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摘要:
利用跨孔雷达走时层析成像和衰减层析成像技术实现钻孔之间地层岩溶发育程度的精细化定量评价，可弥补实际工程中钻探密度的限制。走时层析成像方法可得到岩溶地层中电磁波传输速度分布情况。因雷达波速是地层介电常数的函数，故可推导出岩溶地层的孔隙比，进而得到钻孔之间可溶岩的岩溶率的空间变化情况。衰减层析成像方法可得到岩溶地层中电磁波衰减系数分布情况。对于岩土这种低损耗材料，衰减系数与地层电导率成正比，与地层相对介电常数的平方根成反比，而岩溶地层电导率和相对介电常数与地层岩溶率密切相关，据此可量化钻孔之间岩溶率的空间变化情况。工程实例显示，两种方法得到的岩溶率空间分布结果一致，可相互印证精细评价钻孔之间不同埋深地层的岩溶发育程度，为地质勘察和工程建设提供更详实的数据支撑。
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.
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表 1 研究区地层的电性参数
Table 1. Electrical parameters of the strata in the study area
介质 相对介电常数 电导率/s·m−1 传播速度/m·μs−1 衰减系数/dB·m−1 衰减系数/np·m−1 空气 1 0 300 0 0 岩溶水 81 0.05 33 0.1 0.0115 干石灰岩 7 10−8~10−6 113 0.4 0.0460 湿石灰岩 8 0.018 106 0.4~1 0.046~ 0.1151 饱和黏土 15 0.08 60~90 1~300 0.1151 ~34.5383 
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表 2 测孔信息表
Table 2. Table of borehole information
深度/m 岩土特征 发射孔
ZK126.3 覆盖土层：填筑土（0~1.5 m），粉质黏土 27.5 中风化石灰岩：青灰色，隐晶质结构，中厚层构造，节理裂隙发育 30.5 溶洞：全填充，黄褐色，充填物为粉质黏土及碎石，粉质黏土呈可塑状 40.5 中风化石灰者：青灰色，隐晶质结构，中厚层构造，节理裂隙发育 接收孔
ZK328.7 覆盖土层：填筑土（0~1.7 m），粉质黏土 28.8 中风化石灰岩：青灰色；隐晶质结构，中厚层构造，节理裂隙发育，岩芯呈柱状 30.1 溶洞：全充填，棕红色，充填为粉质黏土，可塑 31.3 中风化石灰岩：青灰色，隐晶质结构，中厚层构造，节理裂隙发育 31.9 溶洞：半充填，黄褐色，充填物为粉质黏土，流—软塑状 45.5 中风化石灰岩：青灰色，隐晶质结构，中厚层构造，节理裂隙发育
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