Interpretation of drill-core and georadar data of coarse gravel deposits

Abstract Pollution in the shallow subsurface has led to an increasing need of understanding how to quantitatively characterize both the heterogeneity of gravel aquifers and the influence of heterogeneity on groundwater flow and solute transport. Models play an important role in decision-making processes, especially in the context of better characterizing and in forecasting the behavior of a given geological system. The objective of the present paper is the derivation of a gradual lithofacies-based interpretation of outcrop, drill core, and ground penetrating radar (GPR or georadar) data of different quality. The presented method allows a probability estimation of drill core layer descriptions and radarfacies types representing defined sedimentary structure types. The method includes a determination of ‘initial structure type probabilities’ for grain-size categories and combinations thereof described in drill core layer descriptions as well as a subsequent differentiation of these structure type probabilities in an iterative process considering ‘additional information’ like main constituent, quantity, fraction, and sorting of single grain-size categories, color, chemical precipitation, layer thickness, and adjacent layer. The radarfacies types are calibrated with drill cores located in the vicinity of georadar sections. The calibration process consists of the assignment of the calculated structure type probabilities from the drill core layer descriptions to the corresponding radarfacies types considering the proportion in thickness between drill core layers and georadar structures. The structure type probabilities can be given for points along boreholes and a grid with arbitrary mesh size along georadar sections. The method is applied to field examples from the Rhine/Wiese aquifer near Basel in Switzerland. The resulting structure type probabilities can be used for conditioning stochastic simulations of geological models. However, the conditioned stochastic simulation of the Rhine/Wiese aquifer is the topic of another paper. The results show the importance of a detailed sedimentological analysis of outcrops and drill cores as well as its significance on the distinction of sedimentary structure types.

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