中国岩溶

Structure and application of SWAT-MODFLOW coupling model for surface-groundwater

LIU Wenchong, ZHAO Liangjie, CUI Yali, CAO Jianwen, WANG Ying, LI Meiling

2023, 42(6): 1131-1139. doi: 10.11932/karst2023y014

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Surface water and ground water is a unified whole in the water circulation system, and there is a close relationship between them. Especially in the areas where karst surfaces such as drop holes and funnels are developed, surface water and groundwater can form a direct connection. The interactions between surface water and groundwater have been a hot topic in research. A great deal of research has been carried out both nationally and internationally. The SWAT-MODFLOW coupling procedure developed by Seonggyu park and Ryan T. Peley is a beneficial tool for studying surface-groundwater interactions. This coupling procedure establishes the link between the Hydrological Response Unit (HRUs) in SWAT model and the spatial grid in MODFLOW through the ARCGIS platform in order to achieve a loose coupling of the model. A number of applications of this program have been carried out abroad. However, no relevant studies have used this procedure to achieve model coupling in different ranges of surface and subsurface. As far as the actual situation is concerned, often the plains are the focus of extraction and their groundwater dynamic field needs to be detailed delineated. However, the scope of SWAT model covers the overall basin, which is not consistent with the scope of groundwater flow model. Considering the running time and the accuracy of the model, it is necessary to explore the model coupling process in different ranges of surface-groundwater. In this study, the SWAT-MODFLOW coupling program of Seonggyu park and Ryan T. Bailey was used to implement the coupling of models at different scales of the surface and subsurface, and to investigate the differences between the groundwater recharge calculated in SWAT and that calculated in the MODFLOW grid output of the coupling program. Then the advantages of the coupling program for relevant studies on surface water and groundwater were analyzed. Taking this coupling procedure model of the Little River Watershed (LRW) in southern Georgia, USA as an example, this study selected Sub-basin 104 divided by the SWAT in the model as the boundary, and built the groundwater flow model with GMS10.4 based on the data of the original example model. According to the coupling model manual, ARCGIS platform and EXCEL platform, four link files required by the coupling model were completed: swatmf_dhru2grid, swatmf_dhru2hru, swatmf_grid2dhru, and swatmf_river2grid. A coupling model of Sub-basin 104 water flow model and the overall SWAT model was further developed, and the format of the model output results was controlled through its link files. On the basis of calculation results of the model, its multi-year equilibrium condition was calculated in EXCEL to judge its rationality so that the model can be calibrated. After the calibration, the accuracy of each equilibrium item was compared between the coupling model and the independent groundwater flow model. Study results show as follows. (1) The coupling procedure enables the coupling of the SWAT model naturally bounded by the surface watershed with the small-scale MODFLOW model divided by the sub-basin boundary. However, because the grid boundary of the groundwater flow model and that of sub-basin cannot be completely matched, there is a difference between MODFLOW and SWAT in terms of the calculation of rainfall recharge for groundwater. In general, the calculation volume of SWAT is larger than that of MODFLOW grid. The calculations clearly indicate that the error margin becomes smaller as the grid gets smaller because the smaller the grid area is, the more exact the match between the grid and the boundary becomes. (2) Each equilibrium item has changed after coupling. Because the river depth in GMS is taken as the empirical value, and it is taken as the confluence evolution value in SWAT, calculations show that the groundwater recharge of the river before and after the model coupling is significantly different, and the total recharge from the river to the groundwater reduces to 15.25% of that before the coupling. On the other hand, the total discharge from groundwater to river increases 19.29% after coupling. The rainfall recharge of the model before coupling is calculated by infiltration coefficient method, and the groundwater recharge after coupling is the seepage of the soil bottom of the SWAT model. The calculations show that the total rainfall recharge increases 17.07% after coupling. The groundwater evaporation before model coupling is calculated by the Avyanov formula, and the groundwater evaporation after coupling was calculated according to the potential evapotranspiration and the actual evapotranspiration calculated by SWAT. The calculations show that the total evapotranspiration is 3.08 times larger than that before coupling. It is found that the coupling model can simulate the surface-subsurface hydrological process more accurately, and can reflect the relationship between precipitation and groundwater, surface water and groundwater transformation.

Research on water flow attenuation process and regulating capacity in karst vadose zone

ZHANG Yibo, WANG Jinguo, LIU Ruitong

2023, 42(6): 1140-1148. doi: 10.11932/karst20230601

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The karst area in China accounts for about 20% of the total land area. The karst groundwater is rich in resources and is an important source of production and living water in karst areas. The annual average rainfall in karst areas of Southwest China exceeds 1,200 mm, but the time distribution of rainfall is extremely uneven, which is likely to cause drought and flood disasters, and hence will seriously restrict the production and life of local people. The karst vadose zone has the function of conserving and regulating groundwater. Therefore, it is of great significance to study the influence mechanism of water flow attenuation process and regulating capacity in the karst vadose zone for rational development and utilization of karst water resources.The high heterogeneity and the anistropy of the karst aquifer system often make the karst vadose zone form a drainage network and become a channel for groundwater. A karst vadose zone is mainly composed of soil zone, epikarst zone and transfer zone, of which the soil zone and the epikarst zone are main water storage space. The water storage function of karst vadose zone is related to the dynamic regulation of groundwater resources in the epikarst zone. Rainfall infiltration is transferred from the transfer zone to the phreatic aquifer through the regulating and diversion of the epikarst zone. In the karst vadose zone, the low flow velocity in diagenetic pores, micro fissures and carbonate rock matrix shows its feature of slow flow, while the fast flow velocity in large fissures and sinkholes is characterized by rapid flow. Therefore, the heterogeneity of karst aquifer system and the complexity of runoff path in karst vadose zone increase the difficulty of the study on groundwater flow process.In previous laboratory tests on water flow attenuation process and regulating capacity in karst areas, a two-dimensional fissure-conduit model built of polymethyl methacrylate was mostly used. Although it can simplify the structure of aqueous medium, it is quite different from the actual structure, and the soil zone is not considered. Meanwhile, the current studies on the regulating capacity of karst areas are mainly focused on the epikarst zone, and the studies on the influence of the transfer zone on the regulating capacity are very few. In this study, a laboratory-scale three-dimensional model of karst vadose zone was built with limestones. The attenuation curve was analyzed by superposed exponential decay equation, and the regulating coefficient was calculated by formula. The influence of rainfall intensity, fissure development degree and thickness of transfer zone on the attenuation process and regulating capacity of water flow in karst vadose zone were explored.In this study, three tests were set up to explore the influence of the above factors on the attenuation process of water flow and the regulating capacity in karst vadose zone. In Test 1, five control groups with different rainfall intensities were designed, including 5 mm·h⁻¹, 10 mm·h⁻¹, 20 mm·h⁻¹, 30 mm·h⁻¹ and 50 mm·h⁻¹, and the attenuation coefficient and regulating coefficient under the corresponding rainfall intensity were obtained respectively. The analysis of test data shows that the greater the rainfall intensity is, the faster the attenuation speed is, and the influence on the attenuation coefficient of fast flow is greater than that on the attenuation coefficient of slow flow. When the structure of karst vadose zone is fixed, the regulating capacity gradually decreases with the increase of rainfall intensity, but this effect is limited: when the intensity increases to a certain extent, the regulating coefficient is basically unchanged. In Test 2, the number of fissures in the transfer zone is designed as 1, 2, 3, 4 and full fissures. It is concluded that the attenuation speed of fast flow accelerates and the attenuation speed of slow flow slows down with the increase of fissure development in the transfer zone. The data also demonstrates that the higher the fissure development is, the weaker the regulating capacity of the aquifer system is. In Test 3, five groups of control tests with different thicknesses of transfer zone were designed, and the thickness increased from 40 cm to 120 cm, with a difference of 20 cm between each group. Results indicate that the increase of the thickness will lead to the decrease of the attenuation coefficient and the increase of the regulating coefficient, but the influence of thickness on the latter is not very obvious.

Analysis of water environment dynamic characteristics and travertine deposition capacity in Huanglong core scenic spots

LIU Xinze, ZHANG Qingming, SUN Dong, TANG Shu, TIAN Changbao, HUANG Heping, ZHOU Yaping, XIONG Yuxia, LIANG Xinyu, FAN Jiajun, FAN Ming, CHEN Hongwei

2023, 42(6): 1149-1160. doi: 10.11932/karst20230602

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The Huanglong scenic area, a World Natural Heritage, is located in the east of Songpan county, Aba Xizang and Qiang Autonomous Prefecture, Sichuan Province. It is a typical landscape plateau of cold-water travertine. Water in travertine landscape, as the most important carrier, controls the formation and evolution of travertine. Yuan Daoxian, Liu Zaihua, et al., from Institute of Karst, Chinese Academy of Geoscience, studied the source, sedimentary mechanism and influencing factors of the sedimentary process of travertine, which laid the foundation for the geological study of Huanglong travertine. In recent years, affected by the increasing human activities, the water environment in the Huanglong scenic area has deteriorated, resulting in local degradation of travertine landscape. Therefore, this study has been conducted to effectively assess the hydrochemical environment and the deposition capacity of travertine in Huanglong core scenic spots, and to analyze the change characteristics of hydrochemical field in these spots, thus providing corresponding support for the subsequent research on technology of travertine landscape conservation. The research area covers all the core scenic spots of Huanglong, from Zhuanghua pool to Fujiang river. In this study, field investigation, water quality monitoring, flow monitoring and analysis of historical monitoring data were used to evaluate the water environment and explore its influence on travertine deposition. A total of 14 monitoring sites covers all springs and surface water related to the water cycle. The indexes such as pH, temperature, Ec, Ca²⁺ and HCO$_3^{−}$ were measured on site. Other indexes of water quality such as PO$_4^{3-}$, Cl⁻, SO$_4^{2-}$, NO$_3^{−}$, NO$_2^{−}$, NH$_4^{+}$, K⁺, Na⁺, Mg²⁺ and Ca²⁺ were collected and sent to the laboratory for testing according to requirements. The rainfall and water quantity data before 2017 were provided by Huanglong Scientific Research Office, and the data from 2018 to 2021 were obtained by the authors through long-term monitoring. Research findings show: (1) The Zhuanhuachi springs are the main calcium and carbon source of travertine deposition in the area. They, together with surface water in Huanglong valley, constitute the water source boosting the development and evolution of travertine landscape. The main sources of water recharge for Huanglong valley and Zhuanhuachi springs are rainfall and meltwater of snow. (2) Since 1999, the flow rate of Zhuanhuachi springs has increased steadily and then to the maximum. The water quality is extremely stable with average pH of 6.6, average water temperature of 6.7℃, average Ca²⁺ of 208 mg·L⁻¹. The SIc is generally in an unsaturated state. The historical maximum water volume in the past 30 years appeared in 2020, with an average daily water volume of 9,998.09 m³·d⁻¹ from April to November. The water volume has experienced a slow downward trend since 2021. From 1999 to 2017, the annual average daily water volume of Zhuanhuachi springs was 7,817 m³·d⁻¹, and the annual variation of the average daily water volume was about ±1,500 m³·d⁻¹. The annual average daily flow from 2018 to 2020 was 9,368 m³·d⁻¹. (3) In 2018, linear TOC anomaly zones appeared in the area of Zhuanhuachi springs, and the main anomaly points occurred at the spring mouths of No.1, No.2, No.4 and No.6. The main reason was the grazing activities from Sandao lawn to Toudao lawn in the upper reaches of Huanglong valley. The manure produced by cattle contained a large amount of organic carbon (about 15%), which dissolved in water under the effect of rainfall leaching. As the water circulation entered the groundwater and migrated to the area of Zhuanhuachi springs, the deposition rate of travertine decreased. The TOC anomaly approximately disappeared at the north of the Zhuanhuachi springs. (4) The core scenic spots of Huanglong experienced rapid degassing from upstream to downstream. SIc increased significantly, and the concentration of HCO$_3^{−}$ decreased considerably in the first cycle. But there was little change in the north of Yuye spring. The decrease of Ca²⁺ can be divided into two stages. In the first cycle section (from Zhuanhuachi springs to Yuye spring), the concentration of calcium ions decreased obviously, and there were more travertine deposits. The deposition rate of travertine in the lower reaches of Yuye spring was stable, and the ions concentration decreased linearly. (5) Landscape water in each water cycle segment in the area still shows a certain depositional capacity, because the dilution of heavy rainfall reduced the depositional capacity of travertine. The SIc value decreased in 2019. The water environment of Huanglong core scenic spots is stable with a good capacity of travertine deposition, which maintains spring overflow, strong degassing and rapid deposition. Influenced by animal husbandry and rainfall, the deposition capacity of travertine decreased slightly. It is suggested to strengthen the protection for the conservation area of travertine spring source and reduce the influence of grazing activities on it.

Evaluation system for the suitability of reservoir construction for pumped storage and optimization of its site selection in karst depressions

ZHANG Tao, ZUO Shuangying, SHEN Chunyong, ZHENG Kexun, CHEN Shiwan, ZHANG Qing, LEI Lin, WU Zongqin

2023, 42(6): 1161-1172. doi: 10.11932/karst20230603

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In the planning policy on modern energy system in China's "14^th Five-Year Plan", it is proposed to accelerate the improvement of system for energy production, supply, storage and marketing, and to promote the large-scale and high-proportion development of renewable energy, so as to achieve the goal of "carbon peaking and carbon neutralization". Pumped storage is currently recognized as the most mature, reliable, clean and economical means of energy storage. However, the site selection is imminent in order to construct power stations for pumped storage. Taking Guizhou Province as an example, the current planning of site selection and the construction technology of power stations for pumped storage are seldom studied. Karst is distributed in more than 77% of Guizhou's land area, with the peak-cluster depression as the main geomorphological type. As a kind of natural negative terrain, the peak-cluster depression has hardly been excavated. But its excellent surrounding sealing, and wonderful geometric and engineering characteristics can save much investment in excavation support. Thus, the peak-cluster depression is an ideal place for building a reservoir. Taking the karst depression as a research object, this study focuses on the practical engineering problem of building a power station for pumped storage in the karst depression. The factors and their degrees affecting the suitability of reservoir construction in the depression are obviously different in different regions; therefore, some typical factors can only be selected as the evaluation indicators. These factors involve those having an important impact on the stability of the reservoir area, the leakage problem and the benefit of reservoir construction.In this study, samples of karst depressions were automatically extracted, and the attribute analysis and calculation were carried out based on DEM data and ArcGIS platform. According to the engineering practice, experts were organized to discuss and score so as to determine the influence factors of the evaluation model. Then the evaluation was conducted respectively in terms of terrain conditions, geological conditions, engineering conditions, environmental conditions, etc. The model of fuzzy comprehensive evaluation and the model of partial least square path have been constructed based on the seven key influencing factors of the decision-making of reservoir construction, including slope, reservoir capacity, petrofabric type, fault, hydrogeology, vertical distance between the upper and lower reservoirs and the ratio of distance to height. Taking the red line of ecological protection and the autothermal protection area as controlling variables, 488 candidate depressions in the study area were preliminarily screened and sorted in combination with the geological conditions of the supporting site. The results show that vertical distance, storage capacity and distance-height ratio play key roles in site selection. The application of GIS map overlay technology can eliminate the infeasible sites limited by environmental factors and the candidate depressions in the eco-environmental protection area. The geological conditions of the proposed lower reservoir and water conveyance power house were considered in the selection of the optimal candidate depressions, which effectively avoided the subjectivity and limitations of decision makers. A total of 6 of the top 10 selected depressions by the two models are consistent. Their good topographic conditions, suitable geological conditions, superior engineering conditions and good consistency indicate strong applicability and high reliability of the evaluation model. The two models are mutually matched. The depression selected is basically consistent with the quantitative standard of the index system, which has been well verified in engineering practice. No.1 depression of Baishui pond is the most suitable for the construction of power station for pumped storage. The research methods and results can further promote the resource utilization of karst depressions.

Characteristics of karst development and water abundance of the Beiya Gold Deposit in Heqing county of Yunnan

HE Xiang, DONG Xuelan, YANG Chao, LIU Peng, XUE Boqiang

2023, 42(6): 1173-1182. doi: 10.11932/karst2023y025

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The Beiya Gold Deposit is a typical karst water-filled deposit. As the hydrogeological conditions and water environment are expected to be changed by future deep mining, the water inrush risk will increase if the roadway is exposed or connected with the karst zone, the underground river and Guochang river. In this study, comparing the field investigation of regional hydrogeology and the geological exploration data with the actual production, we analyze the karst development and water abundance characteristics from the aspects of lithology, structure, landform, etc. The study results show as follows. (1) The karst forms mainly include stone forest, drainage cave, karst depression, solubilization trough, solubilization gap, karst cave and underground river. (2) Lithologically, karst in the second segment of the Middle Triassic Beiya Formation is the most developed, with the purest fine crystalline limestone containing 41.52%–54.77% of CaO. Karst development in the first segment of the Beiya Formation is restricted by limestone and siltstone. Karst is developed in the contact zone on the side of the second segment, while not developed on the side of the first segment. (3) Karst development is mainly controlled by structure, and the direction of karst development is consistent with the NS, EW and NW tectonic lines. Karst is more developed in the fold axis and the warped end, and is relatively developed in the zone affected by fault and the contact zone of rock mass. (4) Karst development shows the characteristics of vertical zonation. Funnels and sink holes are developed at the level of 2,500 m to 3,500 m, grikes and solution grooves at 1,800 m to 2,500 m. Slope valleys, karst caves and underground rivers are developed at the level of 1,694 m, while karst is still relatively developed below the base level of erosion. The lowest control elevation of the cave is 1,479.54 m, and the depth of karst development below the base level of erosion is more than 150 m. (5) The karst water abundance shows its synclinal confluence pattern. The Beiya synclinal water storage structure plays a key role in karst water enrichment. The zone affected by fault and the contact zone between rock mass and its surrounding rock are also conducive to groundwater migration and enrichment. (6) The spatial variation of water abundance in karst aquifer is large. From rock mass as the center to its outward direction, radial water abundance and permeability can be listed in the order of weak-medium-strong or extremely strong. The rock mass and its proximal annular ore-bearing alteration zone shows weak-medium water abundance, while the distal end shows medium-strong water abundance. Some parts of the area are extremely strong in water abundance. (7) The underground river is developed along the axis of the Beiya syncline at the level of 1,740 m to 1,694 m. The Beiya Formation is composed of sandy limestone. The possibility of karst cave or water inrush of the underground river will increase if the mining passes through the fault zone and the syncline core.

Equilibrium analysis and prediction evaluation of the Heilongtan groundwater system in Lijiang of northwestern Yunnan Province

REN Shichuan, YANG Xiaoyan, YANG Yingbin, LIU Haifeng, YANG Fan

2023, 42(6): 1183-1192. doi: 10.11932/karst20230604

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The karst water system is a dynamic system controlled by the degree of karst development and the underlying geological conditions. It constantly evolves with changes in climate, human activities, and many other factors. As one of the main driving forces of environmental change, human activities have caused the change at the rate and intensity far exceeding that of natural succession in a short period of time. The environmental effects and induced hydrogeological problems resulting from human activities are extremely difficult to distinguish and quantify, and hence have been the working focus of local governments and scholars at home and abroad. The Lijiang basin is a typical covered and semi-covered karst water system developed in the strong uplift zone of the late Cenozoic crust, which has given birth to the Heilongtan spring, the Xiguanlongtan spring, the Jiudinglongtan spring and other karst springs (spring groups). These springs are not only important water sources for Lijiang City, but also the city's highlights of tourism resources. Among them, the Heilongtan groundwater system is the most important karst water subsystem in the groundwater system of the Lijiang basin. However, the primary discharge point of this system, the Heilongtan spring, has currently experienced a significant reduction in flow and increasing intermittent drying up, which have become important factors restricting local economic and social development. Based on the comprehensive analysis of data over the past 70 years, the investigation of hydrogeological conditions in the Lijiang basin area, and the dynamic monitoring of the Heilongtan groundwater system, we quantitatively evaluated the dynamic flow change of and its influencing factors on the Heilongtan groundwater system with the method of water equilibrium. Fully considering the impact of human engineering activities in socioeconomic development, we summarized factors for the drying-up of the Heilongtan groundwater system as follows. (1) From 1951 to 1990, human engineering activities that replaced and reduced regional vegetation increased natural evaporation capacity and continuously increased evaporative discharge by 10,827,400 m³, accounting for 87.00% of the total decrease in the resources of the Heilongtan groundwater system (12,445,400 m³). This is the main reason for the drying-up probability and the increasing span of Heilongtan spring. (2) From 1991 to 2018, with the recovery of vegetation, the evaporative discharge continuously decreased by 18,303,400 m³, compared with that in the 1980s, which was beneficial to the recovery of Heilongtan spring. However, the factors such as human mining, leakage from the surface river system, and urban development and rereconstruction have caused a total decrease of groundwater resources by 50,586,900 m³, which is unfavorable to the recovery of the Heilongtan spring and is also the cause for its drying up. Among these causes, urbanization and human groundwater extraction reduced groundwater resources by 45,805,200 m³, which accounted for 90.54% of the overall decrease and was another primary cause of the drying-up. According to the change of influencing factors of the Heilongtan spring in recent years, the future changes of its flow predicted by the method of water equilibrium shows that: (1) From 2021 to 2030, the Heilongtan spring will change into a seasonal spring and become drying up during median water year and low flow year. (2) After 2030, the Heilongtan spring groups will remain drying up for a considerable amount of time with a very slim chance of reproduction. This study research can serve as a foundation for the dynamic recovery of Heilongtan groundwater, and the development and environmental protection of the groundwater resources in the urban area of Lijiang.

Characteristics of temperature field of the Heilongtan groundwater system in Lijiang

CHEN Xin, ZHENG Kexun, HAN Xiao, TIAN Mao

2023, 42(6): 1193-1201. doi: 10.11932/karst20230605

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The groundwater temperature is an important indicator of groundwater dynamic change. Therefore, comprehensively monitoring groundwater temperature, understanding its temporal and spatial distribution characteristics, and studying its change rule can not only reflect the hydrogeological characteristics of groundwater system, but also provide a basis for its identification and division. With an elevation of 3,500 m–2,400 m, the Heilongtan groundwater system of Lijiang, Yunnan Province is a typical mountain and basin landform with wide distribution of soluble rocks, strong development of karst, and combination of faults and folds. Its hydrogeological environment is complex, challenging the delimitation of spring area and groundwater system. Because the study area is located in the plateau mountainous area, and the temperature field of karst groundwater can also reflect the variation of groundwater temperature at different elevations in the mountain and canyon areas on the plateau, this study has a great referential significance for the research of groundwater in other similar areas. Based on previous research results as well as a large number of monitoring data on groundwater temperature obtained in the study area from June to November, 2018, the spatial-temporal variation characteristics of groundwater temperature in the spring area and the variation characteristics of borehole water temperature were compared and analyzed, and the indicative significance of these two types of variation characteristics were discussed. Meanwhile, the feasibility of system division of the Heilongtan spring area was verified. The results show that there is a good spatial correlation between the elevation of groundwater level and temperature, and the groundwater temperature in the recharge area at high altitude is low. From the recharge area to the discharge area, the water temperature experiences a gradual upward trend. The water temperature in the discharge area can also indicate the circulation of groundwater runoff. The lower the water temperature in the discharge area is, the smoother groundwater becomes. In terms of time, as groundwater receives a large amount of rainfall recharge during the flood season, the circulation of groundwater flow increases, which may result in a short storage time of water and the phenomenon that the groundwater temperature in the discharge area is closer to that in the recharge area and that the overall water temperature decreases. At the end of the flood season, as rainfall decreases, the circulation of groundwater flow weakens, and the underground water temperature in the discharge area rises in different degrees. In a certain depth below the surface, the groundwater temperature does not completely conform to the theoretical law that the temperature rises gradually with the increase of depth, On the contrary, the groundwater temperature will remain unchanged or decrease gradually with the increase of depth, the reasons of which can be summarized as follows. (1) The temperature of surface water formed by precipitation is higher than that of groundwater, and the groundwater will be heated during infiltration and recharge. (2) When a deep karst pipeline is developed, the groundwater will "wash" away the heat from the deep, and the groundwater temperature will decrease with the increase of depth. (3) If the vertical movement of groundwater is intense, with large up-and-down disturbances and good temperature exchange conditions, the temperature value will become relatively stable in this active zone. This phenomenon is found in all monitoring points in the discharge area, especially in the period of smooth groundwater flow. (4) Under the influence of solar radiation energy, the surface temperature shows diurnal and seasonal variations. The diurnal variation affects 1–2 m below the surface, and the seasonal variation affects 30 m below the surface. This study shows that there are similarities and differences in the groundwater temperature in the study area, and the groundwater temperature is a good indication for the delimitation of groundwater system. This study also verifies the rationality of the existing delimitation system in the study area.

Screening and isolation of carbonic anhydrase-producing microorganisms from rocky karst habitats

CHANG Kaiyun, WANG Zhongcheng, WEI Xiaomeng, LIU Qiumei, ZHAO Jin, ZHAO Jie, HE Xunyang

2023, 42(6): 1202-1212. doi: 10.11932/karst20230606

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In China, the total area of rocky desertification covers 1.01×10⁷hm², accounting for 22.3% of the karst area. Nowadays, rocky desertification has become one of the three major land degradation problems in our country, which has seriously affected our economic development and environmental governance. Naturally, rock weathering in the karst area is so long that it takes from a few decades to a hundred years to form a centimeter think of red soil. Because the karst area is rich in calcium and barren in soil, ecological restoration of this area is urgent in both rocky desertification control and karst carbon sink. Carbonic Anhydrase (CA), one of the fastest catalyzing enzymes, can promote the weathering of carbonate rocks by accelerating the hydration reaction of CO₂, thus providing soil matrices for plant colonization. At the same time, CA can also be used to absorb CO₂ in the atmosphere to fix the carbon source, thereby participating in the carbon cycle process of karst dynamic system. The CA-producing microorganisms are considered to have good application prospects in the restoration of degraded karst habitats. However, for the target strains isolated from favorable habitat, their weak adaptability to the extreme environment may constrain them from exerting their expected effects.In order to select the strain with high CA activity in the extremely degraded karst habitat, CA-producing strains were isolated from the samples of severely degraded rocky karst habitats in this study. The study area is located in Hechi City, Guangxi Zhuang (23°41'–25°37' N, 106°34'–109°09' E). It has a subtropical monsoon climate, with an average annual temperature of 16.9–21.5 ℃. The area is typically developed with karst, scattered with thin soil and highly exposed with rocks. From September to October 2021, four different habitat samples were collected, including lichen in the area with extreme rocky desertification weathered materials under moss, soil under moss, and weathered materials under moss in karst native forest. The source of isolation was a mixture of the four different habitat samples, 1g of each. CA-producing strains were isolated and screened from karst rocky habitats by inoculating suspension of isolation source in calcium carbonate medium with a sprayer. The field emission scanning electron microscope was used for morphological identification of the strain. The physiological and biochemical characteristics of the strain were confirmed by carbohydrate decomposition test, V-P test, methyl red test, citrate utilization test, nitrate reduction test, starch hydrolysis test and contact enzyme test. Strains were identified by 16S rRNA sequence analysis. The CA activity of single strain and mixed strain was determined by electrode method. The sequence alignment was performed in the National Center for Biotechnology Information Database. In MEGA6, Neighbor-Joining method is used to construct the phylogenetic tree. SPSS software was used to analyze whether there were significant differences in CA activity among different strains/microflora.The results show that six CA-producing strains were isolated by calcium carbonate medium, namely, Flavobacterium resistens, Delftia acidovorans, Stenotrophomonas rhizophila, Pseudomonas oleovorans, Agrobacterium cavarae and Bacillus albus. The CA activity of Stenotrophomonas rhizophila was the highest, up to 4.27 U·mL⁻¹·OD₆₀₀⁻¹, while that of Bacillus albus was only 0.46 U·mL⁻¹·OD₆₀₀⁻¹, which indicated that although CA commonly existed in prokaryotes, the activity of different species of bacteria varied greatly. Mixed culturing of Stenotrophomonas rhizophila whose CA activity is high with other strains led to a decrease in CA activity, indicating the great potential application of Stenotrophomonas rhizophila in the extremely oligotrophic environment.The results provide a new bacterial source for ecological restoration of extremely degraded karst habitats. However, all the CA activity decreased when the isolated Stenotrophomonas rhizophila strain N6 with high CA activity is mixed with other 5 strains. Because of the complex living environment, the competition of indigenous microbial community will reduce the survival efficiency of the added microorganisms, make it difficult to form a stable community structure, and then reduce the repair effect. In restoration of degraded karst habitat, whether there is a kind of dominant bacteria that is symbiotic with Stenotrophomonas rhizophila N6 to jointly promote carbonate dissolution is a research focus. Therefore, future studies should identify the core species that drive the restoration of degraded habitats, and explore the relationship between the isolated and culturable CA dominant bacteria and the core species. These study focuses will promote the research and development of biological fertilizer for the rapid restoration of degraded karst habitats.

Stoichiometric characteristics of soil carbon, nitrogen and phosphorus in different stages of vegetation succession at karst graben basin of Yunnan Province, China

HU Lin'an, QIU Jiangmei, LI Qiang

2023, 42(6): 1213-1223. doi: 10.11932/karst20230607

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The contradiction between humans and land is prominent in the karst area of Southwest China. Human destruction of vegetation eventually leads to serious water loss and soil erosion, resulting in a large number of rocky desertification. In order to control the expansion of rocky desertification, the Chinese government has carried out a series of projects, such as returning farmland to forest, natural forest protection and Yangtze River shelterbelt projects, which have caused corresponding changes in land use and vegetation cover. As one of the most important types of rocky desertification control in China, karst graben basins are usually characterized by drastic changes in basin-mountain topography, great climate contrast, severe soil flow/loss, poor vegetation site conditions, and the slow recovery of community ecosystem. Therefore, the study on the comprehensive control of rocky desertification at the karst graben basin became urgent. The ecological stoichiometry is a state-of-the-art science that analyzes the regional circulation patterns of biogeochemical elements from the perspective of element metrology, which provides a new insight to solve the problem of nutrient supply/demand and circulation in ecosystems. It has important application value in the study of soil nutrient circulation and balance mechanism, and is one of the hot spots in current ecological research. However, domestic studies on ecological stoichiometry mainly select desert steppe, Loess Plateau and Minjiang River estuary as research areas in recent years. There is a paucity of systematic studies on soil metrology characteristics in different stages of vegetation succession at karst graben basins, and the response of soil nutrients and ecological stoichiometry to different types of vegetation restoration is still unclear.In order to elucidate the content and stoichiometric ratios of C, N and P elements in the soils in different stages of vegetation succession at karst graben basins, Santang, located in Xiaojiang karst basin, Luxi county, Yunnan Province, was selected as a study area. This area is dominated by a subtropical monsoon climate, with the average annual temperature of 15.2 ℃, the average annual sunshine of 2,122 h, and the average annual precipitation of 1,000 mm. The main tree species in the study area include Platycladus orientalis (L.) Franco, sticky alder, Yunnan pine, and so on. In January 2018, by the "space for time" approach, the soils from 5 different stages of vegetation succession (corn land, grassland, shrub land, planted forest and primary forest) were collected from different soil depths (0–10 cm and 10–20 cm) according to the sampling method of profile excavation. The soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) contents were determined by potassium dichromate oxidation with an external heating method, Kjeldahl method, and molybdenum antimony anti-colorimetric method. Excel 2019 software was used for data statistics. One-Way ANOVA, Duncan's new multiple range test method (LSD), Pearson correlation and Two-way ANOVA in SPSS 25.0 software were used to study the variance and correlations of soil ecological stoichiometric characteristics. The study results are expected to further reveal nutrient restriction and nutrient change patterns in the vegetation succession process, meanwhile, providing the scientific basis for vegetation restoration management and rational use of land resources in karst graben basins.There were significant differences in soil nutrient contents in different succession stages, mainly manifested in the 0–10 cm soil layer. SOC and TN contents showed an overall increasing trend with the increase of succession years, while TP content fluctuated and showed no obvious change pattern. Soil C and N contents differed significantly among soil layers. Specifically, SOC and TN contents in 0–10 cm soil layer were higher than those in 10–20 cm soil layer at each succession stage. Compared with C and N contents, soil P content did not differ significantly among soil layers at each succession stage, except for the content in the primary forest. In 0–10 cm soil layer, C/N, C/P and N/P showed significantly positive correlations with SOC content; C/P and N/P indicated significantly positive correlations with TN content; soil TP content was not significantly correlated with C/N, C/P and N/P. There was no significant correlation between soil physicochemical properties and soil stoichiometric ratio in the 10–20 cm soil layer. In the present study, the N/P ratios of 0–10 cm soil in different succession stages were between 2.71 and 5.93, and those of 10–20 cm soil were between 3.11 and 4.92. Essentially, all of the N/P ratios of soil at different depths were lower than 10, indicating that vegetation growth in the study area was more likely to be limited by N. However, given the fact that P content in karst soil is relatively low, vegetation growth should be first limited by P rather than N. Moreover, vegetation types and soil depths significantly affected SOC, TN and TP contents and their stoichiometric ratios in the study area. In summary, soil nutrient and stoichiometric ratios at karst graben basins change significantly with vegetation restoration chronosequence, and soil physicochemical properties have an important impact on soil stoichiometric ratios. These results are of great significance for further research on soil nutrient balance and limiting factors in karst graben basins.

Analysis of water inflow conditions and prediction for water inflow of deep-buried tunnels in the karst area of Southwest China: Taking Dapozi tunnel of central Yunnan Water Diversion Project as an example

LUO Yiming, CHENG Jianmei, XU Wenjie, BA Jinghui, HUANG Shengcai, DUAN Tianyu

2023, 42(6): 1224-1236. doi: 10.11932/karst20230608

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The karst area in Southwest China is characterized by complex terrain and karst development, with a large area of exposed carbonate rocks and a wide distribution of karst depressions and valleys on the surface. Therefore, this area is highly subject to water inflow in tunnel construction. The Dapozi tunnel area of the central Yunnan Province is a deep-buried tunnel in the karst area, in which exist many faults and very complex hydrogeological conditions. Therefore, it is necessary to predict the tunnel inflow during construction by analyzing the water inflow conditions and identifying its sources, which can ensure the safety of tunnel construction. Currently, numerical and analytical methods are commonly used to predict water inflow. The analytical methods mainly calculate the water inflow of tunnel through empirical calculation formulas. However, most of these methods are based on some assumptions and specific boundary conditions, which may limit their applicability in complex cases. The numerical method can simulate complex geological conditions by obtaining accurate data about geological characteristic, but its accuracy is restricted by the accuracy of borehole data in numerical calculation. Therefore, the mutual supplementation of numerical and analytical methods can significantly improve the efficiency and accuracy of water inflow prediction. Besides, most of the previous studies only predicted the water inflow of tunnel in the survey and design stage. But they did not describe the actual construction progress in the model, and the accuracy of prediction results conducted separately with numerical or analytical methods were not verified by the actual water inflow data. By analyzing the stratigraphic lithology and geological structure of the strata in the study area, we preliminarily identified the construction sections with the risks of water inflow. Based on hydrogeological surveys, we collected water samples from tunnels, springs, and wells in the study area to measure the geochemical characteristics of groundwater. Furthermore, we analyzed the sources for tunnel inflow and degrees of karst development, using Piper trilinear diagrams and Gibbs diagrams. Based on the procedures above, we used analytic and numerical methods to calculate the maximum and average water inflow of each tunnel unit, and compared the predicted values with the actual ones. We have also built a numerical groundwater flow model based on FEFLOW, which couples the simulation of regional macroscopic groundwater distribution and employs a method that combines multiple time series with the comprehensive assignment of various internal boundaries to depict the dynamic construction process. The results show the study area is developed with three main faults in. Although the permeability of these faults is weak, the gullies formed by these faults will gather atmospheric precipitation and surface water, increasing water inflow risk during tunnel construction. The TDS of groundwater in the area gradually increases from south to north, indicating that groundwater flows from the south to the north of the study area. The δD and δ¹⁸O isotope results prove that the water inflow in tunnel is from the low altitude; the distance of the groundwater runoff path is medium; atmospheric precipitation is the primary supply source. Gibbs diagram shows that the ions in the groundwater are mainly from rock weathering, suggesting that there may be karst fissures in carbonate rocks due to water-rock interaction within the area. In addition, the analytical method can efficiently calculate the water inflow of tunnel in the preliminary survey and design stage of the project. However, this method cannot dynamically predict the change in the water inflow. In essence, it is an analytical formula derived from the theory on the stable movement of the phreatic aquifer to the complete well. Therefore, the groundwater table dramatically affects the result, and the prediction accuracy in the section of the high groundwater table of analytical method is lower than that of the numerical method. The numerical method focuses on the high-accuracy model of engineering scale in the karst system, which is controlled by the macroscopic groundwater distribution. It pays attention to the description of the dynamic process and the conditions of the project area. Specifically, it describes the dynamic construction process by multi-time series. The working conditions of the excavating and lining are described by "GAP" in FEFLOW, which can accurately predict the change of water inflow in construction. Therefore, based on the actual hydrogeological conditions, the analytical-numerical method can significantly improve the efficiency and accuracy of water inflow prediction. The methods and models used in this paper are significant for preventing and controlling water inflow disasters in high-risk tunnels.

Application of different time series models to the prediction for mine water inflow in karst mountainous areas

ZOU Yinxian, CHU Xuewei, DUAN Xianqian, LIU Pu, WANG Zhongmei, WANG Yiwei

2023, 42(6): 1237-1246. doi: 10.11932/karst2023y031

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Coal resources are one of the important mineral resources in China. In the process of coal mining, due to the complex hydrogeological conditions in the mining area, and ineffective water exploration and discharge, accidents of mine water inrush occasionally occur, which may seriously restrict the safe production of coal resources. According to statistics, from 2000 to 2017, there were 1,173 accidents of coal mine flood in China, with 4,760 deaths. Therefore, the prediction reliability of mine water inflow plays a vital role in the safety of coal mining. A time series model is specifically designed to simulate and predict a time-sequential, time-varying, and interrelated data series. Most time series models require that the data must be stationary and the time series must follow a normal distribution. Taking Laoyingshan coal mine as an example, this study establishes a model of Seasonal Auto-Regressive Integrated Moving Average (SARIMAX model) and a model of Seasonal Auto-Regressive Integrated Moving Average with eXogenous factors (SARIMA model), compares the fitting and prediction results of these two models, and evaluates their adaptability in prediction of mine water inflow in karst mountainous areas. Based on the monthly average rainfall and monthly average water inflow from October 1994 to December 2014, a SARIMA model for univariate seasonal time series and a SARIMAX model for multivariate seasonal time series have been established. To establish a corresponding mathematical model, it is necessary to perform a parameter significance test on each model, analyze the model fitting goodness and model fitting accuracy, and determine the optimal model. The test parameters can be selected from the coefficient of determination R2 of the sample, the Nash efficiency coefficient (NSE), the mean absolute percentage error (MAPE), the deviation, the root mean square error (RMSE), the AIC value, the BIC value and other indicators to test. Since NSE, RMSE, R2, and MAPE standards are correlated in some degree, the NSE, AIC and BIC values are selected as the criteria for validating the quality of the model. The above two different models are used to predict the average monthly water inflow of the mine in 2015. The model prediction results show that, except for the SARIMA model in November 2015 and the SARIMAX model in July 2015, the MAPE is greater than 40%. According to Qian Xuepu's classification of the prediction accuracy of mine water inflow, these prediction results can reach the mine water inflow of Level B. According to the relative error of the prediction results, the MAPE errors of the SARIMA model in the first 4 months and those of the SARIMAX model in the first 5 months are both within 25%, and the subsequent errors experience a maximum value or a large fluctuation with the step size, indicating good short-term prediction but poor adaptability for long-term prediction of these two models. Even so their prediction accuracy can still reach the mine water inflow controlled by Level C. In terms of the prediction accuracy, the SARIMAX model is more accurate in prediction than the SARIMA model. The main reason is that the SARIMA model is a univariate prediction model, which predicts the later changes only based on the time series changes of the water inflow itself, but ignores the external factors caused by the water inflow. The SARIMAX model only introduces the influence of rainfall on water inflow, but as one of the important factors affecting water inflow, rainfall plays an obvious role in improving the prediction accuracy. The correlation between rainfall and mine water inflow indicates that atmospheric precipitation is the main source for water recharge, and mining fissures are the main recharges channels in the mining area. The change of water inflow has a certain hysteresis effect relative to rainfall. With the extension of mining level, the increase of the mining area and the backfilling of the water-conducting fissures caused by the goaf collapse, the hysteresis effect of rainfall becomes increasingly obvious. The correlation between rainfall and inflow in the same month and the first previous month radually decreases, while it gradually increases from the second and fifth months. Based on the mine water inflow and rainfall, the SARIMA model for univariate seasonal time series and the SARIMAX model for multivariate seasonal time series have been established to predict the mine water inflow. The prediction results show that 91.7% of the predicted values of the two models reach the mine water inflow of Level B, and the prediction accuracy is high. The MAPE of SARIMAX model is 18.57%, less than that of SARIMA model (25.27%), indicating higher accuracy of SARIMAX model.

Inversion of the fine in-situ stress field for deep-buried tunnel: A case study on the Jiangranshan tunnel of the Shuangjiang-Cangyuan express way in southwest Yunnan

LI Hongming, SUN Weifeng, ZHANG Hongri, QIN Xianghui, FENG Jian, WEI Jianhai, LAN Sulian, ZHANG Chongyuan, SUN Dongsheng

2023, 42(6): 1247-1257. doi: 10.11932/karst2023y029

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Under complex geological conditions of fault system, stratum and terrain, an accurate inversion of the in-situ stress field is a challenging and hot issue in the study on engineering geology. Focusing on the accurate inversion of fine in-situ stress field for deep-buried tunnel under complex geological conditions, and the influence of major geological factors on the stress state, we took the Jiangranshan tunnel of the express way from Shuangjiang to Cangyuan in southwest Yunnan as a case to perform the study. We first established the fine geological model of the Jiangranshan tunnel area by integrating the fine DEM data and the geological survey data. Then, taking the measured in-situ stress data in the tunnel site and the measured GPS data of velocity field as the integrated constraints, we carried out the inversion of fine in-situ stress field in the engineering region of Jiangranshan tunnel. On the basis of the inversion results, we analyzed the characteristics of fine in-situ stress field in the tunnel site and the influence of main geological conditions on the current in-situ stress field. Finally, we estimated the engineering geological stability of the surrounding rock mass of the Jiangranshan tunnel. The results show that the simulated displacement velocity field in the Jiangranshan tunnel area is basically consistent with the GPS observation results, revealing that the inversion model adopted in this study can well reflect the current tectonic stress environment of the engineering region. The simulation results show that the level of the in-situ stress field in the western part of the study area is relatively high, while it is low in the eastern part. The direction of the maximum principal stress shows partial deflection in the study area. The influence of the nearly E-W-striking Xiaoheijiang fault—the largest fault in the study area—on the in-situ stress is that it causes the slight deflection of the direction of the maximum principal stress; however, it does not cause abrupt change of the in-situ stress magnitude. The secondary faults and topography show little effect on the in-situ stress field, which is limited to very small area and does not cause notable disturbance of the in-situ stress field. The results reveal that the maximum, intermediate, and minimum principal stresses along the tunnel mainly distribute in 7.47–27.23 MPa, 1.59–15.12 MPa, and 0.01–6.71 MPa, respectively, and does not show obviously abnormal characteristics. The engineering geological stability of the surrounding rock mass of the Jiangranshan tunnel is estimated by the stress intensity index (i.e., the ratio of the maximum tangential stress to the uniaxial compress strength of rock). The estimation results, which are determined by integrating the stress field simulation results and the mechanical properties of the typical rocks obtained from the laboratory tests, show that the stress intensity indexes of the Jiangranshan tunnel mainly range between 0.20–0.48. It indicates that the surrounding rock masses of the Jiangranshan tunnel are mainly in a stable state or in a slight rockburst risk under the current in-situ stress conditions. The conclusion can be drawn from the case study that under the complex geological conditions, the fine geological model suitable for the scale of engineering area can be established by using the fine DEM and actual fault and strata data, which can effectively reveal the characteristics of the in-situ stress field in engineering area and the influence of the main geological conditions. This study provides not only the profound understanding of the in-situ stress field of the Jiangranshan tunnel area, but also the implications for the fine inversion of in-situ stress field under complex geological conditions in Yunnan and other similar areas. In addition, this study can directly support the stability evaluation of the surrounding rock of the deep-buried Jiangranshan tunnel.

Numerical simulation on uniaxial compressive mechanical properties of karstified rock mass in epikarst zone

LENG Deming, SHI Wenbing, LI Hua, LIANG Feng

2023, 42(6): 1258-1269. doi: 10.11932/karst2023y015

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The research of mechanical properties of karstified rock mass is helpful to the study of slope stability, foundation bearing capacity and mechanism of karst-induced disasters in karst area. However, it is difficult to obtain suitable rock samples for mechanical tests of karstified rock mass, or there exist experimental results with large differences. Nowadays, many scholars have studied mechanical properties by numerical simulation and have achieved greatly. In this kind of research, the stochastic method to simulate the karstification process is mainly adopted, whose algorithm is relatively simple but with less accurate response to the kasitified characteristics of rock mass, thus leading to the limitations of the results. Relevant literature shows that the epikarst zone presents characteristics of negative exponential relationship between the dissolution rate and depth, uneven distribution of dissolved pores caused by the difference of dissolution morphology, and more vertical karstification than horizontal karstification. In this study, two parameters of the karstification characteristics—the dissolution rate (the quantitative index of dissolution degree of rock mass) and dissolution uniformity coefficient (representing the uneven distribution of dissolution fractures)—have been set to analyze its characteristics in the epikarst zone. According to the characteristics of limestone joint in a typical karst area in Guizhou, the equivalent model of jointed rock mass has been established by using the Flat Joint Model (FJM), Smooth Joint Model (SJM) and Discrete Fracture Network (DFN). Based on the cellular automata algorithm and the dissolution of the equivalent model of jointed rock mass, the kastified rock mass model with different dissolution rates and different dissolution uniformity coefficients has been obtained. According to the numerical uniaxial compression test and the acoustic emission events in the process of loading rock mass. A total of 48 groups of test samples were collected in terms of 8 grades of dissolution rate and 6 groups of dissolution uniformity coefficient. The results show that the loading process of karstified rock mass can be divided into six stages: (1) stage of compaction; (2) stage of elastic deformation; (3) stage of stable fracture development; (4) stage of unstable fracture development; (5) stage of post-peak strain softening; (6) stage of post-peak rapid strain softening. The karstification reduces the cementation between rock masses and destroys the skeleton of rock mass, which leads to the following effects. With an increase of dissolution rate, the uniaxial compressive strength decreases. The curve shortens at the stage of elastic deformation. The number of acoustic emission events decreases significantly when the rock mass is damaged. The failure mode of rock mass gradually changes from brittle failure to ductile failure. The strength after failure mainly comes from the locking effect of rock bridge and the friction between fracture surfaces. At the same time, with the continuous increase of dissolution rate, the compressive strength of rock mass declines from rapid to slow rate. The higher the dissolution uniformity coefficient is, the smaller size the dissolution pore and the more uniform distribution of dissolution fractures may become. Under these conditions, the formation and penetration of macro cracks need to go through a longer process, and the curve of unstable fracture development increases. The internal stress of rock mass is more divergent, and the rock mass changes from local failure to overall failure. The fitting of uniaxial compressive strength and karstification characteristic parameters (k, U) indicate their negative exponential relationship, and the exponential constant C is related to the uniformity coefficient u. The relationship can be used as the basis for determining the strength of karstified rock mass. In practical engineering, the deformation state of karstified rock mass can be judged according to this acoustic emission characteristics, and thus corresponding treatment measures can be taken. In the evaluation of rock mass quality, the dissolution uniformity coefficient can be used as an auxiliary evaluation basis, in addition to the dissolution rate as the main feature of dissolution development.

Simulation analysis of mechanical influence of water level fluctuation in water-eroded groove on tunnel lining

CAI Zhiyan, WU Yimin, XU Peng, CHI Zuoqiang, HUANG Min, WU Haoran, ZHANG Jiawei

2023, 42(6): 1270-1281. doi: 10.11932/karst20230609

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Karst tunnels are usually faced with the risk of water disaster caused by seasonal fluctuation of water level in water-eroded groove. Due to seasonal precipitation, water supply and other reasons, water storage structures such as karst caves and underground rivers are in the state of low water level in the dry season. When the rainy season comes, the groundwater level rises rapidly due to water supply of these water storage structures, and water accumulates behind the tunnel lining through water-eroded groove or karst fissure (karst pore). The water pressure of tunnel lining increases sharply, which seriously threatens the safety of lining structure. Through relevant investigation, the authors learn that a heavy rainfall caused a section of lining to break, and this section had to be repaired during the construction of Yunwushan tunnel on Yiwan railway. Besides, during the construction of Yinshan tunnel in Guizhou Province, a rare rainstorm occurred, and the vertical side wall of the tunnel was crushed by sudden karst water, with the destruction length of the side wall reaching 20 m. Another example is when the construction of Jijiapo tunnel passed through the karst area in Hubei Province, the water pressure behind the lining increased sharply during the heavy rainstorm, which resulted in serious fracturing of the second lining and the bottom plate as well as water gushing due to bottom drum rupture of the construction joint. Macro influences of karst groundwater on tunnel lining structure have been studied by a large number of scholars through numerical simulation and field test. However, most of them are qualitative analyses insufficient in the study on external water pressure of tunnel lining in karst areas, and do not highlight the distribution characteristics of external water pressure of lining under fluctuating water level of water-eroded groove. Therefore, these studies can hardly reflect the actual influence of karst groundwater on lining structure in terms of the calculation with load-structure method. Based on the engineering case of a highway tunnel in the karst area of Southwest China, this study quantitatively analyzes, through numerical simulation, the influence of different groundwater levels and different locations of water storage in water-eroded grooves on the stress of tunnel lining. The research process is shown as follows. According to the results of geological exploration and groundwater connectivity test under on-site rainfall conditions, the typical cross-section of the tunnel was firstly selected. Subsequently, the external water pressure of tunnel at different water levels was calculated through seepage software, which was followed by the calculation of the surrounding rock pressure according to the specifications. Finally, the internal force of the lining was calculated through the "load-structure" method. According to the careful analysis and scientific judgment, the occurrence mechanism and evolution characteristics of the tunnel water hazard risk were revealed. The main conclusions indicate as follows. (1) The water level fluctuates in water-eroded groove under seasonal heavy rainfall, and the pressure of water outside the tunnel changes frequently, which results in significant changes of internal force of tunnel lining. When the water level rises, the structural stress increases and the safety will greatly reduce. The arch and side walls are still loaded in the mode of small eccentric compression, while the mode of loading at the bottom of the tunnel gradually develops from small eccentric compression to large eccentric compression. There exists the risk of cracking and breakage of lining structure at high water level. (2) The water storage of water-eroded groove at the side wall of the tunnel results in bias water load on the tunnel, and the safety factor of the side wall decreases by up to 1.1. The safety factor of tunnel lining at the same water level is less than that of the corresponding position on the other side, with the difference of safety factor between 0.1 and 0.3. With the gradual increase of groundwater level, the difference shows a decreasing trend, i.e. from bias water load to even water load, and the influence on structural force difference gradually reduces. (3) Under the condition of high groundwater level caused by seasonal heavy rainfall, although the tunnel drainage system has played a certain role of pressure relief, the tunnel lining still bears high water pressure. There is a possibility that the structural safety cannot be satisfied, especially at the side wall and the bottom of the tunnel. Therefore, when seasonal heavy rainfall comes, the drainage system should be conducted smoothly. Strengthening drainage and pressure relief is the key to the problem that excessive water pressure caused by water head rise will roughly destroy lining. Besides, the safety of lining structure of side wall and tunnel bottom should be paid close attention to. In this study, the numerical simulation is used to quantitatively analyze the stress response and difference of lining structure under the condition of water level fluctuation in water-eroded groove, so as to provide theoretical support for disaster prevention of karst tunnel.

Analysis of critical safety distance between tunnel and concealing filled karst cave in the karst area

WANG Wanfeng, YANG Yongtai, LU Yuquan, ZHAO Chujun, CUI Pengjie, QIU Junling

2023, 42(6): 1282-1293. doi: 10.11932/karst20230610

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If the tunnel construction work is done in the karst area, the tunnel shall keep a certain distance from the karst cave to ensure the stability of the surrounding rock and support. If the distance between the tunnel and the filled karst cave with high water pressure is short, it is easy to cause the instability and damage of tunnel face or the circumferential surrounding rock, thus resulting in karst water inrush, mud inrush, collapse and other disasters. Especially, the karst cave concealing around the tunnel is often difficult to be accurately predicted. Therefore, the study of the critical safety distance between the karst cave and the tunnel plays a vital role in the evaluation and prevention of disasters caused by concealing filled karst cave. Studies on the critical safety distance between tunnel and karst cave can be divided into qualitative research, semi-quantitative research and quantitative research. Qualitative research, such as judging the influence degree of karst cave on tunnel through numerical analysis, is generally conducted to comprehensively analyze the factors affecting the stability of structure for water burst prevention and obtain the empirical critical safety value. Semi-quantitative research mainly focuses on theoretical calculation, which can be roughly sub-divided into three types: simplified beam slab model based on strength theory, catastrophe theory model and the model of crack tension-compression shear based on fracture mechanics. Mainly focusing on numerical simulation, quantitative research is conducted to set up multiple groups of numerical tests for calculation, and to obtain the calculation model of safety distance through linear regression. The safety distance between the tunnel and the filled karst cave can be explored by theoretical calculation and numerical simulation test. Based on the bending and shear strength theory, the mechanical calculation models of the filled karst cave at the top, bottom and side of the tunnel are established by using the fixed beam model at both ends. Meanwhile, the self-weight of the internal filler and the pore water pressure are taken into account. Given the influence of the surrounding rock pressure above the karst cave on the waterproof rock stratum, the critical safety distance between the tunnel and the karst cave can be explored, and the calculation formula of the distance in different circumferential positions can be deduced. However, this formula does not consider the excavation effect of the tunnel and the process of support, nor can it reflect the changes of the displacement, stress and plastic zone of the surrounding rock. Therefore, it is difficult to obtain a universal formula to express relationship. It is necessary to supplement the predicted safety distance with the help of numerical simulation. In this study, the numerical model of the critical safety distance between the filled karst cave and the tunnel at different circumferential positions was established by FLAC 3D software. Based on the method of orthogonal experimental design (a total of 48 numerical test schemes), the stability of surrounding rock has been evaluated by the distribution range of plastic zone of surrounding rock caused by tunnel excavation. If the plastic zone connects the tunnel with the karst cave, the distance between these two indicates that the surrounding rock is unstable, and thereby the critical safety distance can be calculated. The results of critical safety distance under different working conditions were also analyzed by range analysis and variance analysis. Besides, the law and significance of three influencing factors—the level of surrounding rock, the water pressure in karst cave and the karst cave size—on the critical safety distance were explored. Through the nonlinear multiple regression analysis of the orthogonal test results, the prediction formula of the critical safety distance between the filled karst cave and the tunnel at different circumferential positions was established respectively. The results show that the critical safety distance increases with the increase of surrounding rock level, pressure of karst cave water and the karst cave size. The comprehensive influence degree of the three factors can be ranked from the strongest to the weakest as follows: the surrounding rock level, the pressure of karst cave water and karst cave size. Finally, the research results were applied to Yangzong tunnel project to verify the rationality and applicability of the prediction model of critical safety distance. The results show that the safety distance predicted based on the strength theory is relatively conservative, and the predicted results based on the numerical test are close to the reserved distance. The predicted results have a certain reference for the project of karst tunnel with water abundance.

Study on susceptibility of karst collapse based on normal cloud model in Yonghe town, Liuyang City

LU Yulong, YE Gaofeng, YANG Xian, LU Zhilin, LIU Yang, ZHANG Lianzhi, LI Ganlong

2023, 42(6): 1294-1302. doi: 10.11932/karst2023y027

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Ground collapse is a common geological disaster in karst area, which would bring great harm to people's life and property. The zoning evaluation on karst collapse susceptibility is beneficial to the classification and treatment of the disaster in order to ensure safety and economy. Karst ground collapse is characterized by suddenness, concealment, multi-factor, randomness and fuzziness; therefore, it is difficult to be fully quantified. The normal cloud model could effectively reflect the fuzziness and randomness of objective things, and integrate them to form the mapping between qualitative and quantitative analyses. In this paper, based on the normal cloud model, the study on susceptibility of karst collapse in Yonghe town, Liuyang City has been conducted in order to provide a basis for the classification and treatment of ground collapse in this area, and also provide a reference for the susceptibility evaluation of karst collapse in other areas.The study area is located to the northeast of Liuyang City, 24 km away from it, and its administrative division is located in Yonghe town, Liuyang City, Hunan Province. Ground collapses in this area are all developed in the distribution area of soluble rock layer. The most developed layer is the second member of the Lower Permian Qixia Formation (P₁q²) with thick-layered carbonate rocks. Secondarily, ground collapses are distributed in the middle and upper Carboniferous Hutian Group (C₂₊₃ht). The study area covers an area of 14 km², and 38 karst collapses have occurred in this area so far. Among these collapses, 22 have been surveyed and filled. Sixteen collapses, four of which have been filled, have been investigated in detail. The collapse sites are mainly distributed in the region of soluble rock where faults are developed and surface water and groundwater are closely connected. Geographically, the collapse sites are mainly distributed in Yueshan Formation-Oujia Formation-Dahe Formation, Yongfu village, Yonghe town, Juxiang community-the old street of Yonghe-Huayuan village, Yanxi town, Nanshan Formation-Lizhen Primary School (old)-Xinwan Formation-Xinping Formation.Six evaluation indexes of karst collapse susceptibility were selected in this study, including karst development degree (dissolution rate of borehole), distance from the fault in the area, thickness of overlying soil layer, characteristics of karst water, distance from the pumping funnel center and current situation of ground collapse (ground collapse density). Firstly, according to the detailed exploration results of existing subsidence pits, the weights of six evaluation indicators of 16 existing subsidence pits were calculated and assigned based on entropy weight method. The calculation results indicate that the weight of development density of existing ground collapses is the largest, and that of the fluctuation amplitude of groundwater is the smallest. Secondly, the scoring standard for the risk level of normal cloud model was determined, and the susceptibility of karst collapse in each unit area in the study area was evaluated combined with the weight of each evaluation index. The evaluation results show that all the karst collapses that occurred are distributed in the area highly subject to karst collapse, indicating that the zoning in the evaluation of this study is reasonable.When the normal cloud model is used to evaluate the susceptibility of karst collapse, the size of grid cells could be controlled by the cloud similarity of comprehensive risk evaluation of karst collapse. For areas with high similarity and small cloud droplet dispersion, the unit area of each evaluation could be appropriately increased in order to reduce the evaluation workload. In addition, the high risk area calculated by analytic hierarchy process is 10.8% larger than that by normal cloud method, but three existing collapse pits distributed in the area at medium-level risk of karst collapse, indicating that normal cloud model takes more advantages than analytic hierarchy process in dealing with fuzzy and random problems such as the risk evaluation of karst collapse.

Control action of thrust fault to oil and gas accumulation in Hade block of the Fuman oilfield

LI Shiyin, LUO Xiao, WANG Peng, LI Huiyuan, QIN Hanlian, LI Jingrui

2023, 42(6): 1303-1311. doi: 10.11932/karst20230611

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The Tarim Basin, renowned for its super large Ordovician marine carbonate oil and gas reservoirs, is a key area in the field of deep oil and gas exploration and development in China. Especially, the Fuman oilfield has become an important area in oil and gas exploration and development in recent years, largely owing to its extensively distributed fault-controlled gas reservoirs. Nevertheless, Hade block of the Fuman oilfield faces the complex interaction of thrust faults and strike-slip faults, which poses severe challenges. Due to the lack of comprehensive understanding of the impact of thrust faults on reservoir development, many wells with low yield and low efficiency had been drilled, which seriously hindered the effective exploration and development of oil and gas in this area. Therefore, an exhaustive geological examination of the Fuman oilfield in the Tarim Basin was meticulously conducted. This analytical endeavor was directed towards illuminating the intricacies underlying the synergy of thrust faults and strike-slip faults, and their consequential influence on oil and gas reservoirs. Methodologically, a methodical exploration was undertaken based on state-of-the-art 3D seismic data, thereby offering an all-encompassing perspective on the spatiotemporal characteristics of these fault structures in Hade block. The scrutiny further encompassed the delineation of the planar distribution of these faults, the elucidation of vertical crosscutting relationships, and the ascertaining of fault activity periods. The research findings were subsequently amalgamated with a comprehensive investigation of the regional temporal periods underpinning oil and gas accumulation, thereby proffering a lucid perspective on the dominion of thrust faults in this context. The investigation results display the key information on the endemic geological evolution of the Fuman oilfield. Foremost, the research findings determine that these fault structures originated from the late Middle Ordovician, especially the Middle Caledonian period. Noticeably, thrust fault activities peaked during the subsequent Hercynian period, i.e. after the appearance of strike-slip faults. This temporal distinction between the fault types is of profound significance for understanding their respective roles in oil and gas accumulation. The Fuman oilfield is divided into three different stages of oil and gas accumulation, namely, the late Caledonian, late Hercynian, and Himalayan stages. The results show that the oil and gas accumulation in the late Caledonian period is mainly affected by strike-slip faults, while the influence of thrust faults is great in the late Hercynian and Himalayan periods. The understanding of the interplay between fault types and oil and gas accumulation is of great significance for accurate exploration and development of benefit model in oil areas. In addition, this study unveiled three distinct modes of convergence between thrust faults and strike-slip faults, based on which three different modes of oil and gas accumulation and conveyance steered by thrust faults in Hade block were delineated. It is concluded that when thrust faults intersect with non-petroleum-endowed strike-slip faults in the Upper Cambrian-Yijianfang Formation, the reservoir is mainly formed in the late Hercynian period. When thrust faults and petroleum-endowed strike-slip faults intersect in the Middle Cambrian, the reservoir is mainly formed in the late Caledonian period. The intersection of thrust faults and petroleum-endowed strike-slip faults in the Upper Cambrian-Yijianfang Formation is most favorable for oil and gas accumulation, with characteristics of multi-source hydrocarbon supply and multi-stage accumulation. These conclusions provide valuable experience of and incisive insights into the exploration and development of similar oil and gas reservoirs, which is conducive to efficient decision making in the ongoing search for energy resources.

Experiment for the differential dissolution of dolomite of Sinian Dengying Formation in the Gaoshiti–Moxi area, the Sichuan basin

LUO Wenjun, JI Shaocong, LIU Xixiang, DAN Yong, LIANG Bin, NIE Guoquan

2023, 42(6): 1312-1321. doi: 10.11932/karst20230612

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In recent years, important discoveries have been made in natural gas exploration of Dengying Formation in Gaoshiti–Moxi area of the Sichuan basin. The gas-bearing reservoirs are mainly located in the fourth member of the Dengying Formation, and the reservoir rock types are mainly algal agglomerate dolomite, algal arenaceous dolomite, and algal-laminated dolomite. Though many previous studies on the dolomite of Dengying Formation in the Gaoshiti–Moxi area of the Sichuan basin have been conducted, they mainly focus on reservoir characteristics, paleogeomorphology characterization, gas reservoir productivity, etc. There are relatively few studies on simulation experiments of dolomite dissolution. The carbonate rock dissolution experiment is an important method to study the favorable conditions and distribution laws of carbonate rock dissolution. Since the 1970s, scholars at home and abroad have successively carried out simulation experiments of carbonate rock dissolution to explore the influence of composition, structure, temperature, pressure, fluid and other factors on dissolution. Early dissolution experiments mainly simulated surface environments, with experimental temperatures below 100 ℃. In the 1980s, scholars at home and abroad mainly studied the dissolution mechanism of carbonate rock in a deep burial environment. The experimental method was the surface reaction between fluid and rock particles or blocks. This study takes the algal dolomite of the Dengying Formation in the Gaoshiti–Moxi area as the research object. The dissolution rate, surface morphology, and microscopic characteristics of the dolomite of Dengying Formation are studied based on dissolution experiments with rock slices and thin sections. Meanwhile, the effects of lithology, structure, and reaction time on the dissolution degree of the dolomite are analyzed. The experimental results can not only display the quantitative indicator of dissolution—the dissolution rate, but also directly demonstrate dissolution characteristics and changes in pore structure after dissolution. The results of the dissolution experiment indicate as follows. (1) All samples indicate high initial dissolution rates in the early stage of the experiment, and as the dissolution time increases, the dissolution rate shows a significant attenuation and then tends to stabilize. (2) All samples undergo a certain degree of dissolution in a weak acid environment, and there are significant differences in the degree of dissolution among samples with different lithology and structure. There are significant differences in dissolution rates among different samples. Dissolution rates of algal-laminated dolomite and algal arenaceous dolomite are the highest, followed by that of algal agglomerate dolomite, and the dissolution rate of algal-layered siliceous dolomite is the lowest. (3) Observation and comparison of the microscopic dissolution characteristics of samples at different reaction times show that samples developed with intergranular and inter-crystalline pores exhibited a higher degree of dissolution along these pores. Samples developed with microcracks exhibit a higher degree of dissolution along these microcracks. (4) Regular recording of experimental data during the experiment can accurately illustrate the formation and evolution of dissolution pores and fractures. Conducting dissolution experiments with rock slices and thin sections can not only provide quantitative indicators of dissolution rates, but also show changes in dissolution structure, enabling a more comprehensive understanding of dissolution laws. (5) The development of algal dolomite reservoirs in the Dengying Formation may be related to the dissolution of algal dolomite, because a large number of dissolution pores developed by the dissolution of algal dolomite ultimately formed the appearance of the current karst reservoirs of Dengying Formation, which were mostly developed in algal dolomite with high algal content. Through dissolution experiments, the differences in dissolution of different dolomites in the study area have been analyzed, which is of great significance for predicting the distribution of high-quality reservoirs and guiding oil and gas exploration.

Application of the method of passive surface wave to the exploration of urban residential area

WU Jianqiang, CHEN Song, XU Junjie, ZHENG Zhijie, LIU Yongliang, WANG Yue

2023, 42(6): 1322-1330. doi: 10.11932/karst20230613

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With the rapid development of cities, people are paying increasing attention to urban ground subsidence and underground space. As a means to directly explore the structure of subsurface media, geophysical methods are increasingly being applied to solve the geophysical and geological problems on shallow surface in urban areas. Due to the various electromagnetic interference produced by the human noise and industrial production in cities, the application of traditional geophysical survey methods (gravity, magnetism, electricity and earthquake) is greatly limited in densely populated urban areas, but the human noise provides an effective source for the method of passive surface wave.The method of passive surface wave has developed rapidly due to its strong anti-interference capacity and little influence by site conditions. Specifically, this method has been used in karst collapse detection, prediction of overburden thickness, detection of velocity structure of underground shear wave, site effect evaluation, geothermal resource exploration, active fault detection, urban geological survey and other fields. Since the study area is located in an urban residential area whose ground had subsided, it is urgent to find out the geological structure of the underground medium. The authors adopt the method of passive surface wave for exploration and application research. The core of this method lies in the extraction of the dispersion curve. There are two most commonly used methods at present. One is the method of frequency-wave number domain (F-K method), and the other is the method of spatial autocorrelation (SPAC method). In this study, the authors mainly adopt the latter, while for irregular arrays, the method of extended spatial autocorrelation (ESPAC) is used.The main exposed strata in this study area lies successively as follows: the overlying clay in light yellow and brick red and gravel in Quaternary Holocene Guiping Formation (Qhg), and the underlying bedrock composed of thick limestone in light gray and gray-white in the upper segment of Devonian Upper Rongxian Formation (D₃r³). There is no fault structure developed in the area. The overburden is characterized by low velocity. The complete limestone shows characteristics of obviously high velocity. When there are fissures or even water-filled caves in the limestone, the high velocity will be significantly weakened, which is the geophysical premise for the application of this method.Due to the narrow space and the criss-crossing buildings in the area, the method of linear passive surface wave was adopted in actual data collection. The SUMMIT seismograph of the German DMT company was adopted as the instrument to collect data, and a vertical geophone of 4.5 Hz was used to receive data. One-time layout at intervals of 2 m was completed, and the whole survey line was received at the same time. Each survey line collected 20 or more records, and the sampling time of each record was 32,768 ms with the sampling frequency of 500 Hz. Before indoor processing, the original data were extracted in an orderly manner according to the spread length of 11 traces. The specific processing flow included preprocessing, extraction of dispersion curve, calculation of apparent shear wave velocity and drawing of apparent S-wave velocity profiles. The apparent shear wave velocity V_x is a physical parameter of surface wave different from the phase velocity V_r and the shear wave velocity V_s. It has a velocity dimension. Because it avoids the influence of human factors such as setting the initial model and selecting the inversion results during the inversion process, the apparent shear wave velocity was chosen.In this study, the characteristics of apparent shear wave velocity in the backfilling area for the original subsidence were firstly interpreted and analyzed, and then the characteristics of other survey lines were interpreted. The profiles and the slice diagrams at different depths of the apparent shear wave velocity show that: (1) The thickness of overburden layer in this area ranges from several meters to more than ten meters, and the thickness varies greatly at or near the sudden change in velocity. According to the comparison with the backfill area of original subsidence, the obvious abnormal features indicate that the upper soil layer is disturbed and developed in the solution groove, and karst caves are developed in the deep part, resulting in soil leakage and then ground subsidence. (2) Survey lines reveal that the karst in the bedrock is mostly developed within 30 m, mainly composed of Zone I—a strong karst development zone. Zone I is mainly distributed under and around Building 10 in a residential area, with a wide range and a certain extension direction. It is speculated to be a strong runoff zone with the main trend of north-south direction. (3) The statistical results of apparent shear wave velocity of multiple survey lines show that the apparent shear wave velocity range of the overburden in this study area is 220–380 m·s⁻¹, and the intact limestone is 580–920 m·s⁻¹. However, due to the development of fissures in broken limestone, its range is 390–510 m·s⁻¹, and the apparent shear wave velocity of water-filled cave is relatively lower at the range of 340–430 m·s⁻¹.

Three-dimensional electrical imaging of urban karst groundwater channels based on electrical resistivity tomography

LIU Daohan, XU Junjie, QI Xin, WU Jianqiang

2023, 42(6): 1331-1338. doi: 10.11932/karst20230615

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In China, approximately 346.3×10⁴ km² of the karst rocks are distributed, accounting for about one-third of the country's land area. Karst groundwater channels are common geological phenomena in concealed karst areas. Various karst caves and underground channels widely developed cause the complex groundwater system dominated by pipe flow, which promotes the transport of groundwater, soil, and rocks, resulting in complex geological issues for urban underground space development and utilization in karst areas. In these areas, ground collapse disasters occur from time to time. In geothermal areas, karst groundwater channels are often accompanied by frequent heat exchange, directly affecting the quality of geothermal development. Therefore, conducting exploration of karst underground channels is of great significance for the development of urban underground space and the prevention of geological disasters in karst areas.Due to the high degree of spatial variability of karst channels, it is a challenge for conventional two-dimensional detection methods to complete the geophysical imaging, which in turn results in the difficulty of conventional karst detection. Geophysical exploration is widely used due to its wide applicability, large detection depth, and accuracy. However, different geophysical methods have different advantages in terms of detection depth, resolution and practicality. The Ground Penetrating Radar has the characteristics of high efficiency and high resolution, but it is mainly applied to the exposed and shallow-covered karst due to factors such as limited penetration depth, irregular terrain, and clay content. Electromagnetic and seismic exploration can generate better results in the exploration of buried karst. In addition, microgravity exploration can be used to detect shallow karst caves and fill-type karst caves. Electrical resistivity tomography (ERT) is the most commonly used geophysical method for karst exploration, and has been applied to various karst geological environments, such as cave exploration, the exploration of water-filled karst groundwater channel, the detection of urban underground cavity, and engineering surveys.In this study, 3D ERT was used for fine detection of concealed karst underground channels in the urban area. Combining with geophysical numerical simulation and application examples, the effectiveness of 3D on karst underground channels with different filling types was analyzed. The research results show that 3D ERT has a significant improvement in data volume and resolution compared to 2D ERT. 3D ERT can more intuitively characterize the three-dimensional electrical structure characteristics of the target body and has advantages in imaging karst groundwater channels. Through the three-dimensional electrical imaging of the karst groundwater channel in Yuanquan village, Wuhan City, the groundwater migration characteristics of the low-temperature hot spring are revealed. The analyses of geophysical prospecting and drilling show that two low-temperature hydrothermal vent groups in this area are directly connected by karst underground channels and connected with north-west fault structures, reflecting a hydrodynamic process. In this process, deep hot water firstly circulates along the fault fracture zone to shallow karst channels, and after its horizontal migration at a certain distance, deep hot water exposes at the surface in the form of spring near the fracture zone, and the contact zone of carbonate rock and schist.In this study, the three-dimensional electrical structure imaging of karst groundwater channel near the hot spring point has been jointly realized by geophysical exploration and drilling, and the trends of fractured fault zone and karst channel have been displayed. Study results show that the low-temperature hydrothermal spring is connected to the karst groundwater system through deep faults. In view of this, it is proposed to carry out systematic geothermal explorationin the area to find out the hydrogeological conditions of hydrothermal spring, the groundwater circulation and heat interaction processes. In addition, the study results may provide support for the temperature increase of the hydrothermal spring and for the development of urban geothermal energy.

2023 Vol. 42, No. 6

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