The medium-to-large circular imaging features of various genetic types and their engineering geological significance

Annular images refer to image features such as circular/elliptical, butterfly-shaped, and unclosed arc-shaped patterns displayed in the form of tones and/or textures on remote sensing images of different resolutions, which are formed by endogenic processes, exogenic processes, or their combined actions. Annular images of different genetic types exhibit distinct implications for engineering geological stability. In this study, seven genetic types of annular images within the scale ranging from kilometer-level to 50 km were selected as research objects, including those formed by fault activity, magmatism, the combined action of faults and magmatism, domes, salt dome diapirs, karst and underground collapse, and weathering. The key study areas were designated as regions with typical annular images, such as the interior and surrounding areas of the Qinghai-Tibet Plateau, the Tianchi Volcano in the Changbai Mountains, and the Xiangshan Volcanic Basin in Jiangxi Province. Utilizing optical satellite remote sensing image data with spatial resolutions of ten-meter, meter/sub-meter levels (e.g., Landsat-8/Sentinel-2, GF2, and Worldview2/3), and based on remote sensing identification indicators (spectral features, morphological features, micro-geomorphic features, and drainage system combination features) as well as specialized remote sensing image processing results, the characteristics of the above seven genetic types of annular images were elaborated.On this basis, their respective roles in engineering geological stability were analyzed during the preliminary site and route selection of proposed large-scale projects, as well as the safe operation period of existing projects. The results are as follows: (1) For annular structures of endogenic origin, such as those formed by magmatic intrusion, domes, and diapirs, the internal engineering geological stability is relatively favorable. During site and route selection, priority should be given to intact granite interiors, and hard rocks within the caprocks of domes and diapirs; (2) Endogenic-related annular structures including annular faults, rotational annular structures, and volcanic edifices are often weak zones of crustal rocks, exerting negative effects on engineering geological stability; thus, they should be avoided as much as possible during site and route selection; (3) During the site and route selection of large-scale projects, it is advisable to keep a safe distance from regional active faults that transversely cut through or are tangent to annular structures, or to align the project route parallel to such faults if unavoidable. For existing large-scale projects constructed near active faults, temporal deformation analysis of regional fault activity should be conducted based on large-scale active fault mapping results and GPS station observation data; (4) Among exogenic processes, special attention should be paid to the destructive effect of collapses caused by underground karstification on deep-buried tunnel projects. For annular images formed by complex endogenic processes (e.g., multi-stage magmatic intrusion, magmatic eruption or effusion to the surface), complex exogenic processes (e.g., collapse caused by underground karstification, differential weathering of surface lithology), or the combined action of endogenic and exogenic processes (e.g., multi-stage magmatic intrusion/eruption coupled with fluvial erosion, glacial erosion/abrasion), on the basis of determining their approximate spatial distribution, it is necessary to integrate multi-disciplinary knowledge including remote sensing geology, geomorphology, structural geology and geophysics, as well as multi-source data acquired by technical means such as remote sensing, geophysics (geophysical exploration), drilling, ground-based radar detection, time-series SAR deformation monitoring and field investigation, so as to accurately analyze the origin, formation stages and spatial distribution range of the annular images, which can provide accurate spatial information support for auxiliary decision-making of the scientific and reasonable site selection and route planning of large-scale projects in complex and harsh regions, as well as the formulation of optimal safety protection measures.