Lichao Nie

A new comprehensive geological prediction method based on constrained inversion and integrated interpretation for water-bearing tunnel structures

Advanced geological predictions and ascertainment of the water-bearing geological structures in front of the tunnel face are effective ways to ensure the safety of tunnel construction. Comprehensive geological predictions are often used to obtain adverse geological conditions in front of the tunnel face, and the comprehensive comparison analysis on various prediction methods can help to reduce multiplicity of solutions and improve reliability. However, in the conventional comprehensive prediction system, the prediction data for each method is separately inverted and processed, so the problem of multiple solutions in detection results for each category actually cannot be solved. Therefore a new comprehensive geological prediction method is proposed in this paper based on constrained inversion and integrated interpretation for water-bearing tunnel structures to solve the problem. Firstly, four preferred prediction methods are selected to meet the needs of the advanced detection of water-bearing structure. They are seismic reflection method, transient electromagnetic method, 3D resistivity method and ground penetrating radar (GPR) method. Then, a constrained inversion method of 3D resistivity is proposed based on the inequality and spatial structural constraints. Inequality constraint characterises the electrical resistivity variation range of the detection area, and its prior information is mainly from the geological inference and core-drilling analysis. Spatial structural constraint characterises the geological structure boundary and shape, its prior information is mainly derived from some more effective methods for geological tectonic boundary recognition such as seismic reflection and GPR methods, and it also can be derived from the disclosure by drilling exploration. The role of resistivity constrained inversion to reduce the multiple solutions of prediction is mainly reflected in data processing, and the role of integrated interpretation to reduce the multiple solutions of prediction is mainly reflected in the result interpretation process. Meanwhile, the responding characteristics of methods of seismic reflection, electromagnetism, 3D resistivity, GPR and others to the water-bearing geological structure are summarised from the differences of elasticity, electrical conductivity and dielectric constant. Therefore, an integrated interpretation guideline for water-bearing structures is proposed. Finally, a new comprehensive geological prediction technology system is established through combining the constrained inversion and the integrated interpretation. Numerical examples and engineering applications show that this method can take full advantage of the prior information of inequality constraints and spatial structures, and together with the integrated interpretation on the comprehensive geological prediction results, it has a significant effect in eliminating the false anomalies of inversion, reducing the multiple solutions of inversion and improving prediction accuracy.

3D electrical resistivity inversion using prior spatial shape constraints

To minimize the number of solutions in 3D resistivity inversion, an inherent problem in inversion, the amount of data considered have to be large and prior constraints need to be applied. Geological and geophysical data regarding the extent of a geological anomaly are important prior information. We propose the use of shape constraints in 3D electrical resistivity inversion. Three weighted orthogonal vectors (a normal and two tangent vectors) were used to control the resistivity differences at the boundaries of the anomaly. The spatial shape of the anomaly and the constraints on the boundaries of the anomaly are thus established. We incorporated the spatial shape constraints in the objective function of the 3D resistivity inversion and constructed the 3D resistivity inversion equation with spatial shape constraints. Subsequently, we used numerical modeling based on prior spatial shape data to constrain the direction vectors and weights of the 3D resistivity inversion. We established a reasonable range between the direction vectors and weights, and verified the feasibility and effectiveness of using spatial shape prior constraints in reducing excessive structures and the number of solutions. We applied the prior spatially shape-constrained inversion method to locate the aquifer at the Guangzhou subway. The spatial shape constraints were taken from ground penetrating radar data. The inversion results for the location and shape of the aquifer agree well with drilling data, and the number of inversion solutions is significantly reduced.

Three-Dimensional Seismic Ahead-Prospecting Method and Application in TBM Tunneling

AbstractThe tunnel seismic ahead-prospecting method is used to estimate adverse geology ahead of the tunnel face. Accurately realizing this geology helps to guarantee safety during construction usi...

Forward modelling and imaging of ground-penetrating radar in tunnel ahead geological prospecting: Forward modelling and imaging of ground-penetrating radar

Adverse geologies are often encountered during tunnel construction, which could seriously endanger the construction. To ensure the safety, it is essential to detect adverse geologies and their water-bearing situation ahead the tunnel face. Ground-penetrating radar is a suitable instrument, but the accurate interpretation of its detection results is difficult. In this paper, at first, an improved back projection imaging algorithm is proposed, which can make reflection waves closer to the real geological boundaries with few artificial clutters. And then, forward modelling of ground-penetrating radar is carried out for typical adverse geologies, such as karst caves, faults, fractured rock masses, fracture network, and water-bearing body. Their corresponding response features are obtained, accumulating experience for geological interpretation. The above two methods provide the basis for target identification and geological interpretation. In the last part, the application of the above two methods in several engineering cases are given, and their effectiveness is verified.

Research on Induced Polarization Relaxation Properties of Sand Sample

The work discuss the main influencing factors of induced polarization effect including porosity, water content and current intensity from the analysis of induced polarization relaxation properties. The experimental device of induced polarization four-electrode method has been used to contrast three groups (particle size 0.6–1.18, 1.0–1.4, 2.36–4.72 mm) of aquifers sand sample. Different current has been supplied into each particle size group of sand simple with different water content (2–24%) to explore the impact of the relationship between aquifers sand sample and the induced polarization relaxation properties.