1 preface
The existing geographic information system (GIS) mainly expresses the graphics and attribute information of two-dimensional surface features, but there is still a gap in expanding to the real three-dimensional geological information system including underground geological structures. A large-scale geological engineering project, from feasibility study stage, preliminary design stage to detailed design stage, and even to the construction and operation stage of the project, often accumulates a lot of geological data. Using three-dimensional model graphics and images to express and explain such huge data is much more effective than relying solely on traditional means such as databases, charts and drawings. Establishing a three-dimensional model of engineering geological body, dealing with the combination relationship between rock interface and structural plane, and realistically reflecting the whole picture of main underground geological structures will provide a brand-new research means and method for engineering geologists to analyze and study engineering geological phenomena and discover and master the structural laws of rock and soil bodies.
The research of 3D geological modeling and visualization abroad has developed rapidly. MicroLYNX, a three-dimensional modeling and analysis software developed by Apollo Science and Technology Group in Canada, processes spatial data such as discrete point sampling, borehole sampling and groove sampling to generate models such as sections, blocks and surfaces, determine the distribution and grade changes of mineral deposits, and calculate mineral reserves. Gemcom software developed by Gemcom Software International Co., Ltd. of Canada shows the distribution of boreholes through data such as boreholes, points and polygons, uses practical graphic editing and generating tools, builds surface and solid models by irregular triangulation, and uses polysemy coils to close the boundaries of rock strata and ore bodies to analyze reserves and grades, providing interactive operation functions, allowing users to draw geological models according to their own experience and expert knowledge, and realize arbitrary cross-sectional observation, intersection and Boolean operation between entities or surfaces. Foreign software is mainly aimed at mining engineering, which can meet the needs of mineral resources exploration and evaluation, design and planning of underground mines and open-pit mines, mineral resources management and mining production management in mining engineering activities. Popular 3D modeling software such as 3D Studio MAX developed by American Kinetix Company, Maya developed by Alias/Wavefront Company, Softimage developed by Microsoft Company, etc. have their own unique features in building industrial and architectural models, animation production, etc., but the interactive query function is weak, and it still has a long way to go to be applied to 3D modeling of engineering geology in combination with engineering survey database.
Zhang Juming and others deeply studied the visualization of weathering zone distribution, multi-layer strata and other geological information, and the expression and display algorithm of fault dislocation strata, which laid a mathematical foundation for the development of three-dimensional visualization software of engineering geology, and realized the display of complex three-dimensional geological graphics with the help of AutoCAD platform. The domestic Turing VRMap GIS software has powerful functions of terrain simulation and object query, but it is not a real 3D geological modeling tool. TITAN 3D modeling software is developed by Beijing Oriental Titan Technology Co., Ltd. Based on the idea of frame modeling, the true 3D solid model of complex objects in 3D space is established by using parallel or basically parallel contour data. But at present, it is only a preliminary three-dimensional modeling and graphics processing engine, and professional modules, such as engineering geology module, need to be added or expanded when facing specific disciplines.
Looking at the research and development status of several softwares at home and abroad, it has laid a good technical foundation for 3D modeling and visualization of engineering geology and provided valuable development experience. However, the modeling and visualization analysis of geological bodies in engineering geology are not targeted enough to meet the professional functional requirements of engineering geology production and research. Therefore, this paper will start with the analysis of key technical problems of 3D modeling and visualization of engineering geology, and briefly describe the author's preliminary development and research achievements in 3D modeling and visualization of engineering geology.
2 Analysis of key technical problems 2. 1 Interpolation and fitting of discrete data
All kinds of geological information in complex geological bodies of engineering geology, including surface topography, groundwater level, stratigraphic interface, faults, joints, distribution of weathering zones, intrusions and equivalent surfaces (lines) of physical and mechanical parameters or data of various geophysical, geochemical and geotechnical bodies, can be regarded as functions in three-dimensional space. Their fitting function should be established according to the actual survey data. The richer the measured data, the more truly it can describe the spatial distribution law of these information. The graphic generation of single-valued surfaces, such as surface topographic survey data and groundwater level survey information, can be attributed to the interpolation and fitting of discrete data of two independent variables. For multivalued surfaces, such as deconvolution and spatial isosurface, other complex methods such as multi-parameter variable interpolation should be adopted. There are several methods to construct interpolation function of space surface, such as weighting method inversely proportional to distance (Shepard method), radial basis function interpolation method [3], plane elasticity theory interpolation method and so on. They are also suitable for single continuous stratigraphic interface, geophysical exploration data, geochemical exploration data and the distribution of geotechnical physical and mechanical parameters in geological space.
2.2 Three-dimensional data structure
Engineering geological bodies are generally irregular bodies. In computer graphics, curves and surfaces always use many tiny straight lines and tiny triangular surfaces to approximate the lithologic boundary and rock surface of strata, that is, the rock interface (as well as geological boundary lines such as curved surface curves and groundwater levels) and rock surface are collections of many tiny straight lines and tiny triangular surfaces respectively. The three-dimensional spatial data structure of geological body is the basis of three-dimensional modeling and visualization of engineering geology, and an effective hierarchical three-dimensional data structure is needed to ensure the realization of human-computer interaction and query.
2.3 surface intersection
There are a large number of layers in geological bodies. When there are unconformity, fault dislocation strata, stratum pinchout and groundwater exposure on the river valley surface, the problem of surface intersection will naturally occur. The upper boundary of the three-dimensional model of geological body is the surface, and the rock water level or groundwater level fitted by mathematical method should not exceed the surface, that is, the excess part should not be displayed. Similarly, when displaying multiple strata, each stratum below should be bounded by the stratum above. Therefore, in order to visualize the stratigraphic interface, it is necessary to solve the intersection problem between the stratigraphic plane and the surface, and between the fault plane and other stratigraphic planes. On the other hand, when drawing a section, the geological boundary is drawn by displaying the intersection line obtained by the intersection of the section (plane) and various geological interfaces (surfaces). Therefore, the intersection of faces includes the intersection between geological interfaces (layers) and the intersection between geological interfaces and sections.
2.4 Three-dimensional topological structure analysis
From the geological point of view, topology is a relational table between geological objects, and the topological table stores stratigraphic relations and geological spatial position relations such as overlying, underlying and intersecting layers (topological expression of fault cutting strata). Topology can also be regarded as a data structure, which allows these geological relationships to be stored reasonably. For example, considering a multi-layer stratum, the bottom surface of the upper stratum and the top surface of the next adjacent stratum are the common or shared boundaries of two entities, and the topological relationship between them is adjacent and the same. When storing data, only the bottom surface of the previous stratum or the top surface of the next adjacent stratum is stored, that is, the boundary surface of the adjacent stratum can be stored as a stratum, which greatly reduces the data storage capacity. The evaluation of geological model system often depends on the topological structure used to describe geological objects [4].
2.5 Visualization technology
Visualization of complex geological bodies in engineering geology is to use computer technology to transform the data obtained from engineering survey into three-dimensional and cross-sectional maps of underground geological structures, which is intuitive and convenient for interactive analysis, and its basis is visualization of engineering data and measurement data [5]. Visualization technology can construct rock strata and structural planes that play a key role in geological engineering from huge geological survey data, such as edge failure stability, deformation and failure of underground caverns, and display their scope, trend and cross relationship, which can help engineering geologists correctly interpret the original data and provide decision support for specific problems in engineering geological analysis.
3. Preliminary development and application of 3D visualization technology of engineering geology 3. 1 research block diagram
The block diagram of 3D modeling and visualization of complex geological bodies in engineering geology is shown in figure 1.
Based on the idea of interpolation and fitting of discrete sampling data, discrete data are transformed into continuous curves and surfaces. The process of 3D modeling and visualization of complex geological bodies in engineering geology is to extract the coordinate positions of various geological information and the physical and mechanical parameters of rock and soil bodies from the exploration database. Through different fitting and interpolation functions, the 3D computer graphic display of geological layers (surfaces) and geological entities is obtained, and the distribution law of geological information in the study area is expressed. After geological strata and geological entities are generated, the established model can be observed from any angle, and a vertical section can be generated according to the specified section strike, dip and dip angle.
3.2 Preliminary Development and Application 3.2. 1 Engineering Survey Spatial Database Management
After collecting and sorting out the field survey data, it is entered into the sub-item data table of the survey space database of a hydropower project in Jinsha River. These data tables not only include the location data of geological information, but also provide attribute data.
Taking the data table of formation lithology as an example, data such as drilling number, formation start depth, formation end depth, layer thickness, lithology (formation name), formation code (formation age), formation strike, formation dip angle, formation dip angle, contact relationship and geological description need to be input. With the progress of engineering survey, it is very convenient to modify, supplement and manage the survey data. Fig. 2 is the management interface of drilling stratum system data table in engineering survey database.
3D browsing
Through fitting and interpolation of discrete data such as orifice coordinates and measurement data, the surface grid of the right bank of the dam site area is drawn (Figure 3), and then virtual reality browsing and observation can be carried out in the three-dimensional graphic environment (Figure 4).
3.2.3 Three-dimensional geological stereogram
According to the engineering investigation data, the three-dimensional geological map of the right bank of the dam site area is established. From top to bottom, the stratigraphic lithologic combination in the dam site area is: Quaternary colluvium, Jurassic mudstone, silty mudstone, argillaceous siltstone, Triassic thick ~ extremely thick layered fine ~ medium grained sandstone, Triassic thin ~ medium thick layered spun yarn, siltstone and Triassic medium ~ thick layered medium grained sandstone. The stereogram obtained through limited engineering survey data can better meet the accuracy of engineering geology. Figure 5 shows the 3D geological map of the right bank of the dam site area.
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