Introduction

The main data sources of 3D geological modeling include: high-precision original data (such as drilling data, profile data, geological attribute data, etc.) from direct observation or exploration; indirect data with different precision and resolution such as geological map (geological boundary, fold, fault), geological structure map, actual material map, etc.(Apel 2006; S Brande et al. 2005; Zhu et al. 2012; Wu 2017; Song et al. 2019; Hao et al. 2019; Wu et al. 2005). In the past three-dimensional geological modeling process, only vertical geological data is used, i.e. borehole or profile drawn by borehole as modeling data source(Velasco et al. 2013; He et al. 2007; Hofmayer et al. 2019; Divya Priya and Dodagoudar 2018; Wang et al. 2017; Jia et al. 2017). In fact, most of the geological information contained in the geological map has been discarded in the process of modeling.

In view of the appeal, this paper puts forward the method of plane section modeling based on multi constraint conditions. The biggest innovation of this method is that it combines the vertical geological data and horizontal geological data to participate in the modeling work. The vertical geological data mainly refers to the geological section, and the horizontal geological data refers to the plane geological map. The surface part of the 3D geological model constructed by this modeling method is completely consistent with the plane geological map. Moreover, this kind of comprehensive constraint modeling with both vertical and horizontal data sources greatly improves the accuracy of the model.

In this paper, taking the kangyangju area of Huaying Mountain in Guang’an City as an example, we use the multi constraint plane section modeling method to carry out the three-dimensional geological model construction of kangyangju area, hoping to provide reference for the peer or related field research.

Overview of the study area

Guang’an City is located in the east of Sichuan Province, with an east longitude of 105 ° 56 ′ - 107 ° 19 ′, and a north latitude of 30 ° 01 ′ - 30 ° 52 ′. Jialing River and Qujiang River flow into the Yangtze River in a zigzag loop. Huaying Mountain, Tongluo mountain and Mingyue Mountain are parallel distributed in the east of the city, with a total area of 6339.22 km2, under the jurisdiction of Guang’an District, Qianfeng District, Yuechi County, wusheng County and Linshui County, and take charge of Huaying City. Huayingshan fold belt belongs to compound anticline, with an area of about 770km2 in Guang’an City, distributed in Huayingshan area in the west of the county, and composed of 8 groups of secondary anticlines and synclines. Kangyangju is located in the south of Huaying Mountain and the core of Huaying Mountain anticline. The exposed strata are mainly the Feixianguan Formation of Lower Triassic, Changxing Formation of Upper Permian, Longtan Formation, Huanglong Formation of Middle Carboniferous, Hanjiadian formation of middle Silurian (Yao et al. 2017; He et al. 2019; Liu et al. 2019), as shown in Fig. 1.

Fig. 1
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Modeling area of Huaying Mountain Kangyangju in Guang’an City

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Research on 3D geological modeling method based on multiple constraints data source of 3D geological modeling

The modeling area of kangyangju area is about 22 km2, and there are about 10 drilling wells in the whole area. However, the drilling hole can not control the formation form of fold formation, so it can only control the formation form of kangyangju through the profile. The modeling data of kangyangju area mainly include geological section, 1:25000 geological map and 1:10000 topographic map. The modeling depth is 200 m below the surface line, and the bottom of the model is plane.

The three-dimensional geological modeling of kangyangju area mainly adopts the “parallel section modeling method with multiple constraints”. The main data sources required for modeling are geological section, geological map and DEM elevation data. Before modeling, additional attributes should be added to the section area and the section stratigraphic line, so that they can find the same attributes in the automatic modeling process. There are 9 sections in kangyangju modeling area, 6 sections are perpendicular to kangyangju structural strike, 3 sections are parallel to structural strike, the section stratum is divided into sections, and the attribute of section stratum line is added to meet the requirements of parallel section automatic modeling (Fig. 2).

Fig. 2
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Profile location distribution

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Plane section modeling method based on multi constraint conditions

Kangyangju modeling area is located in the core of the fold. The modeling needs to show the fold shape and complete the modeling under the constraint of geological map. Therefore, the 3D geological modeling of kangyangju area adopts the parallel section automatic modeling method, which is constrained by geological map and topographic map to restore the real shape of the fold to the largest extent. The technical process of modeling is shown in Fig. 3.

Fig. 3
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Technological process of 3D geological modeling based on multiple constraints

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“Multi constraint conditions” refers to the process of modeling, adding geological profile, plane geological map, surface DEM and other data sources as constraint data for modeling, and multiple data source constraints can ensure to build a 3D model with higher accuracy. Among them, the drawing process of the geological section is based on the engineering geological drilling, which integrates the plane geological map, structural outline map, surface DEM and other data sources, and is drawn by geologists with the help of professional profile drawing tools. Therefore, the original data of establishing 3D geological model are in order of importance: borehole data, geological map, surface DEM and structural outline map.

Based on the three-dimensional geoscience modeling tool, a set of customized modeling method is developed, which integrates the geological conditions, modeling business requirements and actual data conditions of kangyangju area. This method makes full use of all kinds of data produced in geological exploration, advanced professional 3D geological modeling tools, and the geological expert knowledge of professional geologists, and can build a high-quality 3D geological structure model.

The biggest innovation of this method is also a difficulty of this method. Because the production process of plane geological map and geological section is independent, the production process of the two is also accompanied by a large number of subjective judgments of different geologists, so the two must be inconsistent in space, there are a lot of contradictions and conflicts to be reconciled. Moreover, one of the two data sources is vertical and the other is horizontal. In order to adjust the contradiction between the two, it is necessary to edit the spatial data in the three-dimensional space, so as to realize the construction of the three-dimensional geological model based on multi constraint conditions. This modeling method significantly improves the accuracy of the model and also improves the beauty of the model, as shown in Figs. 4, 5 and 6.

Fig. 4
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3D geological model of kangyangju area (including coordinate grid)

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Fig. 5
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Fold formation (Longtan Formation)

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Fig. 6
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3D section extraction

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The 3D geological modeling of Huayingshan kangyangju area in Guang’an City adopts the method of plane section automatic modeling. Combined with the idea of dynamic modeling, it can realize the local automatic updating of the model when the data source is updated, and it can also quickly update the model when the modeling rule algorithm needs to be updated. In the process of building the model, all the points, lines, surfaces and topological information have been saved. The updating and maintenance of the model is very difficult and fast. Later, the section information is added to allow local updating of the model, and also support rapid updating of the model as a whole when the modeling rules are updated.

Fold structure modeling method based on structural axis constraint

Section data is the most intuitive and effective modeling data source to express fold structure, but the effect of modeling fold structure only using section data as modeling data source is often not ideal. The main reason is that the sections are independent of each other, and there is no concept of structural strike between sections. When the computer interpolates to construct the ground plane, there is no constraint of structural strike. When only the boundary contour is used as the interpolation sample point for modeling, only the “saddle” ground plane will be constructed, and there is no fold structure at all.

Based on the analysis of the causes of the problems, this paper creatively puts forward that the structural axis is used as the constraint data for modeling, and the structural axis is used as the constraint data for modeling when it is used for modeling. The formation surface constructed directly has the morphological characteristics of fold structure, which is in good accordance with the knowledge of experts. See Figs. 7 and 8.

Fig. 7
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Comparison of fold modeling effects with or without structural axis constraints1

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Fig. 8
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Comparison of fold modeling effects with or without structural axis constraints2

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Model application direction

For geological professionals

By using the geological model, we can dynamically simulate the step excavation and the foundation pit excavation, form the foundation pit excavation model, grasp the geological conditions within the excavation scope, understand its suitability, and provide the basis for the foundation pit design and construction. It can simulate the excavation of underground tunnel, roam the tunnel, and understand the relevant stratum properties. It can extract the drilling information, query the stratum stratification, and predict the possible geological problems. Virtual drilling can also be created on the model to obtain the details of the strata at the virtual drilling, which can be used as a reference for project planning, scientific research and preliminary exploration, and save the drilling cost of the project.

Provide histogram

By drawing the drilling histogram, we can know the distribution of the underground bearing stratum and the distribution of the bad engineering geological phenomena, and reflect the change of the lithology of the vertical upper stratum. It can help to automatically generate drilling histogram, which greatly reduces the workload of professionals and improves work efficiency.

Provide profile

In the actual project planning and design, the construction technicians often arrange a group of exploration boreholes along the project line to understand the continuous change of the lithology of the vertical upper ground layer and check the distribution law of the bearing layer and the bad soil layer. The spatial analysis function of the three-dimensional model can directly and automatically generate the geological profile map and the three-dimensional geological profile map along the project line for the engineers and technicians For professional drawings.

On the premise of protection and rational utilization of geological resources, the utilization of geological resources should pay attention to comprehensive development, layered development and improvement of underground environmental conditions, etc. the previous planning has little consideration of geological factors. The evaluation and analysis function of the system can support to carry out suitability evaluation and obtain evaluation zoning map.

Facing government decision-making departments

In the planning of key projects, the government departments determine the area through the overall planning combined with the relevant geological conditions. The main consideration factor is economy, and the geological consideration is relatively less. There is also a lack of available tools and standards for consideration. A three-dimensional geological structure model of the study area is established, which can analyze the influence of geological structure on the construction and development of the area in a more comprehensive, intuitive and multi perspective way, quickly understand the geological situation of the planning project, evaluate the feasibility of the project, and provide basic engineering geological data for urban planning and construction and major project site selection According to this, to avoid decision-making mistakes in major project construction, the underground geological body and relevant engineering exploration data in the planning area can also be analyzed to provide results for reference and assist in project site selection.

Conclusion

Based on the comprehensive use of vertical geological data and horizontal geological data, as well as the fold structure modeling method based on the constraint of structural axis, it not only gives full play to the role of all geological data, but also improves the accuracy and beauty of the three-dimensional model, and conforms to the knowledge of geological experts.

This time, the plane section modeling method based on multi constraint conditions and fold structure modeling method based on structural axis constraint were used in the kangyangju area of Huaying Mountain, Guang’an City, with good effect, providing technical means and data support services for decision-making and deployment of government departments and professional application of relevant geologists.