Sketch-based modelling and visualization of geological deposition
Introduction
Geologists are interested in better tools for externalizing their ideas on the earth׳s behaviour. They want to do this in an expressive and simple way, which is particularly important for communicative purposes. Current modelling tools in geology have a high learning curve and are tedious to use (Caumon et al., 2009). We present a simple sketching interface and a data structure for compact representation and flexible rendering of subsurface layer structures. This is useful for representing mountains, basins, lakes, rivers and deltas. Rivers and deltas change the morphology of a terrain through erosional and depositional processes. This is left as imprints in layers of the terrain. Our technique provides a way to sketch and visualize such layers (Fig. 1). Our representation is well suited to obtain interactive layer-cake visualizations, resulting in geological illustrations that are helpful for education and communication. Conventional hydrocarbon reservoirs (and aquifers) are found in porous bodies of rock. Examples of such rock bodies include sandstone, which is found in sedimentary basins and has a high preservation potential (Hinderer, 2012). The sandstone might, under favourable circumstances and the right basin development (where hydrocarbon source rock and reservoir seal is present), become a reservoir for hydrocarbons. This is, in a crude sense, the reason why these rock bodies receive a lot of focus in geology, and a motivation to try to understand them to the full extent that data allows.
Available data (e.g. seismic, well logs or core) can often be of limited resolution and extent. In these cases, geologists have to develop conceptual ideas to describe the shape of rock bodies, and, from that, which processes were involved in their deposition. The processes involved in the deposition of sandstone bodies are known to vary between different depositional environments, and can thus be differentiated based on observations and interpretation (Reading, 1996). To develop good conceptual models for the reservoir sandstone, their horizontal and cross-sectional characteristics are often highlighted by schematic block diagrams (e.g. Gani and Bhattacharya, 2007, Pore¸bski and Steel, 2003). In Fig. 2(left), an example of a hand-drawn block diagram depicting a meandering river channel is shown. It illustrates the aerial and the cross sectional expression of sandstone point bars (sediments deposited along the inner bank of a meandering stream) and how one sandstone body is overlaid by another. Internal architecture is important in the sense that it tells how the depositional element (e.g. channel or delta) evolved, and subsequently how small-scale heterogeneities such as mudstone might be distributed within the overall sandy body. This can have direct implication for hydrocarbon fluid extraction. Our approach offers a new way of producing illustrations by performing interactive erosion and deposition that lets the illustrator mimic processes that she interprets to have been the cause for the sandstone deposition. An additional consequence of our 3D sketched models is their manoeuvrable cutting planes that enable multiple cross-section visualizations. This helps in understanding complex internal layering within the sandstone, otherwise not intuitively apprehensible (Bridge, 1993). Among the depositional systems that may result in hydrocarbon accumulation, rivers and deltas are central.
In summary, our overall contribution is a sketch-based system that makes it possible to quickly build 3D interactive geological illustrations from scratch. Sequentially defining alterations on a model is less laborious than drawing 2D illustrations; this opens up for discussions, fast hypothesis testing and creation of time-series illustrations. A novel central aspect lies in the proposed data structure (each stratigraphic layer corresponds to a composition of one or more heightmaps) and the way it is processed to render volumetric models. The main features that characterize our tool consist of operators that interpret each sketch to generate deposition and erosion processes; in particular, rivers and channels, mountains, basins, deltas and intermediate stages of their evolution.
Section snippets
Related work
Little work exists on modelling and visualizing deltas in computer graphics. For modelling rivers and erosion in a geological setting, most of the methods are based on fractal noise generation and physical based processes (for instance the works of Benes et al., 2006 and Stava et al., 2008). Such procedural approaches reduce the degree of control over the landscape development. Other sketch-based techniques have been introduced to model landforms on a terrain (Hnaidi et al., 2010, Gain et al.,
Description of our approach
The entire approach is based on two synchronized data structures; the relative layers and the absolute layers. The relative layers let us keep geological processes independent of each other with respect to time, allowing us to rearrange layers in any order without further computations. The absolute layers is derived from relative layers and is used for fast rendering. The i-th absolute layer is defined in each of its points by the height of the top surface of the layer in the
Results
The examples used in this paper are generated through a sketch-based technique to acquire a shape definition of the different relative layers, as in Fig. 9, where the shape of each lake is defined by a single user stroke.
Several geological features can be generated with our method. A river with its sedimentary history can be achieved by drawing the initial and the final configuration of its shape evolution, as illustrated in Fig. 3. A delta representation is obtained by sketching a closed
User evaluation
To evaluate the usability and the generality of our tool, we performed two separate user studies on domain experts. One study consisted of 6 geological domain experts (users A–F), who individually watch and give feedback on a tutorial video (Video 1 in supplementary material) (accessible here Natali, 2013). The video demonstrates how to create each of the geological features supported in our approach followed by a more complete example in the end. Feedback was given by means of grading a list
Conclusion
We have identified sketching needs by geologists who model fluvial systems based on interpretations. To address these needs we have constructed a sketch-based interface, a layered data representation and a rendering approach. The data structure leads to an intuitive definition of the geological process of deposition and erosion and requires only basic arithmetic calculations for realizing the model. This results in a volumetric model which relieves us from topology and intersection testing
Acknowledgments
The authors would like to thank all reviewers who gave comments and suggestions for improvements, and Ivan Viola for fruitful discussions. This work is funded by the Petromaks program of the Norwegian Research Council through the Geoillustrator project (#200512).
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