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954Fig. 6 Test profile used todetermine the set-up of constraints.In a it is the twodimensionalinversion ofCVES data by Res2dinv isshown, in b it is the onedimensionalLCI of CVESdata, in c it is an abandonedattempt of LCI of TEM andCVES data, whereas d is theLCI with the model as chosen.A residual of 1 corresponds toa fit at the uncertainty appliedto the data. Hence, data arefitted within the datauncertaintystructure below. At depths greater than 50 m, Res2dinvshows vertical variations in resistivity, where the LCIvaries laterally with similar variations to obtain the sameintegrated resistivity.The TEM data are added to the LCI system in Fig. 6c.The conductive layer at great depth is severely constrainedfrom model to model. To ensure continuity from TEMmodel to TEM model, the constraint needs to be narrow,as 20 CVES models are present between two TEMmodels. The CVES models have a tendency to increasethe resistivity and push the layer down (not shown). Thisresult may be due to anisotropy as discussed above.Based on the results shown in Fig. 6c, it was decided toconstrain the TEM models to the CVES models only bythe bottom layer boundary, as the CVES data have thehigher surface-near resolution. The resulting model is seenin Fig. 6d. This model has the near-surface resolution ofthe two-dimensional CVES interpretation, e.g. note theshallow resistive bodies at 1,600 and 1,800 m. The lowresistivebottom layer is necessary to explain the TEMdata and it confines the lower boundary of the anisotropicbasalts. The bottom layer prevents the overshoot ofthickness and undershoot of resistivity due to electricalanisotropy (Fig. 6a and b). It is believed that the LCIprovides a better estimate of the properties of the basaltand can be used for assessing the degree of fracturing andweathering that are important for recharging underlyingaquifers.Measured and model data for three soundings along theprofile are shown in Fig. 7. Measured data are representedby error bars, whereas the model data are presented bydots. For large electrode spacing, two data points arepresent for the same electrode spacing. This result is dueto the presence of both Wenner and Schlumberger datawith the same array midpoint. Considering a onedimensionalearth, the two data points would coincide.Because the two data points do not coincide, theinadequacy of the one-dimensional model is realized.Generally the fit to data is acceptable for both DC and TEM.Results and interpretationLine 2The inversion results of line 2 are given in Fig. 8. Themodel sections are described below. A model section ofone-dimensional inversions of line 2 TEM data down toan approximate depth of 200 m using a multi-layer modelHydrogeology Journal (2007) 15: 945–960DOI 10.1007/s10040-007-0191-z
Fig. 7 Measured and modeldata for three soundings on thetest profile. Error bars markthe measured data and theapplied data uncertainty,whereas the dots are the modeldata. Note the good data fitRhoa [ohmm]100DC sounding, x=1195 m,Data residual = 0.57100TEM sounding, x=1200 m,Data residual = 0.70100DC sounding, x=1205 m,Data residual = 0.329551010 100Array midpoints [m]100.11Time [ms]1010 10 100Array midpoints [m]is shown in Fig. 8a. The model section shows a highlyconductive bottom layer with resistivities around 10 Ωmat depths varying along the profile from 60 to 150 m withthe minimum depths in the centre, between 4,400 and5,100 m. Above a depth of 4,400 m, at least two layers ofhigher (a few hundred Ωm) and intermediate (a few tensof Ωm) resistivity can be found. The major units deducedfrom the inversion results are, pending additional drillinginformation, attributed to weathered basalt (intermediateresistivity: a few tens of Ωm), fresh basalt (high resistivity:a few hundred of Ωm), and Karoo sediments (10–15 Ωm).In the left third of the profile (Fig. 8a), the top layer hasan intermediate resistivity, and corresponds to an areawhere surface observation has identified soils derivedfrom the weathering of basalt and sandstone (see the localgeologic map in Fig. 3). Gradually higher resistivityappears in the interval from about 3,000 to 5,200 m. Thehigher resistivity corresponds partially to outcrops of freshbasalt observed in the interval from 4,300 to 5,300 m.From profile coordinates above 5,500 m, at least fourlayers can be distinguished. The top high resistivity layer (afew hundred Ωm) is followed by an intermediate resistivitylayer (a few tens of Ωm) and then to a high-resistivity thirdlayer (a few hundred Ωm) to a low resistivity basal layer(around 10 Ωm). The geological interpretation for theseresults is the presence of fresh basalt at the surface,confirmed by outcrop evidence in the river bed, thenweathered basalt and/or sandstone, a second flow of freshbasalt and, lastly, Upper Karoo Group sediments at thebase of the profile. The low resistivity of the UpperKaroo Group sediments is due to the interbedded layersof mudstones, and the presence of conductive groundwater(Fig. 4). Between 4,000 and 5,500 m, the KarooSupergroup sediments dome towards the surface, and thisresult is interpreted as an upfaulted block.The Lower Karoo Group, consisting of mudstone,shale, and coal overlying coarser basal sandstone units atthe bottom, is likely to be more conductive than thepredominantly sandstone units that compose the UpperKaroo Group. Moreover, the groundwater in the LowerKaroo Group has a significantly higher conductivity thanthe more readily recharged groundwater in the UpperKaroo Group. Hence, a layer of lower resistivity likely ispresent below the Upper Karoo Group, but cannot beFig. 8 Inverted model sectionsfrom line 2 at Sawmills:a is the one-dimensionalmulti-layer inversion based onTEM data, b is the two-dimensionalrobust inversion ofCVES data, c is the onedimensionalmutually constrainedinversion of TEM andCVES dataHydrogeology Journal (2007) 15: 945–960DOI 10.1007/s10040-007-0191-z
Page 1 and 2: Geophysical and hydrogeologic inves
Page 3 and 4: geological questions developed for
Page 5 and 6: Fig. 3 Field and regionalgeology ma
Page 7 and 8: Two other deep boreholes at Sawmill
Page 9: parameterized inversion of several
Page 13 and 14: 957Fig. 9 Inverted model sections f
Page 15 and 16: overestimated and its resistivity u

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