Creation of virtual geological outcrops of the outskirts of Lviv
Short description of the teaching package.
Geology of the city of Lviv and its environs
Fig. 1 Geological map of Lviv city
Tectonically, the city of Lviv and its environs is located within the southwestern edge of the Eastern European platform, namely within the Podil plate, which geomorphologically corresponds to the Lviv plateau. The geological structure of Lviv was studied by A. Alt, M. Lomnytsky, O. Vyalov, V. Goretsky, L. Kudrin, I. Kruglov, O. Kruglov, P. Voloshyn and others.
Within the platform there are two structural elements: the crystalline basement – represented by ancient Archean and Proterozoic sediments, the cover – represented by Paleozoic formations (Cambrian-Lower Silurian, Upper Silurian, Devonian, Lower, Middle Carboniferous, Mesozoic and Tertiary), which are very gently immersed to the southwest under the Carpathian folded-covering structure.
The rocks of the crystalline basement lie at a considerable depth, so they are poorly studied, opened by parametric wells (1-Brody, 1-Sushne, 1-Gorokhiv, 1-Berestechko) at depths of 2836–4201 m. Insufficient amount of factual material does not allow to thoroughly cover the composition and conditions of formation of the crystalline basement in the region in the Archaean and Early and Middle Proterozoic. We will describe in detail the geological structure, based on what deposits are exposed within the city and its environs (Fig.).
The geological structure of Lviv and its environs involves deposits of Cretaceous, Neogene and Quaternary systems of different ages. Different parts of Lviv differ significantly in the completeness of the sections, the sequence of layers, thickness, rock composition, age, and the presence of fossil remains (Fig.1). All deposits lie horizontally or close to horizontal. The youngest – Quaternary deposits,cover of almost the entire territory and play an important role in the formation and development of modern landscape systems.
The formations of the Cretaceous system are represented by the terrigenous-carbonate formation of the epicontinental shelf. On the territory of the city of Lviv and its environs there are Upper Cretaceous deposits, namely the Maastricht tier. The most common in the northeastern part of the city and its environs, where they are exposed in the lowest parts of streams, ravines. The sediments are represented by dense marls of light gray color with a brownish tinge, sometimes cracked (abandoned quarry on the Glynyansky Tract, the area of the Honey Cave). Marls have a pelitic structure with an admixture of psammitic (up to 50%).
The thickness of these deposits is unknown here, because the well drilled in 1894 in Stryj Park to a depth of 501 m did not come out of the Upper Cretaceous marls. Comparisons with the wells of neighboring districts suggest that in Lviv the rocks that line the chalk lie at a depth of 800-900 m.
The formations of the Neogene system are quite widely developed in the study area and are represented by Miocene Baden deposits. The Baden region consists of sediments of different ages, which correspond to different facial complexes. They occur with stratigraphic inconsistency on the blurred surface of the upper chalk. These deposits can be observed within the main watershed, namely in the outcrops at the foot of Pishchana, near the street. Honey Cave, in the area of Pohulyanka. They are represented by dense sandy limestones with deposits of fossilized lithotamnium algae.
Virtual outcrops using laser scanning
Virtual outcrops using digital photogrammetry
In this teaching package data and workflow for creation virtual outcrops are presented for two chosen objects.
First is Honey Came in Lviv and second one is natural outcrops of Badenian sandstones in territory of Lviv (Vinnytsia Str).
1 step – Selection of an object for surveying
2 step – Selection of the methods for the surveying (Laser scanning or Digital Photogrammetry).
3 step – Checking the equipment
Workflow of terrestrial laser scanning
The generally accepted technological scheme of shooting and creation of digital models of objects is shown in Fig.2
Fig.2 Technological scheme
Creation a technical project. At this stage of the project is determined by the accuracy of construction of a three-dimensional model or plan, their content and detail, the format of saving scan data, estimates of work.
Reconnaissance of the area. Taking into account the conditions of the object under study, choose the location of special brands, specify the time of work, outline the location of the scanner.
The number and location of scanning stations are designed based on the requirements to ensure the accuracy of the final product.
The choice of scanning stations directly depends on the goal. It should reflect the maximum area of the study area on one scan.
3D terrestrial laser scanning. Work on the scanning station includes the following stages:
1) installation of the scanner at the projected point on a tripod at a height that will provide maximum coverage of the studied objects on one scan;
2) placement of the scanner of special brands, which are the points of the working survey substantiation. Special brands are shown in Fig. 1.11. Flat marks are used to orient scans relative to the external coordinate system. Volume marks are used for stitching scans;
3) obtaining the coordinates of the centers of the marks from the points of the main reference network. Determining the accuracy of the construction of the working justification is performed by multiple measurements of the centers of special brands or determining the coordinates of the same brands from different points of the main survey.
4) scanning of the studied objects around the scanner. Provided that the scanner is accompanied by a digital camera, then digital shooting is carried out;
5) determining the approximate coordinates of the centers of special brands for further rapid obtaining of their location on the scan. These operations are performed on the basis of a scan or digital image.
6) scanning of special marks with the maximum resolution that allows to define their coordinates with the maximum accuracy for the scanner model;
7) go to the next point of scanning and performing the operations described in steps 1-6.
Terrestrial laser scanning data processing. TLS management and pre-processing of research results is carried out using a software product of the company that produces scanners. This process begins with data filtering. At this stage, points that do not belong to the subject are removed. To do this, use different filters – to eliminate noise, averaging data, low-frequency and high-frequency and filtering.
The next step is the external orientation of single scans to bring them into the appropriate coordinate system.
Evaluation of the accuracy of the external orientation of the scans is performed using the root mean square error of the unit of weight, calculated using algorithms that are implemented in the software.
Export of externally oriented scans. Because there are software products that use an internal format (extension), you must export the data from the program that controls the scanner. The result is a new file that displays a single point model.
2.Scanning the Honey Cave
Fig. 3 Location of the Honey Cave, Lviv city
The following equipment was used to conduct research on terrestrial-based laser scanning of the Honey Cave:
Set of GNSS-receivers Trimble R7 (Fig.4.). Technical characteristics of the equipment are given http://www.eacampos.pt/fotos/editor2/topografia/sistemas_gps/r7gnss_ds_0213_lr.pdf
Fig. 4 Trimble R7 GNSS Receiver Kit
Fig. 5. Faro Focus 3D 120 terrestrial-based laser scanner
Technical characteristics of Faro Focus 3D 120 are given https://onesurveying.com/3d-laser-scanner/faro-focus-3d-s120-laser-scanner
Fig. 6. Volume brand
Laser scanning of the Honey Cave chambers was carried out using a Faro Focus 3D 120 scanner. (Fig.5)
Fig.7 Scanning of the Honey Cave.
Measurements were performed from 4 points using 6 reflector marks to combine individual scans into a single cloud of coordinated points. In addition, the coordinates of some brands were determined in the terrestrial coordinate system, which allowed to transform the obtained point 3D model of the Honey Cave into this coordinate system.
Further processing of the data obtained by laser scanning of the cave was performed using software.
We used Faro Scene 5.0 software to sew the obtained scans.
We carry out the following actions in the software:
– download object scans;
– denote flat marks and spheres;
– register scans on certain points.
Upon registration, the program automatically connects the marked brands. As a result, we obtain point clouds in a single coordinate system, which is a point 3D model of the object under study. For best results on the accuracy of combining scans, you should analyze the accuracy of their combination for each brand.
Fig. 8. The results of the analysis of the accuracy of the connection of brands
The obtained accuracy of scan registration is 0.0035 m. The maximum deviation between flat marks is 0.0096 m, the minimum is 0.0012 m. The maximum deviation between the volume marks is 0.0086 m, the minimum – 0.0005 m.
Fig. 9. Point model of the Honey Cave. You can see the location of the markers
The resulting cloud of points describing the internal structure of the Honey Cave will be used to build a number of models.
Laser scanning data processing and construction of a 3D model of the Honey Cave using Move software
Further constructions of Honey Cave models were carried out using Move software, developed by Midland Valley, a world leader in software for structural geology and geological mapping. Among the huge number of functionalities, we used a 3D block and a block to build sections.
First of all we import the model of a cloud of points created as a result of laser removal, the Model can be presented in many formats. We use * .obj. It is most convenient to present it in the form of a textured model. It should be remembered that the new project must have the same coordinate system as the model.
Since we are exploring the cave, the surface, which is located on all sides, it is convenient to divide the model into two parts, such as top and bottom. It will be convenient also at visualization.
Fig. 10. The lower part of the Honey Cave model is loaded into the 3D module of the Move program
Similarly, load the upper part of the model.
To build the contour of the Honey Cave, select the Map View function.
Fig. 11. The upper part of the Honey Cave is loaded into the Move program
As we can see, these models represent only the surface of the cave. We use two approaches to show the internal structure. The first – “cut the model” into two parts. See Figure 11.
Fig. 12 The structure of the bottom of the Honey Cave
The second way to represent the internal structure of the cave is to build a section. To do this, use the Section tool located in the 3D module Fig. 13.
Fig. 13 Intersection through the Honey Cave, along the chosen direction.
To show the internal structure, we use the Creation Multiply Sections function, which allows to build a series of sections along selected directions at the same time. We can choose not only the number of sections, their location, but also the distance between them. Usually the planes of these sections are oriented vertically, but if necessary, they can be placed at any angle. In fig. 2.26 shows the location of the sections of the Honey Cave, and Figure 14 the same sections.
Fig.14 Location of sections of the Honey Cave
Fig.15. The internal structure of the Honey Cave, shown as a series of sections.
Based on the obtained model, a detailed plan of the cave was built, using the Map block. It has a large set of tools for cartographic purposes. For example, you can select the interval of horizontals, their parameters, and so on Fig. 16.
Fig. 16. Honey Cave Plan.
The model described above will not be complete if the surface under which the cave is located is not shown. To show the surface, we used the results of aerial photography, which was conducted by a group of researchers from the Institute of Geodesy. The resulting surface model was loaded into the Move program to an existing Honey Cave model. On the basis of these models the section on which the surface over a cave is already shown is again constructed, Fig. 17.
Fig. 17 Cut through the Honey Cave and the surface under which it is located
The results can be used by geologists, geomorphologists, speleologists for further research. During the work, the high efficiency of works, their speed and quality were demonstrated. We recommend using this approach to study other caves in Ukraine. The results of the work can also be used during excursions and in the educational process for students of higher educational institutions majoring in Earth Sciences.
For data to build the presented model, please contact
As.Prof. Ihor M. Bubniak email@example.com