Preface

Imaging geology in 3D

Realistic representations of geological complexity are important to address several engineering and environmental challenges. The spatial distribution of properties controlling physical and geochemical processes can be effectively described by the geological structure of the subsurface. In this work, we present an approach to account for geological structure in geostatistical simulations of categorical variables. The approach is based on the extraction of information from a deterministic conceptualization of the subsurface, which is then used in the geostatistical analysis for the development of models of spatial correlation and as soft conditioning data. The approach was tested to simulate the distribution of four lithofacies in highly heterolithic Quaternary deposits. A transition probability-based stochastic model was implemented using hard borehole data and soft data extracted from a 3-D deterministic lithostratigraphic model. Simulated lithofacies distributions were also used as input in a flow model for numerical simulation of hydraulic head and groundwater flux. The outputs from these models were compared to corresponding values from models based exclusively on borehole data. Results show that soft lithostratigraphic information increases the accuracy and reduces the uncertainty of these predictions. The representation of the geological structure also allows a more precise definition of the spatial distribution of prediction uncertainty, here quantified with a metric based on Shannon information entropy. Correlations between prediction uncertainties for lithofacies, hydraulic heads and groundwater fluxes were also investigated. The results from this analysis provide useful insights about the incorporation of soft geological data into stochastic realizations of subsurface heterogeneity, and emphasize the critical importance of this type of information for reducing the uncertainty of simulations considering flux-dependent processes.

Computer programs for an appropriate visualization of crystal types and morphological developments of accessory zircon are not available hitherto. Usually, typological computations are conducted by using simple calculation tools or spread-sheet programs. In practice, however, high numbers of data sets including information of numerous zircon populations have to be processed and stored. The paper describes the software ZIRCTYP, which is a macro-driven program within the Microsoft Access database management system. It allows the computation of zircon morphologies occurring in specific rock samples and their presentation in typology diagrams. In addition, morphological developments within a given zircon population are presented (1) statistically and (2) graphically as crystal sequences showing initial, intermediate, and final growth stages.

An algorithm has been designed and tested which was devised as a tool assisting the analysis of geological structures solely from orientation data. More specifically, the algorithm was intended for the analysis of geological structures that can be approached as planar and piecewise features, like many folded strata. Input orientation data is expressed as pairs of angles (azimuth and dip). The algorithm starts by considering the data in Cartesian coordinates. This is followed by a search for an initial clustering solution, which is achieved by comparing the results output from the systematic shift of a regular rigid grid over the data. This initial solution is optimal (achieves minimum square error) once the grid size and the shift increment are fixed. Finally, the algorithm corrects for the variable spread that is generally expected from the data type using a reshaped non-rigid grid. The algorithm is size-oriented, which implies the application of conditions over cluster size through all the process in contrast to density-oriented algorithms, also widely used when dealing with spatial data. Results are derived in few seconds and, when tested over synthetic examples, they were found to be consistent and reliable. This makes the algorithm a valuable alternative to the time-consuming traditional approaches available to geologists.

In order to quantify low amplitude deformations in sedimentary basins, fault offsets are modelled, restored and quantified in 3D. We studied a field example where such faults where blanketed by a small angular unconformity between Upper and Lower Cretaceous in the central area of the Allauch Massif (SE Provence, France) to which we applied sub-surface structural modelling techniques (Gocad). We gathered mapping, structural and sedimentologic data in the field. Fault and stratigraphic surfaces were interpolated with gOcad for constructing a 3D structural model of the Allauch Massif. In order to restore the original geometry of the erosional wedge and low amplitude fault offsets, we restored the post-unconformity Pyrenean-Alpine deformation. We obtained a 3D structural model of the angular unconformity at the pre-Upper Turonian times.

The angular unconformity was due to a 5° fault-induced tilting of the Lower Cretaceous and underlying layers, which implied a 3° erosion wedge and a differential erosion of ∼140 m over 4 km in the Valanginian and Hauterivian layers. Bauxites were deposited during the time interval between Early and Late Cretaceous. The restored geometry suggests that the bauxite was preserved in topographic steps aligned parallel to the strike (∼NÑW-SSE) of the Lower Cretaceous. This strike is due to tilting synchronous with normal faults.

The Geological Surveying and Investigation in 3 Dimensions (GSI3D) software tool and methodology has been developed over the last 15 years. Since 2001 this has been in cooperation with the British Geological Survey (BGS). To-date over a hundred BGS geologists have learned to use the software that is now routinely deployed in building systematic and commercial 3D geological models. The success of the GSI3D methodology and software is based on its intuitive design and the fact that it utilises exactly the same data and methods, albeit in digital forms, that geologists have been using for two centuries in order to make geological maps and cross-sections. The geologist constructs models based on a career of observation of geological phenomena, thereby incorporating tacit knowledge into the model. This knowledge capture is a key element to the GSI3D approach. In BGS GSI3D is part of a much wider set of systems and work processes that together make up the cyberinfrastructure of a modern geological survey. The GSI3D software is not yet designed to cope with bedrock structures in which individual stratigraphic surfaces are repeated or inverted, but the software is currently being extended by BGS to encompass these more complex geological scenarios. A further challenge for BGS is to enable its 3D geological models to become part of the semantic Web using GML application schema like GeoSciML. The biggest benefits of widely available systematic geological models will be an enhanced public understanding of the sub-surface in 3D, and the teaching of geoscience students.