During tunnel excavation, water hazards in faults, especially steep water rich faults, pose a serious threat to safe construction in some complex mountains, which leads to low economic growth and development in these areas. Direct current resistivity method, which has high resolution and sensitivity to the low resistivity body is widely used to predict the water-bearing structures in the front of tunnel face. The current prediction models are based on the resistivity isotropic medium, however, the resistivity of water bearing fault is often anisotropic due to rock fracture. The prediction model neglecting the anisotropy is obviously inaccurate, which brings potential threats to safe construction. We develop a three-dimensional resistivity modeling for anisotropic media using unstructured finite element method. The algorithm is proved to be accurate by comparison of numerical results and analytical solutions for a whole-space model. Another classical anisotropic model also demonstrated the reliability of our code from a physical point of view. Then we propose a prediction equation to predict the position of a vertical fault with anisotropic resistivity in the front of tunnel face by the finite element simulations. The parallel Monte Carlo method is used to test and evaluate the quality of our prediction equation by simulations of 10000 random vertical fault models, results counted by the histogram showed 85.36% of the results are predicted within 10% of the error. Besides, 93.17% of the results are predicted within 15% of the error using the equation for random faults with 75 degree dip angle, which shows that our prediction model can effectively forecast steeply dipping water-rich faults or fracture zones.

防渗墙广泛应用于堤坝防渗处理,为确保其抗渗能力,需对墙体底界进行检测。为提高对防渗墙底界的判定能力,提出了动态高密度视电阻率测试方法。构建了防渗墙物理模型,并在模型两侧分别布置了高密度电法测试系统和钻孔,通过测试钻孔注入盐水前后的动态电性参数,发现视电阻率及其变化率对受注水影响的地质体敏感,初步确定注水后的地质体视电阻率变化率约为15%时可作为模型底界判定的依据。工程实践进一步证实了研究方法的有效性和可靠性。

Water inrush during tunnel excavation severally threatens the mining safety as blind water-bearing structures may develop in front of the working face. The transient electromagnetic method (TEM) has been widely applied in the advanced detection of tunnel water-bearing structures. However, the metal interference of both supports and tools in the tunnel has become a bottleneck that reduces the forecast accuracy of this method. In this paper, we analyse the effect of metal interference on TEM data and propose a novel set of an observation and correction method under metal interference based on the ratio of anomalous and background apparent resistivity. Flume model experiments both with and without metal interference are carried out, showing that this interference can affect TEM measurements significantly and result in false anomalies, and that our proposed method can remove this ambient noise caused by metal interference appropriately. The practical application further proves that this method can effectively reduce low-resistivity interference introduced by the support and other metal tools inside the tunnel. By applying this correction method, the location of water-rich anomalies can be detected more precisely during the excavation process of the same tunnel, which is of high application value of reducing exploration difficulty and tunneling risk.

Silty-clay reservoirs are a weak point in sand production and sand control studies due to their low economy. However, China’s marine natural gas hydrates (NGH) mostly exist in silty-clay sediments, which restrict the sustainable and efficient development of NGH. In order to study the sand production of hydrate silty-clay sediments, hydrate production experiments in vertical wells and horizontal wells were carried out using a self-developed hydrate sand production and sand control simulation device. The results showed a great difference between the hydrate silty-clay sediments and hydrate sand sediments. The significant differences in production pressure and production temperature between the different layers indicated the low permeability and low heterogeneity of the hydrate silty-clay sediments. The sliding settlement of the overall depression in the horizontal well and overall subsidence in the vertical well of the hydrate silty-clay reservoir would easily lead to silty-clay flow and large-scale sand production. When water rates decreased, the property of "silty-clay sediment filtration and wall building" was found, which formed a "mud cake" around the wellbore. The good strength of adhesion and fracture permeability of the "mud cake" provided ideas for reservoir reformation. This study further discusses sand production and sediment reformation in hydrate silty-clay sediments. © 2019 by the authors.

The effects of sediments with different clay contents on the mechanical properties of hydrate deposits were studied using a high-pressure, low-temperature triaxial apparatus with in-situ synthesis, as well as the mechanical properties of self-developed hydrate sediments. Through multi-stage loading, triaxial compression tests were conducted by adding quartz sand with different clay contents as the sediment skeleton, and the stress–strain relationship of the shearing process and the strength of sediments with different clay contents were determined. Volumetric changes were also observed during shearing. The results show that the strength of hydrate sediments decreases with the increasing clay content of sediments; in the processes of depressurization and shearing, the hydrate samples exhibited obvious shear shrinkage, regardless of the sediment particle size. © 2019 by the authors.

The geophysical electromagnetic (EM) theories are commonly based on the assumption that the conductivity of underground media is frequency independent. However, due to the existence of induced polarization (IP) effect, many earth materials are dispersive, and their electrical conductivity varies significantly with frequency. Therefore, the conventional numerical techniques are not proper for EM forward modeling in general dispersive medium. We present a new algorithm for modeling three-dimensional (3D) EM data containing IP phenomena in frequency domain by using an edge-based finite element algorithm. In this research, we describe the dispersion behavior of earth media by using a Cole–Cole complex conductivity model. Our algorithm not only models land and airborne EM surveys but also provides more flexibility in describing the surface topography with irregular hexahedral grids. We have validated the developed algorithm using an analytic solution over a half-space model with and without IP effect. The capabilities of our code were demonstrated by modeling coupled EM induction and IP responses in controlled-source audio magnetotelluric (CSAMT) and airborne electromagnetic (AEM) examples. This algorithm will have important guiding significance for survey planning in the dispersive areas, and it could be taken as a forward solver for practical 3D inversion incorporated IP parameters. © 2019, Institute of Geophysics, Polish Academy of Sciences & Polish Academy of Sciences.

介质的各向异性会引起大地电磁观测曲线的畸变,对大地电磁数据解释影响很大。这里从麦克斯韦方程组出发,推导了基于二阶插值基函数的二维大地电磁非结构有限元公式,实现了任意地形起伏条件下任意各向异性二维大地电磁响应的数值模拟。各向异性断层模型的数值模拟结果与解析解吻合一致,复杂各向异性模型的有限元模拟与已有的有限差分计算结果也相符合,验证了算法的正确性。在此基础上探讨了各向异性参数对二维大地电磁响应的影响。

针对三维直流电法正演模拟中大型稀疏线性方程组求解,在GPU(Graphic Process Unit)集群上实现了并行预处理共轭梯度(conjugate gradient,CG)算法。矩阵预处理使用可并行的对称逐次超松弛近似逆预处理(symmetric successive over relaxat

＜正＞高密度电法可以高效地获得大量地电观测数据,使得地下精细结构的电阻率三维反演成像成为可能,并迅速成为该领域国内外学者关注的前沿课题。然而目前电阻率三维反演及其应用多基于平坦地形,实际勘探中地形影响不可避免,其对电阻率反演结果可能造成难于预料的偏差。已有结果表明:做地形改正只能是近似的,地下结构稍复杂,这种改正误差就很大。只有将地形同时带入反演算法中,才能精确消除地形影响及其对反演结果的偏差。但是

The observation data of seven circumjacent terrestrial resistivity stations before two strong historical earthquakes in China were studied through performing the method of normalized monthly variation rate. We analysed the relationship between anisotropic variations of apparent resistivity and the corresponding maximum principal stress's direction of the focal mechanism solution. The results may demonstrate that significant apparent resistivity variation occurs in the direction nearly perpendicular to the azimuth of the maximum compressive principal stress, while only small fluctuations were recorded in nearly parallel direction. In the first case, a decrease in 1 yr scale and a short-impending increase of apparent resistivity were clear. We also suggest that anisotropic variation of apparent resistivity is more substantial in (or near) the seismic fracture zone than at two sides of the fault. We conclude that the most effective way to estimate the epicentre of an earthquake is to increase the density of observation networks within the earthquake fault zone and to reasonably dispose electrode orientations based on this physical phenomenon.