中国岩溶

A study of reservoir architecture of Dengying formation in Gsoshiti-Moxi area, Sichuan Basin

HU Xiuquan, LU Hongjiang, YI Chi, LI Jianghan, CHEN Keyong, LU Jie, XIAO Chenjing

2022, 41(6): 847-859. doi: 10.11932/karst20220601

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There is a largest complete carbonate gas reservoir so far found in the mid of Sichuan Gaoshiti-Moxi area. Influenced by multi-period tectonic movement, this reservoir presents complex characteristics of diagenesis and reservoir formation. Combined with the field, lithologic, logging and production dynamic, the reservoir architecture can be divided into four types. The FMI of layered cavitation configuration shows layered distribution with dark spots on light background; the FMI of grape-lacelike cavitation configuration shows sporadic distribution of dark patches on light background; the FMI of the inter-breccia cavitation configuration shows clutter distribution of dark patches and short line images; and the FMI of cavitation configuration combined with crevices shows distribution of dark sinusoidal line images and patches on light background. The layered cavitation configuration is supposed to be formed by the dissolution of external acidic fluids during the burial period. The inter-breccia cavitation configuration and cavitation configuration combined with crevices are supposed to be formed by supergene karstification, and the grape-lacelike cavitation configuration is presumed to be a secondary cause—When the sea level rises, seawater fills the cracks and holes, forming grape lace cement. Different reservoir configurations are also different in conventional logging characteristics. The establishment of a corresponding discriminant model by multiple logging series can, to a certain extent, distinguish these four types of reservoir configurations. In this study, the single well logging, the variance function analysis, and the sequential indicator simulation method based on pixel are adopted to carry out the reservoir structure modeling of Dengying formation under the condition of seismic inversion constraint. The distribution law of reservoir configuration in No.4 segment of Dengying Formation in Gaoshiti-Moxi area is discussed. The method of reservoir structure analysis that enriches the methods and studies of carbonate rocks has a wide range of application prospects. The results of drilling gas test show that the thickness of layered cavitation configuration has a certain control effect on oil and gas productivity, and the areas where the layered cavitation configuration is relatively developed are of great exploration potential.

Reconstruction of karst paleo-geomorphology and paleo-water system on the top of Maokou formation in Yuanba area, northern Sichuan Basin

ZENG Tao, JI Shaocong, XIA Wenqian, ZHANG Qingyu, BA Junjie, DONG Hongqi, NIE Guoquan

2022, 41(6): 860-868, 879. doi: 10.11932/karst20220602

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The carbonate karst reservoir of Maokou formation is developed in the Yuanba area, northern Sichuan Basin. However, it is difficult to predict this set of reservoir because of its strong heterogeneity. Therefore, for the guidance of future oil and gas exploration and development, it is necessary for us to further restore karst paleo-geomorphology and paleo-water system so as to master the law of reservoir distribution. In order to better depict the micro karst landform, the ''the trend surface of residual thickness and impression residual" are combined to restore the ancient landform. According to the characteristics of karst paleo-geomorphology on the top of Maokou Formation in Yuanba area and the thickness from the top of Maokou formation to the top of its first section in Yuanba area, an index system for the classification of karst paleo-geomorphology types is established. Based on the thickness mentioned above, the paleogeographic environment and paleo-hydrodynamic conditions, the study area is divided into four second-tier geomorphic units—karst platform, gentle karst slope, karst plain and karst basin. Then, in terms of modern karst classification, a third-tier geomorphic units are subdivided into six types—micro hill depression, hill-cluster depression, micro hill trough, cluster-peak depression, hill-cluster ridge valley and monadnock plain, according to the micro geomorphic combination form of karst paleo-geomorphology on the top of Maokou Formation in Yuanba area. Based on the characteristics of paleo-topography as well as the plane distribution and mutual configuration relationship of the six third-tier geomorphic units, the surface water system of the paleo-karst surface at the top of Maokou formation in Yuanba and its adjacent areas is constructed. In general, controlled by the ancient topography, the surface runoff is mainly the scattered runoff from the karst platform to both sides. The ancient water system is not well developed with no centralized runoff system, and only small gullies and karst lakes are locally developed. On the whole, the ancient karst landform on the top of Maokou formation in the study area belongs to the initial stage of the formation and evolution of karst landform. The relative elevation difference of the regional terrain which is slightly undulating is generally less than 120 m. The relative elevation difference of hills (peaks) and depressions is generally less than 10-30 m, which belongs to the initial stage of tectonic uplift and denudation and is characterized by coastal (island) karst landform, and the surface water system is not fully formed. The karstification time of the study area is relatively short, belonging to the initial stage of epigenetically exposed karst which is characterized by the joint karstification of atmospheric fresh water and seawater. The karstification mode is mainly the leaching and infiltration of atmospheric fresh water, which is difficult to form concentrated lateral runoff. Karstification mainly occurs in the shallow part, and karst is dominated by dissolution holes. Large-scale karst fractures and caves have not been formed. There are great differences in karst development in different geomorphic locations, among which the karst platform belongs to the groundwater recharge area; the atmospheric precipitation is mainly vertical infiltration; and the lateral runoff of groundwater is slow. The karst plain belongs to groundwater runoff and discharge area, where groundwater runoff is slow, and the intensity of karstification is relatively weak. The karst basin belongs to groundwater drainage area with long water-rock interaction cycle and weak karstification intensity. The gentle karst slope belongs to groundwater runoff area with the strongest hydrodynamic force and the most developed holes, which is suitable for the future reservoir exploration.

Describing carbonate sedimentary facies by use of the radio of shoals to the whole strata :Taking the layer a of Mao2 submember in eastern Sichuan for example

XIAO Chenjing, HU Xiuquan, SHI Zejin, TAN Qian, LI Jianghan, YI Chi, LI Shilin, XU Fabo

2022, 41(6): 869-879. doi: 10.11932/karst2022y03

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The accurate description of the distribution of carbonate reservoirs requires increasing precision for the characterization of beach facies sedimentation. In view of the difficulty in calculating the real thicknesses of granular beaches on Maokou formation in eastern Sichuan due to uplift and erosion of strata, we characterized the types and distribution range of carbonate beaches based on the ratios of shoal thicknesses to the whole stratum thickness, and then described the carbonate sedimentary facies. Based on abundant drilling data, we conducted a case study by the establishment of identification marks of beach facies, the correlation of inter-well sedimentation, ratios of shoal thickness to stratum thickness and the planar distribution of sedimentary facies. This study indicates that the distribution of sedimentary facies in layer a of Mao2 submember from southwest to northeast in eastern Sichuan is respectively open platform, platform margin, slope and basin, among which the platform margin is approximately east-west banded in the area of Linshui-Fengdu-Zhongxian. The low-energy beaches both in the platform and in the slope are mainly distributed in the area with ratios of shoal thickness to stratum thickness of 0.3-0.5; the high-energy beach in the platform is developed in the area with ratios of 0.5-0.6; and the marginal platform beach is mainly distributed in the area with ratios of 0.6-0.8. The results of well gas test show that beach facies have a certain controlling effect on oil-gas productivity, and the marginal platform beach has more exploration potential than the high-energy beach in the platform. The study shows that in the area where carbonate strata are eroded, compared with the thickness of beach, the use of the ratios of shoal thickness to the whole stratum to accurately analyze the type and distribution range of granular beach so as to judge the development of granular beaches in the layer section.

Origin and trend analysis of karst collapse in Huangqi and Haibei communities, Foshan City

ZHANG Zongsheng

2022, 41(6): 880-894. doi: 10.11932/karst20220603

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The study area is located in the eastern part of Michong village, north Huangqi community, Dali town, Nanhai district, Foshan City. Situated in the southern margin of Guanghua basin and the transitional zone between hilly region and plain region in the northern Pearl River Delta, the area is flat in terrain and convenient by transportation. The study area is characterized by wet spring, long hot summer, warm autumn and winter without frost, with south subtropical monsoon climate. The annual average temperature in this area is 21.9 °C with an annual average precipitation of 1,400-1,900 mm and a multi-year average of 1,613.5 mm. Rainfall is concentrated from April to September, accounting for about 80% of the whole year, and the multi-year average evaporation is 1,153 mm, which is generally less than the rainfall. In recent years, intensive human engineering activities such as the construction of high-speed rail, underground engineering of subway and pile foundation construction in the area have induced a large number of karst ground collapses and ground subsidence. Especially from February 2008 to September 2009, the affected area reached 1.2 km², and 15 karst ground collapses occurred successively in the area, with the maximum land subsidence amounting to 87 cm. Damage, in different degrees, to Huangqi No.2 Middle School, DHL logistics Center, some factories and residential buildings was caused by the collapse and subsidence, in which more than 2,000 people were directly affected, and the direct economic loss was nearly 200 million yuan. The only exposed stratum in the study area is Holocene Guizhou formation (Q_hg), but the hidden strata include lower Carboniferous ShiDengzi formation (C₁s) and Ceshui formation (C₁c), upper Carboniferous Hutian formation (C₂h), lower Jurassic Jinji formation (J₁j), upper Cretaceous Dayangshan formation (K₂dl), and Quaternary residual layers (Q^el). There is one fault zone with 5 groups in the study area, belonging to Shabei fault zone extending from the south-west direction of the fault zone. The fault zone provides a good channel for groundwater migration and favorable conditions for the development of karst caves, and then controls the development trend of karst in the area. The rock and soil mass in the study area is divided into 15 engineering geological layers from top to bottom, and the groundwater can be divided into four types, pore water of loose rock, fissure cave water of overlying carbonate rock, fissure water of overlying bedrock and fissure water of overlying red bed. Quaternary soft soil and sandy soil are relatively developed. Hidden karsts are developed in the study area. Of a total of 266 boreholes collected in the area, soil caves in 16 boreholes were exposed with a cave-spotting rate of about 6.35 %, and karst caves were exposed in 44 boreholes with a cave-spotting rate of 22%. In the study area, there have been 15 ground collapses accompanied by geological disasters such as ground subsidence over a large area. Though the collapse and subsidence did not cause casualties, they brought about damage to Huangqi No.2 Middle School, DHL Logistics Center, some factories and residential buildings. Huangqi No.2 Middle School was relocated on December 3,2008, and 1,300 teachers and students were evacuated. The disaster was then estimated as an oversize one, posing a huge impact on the economy and society of the study area at that time. The development characteristics and causative mechanisms of karst ground collapse in the study area, show that the development of karst caves and soil caves is the prerequisite; the development of water-conducting fractures is the necessary condition; and the construction precipitation is the main trigger factor. The formation mechanism is closely related to the fluctuation of groundwater level, whose types can be divided into "continuous groundwater decline decline", "continuous groundwater decline + rainfall infiltration", "recovery of rapid groundwater rise" and "rainfall infiltration". There is still possibility of karst collapse near F1 fault zone in the study area. Controlling the groundwater level elevation within −3.0 meters and the fluctuation range of groundwater level within 1.5 meters can greatly reduce the probability of karst collapse, which can be used as the warning threshold. The research results can provide a basis for prevention and control of karst ground collapse in urbanization of Foshan City, and also provide a reference for disaster prevention and reduction in other similar areas.

Analysis of 6.10 water and mud inrush incident in the exit of parallel pilot tunnel of Chaoyang tunnel of Guiyang-Nanning high-speed railway

FU Kailong, ZHOU Yu, WEI Zhengxiong

2022, 41(6): 895-904. doi: 10.11932/karst20220604

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A high-speed railway running 350 km·h⁻¹ is being built between Guiyang in Guizhou Province and Nanning in Guangxi. Chaoyang tunnel is a double-track single-hole tunnel located in southeastern Guizhou. The tunnel clearance is 100 m²with the total length of 12,734 m (including more than 7-km-long soluble rock), and its maximum burial depth is 386 m. The railroad track elevation of the tunnel is 782-468 m, and it gradually decreases from the entrance to the exit, with a slope of −22.7‰ to −25‰. An auxiliary tunnel scheme of "2 transverse tunnels +2 parallel pilot tunnels +1 inclined shaft" has been adopted, with the parallel pilot tunnel on the left side of the main tunnel. The midline of the parallel pilot tunnel is 30 m away from the main tunnel, and 3 m lower than the main hole. The inner headroom size is 5.0 m (width) ×6.0 m (height). Construction of the tunnel began in early 2016 and will be completed in 2022. On June 10, 2018, karst water and mud inrush, lasting about 1 hour, occurred at the exit of parallel pilot tunnel of Chaoyang tunnel. The total amount of water and mud inrush was about 1.6×10⁶ m³. In order to complete the construction tunneling scheme and drainage scheme, it is necessary to analyze the causes of water and mud inrush, evaluate its risk in the subsequent construction, and calculate the water inflow of the tunnel. In this study area, the exit section of the tunnel is in the sloping medium-and-low mountainous area, belonging to the karst peak cluster landform, and the terrain fluctuates greatly. The tunnel passes th exit section of the tunnel is in the sloping medium and low mountainous area, belongs to the karst peak cluster landform with relatively sharp topographic relief. The tunnel passes through a regionally wide and gentle symmetrical anticline, and the exit of the tunnel is located on the southeastern wing of the anticline widely distributed by thick-to-extremely-thick bedded limestone with pure rock texture. Extreme development of surface karst funnels, dissolution basins, karst drop holes, karst caves and other phenomena provides favorable conditions for the collection, infiltration and runoff of surface water, but there is no long-term water system on the surface. After rainfall, the catchment of the non-soluble rock segment is mainly infiltrated through the contact zone between the soluble rock and the non-soluble rock, but the water catchment of the soluble rock section penetrates directly into the underground karst water system through the surface negative topography. Controlled by topography, geomorphology and transverse faults, except for the direction of groundwater seepage along the tectonic line in some sections, the infiltration groundwater is mainly discharged from the Zhangjiang river, which is basically perpendicular to the axis of the anticline and to the southeast wing (the exit of the tunnel). As a result, a series of transverse gullies and underground rivers cutting through the main structural lines were formed. The present study is mainly focused on the following aspects: analyzing the topography, engineering geology and hydrogeology of the tunnel, introducing the implementation of advance geological prediction on the face of the parallel pilot tunnel, and describing the process of water and mud inrush. In this study, the engineering geology and hydrogeology conditions in the area affected by the disaster were investigated and a one-year dynamic observation of the relationship between water inflow and rainfall was also completed. The research finding shows that a huge cavity and pipeline system with a water head of up to 84 m in front of the tunnel face caused the water and mud inrush. After excavation and exposing the bottom of the cavity, the pressurized water flow filling in the whole karst water system carried mud and sand rapidly into the tunnel face and rushed out of the parallel pilot tunnel with a large kinetic energy, resulting in the water and mud inrush event on June 10. In the process of this event, the surrounding karst water system was supplied in time, and the amount of water and mud inrush was much larger than the volume of large dissolution cavity near the tunnel face. The process of "water gushing-water shutoff-water gushing-water shutoff after dredging" occurred many times, indicating that the deposition in the lower part of the cavity is very serious and the drainage is not unobstructed. If the groundwater in the saturated cavity is not discharged effectively, the blasting excavation will cause serious water and mud inrush disaster. Long-term rainfall and water inflow monitoring shows that the karst water system at the exit of the tunnel receives rainfall infiltration recharge and the runoff is unobstructed. The catchment area corresponding to the water inrush to the tunnel is 6.423 km², and the maximum water inrush in the tunnel after the extreme rainstorm is 5×10⁴ m³·h⁻¹. After the occurrence of water and mud inrush, the static reserves in the mountain body have been fully released, and the groundwater level has dropped to the elevation of the tunnel floor. On the premise of ensuring the unobstructed drainage during the tunneling process, the risk of encountering water and mud inrush in the subsequent construction is low.

A study on collapse law of soil cave with different rainfall rates based on FLAC3D

XUE Mingming, CHEN Xuejun, SONG Yu, GAO Xiaotong, LI Hui, GAN Xiaohui, ZHANG Mingzhi, PAN Zongyuan, TANG Lingming

2022, 41(6): 905-914. doi: 10.11932/karst20220605

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Frequent heavy rainfall can lead to large-scale collapse of karst soil caves. Quaternary strata are widely distributed in the karst area of Lingui district, Guilin City, among which the subsidence area of the silty clay overlay with a one-dimensional structure formed by the Upper Pleistocene alluvial-pluvial layer Q₃^al-pl under the action of heavy rainfall is the largest in the collapse area of the entire study area. This subsidence area accounts for the largest proportion, posing a huge risk on security to the people. Heavy rainfall has aroused extensive attention and research on the collapse law of karst soil caves. Many scholars have revealed the mechanism of rainfall collapse, the effect laws of erosion, and water level fluctuations in the process of karst collapse from qualitative and quantitative perspectives. In order to better explore the collapse law of soil caves of different sizes with different rainfall rates, a generalized geological model was constructed for the study area, and the calculation models of karst soil caves of different sizes were established in the modeling software Rhino. Besides, the FLAC3D finite difference software was used to simulate the infiltration process of heavy rainfall with different rainfall rates. In this study, a karst calculation model—Mohr-Coulomb constitution and the parameters of physical and mechanical properties of the overlaying soil layer were firstly assigned. Then, the boundary conditions, the rainfall rate, the superimposed force field and the flow field were set to reproduce the cloud map of changes in stress, displacement and plastic zone during the evolution of karst soil caves in the process of heavy rainfall infiltration. The numerical simulation results show that, (1) Under the condition of heavy rainfall, the maximum displacements of soil caves with different diameters all appear at the top of the cave. With the same rainfall rate, the larger the diameter of the soil cave is, the faster the growth rate of vertical displacement becomes. The increase of rainfall rate may result in obvious increase of vertical displacement, and the vertical displacement of the cave top is positively proportional to the size of the soil cave. (2) Given the same rainfall rate, the expansion of the soil cave diameter has caused the further expansion of the shear failure area at the bottom of the soil cave. The overlying soil layer is mainly subjected to subsurface erosion in the early stage of heavy rainfall. When the rainfall rate is accelerated, the water-hammer gas explosion of the overlying soil caused by the violent fluctuation of the water level at the bottom of the soil cave becomes the dominant effect. The shear failure rate is accelerated, and the shear strain at the toe of the cave increases significantly. When the diameter of the soil cave reaches 3 m, the water level fluctuates more and more violently, which accelerates the destruction of the overlying soil layer. (3) The change of rainfall rate poses, in different degrees, the influence on the development range of plastic zone in the soil cave. In the soil cave with a larger diameter, the development range of plastic zoon is significantly expanded with the accelerated rainfall rate. The size of the soil cave and the speed of rainfall have a great influence on the stability of the overlying soil layer. These results provide a basis for the quantitative study of the relationship between heavy rainfall and collapse of the overlying soil, which is crucial to effective and reasonable warning to karst collapse.

A study on poverty caused by rocky desertification in karst areas of China

ZUO Taian, ZHANG Fengtai, YU Shijie, LI Jiao, FAN Hao, YE Dan

2022, 41(6): 915-927. doi: 10.11932/karst2021y40

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Rocky desertification is a land degradation process that changes a karst area covered by soil and vegetation to a rocky, desert-like landscape, which causes not only ecological problems but also serious social problems. Its negative impact on the local economy may not be apparent when land resources are abundant. However, in Southwest China, where the population density is already high, the expansion of rocky desertification has added pressure to people's lives. Since the issue of rocky desertification is put forward by scholars, poverty caused by rocky desertification has been a hotspot in the academic circle, which refers to the rural poverty and exacerbated by rocky desertification because the desertification deprives farmers' opportunities to improve their life by agricultural production. Land degradation (the exposure of basement rock or the thinning of soil layer) and ecological degradation (deforestation, the fragility of ecological environment and the reverse evolution of ecological system) worsens the endowment of land resources and even ecological resources, and thus results in a declining crop output but rising costs, which in turns will reduce farmers' income. Once the largest areas of poverty as well as the areas highly subject to the returning of poverty in China, advancing rural revitalization in the karst areas of Southwest China is both the government's and academia's working priority. Therefore, it is necessary for us to summarize the achievements and lessons of lifting out poverty caused by rocky desertification in karst areas, so as to consolidate poverty alleviation and build the theoretical basis of rural revitalization. This paper systematically reviewed the concepts, key historical trends and research progress of poverty caused by rocky desertification. Next, it summarized main research findings in four aspects—the relationship between rocky desertification and rural poverty, the causes of poverty by rocky desertification, the strategies for lifting out poverty, and the anti-poverty evaluation of rocky desertification control projects. And then relevant challenges were identified and put forward. The research shows that research on poverty caused by rocky desertification has undergone four stages since the 1980s, i.e., start-up, poverty alleviation and ecological construction, comprehensive control and green development. An interesting finding is that the research progress and Chinese scholars' research findings appear to be directly related to the Chinese government's major decision-making and growing policy support. Secondly, although much research has been made in this field, studies are achieved from different perspectives without a systematic theoretical system. However, ever since rocky desertification becomes a problem of concern in the academic circle, the strategies aiming at lifting out poverty caused by rocky desertification have drawn much attention from the government and academic circle. The research achievements mainly focus on the strategies adopted to alleviate poverty in the rocky desertification areas. Thirdly, the ambiguous boundary between poverty caused by rocky desertification and poverty in rocky desertification areas may be the main reason why control projects of rocky desertification are detached from projects of targeted poverty alleviation. Meanwhile, there are many factors for the poverty in rocky desertification areas, including the natural resources, population pressure, infrastructure, industrial structure, natural disasters, urbanization level, technological level, cultural background, education, location and policies. However, as only one type of poverty in rocky desertification areas, poverty caused by rocky desertification is attributed to the land and ecological degradation in karst mountainous regions is. Therefore, it is necessary to highlight the characteristics of rocky desertification and enhance research on the causes of poverty by desertification. Fourthly, there remains a harsh reality that the theoretical foundation lags behind practical work. The main reason of the decrease of rocky desertification areas is that the Chinese government has attached great importance to the rocky desertification problem, implementing many ecological construction projects. However, a majority of these ecological construction projects have not been targeted at restoring rocky desertification; instead, measures have been taken for water and soil conservation, and hill closing for forestation and forest planting. It is necessary to strengthen the study of rural man-earth relationship and relative poverty in rocky desertification areas, and reveal the restriction mechanism of rocky desertification hindering rural revitalization. Finally, the ecological environment in karst areas of China is improving. Although research achievements of anti-poverty evaluations of rocky desertification control projects have been found, the anti-poverty evaluation has not yet been assessed to the theoretical level of post-project evaluation. It is necessary to assess the impact of projects on improving the living standards of local farmers, summarize the relevant technical and economic activities, learn lessons from the experience, and put forward suggestions for decision-making.

Analysis of land use changes and their driving force in "Two Lakes and One Reservoir" area of central Guizhou Province based on the concepts of "Source and Sink"

LI Qingsong, SU Weici, LYU Sisi

2022, 41(6): 928-939. doi: 10.11932/karst2021y34

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"Two Lakes and One Reservoir", the main drinking water source of Guiyang City and even the urban agglomeration in central Guizhou, are typical karst plateau lakes. But the ecological environment around the lakes and reservoir is extremely fragile, where human activities are intense; urbanization is expanding rapidly; and land use changes are significant. Therefore, the land use changes have an important impact on the regional ecological security/water supply security and social and economic development. Based on the concepts of "source and sink" and the 4-phase interpretation data of remote sensing in "Two Lakes and One Reservoir" area in central Guizhou Province from 1998 to 2018, the dynamic model of land use was used to analyze the dynamic evolution law of land use in the study area in the past 20 years. Besides, quantitative analysis and qualitative analysis were combined to explore the driving force of land use changes. Results show that, (1) From 1998 to 2018, forest land and arable land were always the most important types of land use in the "Two Lakes and One Reservoir" area, with their respective proportions exceeding 35% in each period. (2) During the study period, the areas of construction land, forest land and unused land (mainly temporary unused land) increased continuously; the fluctuation of water area decreased; and the areas of arable land and grassland decreased continuously, among which changes of construction land and unused land presented most actively in each period. (3) The annual average dynamic degree of comprehensive land use in the study area increased repectively from 1.33% in 1998-2008 to 2.4% in 2008-2013 and 2.3% in 2013-2018. In the past 10 years, under the background of accelerated regional industrialization, urbanization and agricultural and rural modernization, the mutual conversion between various categories of land use has been more frequent than 10 years ago. (4) From 1998 to 2018, the main "source and sink" conversion relationships of land use in the study area were shown as the conversion of arable land into forest land and construction land, and grassland into forest land and arable land. Arable land was the main "source" of construction land increase; forest land was the main "sink" of arable land; and arable land and forest land were the main "sink" of grassland. (5) The "source and sink" coefficients of variation of mutual conversion among different categories are quite different, among which the "source" coefficient of variation of unused land converted to grassland is the largest (110.14%), and the annual average conversion is the most unstable. The "source"coefficient of variation of grassland converted to arable land is the smallest (19.84%), and grassland is the most stable source of farmland. (6) The conversion from various types of land into construction land has been accelerated; the area of the built-up area has expanded rapidly; and the water supply pressure of "Two Lakes and One Reservoir" has further increased. Population pressure and economic development are the main driving factors for the expansion of construction land in the study area. Policy factors such as Returning Farmland to Forest Program and the establishment of the "Two Lakes and One Reservoir"Administration Bureau have an important impact on the change of forest land and water area. In the future, it is necessary for us to focus on and solve the contradiction between the rapid urban expansion and the protection of arable land and water sources in the "Two Lakes and One Reservoir"area. The research results can provide a reference for the land use planning and social-economic-ecological coordinated development in the "Two Lakes and One Reservoir" area.

Correlation between the composition and diversity of community species and soil factors under two geological conditions-A case study of Mao village in Guilin

ZHANG Xunyuan, ZHOU Yunchao, HUANG Mei, BAI Yunxing, ZHANG Chunlai

2022, 41(6): 940-951. doi: 10.11932/karst20220606

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In order to compare the differences of species composition and diversity between karst and non-karst areas, and to explore the influence of different soil factors under these two geological conditions on species diversity, we conducted a field investigation in Maocun of Guilin that is composed of both karst and non-karst areas. Results show that a total of 123 species is recorded in the study area, including 87 species, 46 genera and 35 families in karst area and 95 species, 61 genera and 48 families in the non-karst area. The number of species composition in non-karst area is significantly higher than that in the karst area. In addition, the quadrat of species diversity in karst area is listed as: herb quadrat>shrub quadrat>tree quadrat. In the non-karst area, it is listed as, shrub quadrat>tree quadrat>herb quadrat. Finally, different types of geological background contribute to different soil factors, among which the total nitrogen content of soil has the most significant effect on species diversity. In conclusion, the species composition and diversity in the karst area is significantly lower than that in the non-karst area, and there is a correlation between soil factors and species diversity. Total nitrogen of soil has the most significant effect on species diversity, and the soil depth weakens the correlation between soil factors and species diversity. Comparing the relationship between soil factors and species in karst and non-karst areas provides a theoretical basis for the restoration and reconstruction of regional ecosystem.

Spatial accessibility analysis of urban parks in typical karst area based on minimum cost of proximity: A case study of Guiyang City

AI Bing, HUANG Wei, LI Yangbing

2022, 41(6): 952-961. doi: 10.11932/karst20220607

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Urban parks are important to urban residents' physical and mental well-being, social harmony, and sustainable urban development. Constrained by landform features such as high mountains, steep slopes and inadequate flatlands, cities located in karst mountainous regions often face challenges of inadequate land available for construction, high population density in built-up areas, and small areas of park space per capita. With accelerated urbanization in recent years, some cities have built urban parks in natural karst hills as a means to increase both the quantity and quality of park service. However, some of these parks featured with tower karst or cluster-peaks are subject to limited tourist capacity and low-level barrier-free access and thus fail to meet visitors' basic need for recreation. Therefore, an appropriate design and layout of urban park system in a karst mountain city should be conducted based on such key elements as the size of park patch, the topography of the park site, and the features of population distribution in the city. With the principle of minimum cost of proximity, this study takes Guiyang, a typical karst mountain city, as an example and discusses strategies to improve urban park accessibility in karst mountain cities. In this study, we first set topographic coefficients. The percentage of park space with a gradient less than 15 degrees was determined to indicate the topography of the park. 923 urban parks in Guiyang were then categorized into "comprehensive parks" and "parks in residential areas" based on the sizes of the park patches. They were also classified into "parks with flat landform" and "montane parks" based on their topographic coefficients. Subsequently, with the principle of minimum cost of proximity, we measured the distances of 2,616 residential communities to their nearest and to other different types of parks. These distances were then used to assess the accessibility level of different park types. Thirdly, we conducted the spatial autocorrelation analysis to examine whether there would be spatial aggregation in various types of parks with different accessibility levels and what the pattern would be for their spatial distribution. In this study, we have made the following findings. Firstly, the total amount of urban parks in the study area is large, accounting for 7.54% of the study area and a per capita park area of 28.98 m². Secondly, features of park patches are evidently dichotomous in terms of their areas. 93.17% of the parks cover an area smaller than 10 hectares, occupying 10.52% of the total park area, whereas 25 large-scale parks with an area larger than 50 hectares account for approximately 80%. Thirdly, the terrain of most parks is mountainous. 53% of the parks are located at a gradient larger than 15 degrees, and 6.73% of the parks in residential areas present a topographic coefficient below 0.6, some of these parks are completely built in tower karst or small cluster-peak areas where service capacity is limited. Fourthly, the accessibility of parks is generally good, with a good match between the park's service area and the population distribution. The minimum cost of 91.89% of the communities to a park is less than 500 meters. There are differences in the accessibility level of different park types, but within a certain distance, accessibility levels are still able to meet the varied needs for park services. For more than 80% of the residential communities, their minimum cost to a comprehensive or a montane park is less than 1,500 meters. Lastly, spatial distributions of various park types are relatively balanced in the central part of residential areas in Yunyan, Nanming, and Wudang. In the central part of Guanshanhu residential area, however, there are less montane parks. In addition, 10% of the residential blocks have high accessibility to montane parks but low accessibility to parks in flat areas. The fact that park service may not cover these blocks caused by karst landform results in residents' difficulties in accessing parks with large open space and high-level barrier-free paths in the vicinity. In order to improve park accessibility and service quality, the areas without the coverage of park services should firstly be provided with the services. Small parks should be built in small-scale residential areas scattered along the edge of the city. In the residential areas close to montane parks but far from parks with flat landform, parks can be built on small patches of smooth terrain. Secondly, the working focus should be placed on enhancing the quality and diversity of park services after satisfying the basic need for park services. In old urban areas where it is difficult to add large parks, efforts should be made to increase small park patches and the park service capacity per unit area. In the new urban development areas, considerations should be given to principles of proximity and quality in distributing different types of parks. Natural features of karst hills can be used to good advantages in building a unique park service system for karst cities. In addition, based on the features of population distribution and spatial patterns of park types in the city, park design in karst hills should proceed appropriately so as to avoid damage to the ecological environment and prevent parks from falling into disuse.

Research progress of soil erosion in karst areas of southwest China

LI Chengfang, WANG Zhongcheng, LI Zhenwei, XU Xianli

2022, 41(6): 962-974. doi: 10.11932/karst20220608

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As one of the largest contiguous karst areas in the world, southwest China is characterized by shallow soil layer and low rate of soil formation. The very low tolerance of soil loss in this area may lead to the extreme difficulty in the recovery of soil layer once lost. Therefore, soil erosion and its effects such as the rocky desertification have become the most serious problem of ecological environment, restricting the sustainable development of the karst area. The purpose of this study is to review the characteristics of soil and water loss at different scales, identify the limitations of different evaluation methods for soil erosion, and to predict the future research on the soil erosion in karst areas of southwest China. Firstly, according to plenty of research data about soil erosion in karst areas at home and abroad, the characteristics of soil and water loss in karst areas are expounded at various spatial scales such as slope, small watershed, and region. Additionally, the characteristics of soil erosion in karst areas and non-karst areas are compared at these three scales. The karst area is featured by a binary three-dimensional erosion system—runoff and sediment production processes at the slope scale and the sediment transport process at the watershed scale. At the slope scale, the effect of rainfall intensity and duration, underground fissure degree, slope, and bedrock exposure rate on soil erosion are investigated at both indoor and field runoff plots. However, because of the big difference between the experimental conditions of indoor rainfall simulation and natural conditions, it is essential to investigate the soil erosion characteristics at the field runoff plots. Meanwhile, the instability of rainfall intensity and the uneven distribution of fissure degree of underground pore also pose a challenge to the accurate quantification of the correlation between rainfall intensity, fissure degree of underground pore and sediment yield in the field runoff plots. Generally, radionuclide tracing technology can quantitatively identify the spatial distribution of intensity and rate of soil erosion. However, due to the high bare rock ratio, karst slopes with high soil homogeneity and continuous soil cover must be selected to avoid the influence of bare rock on the content of radionuclide. At the small watershed scale, monitoring the discharge and sediment concentration at the outlet of watershed is a typical method to analyze the characteristics of runoff and sediment yield in small watershed, and to establish the relationship between sediment yield and runoff or its influencing factors. This method is also used to predict the average annual soil loss and soil erosion intensity in a small watershed, and to analyze the temporal and spatial variation in soil erosion for different land use patterns, altitudes and slopes. But the application of various models is still limited by some factors. For example, RMMF model is generally used within a county territory; the distributed soil erosion model has not been verified, and the influence of initial soil water content and runoff on soil erosion have not yet been considered in this model. In addition, peak-cluster depression is a special geological and geomorphic type in the karst area of southwest China, and its topographic features contribute to the depositing of most eroded sediment in the central karst depression. Radionuclide tracing technology can be applied to analyze the sediment interception rate and sediment transport ratio of different types of karst depressions, river beaches and ponds, to determine the sediment source and to establish the sediment yield model of river basins. At the regional scale, the regional differences and temporal dynamic changes of soil erosion and sediment discharge are investigated, and the dominant factors controlling the sediment discharge are also identified. The relationship between different topographic indexes and regional soil erosion is discussed, and the optimal evaluation index system of regional soil erosion is also determined. Based on the soil erosion model (RULSE), the temporal and spatial variation in soil erosion on the regional scale is investigated. In terms of research advance, it is suggested that future studies should be focused on the occurrence and development of soil erosion as well as the spatial and temporal evolution patterns in karst rocky desertification. It is necessary to explore the occurrence, development and temporal and spatial evolution patterns of soil erosion in karst areas on a larger scale, and to quantitatively evaluate the amount of soil erosion by combining high-tech remote sensing technology, geophysical exploration and different models. According to the correlation between soil loss and its influencing factors, a reasonable model for ecological restoration management of soil and water conservation and a relevant monitoring and evaluation system should be constructed. This study could provide theoretical and technical support for controlling soil erosion in the karst area of southwest China．

Hydrogeological structure model of the fault zone in the karst area of north China

LIU Yuanqing, ZHOU Le, WANG Xinfeng, LV Lin, LU Xiaohui, YU Kaining, ZHANG Weifeng

2022, 41(6): 975-985. doi: 10.11932/karst20220609

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The fault zone has a controlling effect on the mechanical properties and fluid flow characteristics of the crust. It is an extensively distributed and very important structural pattern in the upper crust, and widely participates in the crustal activity process. At present, the structural composition and fluid flow pattern of fault zones are hot and difficult points in foreign research, and the research methods and understandings are different in various fields. In the field of petroleum geology, mature quantitative evaluation techniques for fault sealing have been developed in hydrocarbon migration and accumulation. In contrast, research of hydrogeological properties of faults is still in the stage of qualitative evaluation of mechanical properties on the water conductivity or water resistance of faults. Studies on fault zone structure, permeability anisotropy and other aspects have not yet been carried out. At present, many scholars and teams at home and abroad have done much meaningful research and discussion on the internal structure characteristics of the fault zone and its influence on fluid flow. This paper summarizes the results of foreign studies on the hydrogeological properties of fault zones in terms of structural composition, fault zone evolution, permeability factors, etc. The hydrogeological structure model of the fault zone in the northern part of the Taihang Mountains is introduced by taking the fault that is developed in the carbonates of the Cambrian Zhangxia formation with clastic water-bearing rocks as an example. In previous studies on the composition and permeability structure of fault zone, its composition is divided into fault core and fracture zone. The structure of fault core and fracture zone determines the heterogeneity and anisotropy of permeability structure of fault zone. In the fault zone, the thickness ratio between fault core and fracture zone provides a convenient and generalized framework for describing the hydrogeological characteristics of fault zone. If the permeability of each component of the fault zone is combined with geological maps, geological profiles, or three-dimensional structural models of the fault zone, the permeability structure of the fault zone can be obtained from field outcrops. The permeability characteristics of each component of the fault zone are the most important to study the fluid flow properties of the fault zone. The permeability characteristics are mainly affected by the structural characteristics of the fault zone, the properties of the fluid passing through the fault zone, the tectonic stress, the scale of the fault slip and other factors. The geometric structure and permeability of the core and fracture zone are the main controlling factors to characterize the hydrogeological properties of the fault zone, which is characterized by the type of water-blocking and water-guiding system of fluid flow. According to the thickness ratio of its component parts, the fault zone can be divided into four types: single fault, dispersed deformation zone, local deformation zone and composite deformation zone. Meanwhile, the corresponding permeability structures are local water conduction, dispersed water conduction, local water resistance, and composite water-resistance. In order to accurately reflect the hydrogeological properties of the fault zone, in this study, we should take the different positions of the fault zone into full consideration when constructing the hydrogeological structure model, and to generalize the permeability structure. The structure and hydrogeological properties of the studied fault zone will be different due to the different scale and precision of the selected fault zone and the different development sites. Taking the northern Cambrian carbonate aquifer as an example, there are clastic rocks with different thickness and relative water isolation, where lithology is dominated by thin layer shale. At different positions of the same fault zone, the different distribution characteristics of the water-proof layer and the fracture mechanism result in significant differences in the permeability and hydrogeological characteristics. How to establish the characteristics of the spatial structure and permeability of the fault zone is the basis and premise for the accurate evaluation of the fluid flow characteristics of the fault zone, which requires geologists to have the professional knowledge of structural geology, rock mechanics, numerical simulation and other fields. Through the sharing of data in various fields and the integration of multidisciplinary methods, accurate and typical permeability structure models of fault zone are established so as to promote the study on development of fault zone and fluid flow characteristics.

Water abundance of karst fissure water and its electrical properties in north Taihang Mountains: A case study of mountainous area in the west of Baoding

LU Fang, LUO Xuan, HU Wenguang, YU Lei

2022, 41(6): 986-997. doi: 10.11932/karst20220610

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In north Taihang Mountains, there are mainly located the karst fissure groundwater systems of Juma river, of Puhe river-Caohe river and of Jiehe river-Tanghe river. The water-bearing formation of carbonatite karst fissure in the research area can be divided into the Ordovician and the Jixian water-bearing formations. The Ordovician water-bearing formation is mainly developed with chert zebra dolomite and limestone with holes, solution cracks and fissures and groundwater concentration zones. The Jixian water-bearing formation is made up of chert zebra dolomite with fissures, where water abundance is good. The aquifer medium structures in the research area are divided into corrosion pore type, fault type, fold type and water blocking type of rock mass. Zijingguan fault is a normal fault with relatively fragmented hanging wall. The fissures of dolomite and limestone are developed in the fracture zone with good water abundance. To determine the spatial variation of water abundance of karst fissure water in the northern Taihang Mountains and the electrical characteristics of fissure water, karst fissure water in the northern Taihang Mountains was taken as research object and the data of unit water inflow of 39 boreholes was obtained by pumping tests. According to the characteristic of karst fissure water i.e., obvious induced polarization response, four electrical parameters—resistivity, polarizability, half damping time and deviation degree—were acquired by induced polarization method. Statistical analyses and box-whisker plots were processed by Grapher software. The distribution characteristics of four electrical parameters were studied. The relationships between electrical parameters and water abundance of karst fissure water in different aquifer medium structures, water-bearing formations and groundwater systems were analyzed. Interfering factors of water abundance of karst fissure water in the research area and its electrical response characteristics were discussed. The range delineated by lower box (25%) and upper box (75%) of unit water inflow or electrical parameters reflects the dispersion degree of 50% of the data in the central range, which is indicative of various types of karst fissure water in the research area. Results showed that the structure of aquifer media was arranged in the order of corrosion pore type→fault type→fold type→water blocking type of rock mass. In this sequence, the distribution range of half aging value decreases in turn, while the distribution range of deviation value increases correspondingly. It is difficult to distinguish the four types of aquifer medium structures by resistivity and polarizability. Water-bearing formations of the Ordovician system and the Jixianian system could be identified effectively with resistivity, polarizability, half damping time and deviation degree. It was hard to identify groundwater system types among the Juma river, the Jiehe river-Tanghe river and the Puhe river-Caohe river just by the use of polarizability, half damping time and deviation degree. Water abundance in the Ordovician system is better than its value in the Jixianian system. The best water abundance was found in corrosion pore type, followed by fault type, fold type, and water blocking type of rock mass in sequence.

Nitrate pollution characteristics and identification of nitrate sources in Baotu Spring area of Jinan

ZHANG Hailin, WANG Zhong, LIN Guangqi, XU Yuan, MA Xueying, GUAN Qin

2022, 41(6): 998-1006. doi: 10.11932/karst20220611

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Under the control of the topography and the geologic structure, karst water in Baotu Spring of Jinan City is supplied in the southern mountain area, and flows from south to north along the karst fractures. In front of the northern mountain, karst water runoff is blocked by the hidden igneous rock mass. Therefore, many famous springs have been formed. Karst is developed in the spring area with the aquifer lithology of primary Cambrian and Ordovician limestone. The geological structure of this area is complex with low mountains in the south and the piedmont sloping plain in the north. In recent years, with the rapid development of industry and agriculture, activities such as industrial production, agricultural planting and domestic waste discharge have led to different degrees of pollution of karst water in Jinan. Among these different types of pollution, the surface pollution caused by nitrate, one of the main pollutants in the spring area, is more prominent and related to the safety of water environment and local people's health. In order to effectively identify the sources of nitrate pollution, prevent the pollution, and ensure the safety of local drinking water, we carried out the accurate qualitative and quantitative identification of nitrate pollution sources in this study, according to the traditional water chemical analysis, the values of nitrogen and oxygen isotopes and the quantitative source analysis model. Based on the systematic hydrogeological analysis of Baotu Spring in Jinan, we collected 21 groups of nitrate samples of karst water, 14 groups of nitrogen and oxygen isotope samples, 2 groups of sewage isotope samples and 1 group of isotope samples for atmospheric precipitation from the end of May to the beginning of June, 2016. According to the analysis of test data, the nitrate concentration presents a large degree of dispersion in spatial distribution, with a coefficient of variation of 80%, which shows that the nitrate concentration is greatly affected by hydrogeological conditions and human activities. Because nitrates from different sources have different δ¹⁵N and δ¹⁸O values, combined with the investigation of pollution sources, the main sources of nitrate pollution in karst water in the study area are animal manure and sewage, soil organic nitrogen, and NH $_4^{+}$ in chemical fertilizers. The contribution rate of each pollution source is calculated quantitatively by IsoSource model. Results show that the contribution rate of animal manure and sewage sources is the largest, with an average of 51.07%, followed by soil organic nitrogen and NH $_4^{+}$ in chemical fertilizers, with an average of 25.21% and 23.71% respectively. The study shows that human activities are the main factor of nitrate increase in karst water of this area. The nitrate sources in the east and west of the study area are obviously different. Nitrate in the eastern region mainly comes from animal manure and sewage, with an average of 61.5% occupying more than 50% of the sample. The proportion of NH $_4^{+}$ sources in soil organic nitrogen and chemical fertilizers is relatively small, with an average proportion of 24.33% and 14.16% respectively. The main sources of nitrate in the western region are animal manure and sewage, but the proportion of NH $_4^{+}$ sources in fertilizers is increasing. In southwest mountainous area, the average proportion of soil organic nitrogen is 46%, and the proportion of chemical fertilizer is relatively small. But due to the intensive agricultural cultivation in the piedmont area, more use of chemical fertilizers results in the increasing proportion (an average of 36.2%) of NH $_4^{+}$ sources in fertilizers. From south to north, the source of soil organic nitrogen has been gradually transformed into the source of chemical fertilizers in the piedmont plain.

Development characteristics and prevention countermeasures of karst collapse in Zhongqiao of Yantai

WANG Youzhi, YANG Ning, YU Linhong, LV Lingfeng

2022, 41(6): 1007-1015. doi: 10.11932/karst20220612

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The limestone distribution area of Zhongqiao is a typical representative of the karst landform distribution in Jiaodong Peninsula. With an area of about 30 km², it is located about 25 km southwest of Yantai City, and is the core area of Fushan Economic-Technological Development Zone. In recent years, frequent karst collapse has seriously affected the economic development and local people's life, and even brought some panic to the them. 17 times of karst collapse in the study area occurred in the covered karst area. The collapse began in 2005 (2 times), and then occurred 17 times in 2014-2021. The diameter of the collapse pit varies from 1 m to 10 m, and the depth is mostly 3-5 m. The collapse is mostly elliptical and circular on the plane, appearing in groups, and the section is jar-shaped and funnel-shaped. In terms of space, the collapse is mainly located near the groundwater runoff zone, river bed, gully and terraces on both banks where geological structures are developed. The covering soil thickness of collapse point is mostly between 10-15 m. In order to provide a scientific basis for the future rational planning of groundwater exploitation and monitoring and early warning of karst collapse, and provide a geological basis for the prevention of karst collapse, we have further analyzed the spatial and temporal distribution, the development characteristics, process and triggering factors of karst collapse in the study area to better understand the development mode and evolution law of karst collapse in Jiaodong Peninsula. In this paper, the development characteristics, process and triggering factors of karst collapse are studied from the aspects of mechanics, structural geology and hydrogeology. The research shows that among the many factors of collapse, stratum lithology is the basic condition; the geological structure is the dominant factor; and the hydrodynamic condition is the decisive factor. From the perspective of geomorphology, the karst development is more intense within 300 m from the river, and karst collapse occurs in the V-shaped area between the two rivers. In terms of cap rock and structure, karst collapse is mostly developed in the fragile area of karst area where the thickness of overburden is 10-15 m and the hanging wall of fault is less than 1 km. Given the basic conditions of karst collapse, collapse trigger factors, such as heavy rainfall, excessive exploitation and drainage of groundwater and continuous drought, especially the continuous fluctuation of water level 0-2 m above limestone surface for more than 5 days, are more likely to trigger collapse. In the prevention and control of karst collapse, we must first rationally plan the groundwater mining and drainage scheme, then establish a groundwater monitoring and early warning network in the area, and build a water-stopping curtain around the mine.

2022 Vol. 41, No. 6

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