Study on the microstructure and variation law of carbonate rock dissolution based on scanning electron microscopy and CT imaging technology
The dissolution of carbonate rocks is the most important process of karst development, and it is also the basis for the processes affecting karst geomorphological evolution, hydrochemistry of karst water, karst water system development, karst carbon sinks and global carbon cycle. Scholars have explored the dissolution mechanism of carbonate rocks and the factors that affect the development of carbonate karst through indoor simulated dissolution and in-situ experiments, but they mostly focus on the effect of chemical components of rocks on karst processes. In field surveys, it was found that not only limestone formations, but also some dolomite formations have developed karst. It is not comprehensive enough to explain the solubility of carbonate rocks only by chemical composition. The pore structure of the rock also plays an important role,but the influence mechanism of rock pore structure on karst development is still unclear. In order to study the variation law and control effect of carbonate rock pores under dissolution experiment,in this paper, we selects four types of carbonate rocks (Sinian Dengying Formation-medium fine-grained dolomite, Cambrian Tianheban Formation-oolitic limestone, fine-grained limestone, and Qinjiamiao Formation-dolomitic breccia)in the Three Gorges area to carry out dissolution experiments. At the same time, scanning electron microscopy and CT imaging methods were used to test and analyze the dissolution characteristics and pore structure of rocks before and after dissolution reaction. The results show that, (1)The dissolution of carbonate rocks always occurs in soluble minerals with low lattice energy and develops along the rhombohedral cleavage planes and weak parts of mineral crystals,showing the selective dissolution of soluble minerals and pore structures. Dissolution preferentially dissolves soluble minerals, and insoluble components either fall off or protrude from the rock surface.Due to the control of rhombic cleavage on the surface of dolomite crystals, the dissolution will develop into dissolution fractures along the cleavage, and at the same time, dissolution pores appear on the surface of dolomite crystals, showing a "honeycomb" dissolution characteristic; Lattice defects, cleavage, and the weaker plasmonic and molecular contact forces at the edge of the crystal are the places where the dissolution preferentially occurs, and the dissolution strength at the rock pores is slightly higher than that at other parts. (2) The pore size of carbonate rocks controls the development of dissolved pores and the evolution of dissolved pores during rock dissolution.For oolitic limestone and fine-grained limestone samples, because their pore diameters are mainly concentrated in 18-80 μm and the pore distribution is uniform,after the dissolution reaction, the small pores further increased, only a small amount of pores were enlarged by dissolution, and the area with increased pore size was mainly concentrated in 40-50 μm,and the pores were mostly developed on the surface of the rock. The pores on the surface of the medium-fine-grained dolomite are relatively developed and the pore size is large, about 20% of which are larger than 100 μm, after the dissolution transformation, the proportion of pores with a pore size of more than 50 μm in the rock increased to 78%. It is shown that the dissolution of rocks with small pore size (40-60 μm) mainly occurs on the surface of the sample, and small dissolved pores are often developed. The dissolution of large pore size (>80 μm) rocks mainly occurs in the pore walls and there are signs of dissolution into the rock. After dissolution transformation, the pore throat radius and connectivity of medium-fine-grained dolomite show a certain increase. (3) The dissolution process of carbonate rocks is jointly affected by chemical composition and pore structure, and is less affected by rock porosity. The pore size distribution is the key factor affecting the dissolution rate of carbonate rocks. The porosity of the samples before dissolution is between 1.97 and 10%. Under the experimental conditions (20 ℃, 1 atm, 0.1 mol·L−1 HCl solution), the dissolution rate of carbonate rocks is between 0.497 g·d−1 and 0.598 g·d−1. The mass loss of dolomitic breccia through dissolution is the largest due to the influence of cement shedding; the medium-fine crystal dolomite with more developed surface pores and larger pore size is second;fine-grained limestone with dense grains and small pores has the slowest dissolution rate.Samples with smaller pore diameters are greatly affected by the stagnant boundary layer, and the dissolution medium is likely to form a local saturation state inside the pores; the samples with larger pore diameters can expand into the rock during the dissolution process of the "water-rock" reaction, which is affected by the stagnant boundary, but the effect of the layer is less and the water flow conditions are better than the small pore size samples, so the dissolution rate is higher. This study has certain guiding significance for the understanding of the differential karstification mechanism and karst development law of carbonate rocks.
Application of self-potential and high-density resistivity method to the water exploration in karst terrain of middle-lower reaches of Xijiang River
Based on hydrodynamic conditions of the shallow karst groundwater seepage, we mainly study the natural polarization phenomenon of geological structures in water-enriched karst induced by the action of groundwater seepage and distribution characteristics of the natural electric field therefrom. By comprehensive prospecting with the combination of self-potential method and high-density electronic resistivity method, we analyse and evaluate the application effect of the combination on the exploration of karst groundwater in different hydrogeological environments of middle and lower reaches of Xijiang River. In this study, we also integrate geophysical prospecting results with the data derived from exploration & production wells in the survey project of hydrogeological environment initiated and supported by China Geological Survey. Study results show that induced electrical methods, such as transient electromagnetic method, audio magnetotelluric sounding method, ground nuclear magnetic resonance method, induced polarization method, sonic frequency geoelectric field method, VLF electromagnetic method, etc., are seriously affected by electromagnetic fields like power transmission and communication network. The high-density resistivity method shows a strong anti-interference ability and high signal-to-noise ratio. But its exploration depth is relatively shallow, and it is easily disturbed by ore bodies with high conductivity such as charry limestones or karst minerals. The self-potential method is used in the situation of polarization of geological structures in water-enriched karst caused by groundwater migration. Anomalies of the natural electric field are closely related to the groundwater recharge and migration. The complementation of self-potential method and high-density electronic resistivity method in groundwater exploration can contribute to the mutual corroboration of prospecting results, hence improving prospecting resolution and precision.
Exploration of superficial soil-rock structure for karst area based on frequency domain electromagnetic method
Soil thickness as a key hydrological and ecological factor, is distributed extremely uneven in karst areas. The explorations of the soil thickness and soil-rock interfaces are still challenge. In this study, a 1-D electrical conductivity (EC) inversion model was developed for the frequency domain electromagnetic method (EMI) based on Maxwell's equation system. The visualization of EC distribution for soil profile in karst area could be realized by the model. The inversion model was validated according to the given ideal stratigraphic EC
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
, Available online ,
doi: 10.11932/karst20220609
, Available online ,
doi: 10.11932/karst20220608
, Available online ,
doi: 10.11932/karst2021y40
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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