Integrating aerial geophysical data in multiple-point statistics simulations to assist groundwater flow models Neil E. M. Dickson & Jean-Christophe Comte & Philippe Renard & Julien A. Straubhaar & Jennifer M. McKinley & Ulrich Ofterdinger Abstract The process of accounting for heterogeneity has made significant advances in statistical research, primarily in the framework of stochastic analysis and the develop- ment of multiple-point statistics (MPS). Among MPS techniques, the direct sampling (DS) method is tested to determine its ability to delineate heterogeneity from aerial magnetics data in a regional sandstone aquifer intruded by low-permeability volcanic dykes in Northern Ireland, UK. The use of two two-dimensional bivariate training images aids in creating spatial probability distributions of hetero- geneities of hydrogeological interest, despite relatively ‘noisy’ magnetics data (i.e. including hydrogeologically irrelevant urban noise and regional geologic effects). These distributions are incorporated into a hierarchy system where previously published density function and upscaling methods are applied to derive regional distribu- tions of equivalent hydraulic conductivity tensor K. Several K models, as determined by several stochastic realisations of MPS dyke locations, are computed within groundwater flow models and evaluated by comparing modelled heads with field observations. Results show a significant improvement in model calibration when com- pared to a simplistic homogeneous and isotropic aquifer model that does not account for the dyke occurrence evidenced by airborne magnetic data. The best model is obtained when normal and reverse polarity dykes are computed separately within MPS simulations and when a probability threshold of 0.7 is applied. The presented stochastic approach also provides improvement when compared to a previously published deterministic aniso- tropic model based on the unprocessed (i.e. noisy) airborne magnetics. This demonstrates the potential of coupling MPS to airborne geophysical data for regional groundwater modelling. Keywords Aerial magnetics . Multiple-point statistics . Heterogeneity . Groundwater flow . UK Introduction The inherent heterogeneity of a hydrogeological system is a function of its depositional and in-situ environment, often creating highly complex structures. Distribution of hydraulic properties is therefore challenging to replicate. Heterogeneity of aquifer properties can be estimated from direct observations, requiring high measurement density (rarely available at regional scale), or can be estimated by calculating distributed equivalent values for the area of interest using sampled values (Anderson 1997; Renard and de Marsily 1997; Wen and Gómez-Hernández 1996). Use of deterministic estimations, i.e. where the system is described by interpretation of observations, can be erroneous due to uncertainties/errors in observed data, spatial correlations and human bias in interpretation. A detailed analysis into accounting for heterogeneity is delivered by de Marsily et al. (2005). Appropriate knowledge of heterogeneity is important for accurate numerical groundwater models (Chen et al. 2003). Subsurface characterization and conceptualisation is of paramount concern when creating hydrogeological models with groundwater flow components. Geostatistical and geophysical methods can assist high-resolution subsurface characterization in complementing point observations (de Marsily et al. 2005). N. E. M. Dickson ()) I J.-C. Comte I U. Ofterdinger School of Planning, Architecture and Civil Engineering, Queen’ s University Belfast, David Keir Building, Belfast, Northern Ireland, UKBT9 5AG e-mail: ndickson03@qub.ac.uk J.-C. Comte School of Geosciences, University of Aberdeen, St Mary’ s Building, Old Aberdeen, Scotland, UKAB24 3UF P. Renard I J. A. Straubhaar Centre d’Hydrogéologie, Université de Neuchâtel, Emile-Argand 11, 2000, Neuchâtel, Switzerland J. M. McKinley School of Geography, Archaeology and Palaeoecology, Queen’ s University Belfast, Elwood Avenue, Belfast, Northern Ireland, UKBT7 1NN Published in Hydrogeology Journal 23, issue 5, 883-900, 2015 which should be used for any reference to this work 1