Research on the resolution of cross-hole electromagnetic wave CT method for small karst caves under different working patterns
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摘要: 跨孔电磁波CT法是一种原位无损的探测方法，因其具有较高的分辨率及施工效率，能够直观、清晰地反映出地下局部异常体的空间分布，被广泛应用于溶洞探测、路基注浆质量检测等领域。但目前关于该方法对小型溶洞分辨能力的影响因素研究并不多见。文章利用电磁波数值模拟系统和自编添加电磁噪声程序，通过控制变量的方法分别对不同介质吸收系数、不同定点距、孔间距工作模式情况下，研究了电磁波CT法对小型溶洞探测的分辨能力。结果表明，围岩与探测目标体的吸收系数差异越大，越有利于电磁波CT法对异常体的分辨，但该过程也受环境电磁噪声干扰的影响。随着环境电磁噪声干扰的增强，CT成像的分辨能力会明显降低；定点距越大方法的分辨能力也降低。对小型溶洞进行探测时，为确保成像的精度，建议定点间距最好不要超过4 m；过大的钻孔间距会降低电磁波CT法对小型溶洞的分辨能力，数值模拟结果表明，利用CT法对小型溶洞进行探测时，钻孔间距一般不要超过30 m。Abstract:
The cross-hole electromagnetic wave CT method, as an in-situ and non-destructive geophysical exploration technology, utilizes the propagation characteristics of high-frequency electromagnetic waves between boreholes to intuitively reflect the spatial distribution of underground anomalies. With advantages such as high resolution, efficient operation, and minimal restrictions by surface topography, it has been widely applied in engineering fields such as karst cave detection, roadbed grouting quality assessment, and mined-out area investigation in coal mines. However, in practical applications, the imaging quality of the electromagnetic wave CT method is affected by multiple factors, particularly in the detection of small karst cavities, where resolution capacity is closely tied to the selection of working parameters. Currently, there is a lack of systematic studies on the resolution capability of this method for small cavities, which to some extent restricts its precise application under complex geological conditions. This study adopts a combined approach of numerical simulation and engineering validation to thoroughly investigate the resolving capability of the cross-hole electromagnetic wave CT method under different working configurations for detecting small karst cavities. The research provides a solid theoretical foundation and technical reference for its practical engineering application.The research follows a technical route that integrates theoretical analysis, numerical modeling, and on-site testing. In terms of numerical simulation, a professional cross-hole electromagnetic tomography processing system was used along with a self-developed program for adding electromagnetic noise. Detection models were constructed under varying conditions, focusing on how key parameters-such as differences in medium absorption coefficients, transmission-reception spacing, and borehole spacing-affect the imaging outcome. The interference effects of ambient electromagnetic noise were also systematically analyzed. The model settings were designed to reflect typical engineering conditions. For example, the absorption coefficient of surrounding rock was set to 0.1 dB·m−1, and that of the target body (representing the karst cavity) was varied between 0.2 dB·m−1 and 0.7 dB·m−1 to simulate different filling conditions. The transmitting spacing ranged from 1 m to 10 m, and borehole spacing was controlled within 10 m to 30 m, covering a comprehensive range of commonly encountered engineering parameters. Based on the forward modeling, random electromagnetic noise at levels of 1% to 3% was also added to better replicate real field environments. For the engineering validation phase, a typical karst-developed area near Jingna Road in Guangxi was selected for on-site testing, and the absorption coefficient cross-sections were compared against borehole data for verification. The research results indicate that the resolution capability of electromagnetic wave CT for small karst cavities shows strong dependence on parameter selection. Regarding the absorption coefficient contrast, when the difference between the target body and surrounding rock reaches 0.6 dB·m−1, cavity anomalies can still be clearly identified even under 3% noise interference. However, when the difference is only 0.1 dB·m−1, the imaging quality deteriorates significantly once the noise level exceeds 1%. This suggests that in practical applications, working frequency bands with pronounced electromagnetic contrast should be prioritized, and effective noise suppression measures must be taken. With respect to transmission-reception spacing, the study shows that increasing the spacing leads to reduced ray path coverage density. When the spacing exceeds 4 m, the imaging quality for cavities of 2 m × 2 m in size becomes noticeably worse. At 8 m, the target is almost entirely unresolvable. This indicates that observation systems in practical projects must be designed according to the size of the target anomaly. For small cavity detection, it is recommended that transmitting spacing be kept within 4 m. Regarding borehole spacing, modeling data demonstrate that when the absorption coefficient difference is 0.6 dB·m−1, resolution remains acceptable at borehole spacing up to 30 m. However, when the difference decreases to 0.4 dB·m−1, cavity anomalies become blurred when spacing exceeds 20 m. This suggests that in regions with weak karst development or minimal contrast between the filling material and the surrounding rock, borehole spacing should be reduced appropriately to ensure detection effectiveness. The field validation further confirms the reliability of the simulation conclusions. In the actual survey conducted near Jingna Road in Guangxi, the electromagnetic wave CT method accurately delineated a cavity development zone between depths of 10m and 15 m. The inversion results closely matched the cavity positions revealed by borehole drilling. This study systematically identifies the key influencing factors and corresponding mechanisms that affect the resolution capability of electromagnetic wave CT in detecting small karst cavities. Firstly, it confirms that the absorption coefficient contrast is the fundamental determinant of resolution, providing theoretical guidance for frequency selection. Secondly, it quantifies reasonable values for transmitting spacing and borehole spacing, establishing technical standards for observation system design. Finally, it analyzes the interference mechanisms of ambient noise, offering guidance for improving data acquisition quality control. Based on the research findings, the following engineering recommendations are proposed for typical small karst cavity detection scenarios: transmission spacing should be kept within 4 meters, and borehole spacing should not exceed 30 meters. These results not only enrich the theoretical framework of electromagnetic wave CT method but also offer direct and practical guidance for improving the detection accuracy of karst cavities in real-world engineering contexts. -
Key words:
- electromagnetic wave CT /
- resolution /
- absorption coefficient /
- small karst caves
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图 1 电磁波CT法观测示意图
Figure 1. Schematic diagram of electromagnetic wave CT observation system
图 3 不同吸收系数差异及不同电磁噪声干扰情况下的电磁波CT法的反演成像图
(a)、(b)、(c)吸收系数差异为0.1 dB·m−1，电磁噪声分别为1%、2%、3% (d)、(e)、(f) 吸收系数差异为0.3 dB·m−1，电磁噪声分别为1%、2%、3% (g)、(h)、(i) 吸收系数差异为0.6 dB·m−1，电磁噪声分别为1%、2%、3%
Figure 3. Inversion results of electromagnetic wave CT method under different absorption coefficients and noise disturbances
(a) The difference in absorption coefficients for (b) and (c) is 0.1 dB·m−1, while the difference in absorption coefficients for electromagnetic noise is 1%, 2%, and 3%, respectively. The difference in absorption coefficients for (d), (e), and (f) is 0.3 dB·m−1, while the difference in absorption coefficients for electromagnetic noise is 1%, 2%, and 3%, respectively. The difference in absorption coefficients for electromagnetic noise is 0.6 dB·m−1, and the difference in absorption coefficients for electromagnetic noise is 1%, 2%, and 3%, respectively
图 4 不同定点距及不同噪声干扰情况下电磁波CT法的反演成像图
(a)、(b)、(c)定点距为1 m，噪声干扰分别为1%、2%、3% (d)、(e)、(f) 定点距为4 m，噪声干扰分别为1%、2%、3% (g)、(h)、(i) 定点间距为8 m，噪声干扰分别为1%、2%、3%
Figure 4. Inversion results of electromagnetic wave CT method under different transmitting spacings and noise disturbances
(a) The fixed-point distance for (b) and (c) is 1 m, and the noise interference is 1%, 2%, and 3%, respectively. The fixed-point distance for (d), (e), and (f) is 4 m, and the noise interference is 1%, 2%, and 3%, respectively. The fixed-point distance for (g), (h), and (i) is 8 m, and the noise interference is 1%, 2%, and 3%, respectively
图 5 不同孔间距及不同吸收系数差异下电磁波CT法的反演结果成像图
(a)、(b)、(c)吸收系数差异为0.6 dB·m−1，孔间距分别为10 m、20 m、30 m (d)、(e)、(f) 吸收系数差异为0.5 dB·m−1，孔间距分别为10 m、20 m、30 m (g)、(h)、(i) 吸收系数差异为0.4 dB·m−1，孔间距分别为10 m、20 m、30 m
Figure 5. Inversion results of electromagnetic wave CT method under different borehole spacings and absorption coefficients
(a) The difference in absorption coefficients between (b) and (c) is 0.6 dB·m−1, with hole spacing of 10 m, 20 m, and 30 m, respectively. The difference in absorption coefficients between (d), (e), and (f) is 0.5 dB·m−1, with hole spacing of 10 m, 20 m, and 30 m, respectively. The difference in absorption coefficients between (g), (h), and (i) is 0.4 dB·m−1, with hole spacing of 10 m, 20 m, and 30 m, respectively
图 6 不同频率下电磁波CT反演吸收系数断面图
Figure 6. Cross-sectional diagram of absorption coefficient inversion by electromagnetic wave CT at different frequencies
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