Lava and pyroclastic flows
Although the geological facts behind their genesis are entirely different, both manifestations of volcanic flow deposits can be modeled in a very similar way.
Input data. The outline of a flow deposit has to be digitized from the geological map in a way similar to the procedure described for volcano cones above. In addition, a line must be specified describing the directions of progression. Again, the number of frames has to be chosen in order to have a value for the increments needed in the computation. Finally, the thickness of the flow must be known at any location. These values derive from digital height models describing the surface of the deposits. If those are not available with sufficient quantity and quality, a constant average thickness has to be used. For reasons already mentioned, all input-data (outlines and DEMs) have been generated as or converted into ARC/INFO® formats and provided as ASCII-files.
Modeling. In order to model the progression of the flow, its front part is moved ahead in increments (Fig. 5). To give the movement a smooth appearance, many front lines have to be defined. Since it would be too tedious to digitize all these lines manually, a program was developed to define these front lines as arcs between the left and right outline, considering also the flow directions given with the input data. A polygon, enclosing the area the flow has moved ahead since the last frame, is generated using the current and the last front line arcs, and labeled according to the frame number. Problems arise in sharp curves where the front line arcs may intersect (Fig. 6). This requires some additional editing (e.g. with ARCEDIT®) to get topologically correct polygons.
A point-in-polygon test is now performed in every frame for each polygon labeled with the current frame number. If a DEM grid elevation point is located inside the last increment of a lava flow movement, its elevation will be replaced by the corresponding elevation of the flow's surface. Pyroclastic flows, consis-ting of gases to a large extent, settle when coming to a halt. This effect was modeled using a multiple thickness during the propagation phase and reducing heights to the final surface location afterwards.
Modeling of volcanoes/maars and flow deposits is integrated into one single program. Thus, it can be performed in one step, if all necessary input data are available.
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Figure 5 Polygons for the increments
of a flow propagation. |
| |
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Figure 6 Intersection of arcs
of front lines |
Texture Modelling
General Aspects
Proceeding in a similar way as with DEM modeling, an initial texture image was produced for the first frame of the video. This image was adapted for all following frames according to the changes occurring. Different textures symbolize different vegetation or deposit types. Climatic changes, occurring quite frequently in the time concerned, can thus be conveyed by the appropriate vegetation patterns, such as forest or prairie.
In the modeling procedure, a segmentation is carried out according to the movement of volcanic material, and an (untextured) color code is introduced for every class of texture. For example, red symbolizes a lava flow, green a volcano or blue a volcanic maar. These colors are used to mark the location of an event in the geo-referenced background image preliminarily (see untextured areas in Fig. 7). Prior to the following animation procedure (cf. Fig. 1) the codes are replaced automatically by more realistic textures (produced based on natural patterns using Corel Photo-Paint®) by means of a script program. The texture files are just another input source for animation in Kinetix 3D Studio MAX®.
Figure 7 Segmented areas (untextured)
to mark areas where texture has to be changed.
Maars and volcanic cones
The same set of polygons that was created for DEM modeling (Fig. 3) can be used for the segmentation of the present volcano cone area of every frame. A point-in-polygon test is performed for the points of the image matrix of each frame and color codes are placed accordingly into the geo-referenced image. While processing consecutive frames, all data are kept in main memory and saved for every frame. Many different events occurring at the same time but at different locations can thus be processed simultaneously.
Lava and pyroclastic flows
The procedure for texture overlays for the flows is basically the same as for the maars and volcanoes: Those color code values of the image matrix appearing inside the area covered by the flow are changed to their new values. Cooling-out of lava after sedimentation is considered as well by change of its color.
Final Film Production
When the modeling procedures had been completed, all DEMs and images were animated with 3D Studio MAX® R2.5 and tested with different variations of lights, cameras and atmospheric effects (smoke, vapor, fall-out, haze, eruption cloud) to get a good realistic appearance. After rendering, many thousand single frames could be combined to show the virtual landscape genesis of the Pellenz area in a digital video. For further details on animation and rendering see Boehler, Scherer, Siebold, 1999. The material was used by the "Institut fuer den wissenschaftlichen Film" (Institute for the Scientific Film), Goettingen, Germany, to produce the final film product mentioned above.
Conclusions
Using only simple linear interpolations, it is possible to model complex volcanic mechanisms and the resulting changes in landscape appearance. DEMs and textural overlays can be created for any stage of development, thus accomplishing a virtual visualization of very complex processes on the basis of scientific knowledge.
Figure 8 Sequences from the video. Left: Build-up of a volcanic cone and subsequent lava flow.
Center: Eruption of a St. Helens type volcano (note fall-out). Right: Clash of two pyroclastic surges.
Acknowledgements
The project was planned and carried out in close cooperation with Roemisch-Germanisches Zentralmuseum, Mainz, and predominantly funded by 'Stiftung-Rheinland-Pfalz fuer Innovation', the innovation foundation of the State of Rhineland-Palatinate. Geologists P. Ippach and E. Harms, both from Vulkanpark GmbH, Mayen, accompanied the project with expert advice. At our own institute, G. Heinz was an advisor on many difficult subjects.
References
- Boehler, W., Scherer, Y., Siebold, M.: Visualization of a Landscape Genesis. Internatioanl Archives of Photogrammetry and Remote Sensing, Vol. XXXII, Part 5-3W12, pp. 9-14, Onuma 1999.
- Ippach, P., 1999, 2000. Personal communication.
- Meyer, W., 1994. Geologie der Eifel. E. Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart.
- Schmincke, H.-U., 1988. Vulkane im Laacher See-Gebiet. Ihre Entstehung und heutige Bedeutung. Doris Bode Verlag GmbH, Haltern.
