundwater recharge was calculated to be 9.9 mm/a, or 2.2% of the annual rainfall. Based on the Groundwater Harvest Potential of the Republic of South Africa Map (1999), the harvest potential of the project area is estimated to be 15 000 to 25 000 m3/km2/annum.

Figure 2: Geology map of the project area (Geology map – 2624 Vryburg)

3. AEROMAGNETIC SURVEY During a previous study in 2002, the former Africon Engineering contracted the Council for Geoscience to perform a high resolution aeromagnetic survey (200m flight line spacing) of the Setlagole/Madibogo area. A specialist consulting geophysicist (Dr Edgar Stettler) interpreted the data in order to identify priority areas for groundwater exploration. When interpreting a colour-scale magnetic map a number of points have to be taken into consideration. Since magnetic geological bodies are considered as ‘bar magnets’ they have two poles; a north and south seeking pole. In the southern magnetic hemisphere the magnetic high is equated with the north seeking pole and the magnetic low (valley) with the south seeking pole. The poles are respectively at maximum field strength over the northern perimeter and minimum field strength over the southern perimeter of a magnetic body, and will have a more subdued high on its western edge and a magnetic low over the eastern perimeter indicating the extreme boundaries of a magnetic body. The body is therefore situated between the north and south seeking poles and not solely under the magnetic high as many non-geophysicists believe. The difficulty lies in determining which poles belong to which body and is referred to as the high-low pair of a body. In the southern magnetic hemisphere, for purely induced magnetism the larger part of the causative magnetic body occurs under the magnetic high (hues of orange to red to mauve). However, if remanent or ‘fossil’ magnetism is present in a geological body, the high-low patterns can change and can make it awkward to determine the dip of a body if the magnetic inclination and declination of the remanent magnetism is not known. These can only be determined with confidence in a palaeomagnetic laboratory on physical oriented rock samples taken from the suspected remanent magnetic body. It is also problematic to determine the exact position of the remanent magnetic body. (Stettler, 2012). By using various methods of manipulating the data, a clearer picture of the possible geological structures can be obtained. This data can be used to produce maps where the dip and strike angle of the structure can be determined. Figure 3 shows an example of a total magnetic field image overlain on the 2526AC topographical map. By processing the data, a magnetic tilt angle image (Figure 4) is obtained, giving a

much clearer picture of the data. The data obtained from this map can then be overlain on a base map, such as a topographic map, to produce a structural map of great detail. An example of this is presented in Figure 5, which clearly shows the linear geological structures as interpreted from the aeromagnetic data. Also present in this map is a radially patterned structure. This is a result of the Setlagole meteorite impact. The impact produced structures consisting of nearly concentric circles, radiating outwards from the impact site. The presence of northeast-southwest striking dolerite dykes of three different ages are also interpreted and showed on this map, along with the inferred fault locations. To the east of the impact crater, the Kraaipan Greenstone Belt can clearly be seen. Existing boreholes of which the yields are known were plotted on the interpreted aeromagnetic map. From this map it became clear that the majority of boreholes drilled close to the magnetic lineaments had a significant yield. Only 2 boreholes not associated with a magnetic lineament had a significant yield. Lineaments with a North Easterly to Eastern strike direction normally gave a yield in excess of 2000 l/h and these structures were thus chosen as priority targets.

Figure 3: Total magnetic field (TMF) overlain on the 2625AC topo map.

Figure 4: Magnetic tilt angle map overlain on the 2625AC topo map.

Figure 5: A qualitative interpretation of the magnetic data overlain on the topo map for 2625AC.

Figure 6: Total magnetic field image showing some targets for ground geophysical surveys.

4. TARGET IDENTIFICATION After priority areas were selected, ground geophysics such as the electromagnetic (Geonics EM34-3), magnetic (Geotron G5) and resistivity (ABEM Lund Terrameter SAS 1000) methods were used to identify drilling targets. It was not always possible to conduct these surveys in the preferred areas as surface rights, disputes between communities and vandalism inhibit it. The best location from a geophysical point of view, is not always the optimum location if all these other factors are also considered. Preference was also given to targets located in areas outside of the villages where boreholes would be quite a distance from possible pollution sources such as the large number of pit latrines and cattle kraals. A further advantage of performing the ground geophysical surveys outside of the villages was that cultural noise did not have a significant impact on the magnetic and electromagnetic methods. Figure 7 is an example of a drilling target selected from a ground geophysical profile using magnetic, electromagnetic & resistivity data. From the resistivity data of this traverse, deep weathering in the form of a possible fault or intrusion was intersected, as is indicated by the resistivity low at depth. The magnetic data also displayed a possible anomaly. The borehole was drilled and significant fracturing was intersected. The total blow yield was measured to be 6000 l/h, while the sustainable yield was calculated to be 3600 l/h.

Figure 7: Drilling target selected from ground geophysical profile using magnetic, electromagnetic &

resistivity data.

5. DRILLING RESULTS A total of 66 boreholes were drilled using air percussion drilling rigs. Drilling supervision by qualified geologists/geohydrologists ensured that boreholes were constructed correctly and logged, focusing on fractures zones, waterstrikes, and the general geology. The blow yields of individual water strikes were also recorded. A summary of the drilling results is presented in Table 1. Statistical analyses of the data proved that the data showed a non-normal distribution (skewed by outliers) resulting that the average/mean of the range cannot be used as the best measure of central tendency. In this case the median of the data range was calculated instead as a more accurate representation of the data range (the average is however included in data analysis tables to illustrate the difference). From Table 1 it can be seen that the average/mean and median blow yield differs significantly. The average blow yield was calculated to be 4911 l/h compared to the median which was calculated to be 2350 l/h, which is a more accurate reflection of the blow yields obtained during the study. Although the study area is relatively flat with a small difference in topography, the static water levels measured within the boreholes varied significantly. This can possibly be attributed to water strikes/fracture zones in different geological units with different piezometric heads and ultimately, different aquifers. Table 1: Summary of the drilling results.

66 Newly Drilled

Boreholes

Depth (m)

Static Water Level

(mbgl#)

Waterstrike Depth (m)

Blow Yield (l/h)

Sus Yield (l/h)

NO3-N (mg/l)

Number (n) 66 32 79 46 30 30

Minimum 26 1.54 14 500 250 0.07

Maximum 120 32.58 106 22000 15840 77.67

Average 100 16.53 51 4911 2508 16.20

Median 100 16.96 41 2350 1800 12.22 # meters below ground level

6. DISCUSSION OF RESULTS To put the results of the exploration program into perspective, a number of graphs, histograms and tables were compiled. From Figure 8 it can be seen that 46 of the 66 newly drilled boreholes had a blow yield equal or higher than the lower limit of 360 l/h of a borehole regarded as successful in the occurring geology. The median blow yield of the successful boreholes was calculated to be 2350 l/h, which exceeds the upper limit of 1800 l/h for successful boreholes. As previously stated, a borehole drilled into the occurring geology can be regarded as successful when it has a yield in excess of 360 l/h and boreholes on record within the study area have a potential yield of between 360 and 1800 l/h (1:500 000 Geohydrological Map-2522 Vryburg). The blow yield of individual waterstrikes was recorded and it was found that the majority of water strikes fall within the 0 to 3000 l/h interval, with the highest frequency of water strikes occurring between 20 and 40 meters (Figure 9 & Figure 10). It is interesting to note that there is very little correlation between the depth of the waterstrikes and their individual blow yields (Figure 11).

Figure 8: Measured blow yields in successful boreholes.

Figure 9: Histogram displaying the frequency of blow yields at individual waterstrikes.

Figure 10: Histogram displaying the frequency of waterstrikes at different depths.

Figure 11: Correlation between depth of waterstrikes and corresponding blow yields.

The geology pertaining to each individual waterstrike was recorded. The results were categorized into 4 different units and presented in Table 2 and Figure 12. From this table it can be seen that the majority of waterstrikes occurred within the upper contact between the weathered/fresh rock. This would typically be the depth to which casing would be installed. The median blow yield of this contact was calculated to be 1500 l/h. A significant number of water strikes also occurred within fractured felsic rock. It is interesting to note that the lowest frequency of waterstrikes occurred in the contact zones between the country felsic

rock (granite, etc.) and intrusive mafic geology (dolerite) and mafic igneous geology (basalt). The highest median blow yield occurred within fractures within basalt and dolerite. Note that this does not refer to the contact between dolerite/basalt and country felsic rock. Table 2: Geohydrological descriptions of waterstrikes in succesfull boreholes.

Geohydrological Description of Waterstrikes in 46 newly drilled boreholes

Geohydrological Description of Waterstrikes Waterstrikes (n) Median Blow Yield (l/h)

Contact between Weathered/Fresh Rock 33 1500 Fracture within Felsic (Granite/Gneiss/Schist) 28 1000 Fracture within Mafic (Basalt/Dolerite) 10 1600 Contact between Felsic & Mafic 8 1000

Figure 12: Geohydrological descriptions of waterstrikes in successful boreholes

7. CONCLUSION Based on the findings of the groundwater exploration program conducted in an around the villages of Setlagoli and Madibogo, the following can be concluded:

• The aeromagnetic data proved to be invaluable to this groundwater exploration project where a success rate of 70% (46 out of 66 boreholes yielding 360 l/h or more) was achieved in traditionally low-yielding igneous geology. Furthermore, the achieved median blow yield of the newly drilled boreholes (2350 l/h) exceeded the upper limit (1800 l/h) of the expected yield in successful boreholes.

• The expected borehole yields in aquifers in the project area are a true reflection of what is reported on the 1:500 000 Geohydrological Map (2522 Vryburg). It is however uncertain whether these published yields refer to blow yields or sustainable yields. The median sustainable yield (1800 l/h) calculated from 24 hour constant discharge tests for this project equals the upper limit of the expected yield in successful boreholes.

• Reported nitrate concentrations (>10 mg/l –NO3-N) on the 1:500 000 Geohydrological map (2522 Vryburg) also corresponds with analytical results from this project. A median concentration of 12.2 mg/l NO3-N was calculated from 30 samples submitted for chemical analysis. Initially the reason therefore was not clear as the majority of boreholes were drilled some distance from villages to reduce the likelihood of groundwater pollution as a result of pit latrines, cattle kraals, etc. Upon studying older aerial photos and topographical maps it could be seen that crops were extensively cultivated within the project area up until the early 90’s and that the use of fertilisers could contribute to the increased nitrate concentrations.

• Although the airborne magnetic data proved to be invaluable to this project, it is important to note that the follow-up ground geophysics in selected target areas is imperative. The targets selected from the airborne data may have indicated the position of the geological structure to be targeted; but the ground geophysics has indicated that the anomalies associated with weathering/fracturing are located some distance from the actual igneous intrusion. These targets also proved to be more successful in terms of yield with the lowest frequency of waterstrikes occurring in the actual contact zones between the country felsic rock (granite etc.) and intrusive mafic rocks (dolerite/basalt). This may be as a result of the magnitude of the dykes. When dykes of this magnitude intrude into the country igneous rock, it may be possible that the contact zones are not fractured but is melted and slowly cooled to form pegmatitic zones with little to no fracturing. The drilling targets are thus rather located some distance (50 to 250 m) away from the actual contact, in the form of fractures zones and weathering. This fracturing might be the result of the mechanical forces associated with the intrusion of dykes.

• Utilising airborne geophysical surveys prior to ground geophysical surveys should be considered in large rural groundwater supply projects, especially in traditionally low-yielding aquifers where geological and aerial maps provide little guidance in terms of groundwater bearing structures. Vast areas can be covered in a short amount of time while ground geophysical surveys can concentrate on actual targets. Not only can there be cost and time savings on ground geophysical surveys, but drilling of dry boreholes can be limited which makes up the largest cost component of a groundwater exploration project.

8. ACKNOWLEDGEMENTS The authors would like to thank the Department of Water Affairs for funding the project, as well as Dr Mannie Levin and Dr Giep du Toit for reviewing the paper. 9. REFERENCES Africon (2002). “Setla-Kgobi and Environs Bulk Water Supply Investigation-Pre-Feasibility

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