Raziskovanje udorov na kraških območjih z uporabo integriranih geofizikalnih metod: primer iz okrožja Conghua, mesta Guangzhou, Kitajska

Integrated geophysical methods were used to investigate the geological conditions of karst collapses in Aotou, Conghua District, Guangzhou City. Based on audio frequency magnetotelluric (AMT) soundings, high-density electric signals and microtremors, a series of features, including the soil structure and thickness, faults, and hidden karst, were detected from shallow to deep subsurface regions in three-dimensional (3-D) space. These were then compared with geologic and drilling data. The measurements of micro-vibrations and high-density electric signals revealed that the strata in the collapsed area consists of clay and medium-grain sand, which is approximately 8–15 m thick and arranged as a multiple-element structure. The AMT soundings uncovered a hidden structure in the subsurface, which lies at the core of a syncline and adjacent to a contact zone between soluble limestone and non-soluble granite. Combined with the geologic data, the survey using the high-density electric method also showed that the study area hosts the Shidengzi Formation from the Datang Stage of the lower Carboniferous, which contain carbonaceous micrite and dolomitic limestone, locally intercalated with thin quartz siltstone, with well-developed karst. Collectively, the soil structure, hidden structure in the subsurface, and karst development provide the primary conditions for collapses. The main triggering factors for the karst collapses are seasonal variations in groundwater levels and excessive groundwater withdrawals. These results can provide guidance for the selection and application of geophysical methods for the examination of this issue in other areas with similar geology.Pri raziskovanju geoloških razmer za pojav udorov na območju krasa v Aotouju, okrožju Conghua, mesta Guangzhou so bile uporabljene integrirane geofizikalne metode. Z uporabo avdiofrekvenčnih magnetno teluričnih (AMT) sondiranj, visokofrekvenčnih električnih signalov in mikrotremorjev smo zaznali razne elemente v podzemlju, kot so debelina in struktura prsti, prelomi in nepoznani podzemni prostori. Izsledki so bili primerjani s podatki geoloških kartiranj in napravljenih vrtin. Meritve mikrovibracij in električnih signalov so razkrile, da so formacije na območju udorov sestavljene iz gline in srednje zrnatega peska, v skupni debelini približno 8‒15 m. Magnetno telurično sondiranje je pod površjem razkrilo neznano strukturo, ki leži na jedru sinklinale ob stiku med topnim apnencem in netopnim granitom. Skupaj z geološkimi podatki je metoda z električnimi signali pokazala, da področje študije zajema formacijo Shidengzi, stopnje Datang iz spodnjega Karbona, ki vsebuje mikritni in dolomitni apnenec, ki se lokalno izmenjuje s tankimi plastmi kremenovega meljevca. Na tem območju je nastal dobro razviti kras. Ugodna sestava prsti, neznane strukture pod površjem in zakrasevanje zagotavljajo dobre razmere za nastanek udorov. Glavni dejavniki udora kraških območij so sezonska nihanja nivojev podzemne vode in čezmerna črpanja podtalnice. Pridobljeni izsledki zagotavljajo smernice za izbor in uporabo ustreznih geofizikalnih metod za raziskovanje teh težav v drugih regijah s podobno geologijo

Integrated Geophysical Methods for Groundwater Exploration in a Karst Area with or Without Thin Cover – A Case Study from Tai’an City, Shandong Province, China

pg(s) 255-26

Integrated geophysical methods for groundwater exploration in a karst area with or without thin cover - a case study from Tai\\u27an City, Shandong Province, China NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst

pg(s) 255-261 Because of heterogeneity and anisotropy, it is very difficult to optimize groundwater exploration drilling locations in karst areas using only hydrogeological information. However, the integrated application of the audio frequency telluric method and electrical resistivity tomography has proved to be efficient for groundwater exploration in karst areas with or without thin cover. In the case studies presented here, audio frequency telluric profiling is used to roughly determine the location and strike of a karsted or fractured zone where the cover thickness is less than 30 m, then an electrical resistivity profile perpendicular to the strike of the zone is designed to reconstruct the resistivity structure with a Schlumberger array. By combining the geophysical results with available hydrogeological data, an optimal drilling site can be established. This integrated geophysical approach for karst water exploration has been used in several projects and the results show that the method is reasonable and useful. Open Access - Permission by Publisher See Extended description for more information

Evaluation of Terrestrial Water Storage Changes and Major Driving Factors Analysis in Inner Mongolia, China

Quantitative assessment of the terrestrial water storage (TWS) changes and the major driving factors have been hindered by the lack of direct observations in Inner Mongolia, China. In this study, the spatial and temporal changes of TWS and groundwater storage (GWS) in Inner Mongolia during 2003–2021 were evaluated using the satellite gravity data from the Gravity Recovery and Climate Experiment (GRACE) and the GRACE Follow On combined with data from land surface models. The results indicated that Inner Mongolia has experienced a widespread TWS loss of approximately 1.82 mm/yr from 2003–2021, with a more severe depletion rate of 4.15 mm/yr for GWS. Meteorological factors were the driving factors for water storage changes in northeastern and western regions. The abundant precipitation increased TWS in northeast regions at 2.36 mm/yr. Anthropogenic activities (agricultural irrigation and coal mining) were the driving factors for water resource decline in the middle and eastern regions (especially in the agropastoral transitional zone), where the decrease rates were 4.09 mm/yr and 3.69 mm/yr, respectively. In addition, the severities of hydrological drought events were identified based on water storage deficits, with average severity values of 17 mm, 18 mm, 24 mm, and 33 mm for the west, middle, east, and northeast regions, respectively. This study established a basic framework for water resource changes in Inner Mongolia and provided a scientific foundation for further water resources investigation

Detection and Treatment of Sinkholes and Subsurface Voids Along Guilin - Yangshuo Highway, Guangxi, China

The 66 km long Guilin - Yangshuo highway is located in northern Guangxi, China. The entire highway was built through an active karst area characterized by tower karst, karst valleys, and watershed divides. The highway construction has induced many sinkholes since the construction started in 2004. The primary goals of this research are to detect locations of subsurface voids and potential sinkholes and to treat these voids to prevent future collapses. A 200m section representing typical karst setting of the highway was chosen for a preliminary survey. Ground Penetrating Radar (GPR), multi-electrode electric resistivity, and shallow seismic reflection were used for the preliminary survey. Comparison of the three geophysical methods indicates that GPR can detect subsurface voids in a more effective and timely manner. Therefore, GPR was applied to a 12 km section highway to located subsurface voids in a highly active karst area. 337 subsurface voids, 1-6 m in diameter, 1-5.5 m deep, and density up to 48 per 100 m, were detected using GPR. Further assessment of the study area indicates that water table fluctuations around bedrock and soil interface cause the formation of subsurface voids. Excavation for the foundation of the highway induced more surface water into the karst aquifer and water pumping from the bridge foundation significantly lowered the water table of the karst aquifer. Highway construction changed the natural drainage pattern and caused more hydrodynamic changes of the karst aquifer, which in turn formed more subsurface voids or enlarged existing voids. Finally excavation, graded-filter, and geofabric lining techniques were suggested to treat clustered subsurface voids and grouting was suggested to treat isolated voids. This treatment plan has been accepted by the highway construction company

Evaluation of Groundwater Storage Depletion Using <i>GRACE</i>/<i>GRACE</i> Follow-On Data with Land Surface Models and Its Driving Factors in Haihe River Basin, China

Groundwater storage (GWS) in the Haihe River Basin (HRB), which is one of the most densely populated and largest agricultural areas in China, is of great importance for the ecosystem environment and socio-economic development. In recent years, large-scale overexploitation of groundwater in HRB has made it one of the global hotspots of GWS depletion. In this study, monthly GWS variations in HRB from 2003 to 2020 were estimated using the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) data in combination with three land surface models (LSMs) from the Global Land Data Assimilation System (GLDAS). The results show the following: (1) HRB suffered extensive GWS depletion from 2003 to 2020, which has been aggravated since 2014, with a mean rate of 1.88 cm·yr−1, which is equivalent to a volume of 6 billion m3·yr−1. The GWS depletion is more serious in the plain zone (−2.36 cm·yr−1) than in the mountainous zone (−1.63 cm·yr−1). (2) Climate changes are excluded from the reasons for GWS depletion due to annual precipitation and evaporation being close to normal throughout the period. In addition, GWS changes show a low correlation with meteorological factors. (3) The consumption of groundwater for irrigation and land use/cover changes have been confirmed to be the dominant factors for GWS depletion in HRB. (4) The effects of inter-basin water transfer projects cannot be obviously observed using the GRACE and GRACE-FO; more inter-basin water transfers are needed for recovering the GWS in HRB. Therefore, it is imperative to control groundwater exploitation and develop a more economical agricultural irrigation structure for the sustainability of groundwater resources in HRB

Evaluation of Groundwater Storage Depletion Using GRACE/GRACE Follow-On Data with Land Surface Models and Its Driving Factors in Haihe River Basin, China

Groundwater storage (GWS) in the Haihe River Basin (HRB), which is one of the most densely populated and largest agricultural areas in China, is of great importance for the ecosystem environment and socio-economic development. In recent years, large-scale overexploitation of groundwater in HRB has made it one of the global hotspots of GWS depletion. In this study, monthly GWS variations in HRB from 2003 to 2020 were estimated using the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) data in combination with three land surface models (LSMs) from the Global Land Data Assimilation System (GLDAS). The results show the following: (1) HRB suffered extensive GWS depletion from 2003 to 2020, which has been aggravated since 2014, with a mean rate of 1.88 cm&middot;yr&minus;1, which is equivalent to a volume of 6 billion m3&middot;yr&minus;1. The GWS depletion is more serious in the plain zone (&minus;2.36 cm&middot;yr&minus;1) than in the mountainous zone (&minus;1.63 cm&middot;yr&minus;1). (2) Climate changes are excluded from the reasons for GWS depletion due to annual precipitation and evaporation being close to normal throughout the period. In addition, GWS changes show a low correlation with meteorological factors. (3) The consumption of groundwater for irrigation and land use/cover changes have been confirmed to be the dominant factors for GWS depletion in HRB. (4) The effects of inter-basin water transfer projects cannot be obviously observed using the GRACE and GRACE-FO; more inter-basin water transfers are needed for recovering the GWS in HRB. Therefore, it is imperative to control groundwater exploitation and develop a more economical agricultural irrigation structure for the sustainability of groundwater resources in HRB