Interpretative modelling of a geological cross section from boreholes: sources of uncertainty and their quantification

Abstract. We conducted a designed experiment to quantify sources of uncertainty in geologists' interpretations of a geological cross section. A group of 28 geologists participated in the experiment. Each interpreted borehole record included up to three Palaeogene bedrock units, including the target unit for the experiment: the London Clay. The set of boreholes was divided into batches from which validation boreholes had been withheld; as a result, we obtained 129 point comparisons between the interpreted elevation of the base of the London Clay and its observed elevation in a borehole not used for that particular interpretation. Analysis of the results showed good general agreement between the observed and interpreted elevations, with no evidence of systematic bias. Between-site variation of the interpretation error was spatially correlated, and the variance appeared to be stationary. The between-geologist component of variance was smaller overall, and depended on the distance to the nearest borehole. There was also evidence that the between-geologist variance depends on the degree of experience of the individual. We used the statistical model of interpretation error to compute confidence intervals for any one interpretation of the base of the London Clay on the cross section, and to provide uncertainty measures for decision support in a hypothetical route-planning process. The statistical model could also be used to quantify error propagation in a full 3-D geological model produced from interpreted cross sections.