Time-lapse resistivity and geophysical measurements ...

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Time-lapse resistivity and geophysical measurements at Dayspring Children’s Village Susan J. Webb*, David Ngobeni, Sally-Anne Lee, Obone Sepato, Michael Jones, Tamiru Abiye, University of the Witwatersrand, Wits, South Africa, Madeline D. Lee, McMaster University, Ontario, Canada, Louise Pellerin, Green Engineering, Berkeley, CA, USA, Darren Burrows, Fugro Airborne, Woodmead, South Africa Summary Using an electrical resistivity array we mapped variations in the weathered subsurface at Dayspring Children’s Village and tentatively identify several significant fractures. The weathered zone is significantly thicker to the west. Time-lapse electrical resistivity data were used to map out variations in moisture between the end of the rainy (April) and end of the dry (September) season. In all the time-lapse difference plots, there is a near surface zone between 8-15 m thick that has consistently lost moisture during the dry season. We interpret a significant increase in resistivity in the deeper zone on the line closest to the trees as being due to depletion of moisture along a fracture from which trees are extracting water. Introduction The Dayspring Children’s Village is a school for ‘at risk’ children located ~70 km north west of Johannesburg (Figure 1). The water supply at the school is highly variable and frequently dries up in August or September at the end of the dry winter season.

investigate the relationship between groundwater and trees at Dayspring through a Geoscientists Without Borders® (GWB) project (Webb et al., 2011). This has proved to be an important project as many geophysics students have been able to get hands on experience at a real groundwater project. Due to the proximity to Johannesburg, we have been able to map the site in significant detail. Geological Setting There are no outcrops on the school property; however, there is abundant float. To the north of the site diabase sills were tentatively mapped on the regional 1:250,000 geological map. This area is poorly mapped and evidence of the sills in the form of float appears to extend much further south than is mapped. In thin section and hand sample, these sills can be more precisely identified as feldspathic pyroxenite sills that are associated with the Bushveld Complex to the north. The rest of the property hosts float that can be identified as variably metamorphosed shales, mapped as hornfels. There is also a north-south trending syenite dyke ~50 m west of the property, and some syenite float in the south west corner suggesting the dyke may intrude this corner. According to previous work, ground water is likely to be hosted in fractures associated with these dykes, sills and contacts (Barnard, 2000).

Figure 1. Locality map for Dayspring Children’s Village, South Africa.

Over 40 years ago the school had an ample supply of water and locals report that portions of the property were irrigated. To date, a total of seven boreholes (DS1-DS7) have been drilled to access the subsurface water supply (Figure 2). This water supply, however, has been drying up more frequently at the same time as a large stand of alien blue gum trees has grown to maturity. The plight of the children and the interesting science prompted us to

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Figure 2. Google Earth image of the Dayspring site, with the locations of boreholes and time-lapse resistivity lines collected in April and September, 2011.

DOI http://dx.doi.org/10.1190/segam2012-1353.1 Page 1


Time Lapse Geophysics at Dayspring

At present the only borehole producing clean water is DS1. During the drilling of borehole DS2, chip samples were collected and bagged. These chips have been used to create a basic log (Figure 3). While the depths are only approximate, fresh bedrock lithologies are encountered at a depth of ~35 m. There is also very fine grained crystalline material at depths of 55-60 m and 75-85 m. This is likely to be a very hard, impermeable layer that may create a seal between the upper aquifer and the lower one.

September, 2011 at the end of the dry season. We then determined the difference between these two datasets to determine changes due to the near-surface hydrology. Line 17 (Figure 6) passes closest to the line of trees. This line also has the largest % changes in the upper 8-15 m, where there is a general increase in resistivity. The top layer has become more resistive implying that it has dried out between the two measurements. About half way along line 17 there is a noticeable increase in the % change of ~25%. This corresponds to a possible break between the bedrock units. We interpret this as a possible fracture zone that the trees are using to access water. Thus during the dry season this zone gets significantly dried out and more resistive as the trees access water through this fracture. Towards the end of the line near 350 m, there is a significant decrease in resistivity, suggesting an increase in water. This may also be a fracture bringing water into the area and bears further investigation.

Figure 3. Basic log of chip samples from borehole DS2.

Resistivity Data Resistivity data were collected along a series of profiles located in Figure 2 using the Wenner-Schlumberger array. Profile #17 is closest to the trees and both lines 17 and 15 show the bedrock is close to surface (Figure 4). These data clearly show that the weathered zone is thickening to the west. There are several possible fractures apparent in the bedrock that appear to be consistent between lines. These are not well resolved as the depth extent and resolution of the data are not sufficient to clearly image fractures. Data on line 11 are clearly distinct from the surrounding data and likely due to an old irrigation pipe. This is clearer in the east to west cross lines (Figure 5). The east-west lines also show that the weathered zone is significantly thicker to the west.

Figure 4. Resistivity data of N-S lines shown in Figure 2. Lines run north to south (left to right) and the easterly most line (17) is at the top.

These resistivity data, first collected in April, 2011 at the end of the rainy season, are all very consistent and show reproducible results. Data were then recollected in




Time Lapse Geophysics at Dayspring Line 13 (Figure 7), is closest to borehole DS2 and the April and September data are interpreted to show fresher bedrock at a depth of ~35 m consistent with the log of DS2. The more resistive weathered zone to the south appears to dry out more and is likely to host less water. The less resistive regions appear to have an influx of water, which may be following a fracture at ~200 m along the profile. It is interesting to note that a stand of healthy trees has developed over this region. They may be accessing the water without drying it out as proposed for line 17. This would be a possible borehole site. Similar features are seen on line 7 (Figure 8), but of lower amplitude.

Figure 6. Line 17 plotted north to south with April (top), September (middle) and the difference (bottom).


Figure 5. Resistivity data of E-W line in Figure 2. (Note East is on the left and West is on the right). The small, conductive bull’s eye on each line corresponds with the location of Line 11 and may be distorting the signal.





Time Lapse Geophysics at Dayspring In addition, we have collected gravity (Figure 9) and ground magnetic data (Figure 10). Additional gravity will be collected in April 2012 to examine variation with time. Further ground magnetic data will be acquired along east west lines to better map the pipe.

Figure 10. Ground magnetic data collected over the eastern two thirds of the site. The major powerline to the west prevented the collection of more data. Both the Petronet gas line, striking NW-SE and the proposed irrigation pipe striking NNW and SSE show up well on this dataset. Lines were collected north south at 5 m station interval and 20 m line spacing. Figure 9. Preliminary Bouguer gravity data. These data were collected in early October at the end of the dry season. Plans are in place to collect gravity data in April, at the end of the rainy season.

Conclusions We have been able to map a probable fracture that the trees are likely to be using to source ground water during the dry season. This fracture shows a significant increase in resistivity between April and September likely due to removal of water by the trees. The near surface zone in all lines appears to be delimited between 8-15 m and in all cases shows a significant increase in resistivity consistent with the drying out of this layer. The weathered zone is thicker in the west, making it more difficult to map the fractures. There is likely to be a very strong impermeable layer at about 50 m which isolates the aquifers. Due to the proximity of the probable irrigation pipe to DS2, we suggest that leaks along this pipe may be causing the contamination reported in DS2. This water could be seeping between the casing and ground.


Acknowledgments We thank the SEG Foundation and the Geoscientists Without Borders® program for funding this project. We are especially grateful for the equipment portion of the grant that has been used for this and several other projects including field schools. Fugro Airborne Services provided Airborne EM and magnetic data to the project at no cost. De Beers and the Council for Geoscience loaned us CG5 gravimeters. We thank Prof. Loke for the use of the resistivity inversion software, RES2DINV, and GeoElectrical Consulting for the use of resistivity equipment. Geosoft provided software and support for use in this project. We thank all of the students who have participated and learned about the importance of geophysics and water!



http://dx.doi.org/10.1190/segam2012-1353.1 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2012 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES

Barnard, H. C., 2000, An explanation of the 1:500 000 general hydrogeological map: Johannesburg 2526: South Africa Department of Water Affairs and Forestry. Webb, S. J., D. Ngobeni, M. Jones, T. Abiye, N. Devkurran, R. Goba, L. D. Ashwal, M. Lee, D. Burrows, and L. Pellerin, 2011, Hydrogeophysical investigation for groundwater at the Dayspring Children’s Village, South Africa: The Leading Edge, 30, 434–440.

