Testing the application and limitation of stochastic simulations to predict the lithology of glacial and fluvial deposits in Central Glasgow, UK

Abstract

Glacigenic and fluvial deposits of variable lithological composition underlie many major cities in Europe and North America. Traditional geological mapping and 3D modelling techniques rarely capture this complexity as they use lithostratigraphic designations which are commonly based on genesis and age rather than lithological compositions.In urban areas, thousands of boreholes have been, and continue to be, drilled to facilitate the planning, design and construction of buildings and infrastructure. While these data may provide the basis for geological maps and 3D models based on lithological interpretation, they are too numerous for manual correlation to be undertaken efficiently. In this paper we explore the application of largely automated stochastic modelling techniques to develop predictive lithology models for glacial and fluvial deposits in the city of Glasgow, UK. These techniques are commonly used to assess facies variation in oilfield models and are applied here in an urban setting using over 4000 borehole records.Predictions derived from these methods have been evaluated by removing control data and rerunning the simulations. We demonstrate a moderate improvement in the prediction of lithology when using a lithologically-derived stochastic model compared with a conventionally interpolated lithostratigraphic model. It is possible to report uncertainty within the resulting models, either with probability maps or through a suite of plausible simulations of the lithologies across the study region. IntroductionThe growth and decay of high-and mid-latitude Pleistocene ice sheets has left 8% of the Earth's land surface, including one third of Europe and a quarter of North America, covered by glacigenic and fluvial deposits Gibbard, 2004a, 2004b). These deposits underlie many major cities and much of their associated infrastructure networks, and exert a significant influence on the groundwater system. Increasing urban development, and its demands (e.g. suitable foundation conditions, the need for waste storage, contaminant migration, drainage re-routing) requires that information about subsurface glacial deposits, which are often highly lithologically variable across short distances, is available for those involved in planning and construction (Campbell et al., 2010). A key challenge for the three-dimensional (3D) geological modelling community is therefore to represent these subsurface deposits in appropriate ways across large, city-wide areas (Culshaw, 2005;MacCormack et al., 2005;Kessler et al., 2009).In Glasgow, west central Scotland (Figure 1), the British Geological Survey (BGS), in partnership with Glasgow City Council and other local authorities, have used extensive borehole datasets to develop and successfully apply a suite of 3D Quaternary lithostratigraphic models (Merritt et al., 2007;Campbell et al., 2010) (Figure 2). A key strength of lithostratigraphic modelling is that it brings together the expertise of geologists and known geological relationships, enabling a geologically realistic r...

Improving understanding of shallow urban groundwater: the Quaternary groundwater system in Glasgow, UK

“…The proportion of (i) coarse grained (sand and gravel) and (ii) fine grained (silt and clay) deposits in each geological unit was calculated from lithological analysis of the large number of available borehole logs for central Glasgow (Turner et al 2014; Figure 4C). More detailed lithological analysis was done on data from the same borehole logs by Kearsey et al (2015), with comparable results.…”

“…The most laterally extensive Quaternary deposit across Glasgow is glacial till of the Wilderness Till Formation, which crops out at the ground surface across much of the city (Figure 3), and rests on bedrock across much of the area (Kearsey et al 2015;Monaghan et al 2014). It is typically less than 5m thick, but can be thicker in drumlins, and becomes thinner and sometimes absent on higher ground ( Figure 3, Table 1) (Kearsey et al 2015, Monaghan et al 2014).…”

Examining the geometry, age and genesis of buried Quaternary valley systems in the Midland Valley of Scotland, UK

“…These were rationalized to six dominant lithofacies (Clay, Diamicton, Organic deposits, Silt, Sand, Sand & Gravel) using the approach employed by Kearsey et al . (). The relative proportions of lithofacies were then calculated.…”

“…Initial processing of the data indicated that 147 different lithological descriptions have been used to describe the Quaternary sediments. These were rationalized to six dominant lithofacies (Clay, Diamicton, Organic deposits, Silt, Sand, Sand & Gravel) using the approach employed by Kearsey et al (2015). The relative proportions of lithofacies were then calculated.…”

Using a multiple variogram approach to improve the accuracy of subsurface geological models

“…It is likely that as additional high-quality data become available for the model domain, the performance of the subdivided model variograms will improve further, thus having an even greater impact on the 3D stratigraphic model accuracy and an improved basis for informing hydrogeologic parameter variability. Stochastic modelling approaches could also be applied within submodel zones to visualize and assess multiple model realizations and capture the variability in the model predictions (Kearsey et al 2015). The next step is to integrate this improved representation of the geologic spatial variability with hydrologic parameters to assess the effects on flow system conditions where water balance and hydraulic head distributions from field and model outputs can be evaluated.…”