Application of transient electromagnetism and cross-hole CT imaging to detect karst distribution and morphological characteristics﹕A case study of Jinan,Shandong Province
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摘要:
文章以山东省济南西南裸露—覆盖型岩溶山区为研究对象，根据钻探资料，将区内岩溶发育形态分为小型溶洞与溶孔，并利用瞬变电磁法、电磁波CT成像两种物探方法，探讨岩溶分布、形态特征与地球物理场的关系。对比瞬变电磁剖面与钻探揭露的岩溶发育段，确定出120 Ω·m作为解译岩溶发育区的分界值，以此圈定岩溶分布区域，之后实施钻孔进行验证，符合性较好；在验证孔及附近钻孔进行电磁波CT成像探测，进一步验证并刻画两孔间的岩溶发育特征。两种物探方法中，电磁波CT的探测数据离散程度较高；两种岩溶发育形态中，小型溶洞的地球物理参数离散程度较大，同时小型溶洞发育区视电阻率平均值较小，视吸收系数平均值较大。
Abstract:The study area is located in the southern mountainous area of Jinan City, which is an exposed-covered karst mountainous area with low hilly landform surrounded by mountains on the east, west and south sides. The overall terrain is high in the south and low in the north, and is generally a north inclined monoclinic structure with exposed strata of Cambrian,Ordovician and Quaternary. Precipitation in this area is mainly concentrated in July, August and September. The catchment area of nearby rivers is limited and cut off for a long time.The development of surface karst in the study area is ordinary, but the underground shallow karst is relatively developed. There are two types of groundwater,pore water and karst water. The karst water aquifer corresponds to the limestone of Cambrian-Ordovician Sanshanzi formation in which karst is relatively developed. The depth of study area is 50 m with dolomite as its the main lithology. There are two kinds of karst development in this area,small karst caves with the pore diameter greater than 20 cm, fast footage and low sampling rate and dissolved pores with the pore diameter less than 20 cm, normal footage, and high sampling rate. Caves and pores are basically filled with cohesive soil.This study is aimed to calculate the dispersion coefficient and eigenvalue of apparent resistivity and apparent absorption coefficient in karst development area, and to explore a method to analyze the development degree and morphological characteristics of karst cave. The research methods include Transient Electromagnetic Method (TEM), cross-hole electromagnetic tomography and drilling. In TEM, the central loop device, multi-turn small loop emission and point measurement are adopted. The distance between fixed sending points and measuring points of cross-hole electromagnetic tomography is 1.0 m, and data is collected in the four frequency bands of 6, 8, 10 and 12.In this study, the transient electromagnetic profile is compared with the karst development section exposed by drilling, and 120-ohm meter is determined as the boundary value for interpreting the karst development area, so as to delineate the karst distribution area. Then the drilling is carried out for verification, and its result is in conformity with that of TEM. Imaging detection of the cross-hole electromagnetic tomography in the verification hole and nearby boreholes is conducted to further verify and characterize the karst development between these two types of holes. By comparing TEM with drilling, the distribution range, the average value and the characteristic value of apparent resistivity in karst development area are statistically analyzed to delineate the distribution range of karst. Then the imaging of cross-hole electromagnetic tomography is used to precisely divide the distribution and development form of karst. Results show that apparent resistivity of small karst caves in TEM is relatively scattered, but it is larger and concentrated in dissolved pores. Under the same conditions of karst cave, both average resistivity and maximum apparent resistivity of non-filled karst caves are larger than those of the caves filled with cohesive soil.The results of these two geophysical prospecting methods show the same degree of dispersion. The degree of dispersion of small karst caves data is greater than that of dissolved pores, but the performance of data average value and characteristic value is different,with transient electromagnetic method, the average and characteristic values of apparent resistivity in small karst caves are less than those of dissolved pores; however, with cross-hole electromagnetic tomography, they are greater than those in dissolved pores.
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表 1 地下水类型及富水性分级表
Table 1. Classification of groundwater and grading of water abundance
地下水类型 含水层代号 含水层岩性 富水性/
m3·d−1松散岩类孔隙水 Q4 碎石、粉质黏土、黏土 <500 碳酸盐岩类
裂隙岩溶水Є4O1s 白云岩 1000~5000 
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表 2 不同钻孔揭露岩溶发育段物性参数统计表
Table 2. Statistics of physical parameters of karst development section exposed by different drilling holes
孔号 钻探揭露岩溶埋深/m 视电阻率值/Ω·m
范围值/平均值岩溶发育形态 溶洞填充状态 ZK1 20.3~23.2 55.6~70.3/59.8 小型溶洞 黏性土 ZK2 14.4~16.9
29.7~33.7
41.4~42.539.6~71.9/60.8
73.0~86.3/77.5
96.2~98.2/96.9小型溶洞
溶孔
溶孔无充填
黏性土
黏性土ZK3 19.6~21.8
25.8~27.090.7~100.6/96.4
80.1~84.8/81.9溶孔
溶孔黏性土
黏性土ZK4 21.7~22.6
35.7~37.0115.1~120/117.7
113.0~117.5/115.7溶孔
溶孔黏性土
黏性土ZK5 / / / / ZK6 28.9~30.1
35.7~36.5118.0~139.2/123.5
124.6~129.8/127.4溶孔
溶孔黏性土
黏性土ZK7 25.9~28.1
40.1~40.758.7~63.9/59.4
68.8~73.2/70.4小型溶洞
溶孔黏性土
黏性土ZK8 34.7~36.6
40.6~41.874.8~77.8/75.3
75.4~78.4/77.7小型溶洞
小型溶洞黏性土
黏性土ZK9 28.9~29.8
31.4~32.5103.4~110.7/105.6
100.4~105.6/103.4溶孔
溶孔黏性土
黏性土ZK10 / / / /
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表 3 岩溶发育形态对应视电阻率统计表
Table 3. Statistics of apparent resistivity corresponding to different forms of karst development
岩溶发育形态 视电阻率值范围/Ω·m 视电阻率值平均值/Ω·m 离散系数 特征值/Ω·m 小型溶洞 39.6~78.4 67.5 43.75% 72.3 溶孔 68.8~139.2 109.4 30.84% 113.9
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表 4 不同钻孔岩溶发育段物性参数统计表
Table 4. Statistics of physical parameters of karst development section in different boreholes
孔号 钻探揭露岩溶
埋深/m视吸收系数
范围值/平均值岩溶发育形态 溶洞填充状态 验证孔 16.0~17.7
21.4~23.8
26.8~30.7
35.1~35.94.1~5.9/5.4
5.4~7.6/6.1
3.3~4.2/3.6
2.6~3.6/3.1小型溶洞
小型溶洞
溶孔
溶孔黏性土
无充填
黏性土
黏性土ZK3 19.6~21.8
25.8~27.01.8~3.7/2.1
3.1~3.8/3.4溶孔
溶孔黏性土
黏性土
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表 5 岩溶发育形态对应视吸收系数统计表
Table 5. Statistics of apparent absorption coefficient corresponding to different forms of karst development
岩溶发育形态 视吸收系数范围 视吸收系数平均值 离散系数 特征值 小型溶洞 4.1~7.6 5.1 56.06% 5.85 溶孔 1.8~4.2 2.9 43.65% 3.13
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