Geological contours for groundwater modelling, South Canterbury

20122.0 INFORMATION SOURCESThe information used in this assessment was:• Published geological maps from the GNS Science 1:250,000 ‘QMAP’ (Quartermillion-scalemap) series, Waitaki sheet (Forsyth 2001) and Aoraki sheet (Cox &Barrell 2007), as well as the underlying digital GIS databases (Figs. 2a, 2b);• Unpublished QMAP geological ‘record’ maps, comprising field observations and otherdetailed published geological information hand-drawn onto 1:50,000 scaletopographic maps, held on file at the GNS Science Dunedin office;• Measurements of thicknesses of rock strata from geological exposures (‘measuredsections’) at various places around the region (Field et al. 1989). Indicative locations(some measured section lines extend for several kilometres) are shown in Fig. 3;• Topographic information, in particular topographic contours, from Land InformationNew Zealand digital map images and datasets;• Interpretations and evaluations from recent groundwater geology assessments in thestudy area by Forsyth (2004) and Barrell (2008);• Information from available bore hole records, particularly the Environment Canterburybore hole database (see Fig. 3 for examples);• Information from geophysical surveys in the Cannington basin (Langdale & Stern1998) and lower Waitaki valley (Woodward et al. (2003).3.0 GEOLOGICAL CONTOURINGStructure contours were drawn on the basis of the information sources outlined in Section2.0. The contours are a geological interpretation, based on weights of evidence from thevarious datasets, and in only a few places are constrained by direct observations. Theyrepresent a hypothesis as to the geometries of the targeted subsurface geologicalboundaries. It is anticipated that the contour models should be refined and improved upon inthe future as additional information comes to hand, such as by the drilling of new bores.3.1 MethodologyThe most important inputs to the contouring are surface geological data from maps andmeasured sections, with ground surface elevations obtained from topographic map contours.Subsurface information from bore holes and geophysical surveys, while valuable, has somelimitations. The bore hole dataset has three significant limitations. First, only some of thebores have records (logs) of the geological materials (lithologies) encountered during drilling.Second, the detail and reliability of lithological logs is highly variable; in many cases even ageological specialist may have difficulty in identifying specific geological formations from borehole lithological logs (see discussion in Forsyth 2004). Third, most bores are relativelyshallow, rarely more than 100 m deep, whereas the geological formation boundaries that areGNS Science Consultancy Report 2012/245 2