Research on the resolution of cross-hole electromagnetic wave CT method for small karst caves under different working patterns

CHEN Fengyi; PAN Jianwei; SONG Hongming; YANG Chen; KUI Yulu

doi:10.11932/karst20250211

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.