Three-dimensional visual analysis of rock structure

T.Ando　S.Omori　Y.Ogasawara

（Institute of Earth Science, Waseda University, Shinjuku-ku, Tokyo 169-50, Japan）

J.B. Noblett

（Department of Geology, Colorado College, Colorado Springs, CO 80903, USA）

Abstract Computed tomography based on a series of grinding sections is used to study the structure of rocks Three-dimensional structure. The rock samples were cut into squares of approximately several centimeters, and then manually ground into sections at 0.5mm intervals and taken into color photographs. Based on this set of two-dimensional cross-section images, the visualization software AVS uses two methods to synthesize the three-dimensional rock structure: the cross-section method and the three-dimensional rendering method. This technique was used on two rock samples: one was a metamorphic composite intrusive rock collected from the Hida metamorphic belt in Japan, which unfolded a complex flow structure of mafic and felsic components in the rock; the other was collected from Garnet-rich metaphenocryst eclogite from the Franciscan Terrane, California. For the former sample, the three-dimensional structural image shows a mixing relationship between mafic and felsic, which is a good illustration of the mixing of the two magmas before metamorphism. For the latter sample, the three-dimensional image clearly shows the size, number and distribution of garnet phenocrysts. These data are meaningful for explaining the nucleation formation of garnet phenocrysts in eclogite. The combination of computed tomography and 3D visualization software is useful for analyzing the millimeter to centimeter scale structure of actual rocks, with sample sizes on the order of squares of several centimeters.

Keywords Three-dimensional visualization of rock structure, magma mixed eclogite

1 Introduction

In recent years, great progress has been made in the software and hardware of three-dimensional image processing. Improved methods of understanding and analyzing many phenomena and objects in several specialized areas. Computed tomography (CT) is one of the most successful methods and has been successfully used in medicine, namely X-ray CT.

In petrology, petrologists can usually only observe two-dimensional profiles under a microscope with the naked eye, and rely on imagination to conceive of the actual three-dimensional structure based on two-dimensional information. However, the actual three-dimensional structure of the rock contains more petrogenic information, and we cannot directly observe the internal structure of the sample. It is well known that overreliance on 2D structural information can lead to misinterpretations. To solve this difficulty in rock structure analysis, three-dimensional visualization technology in the field of computer graphics is urgently needed, but its application seems to be difficult. The pioneering work of using three-dimensional X-ray CT to study rock structure was completed by a team led by Professor W.D. Carlson of the University of Texas. They have developed an almost completely automatic rock structure X-ray CT analysis system. Using the three-dimensional visualization software AVS, we tried the observation technology of the three-dimensional structure of rocks established by using a series of grinding sections and cross-section photos. This technology is basically mature and is effective for several types of rocks.

This article uses two examples of composite intrusive rocks and garnet-rich metamorphic eclogites to illustrate the "serial grinding section CT" technology for obtaining three-dimensional images of rock structures, and gives the three-dimensional structures of these rock samples. Composite image.

2 Sample description

2.1 Metamorphic composite intrusive rock

The metamorphic composite intrusive rock used for three-dimensional observation was collected from Hi in the Hida metamorphic zone in the Gifu Prefecture area in central Japan The gashi-Urushiyama outcrop. The rock consists of two parts: dark amphibolite and light-colored metamorphic tonalite. This kind of rock belongs to the late stage of Hida metamorphism and intrudes into the biotite amphibolite gneiss surrounding rock. The age of the surrounding rock belongs to the early stage of metamorphism. Its complex hybrid structure and flow-like structure are shown in Figure 1a. This structure appears to indicate mixing of mafic and felsic magmas. According to the Rb-Sr isochron age determination made by Y.Arakawa. The main mineral components are garnet, omphacite, polysilicate muscovite, blue amphibole and chlorite. A large number of garnet metaphenocrysts occur in this rock. Euhedral garnet crystals with rhombohedral dodecahedrons are generally 2 to 9 mm in diameter and appear reddish brown to the naked eye. Under a microscope, garnet crystals are often replaced by chlorite at the edges. The matrix of eclogite is mainly composed of omphacite and amphibole, both of which are products of the degradation period. The parts of the rock that are rich in omphacite are dark green, while the parts that are rich in amphibole are dark blue-gray. The purpose of three-dimensional observations of this sample was to understand the size, number, and distribution of garnet metaphenocrysts in the eclogite. The sample for three-dimensional observation is a parallelepiped of 5cm×10cm×15cm (Fig. 1b).

3 Three-dimensional image synthesis method

3.1 Sample preparation and image acquisition

The workflow of preprocessing a series of cross-sectional images from photos is shown in Figure 2. The sample observed this time was cut into parallelepipeds about several centimeters in size. Before grinding, all six surfaces of the parallelepiped were photographed so that they could be compared with the computer-generated surfaces.

After specifying the cutting surface, use No. 100, No. 400, and No. 800 emery to grind the film at 0.5mm intervals, and take color photos with film. This process needs to be repeated 200 times, and the error every hundred times is less than 1mm.

3.2 Hardware and Software

Two UNIX workstations were used in the study: Titan 3000 manufactured by Startent Co., Ltd. and Magnum4000 manufactured by MIP Co., Ltd. The three-dimensional visualization software AVS (Actual Visualization System) was developed by Kubota Graphics Technology Company and is used for the synthesis of three-dimensional images. AVS is a well-known 3D visualization software that can run on a variety of computer systems. AVS is characterized by an interactive user interface, consisting of four subsystems - image viewer, graphics viewer, geometry viewer and network editor. The network editor is a visual programming environment. You only need to connect several executable modules with the mouse to complete the design of the application program.

In this study, two applications were designed for three-dimensional observation using a network editor: one is a cross-section method program, and the other is a three-dimensional rendering method program.

Figure 1 Rock samples used in the study

a—Metamorphic composite intrusive rocks collected from the Hida metamorphic belt in Japan; b—Garnet-rich metamorphic rocks collected from the Franciscan terrane in California Phenocrystalline eclogite

3.3 Two-dimensional series image preprocessing

Use a series of cross-section color photos to conduct three-dimensional observations of rock structures. Each photo was input to the computer using an image scanner at a resolution of 75dpi and saved as a file in the X-Window dump file format (xwd). Each xwd file is converted into a two-dimensional field data format using AVS. Then, a complete set of cross-sectional images is converted into three-dimensional pixels of AVS. The conversion program "2D_to_3D" used is written by ourselves in C language.

Figure 2: Workflow from photo acquisition to cross-sectional image preprocessing

3.4 Cross-sectional observation

This method can generate cross-sections of three-dimensional objects in any direction. Sections, and can produce continuous sections in the form of animations. Figure 3a is a network diagram of the AVS module using the cross-section method to observe the three-dimensional structure. The functions of each module are as follows: "Read field data" reads in the AVS field data; "Pruning" changes the size of the field data; "Downward adaptation" changes The size of the voxel data; "Generate color mapping table" generates the AVS color mapping table data structure; "Color mapping" converts the input voxel data into color values; "Map" generates a three-dimensional picture of the data; " Animation" automatically modifies parameters for animation demonstration.

Figure 3 AVS module network diagram a—cross-section method network diagram; b—stereoscopic rendering method network diagram

3.5 Observation using three-dimensional rendering method

This method generates Transparent 3D image. Figure 3b is a network diagram of the AVS module using three-dimensional rendering to observe the three-dimensional structure. Until "Downward Adaptation", the processing flow is the same as the cross-section method. The functions of the remaining modules are described as follows: "Gradient Calculation" and "Gradient Hue" calculate the gradient vector of each point in the three-dimensional field data set; "Stereo Boundary" generates Lines representing the boundaries of a 3D field dataset.

4 Discussion of results

4.1 Metamorphic composite intrusive rock

This sample is to observe the actual three-dimensional structure of the rock, especially the light and dark components in the rock hybrid structure. The image synthesized by the cross-section method and the three-dimensional rendering method successfully shows the three-dimensional structure of the rock (Fig. 4), which is usually represented by a black-and-white grayscale image. The interval between grinding discs is 0.5 mm, which is sufficient for three-dimensional image synthesis of rock samples of several centimeters in size.

Figures 4a to 4c are three-dimensional structural diagrams obtained by the cross-section method. The six surfaces of the actual sample (Fig. 1a) are clearly reproduced in the three-dimensional composite image (Fig. 4a). Figures 4b and 4c are two examples of cross sections in any direction. By continuously viewing cross-sections in one direction as an animation on the screen, you can confirm the mixing relationship between dark and light parts of the rock: some light components are contained within the dark parts, and some dark components are contained within the light components. , as pointed by arrow A in Figure 4c.

Figure 4 Three-dimensional composite image of metamorphic intrusive rocks in the Hida metamorphic belt, Japan

a, b, c - three-dimensional images obtained by the cross-section method. The dark ones are mafic, and the lighter ones are The colored ones are felsic; d, e, f - three-dimensional images of felsic components obtained by stereo rendering method, with the mafic part set to transparent

Figure 4d to 4f are light colors in the rock The three-dimensional composite image of the components is generated by the stereoscopic rendering method. When generating the composite image, the dark components are set to be completely transparent. Using the brightness of the grayscale image to distinguish dark and light components, object recognition of this rock in a three-dimensional image is simple because it consists of only two parts, as long as the appropriate brightness threshold is set.

In the background of dark components, light components appear as tubes and dendrites, and nowhere appear as flat plates. Some relatively small light-colored components become branches of larger branches, while some appear isolated in the dark background as droplets, as indicated by arrow B in Figure 4f. To explain this structure, mixing of mafic and felsic magmas must be introduced.

If the light-colored felsic magma invaded the dark part that had consolidated at a later stage, its shape should be plate-like. The shape of the light-colored components suggests that the dark-colored mafic magma was liquid when the felsic magma intruded. The results obtained by our three-dimensional analysis method clearly illustrate that the metamorphic composite intrusive rocks at the Higashi Urushiyama outcrop were formed by the mixing of mafic and felsic magma, which is the same as that reported in Colorado, USA [12].

4.2 Franciscan Eclogite

The purpose of using this sample is to understand the size, quantity and distribution of garnet metaphenocrysts in the eclogite. The three-dimensional composite image obtained by the cross-section method and the three-dimensional rendering method is shown in Figure 5. Figure 5b is an example of a cross-section in any direction. Comparison with the photos of the eclogite sample shows that all six surfaces of the composite image have been well reconstructed (Fig. 5a).

In this article, due to printing reasons, the three-dimensional structural diagram is in black and white, but the original three-dimensional composite image is in color. The light-colored ones in each photo in Figure 5 are garnet phenocrysts, and the darker ones are the matrix of eclogite, which is mainly composed of omphacite and blue amphibole. Relatively large garnet metaphenocryst crystals (>6 mm) can be distinguished based on hue differences and eclogite matrix, while smaller crystals (<2 mm) are slightly more difficult (arrows in Figure 5b). Also, it is very difficult to distinguish other minerals in the matrix. The main components of the matrix are omphacite and amphibole, whose colors are dark green and dark blue respectively, so it is difficult to distinguish them from each other based on color photos of the polished surface. To identify these minerals, specialized preprocessing of sample surface or cross-section images is necessary.

The change in particle size is easily seen in the three-dimensional image, but the crystal shape of the garnet phenocryst is not shown in these photos. This is due to insufficient resolution of the 0.5mm grinding plate. of. In the synthesized three-dimensional image, the distribution of garnet phenocrysts can be well observed, especially in the three-dimensional rendering obtained by setting the eclogite matrix to be completely transparent (Figure 5c, d) . Relatively small-grained garnets are concentrated in the lower part of the sample (arrow in Figure 5d), while large-grained garnets are mainly distributed in the upper part and sparsely distributed. The three-dimensional distribution data and the chemical composition data of each part are analyzed together to help explain the nucleation of garnet metaphenocrysts in eclogite, but this article will not elaborate more on this.

Another application of the 3D structure is to try to measure the actual mineral content of garnet in the eclogite. The value derived from the 3D model is 24.5% by volume. This value was obtained from the 2D image. Centrally calculated. The original result obtained by counting points under the microscope was 22.5%. This result was obtained from three identical-looking thin sections of the same sample. The results obtained by the three-dimensional method may be more reliable than the numerical point method. By cutting any specified part of the three-dimensional image, the actual garnet content of the sample in that part can be calculated.

5 Conclusion

This paper proposes a three-dimensional observation technology of rock structure using a series of grinding discs and the three-dimensional visualization analysis software AVS. This method can be used to understand the actual structure of rocks at the millimeter and centimeter scale, using samples that are cubes of several centimeters.

Figure 5 Three-dimensional composite image of garnet-rich metaphenocrystalline eclogite in the Franciscan terrane in California

a,b—Three-dimensional images obtained by the cross-section method, the light color is Garnet phenocryst, the dark color is the matrix, mainly composed of omphacite and amphibole; c, d - three-dimensional images obtained by stereo rendering method, the matrix is ??set to transparent

For metamorphic composite intrusive rocks Samples, mafic and felsic can be successfully distinguished from each other, and an animation of the three-dimensional image can confirm the mixing relationship of the mafic and felsic compositions. The three-dimensional image of the felsic component obtained by the stereo rendering method shows its flow-like structure, such as tubes, dendrites and droplets, indicating that the felsic magma intruded into the unconsolidated mafic magma.

For garnet-rich phenocryst eclogite, garnet crystals can be identified from the matrix based on differences in hue, and the size and distribution of garnet phenocrysts can also be easily determined from three-dimensional images. Understand, especially in three-dimensional renderings. Three-dimensional images can also be used to analyze the actual mineral content of garnet in eclogites.

Using the three-dimensional visualization software AVS and using a series of grinding section CT methods to analyze rock structures, it is effective for rock structures ranging from millimeter to centimeter levels, but there are also some shortcomings: ① This method requires a high level of Rock grinding technology; ② After obtaining a complete set of cross-section images, the rock sample no longer exists. The submillimeter precision grinding technology developed in this study will also be used to analyze the chemical composition of garnet metamorphic phenocrysts using computer synthesis of electron probe images. This study is used to determine the actual three-dimensional composition zoning of the rock. .

Acknowledgments Special thanks to Mr. Tatuya Nikkuni of KGT Co., Ltd. for his enthusiastic help in using AVS.

(Translated by Yao Guoqing, edited by Jiang Zuoqin)

References

[1] D.Shelly.Igneous and metamorphic rocks under the microscope.Chapman and Hall, 1993.

[2]M.J.Hibbard.Petrography to petrogenesi.Prentice Hall,1995.

[3]W.D.Carlson and C.Denison.Mechanism of porphyroblast crystallizatin:results from high- resolution computed quantitative textural analysis (abs.).Geo1.Sci.Amer.Abs.with Progs.,1996,28,A-54.

[5]T.Ando, ??S.Omori, Y.Ogasawara and J.B.Noblett.Applications of 3D visualization technique to petrology—An example of magma mingled rock (abs.),AGU Eos Abs.with Progs.1994,75.

[6]T.Ando, ??S.Omori ,Y.Ogasawara.Synthesis of 3D textures of garnet porphyroblasts in Franciscan eclogite.AGU Eos Abs.with Progs.,1995a,76.

[7]T.Ando,S.Omori,Y.Ogasawara and J.B.Noblett.3D observation of rock textures with AVS—An example of magma-mingled rock (in Japanese with English abstract).Bull.Center for Informatics,Waseda University,1995b,20:12～23.

[8]T.Ando, ??S.Omori, Y.Ogasawara and J.B.Noblett.3D observation of rock textures (abs.).30th Int'l,Geol.Congr.Abs.with Progs.,1995,3.

[9]Y.Arakawa.Rb-Sr ages of the gneiss and metamorphosed intrusive rocks of the Hida metamorphic belt in the Urushiyama area,Gifu prefecture,central Japan.Japan Jour.Assoc.Min.Petro1.Econ.Geol .1984,79:431～442.

[10]R.Coleman and M.A.Lanphere.Distribution and age of high-grade blueschists associated eclogites,and amphibolites fromOregon and California.Geol.Soc.Amer.Bull .1971,164:77～91.

[11]M.Cloos.Blueschists in the Franciscan complex of California:petrogenetic ccnstraints on uplift mechanism,Blueschists and Eclogites.Geol.Sci.Amer.Memoir., 1986,164:77～93.

[12]J.B.Noblett and M.W.Stab.Mid-Proterozoic lamprophyre commingled with late-stage dikes of the anorogen San lsabel batholith,Wet Mountains,Colorado.Geology,1990, 18:120～123.