Karst development evaluation based on cross-hole radar tomography

JIA Long; ZHANG Yi; MENG Yan; LI Lujuan; PAN Zongyuan; WU Yuanbin; YIN Renchao

doi:10.11932/karst20250212

Abstract：Cross-hole geo-radar tomography,which belongs to the method of electromagnetic wave tomography between boreholes,is a way of borehole radar detection.Specifically,electromagnetic waves are transmitted form one borehole(transmitting borehole)and the transmitted signals are received by another borehole(receiving borehole).The kinematic(travel time and ray path)and dynamical(waveform,amplitude,phase,and frequency)characteristics of these signals are recorded.By tomography technology,the influence of the medium between the boreholes on the singlals is analyzed to obtain the distribution of physical properties within the strata between the two boreholes,with the goal of inverting the internal structure of the medium.As a result,the geo-radar tomography method can identify underground structures that exhibit significant differences in physical properties,such as dielectric constant and conductivity.These underground structures include solitary rocks,fissures,caves,and other adverse geological phenomena,as well as buried public facilities.This technology can penetrate deep intosubsurface for detection through drilling boreholes,effectively isolating the interference from above ground and compensating for the limitations of drilling density in practical engineering. In geological field surveys,accurately determining true karstification rate of the soluble rock layers poses significant challenges.Consequently,the karstification rate along the drilling line is typically employed as a representation measure,which has inherent limitations.To enhance the quantitative analysis of karst development within strata,this study employs cross-hole geo-radar tomography technology to process radar transmission data.This approach allows for the calculation of numerical changes in karstification rates,facilitating a more precise quantitative evaluation of the degree of karst development between boreholes. In karst areas,the materials filling karst fissures—such as including air,water,and loose soil—are the primary factors contributing to variations in the dielectric constant of rock masses.The velocities of radar waves are a function of the dielectric constant of the geological strata.By measuring the transmission velocities of electromagnetic waves in karst strata through cross-hole geo-radar,the porosity of limestone can be determined,allowing for the assessment of variations in the karstification rate between boreholes.The cross-hole geo-radar tomography method can provide insights into the distribution of transmission velocities of electromagnetic waves in karst strata,and thereby revealing the spatial variations in the karstification rates of soluble rocks between boreholes. When electromagnetic waves propagate in karst strata,their amplitudes exhibit exponential attenuation.In karst areas,the materials filling in karst fissures are also the main factors contributing to variations in electromagnetic wave attenuation.The cross-hole geo-radar tomography method can obtain the distribution of attenuation coefficients of electromagnetic waves in karst strata.For low-loss materials such as rock and soil,the attenuation coefficient is directly proportional to the conductivity of strata and inversely proportional to the square root of the relative dielectric constant of strata.Moreover,the conductivity and the relative dielectric constant of karst strata are closely related to the karstification rates of strata.Based on this,the spatial variations of karstification rates between boreholes can be quantified. Engineering examples demonstrate that,based on the physical parameters—wave velocity and attenuation coefficient—derived from the cross-hole geo-radar travel time tomography and attenuation tomography,as well as the distribution of karstification rates,it is feasible to map the distribution of karst development between boreholes.At a burial depth of 28 m to 32 m,it is inferred that karst caves are filled with flowing-soft plastic saturated clay and are likely to span the strata between the two boreholes.At a burial depth of 37 m to 41 m,it is presumed that karst caves are filled with plastic saturated clay.The spatial results of karstification rates obtained from both methods are consistent and mutually corroborative,providing a refined quantitative evaluation of the degree of karst development in strata at varying burial depths between boreholes.This provide more detailed data support for geological surveys and engineering construction.In addition,the accurately determining the karstification rates of strata based on wave velocity and attenuation of the cross-hole geo-radar requires precise values of all physical parameters of strata.These parameters can be obtained through laboratory tests on rock cores. Theoretical analysis and on-site testing demonstrate that the application of cross-hole geo-radar travel time tomography and attenuation tomography techniques can achieve a precise quantitative evaluation of the karst development degree in the strata between boreholes,thereby obtaining the inter-borehole"surface"karstification rates.This will provide more detailed data for karst geological surveys.In addition,considering the multisolution nature of geophysical survey results and the complexity of karst geology,it is recommended to combine these techniques with imaging detection of borehole radar reflection and other geophysical methods during actual detection.