I. Application Background
The significant impact of surface subsidence caused by mining operations has garnered increasing attention and extensive research. Similarity simulation experiments serve as a primary method for laboratory-based modeling of mining activities; however, traditional observation techniques—relying on physical or mechanical measurements—suffer from drawbacks such as cumbersome sensor installation, heavy workloads, and limited sampling points. Driven by societal development, technological advancements, and the deepening scope of research, there is an escalating demand for data acquisition methods that are simple, efficient, and comprehensive. Consequently, the introduction of a comprehensive and rapid observation method is of particular importance.
II. Research Objectives
Three-dimensional optical measurement technology, which combines photogrammetry with grating-based measurement, enables the rapid and accurate acquisition of data through non-contact means. The Industrial Close-Range Photogrammetry System (XTDP), developed by XTOP3D Technology (Shenzhen) Co., Ltd., supports mobile measurement, facilitating the rapid extraction of key points from complex curved surfaces and enabling full-scale inspections. Widely utilized in sectors such as automotive manufacturing, aerospace, shipbuilding, and construction, the system leverages 3D coordinates to perform quality inspections, deformation measurements, and reverse engineering.
This experiment integrates the XTDP system into the observation of similarity models. This approach not only expands the application scope of the XTDP technology but also provides a convenient measurement solution for similarity model experiments in mining research.
III. System Overview
The XTDP system is a non-contact industrial optical 3D coordinate measurement system capable of precisely capturing the 3D coordinates of discrete target points. As a portable, mobile optical measurement solution, it is suitable for quality control and real-time static deformation analysis of workpieces.
The XTDP system has a nominal single-point displacement accuracy of 0.03 mm to 0.1 mm and is capable of measuring flexible objects ranging from 0.1 m to 10 m in size. It achieves a relative accuracy of 1:70,000 to 1:150,000—equivalent to a measurement accuracy of 0.01 mm for a 1-meter-long workpiece. For a scale model with a 1:200 ratio, an actual observation error of 10 mm corresponds to a 0.05 mm error within the model; given the system's nominal accuracy of 0.01 mm for a 1-meter workpiece, it meets the precision requirements for monitoring such scale models. Consequently, the 3D optical measurement system shows great promise for applications involving the monitoring of scale models used to study mining-induced subsidence.
IV. Hardware Composition of the XTDP 3D Photogrammetry System
A complete XTDP system is utilized for this experiment. The XTDP system consists primarily of a hardware system for image acquisition and a software system for image processing and analysis. The hardware components include a DSLR camera, a flash unit, a calibration bar, coded targets, and uncoded targets, among others. The software system provides functions such as bundle adjustment, self-calibration, 3D reconstruction, and post-processing (as shown in the figure).
V. Experimental Scheme
Scaled physical modeling of coal mining faces is widely used to study strata behavior—such as ground pressure and strata movement associated with mining under buildings, railways, and water bodies ("three-under" mining). Traditional monitoring methods typically rely on physical or mechanical measurements, which are labor-intensive and cumbersome to install. In contrast, the XTDP system overcomes these drawbacks; it offers high precision, captures extensive data, and is convenient to use, enabling real-time monitoring and image-based recording of failure patterns with a high degree of automation.
This system is applicable to scaled models representing various coal seam inclinations; for this experiment, a model representing a horizontal coal seam was selected for analysis. The model scale is 1:200, simulating a mining face height of 4 m, a mining depth of approximately 180 m, and an extraction length of 400 m.
The XTDP system allows for the placement of monitoring points across multiple horizons, maximizing data collection and measurement value. As shown in the figure, eight layers of marker points (non-coded points) were installed at key horizons—including the surface, the topsoil layer, and interfaces between simulated strata layers—with a spacing of approximately 5 cm between adjacent markers within each layer; for thicker strata, markers were spaced at intervals of approximately 10 cm. Coded points were affixed to the external frame to serve as fixed reference points. Because the marker points are lightweight, they exert no additional pressure on the model and thus do not interfere with the model's inherent deformation behavior.
VI. Experimental Procedure and Analysis of Results
Once the model setup and marker placement are complete, a DSLR camera is first used to capture a series of multi-angle images of the model to establish baseline coordinates for the target points. Subsequently, after each excavation stage and once the model has stabilized, additional photographs are taken for comparison. Fixed points located outside the model serve as global control points for coordinate transformation across the multiple imaging sessions; this allows 3D coordinates—initially obtained in different local coordinate systems—to be converted into a unified coordinate system, thereby yielding the subsidence and horizontal displacement values of key points to support model analysis.
By extracting all coordinate points along a specific horizontal observation line from the processed results and comparing the coordinates of these points before and after excavation, the subsidence and horizontal displacement curves for that layer can be derived. These results demonstrate that using this industrial measurement system to monitor a similar-material model yields data consistent with established patterns of mining-induced subsidence.
VII. Conclusion
Compared to other methods, the XTDP system offers distinct advantages for observing deformations in models made of similar materials: it rapidly captures extensive physical and geometric data from the subject—making it particularly suitable for targets with numerous observation points—and functions as a non-contact measurement technique that exerts no additional influence on the model itself. Furthermore, digital photographs provide intuitive, reliable records that allow for verification at any time; they capture the displacement of any point at the moment of measurement and can provide motion trajectories for dynamic targets.
