Decoupled elastic least-squares reverse time migration and its application in tunnel geological forward-prospecting

Accurate seismic imaging can ensure safe and efficient tunnel construction under complex geological conditions. As a high-precision migration method, reverse time migration (RTM) has been introduced into tunnel seismic forward-prospecting. However, the resolution of traditional RTM imaging results may not meet the requirements in a complex tunnel environment, which affects the interpretation of tunnel seismic forward-prospecting results. In this study, we propose a least-squares RTM method based on the decoupled elastic wave equation in tunnels. The Born forward modeling operator and its exact adjoint migration imaging operator are derived to ensure a stable convergence of the conjugate gradient method. Moreover, a pseudo-Hessian based preconditioning operator is adopted to accelerate the convergence. Numerical examples are provided to verify the efficiency of the proposed scheme. A field test in a traffic tunnel construction site is performed to show the good application effect of the decoupled elastic least-squares RTM in practical situations.

Research on Technology and Application of Buried Faults Identification in Urban Underground Space

With theurbanization rate's rising and three-dimensional expansion and development of urban, the identification of underground buried faults has become the key factor of earthquake risk in urban underground space and surface area. As a typical method of detecting blind faults in underground space, shallow seismic prospecting technology plays an important role in judging and avoiding potential risks such as underground faults in the process of urban expansion and site selection. In this paper, shallow seismic prospecting technology is adopted, and optimized processing technologies such as parameter test, tomographic correction, pre-stack denoising, fidelity and consistency processing, correction iteration, migration imaging, and time-depth relationship deduction are adopted. Underground faults are identified and interpreted in the studied urban area, and fault risk assessment is carried out based on fault characteristics, scale, distribution and overlying strata, thus providing suggestions for regional pattern and construction of urban planning.

3-D Multi-Component Reverse Time Migration Method for Tunnel Seismic Data

Migration imaging is a key step in tunnel seismic data processing. Due to the limitation of tunnel space, tunnel seismic data are small-quantity, multi-component, and have a small offset. Kirchhoff migration based on the ray theory is limited to the migration aperture and has low migration imaging accuracy. Kirchhoff migration can no longer meet the requirements of high-precision migration imaging. The reverse time migration (RTM) method is used to realize cross-correlation imaging by reverse-time recursion principle of the wave equation. The 3-D RTM method cannot only overcome the effect of small offset, but also realize multi-component data imaging, which is the most accurate migration method for tunnel seismic data. In this paper, we will study the 3-D RTM method for multi-component tunnel seismic data. Combined with the modeled data and the measured data, the imaging accuracy of the 3-D Kirchhoff migration and 3-D RTM is analyzed in detail. By comparing single-component and multi-component Kirchhoff migration and RTM profile, the advantages of the multi-component RTM method are summarized. Compared with the Kirchhoff migration method, the 3-D RTM method has the following advantages: (1) it can overcome the effect of small offset and expand the range of migration imaging; (2) multi-component data can be realized to improve the energy of anomalous interface; (3) it can make full use of multiple waves to realize migration imaging and improve the resolution of the anomalous interface. The modeled data and the measured data prove the advantages of the 3-D multi-component RTM method.

Study of Geostatistical Inversion in the Lithologic Distribution and Velocity Modeling of Thick Igneous Rock in the FY Area, Northern Tarim Basin, China

In the northern Tarim Basin, a large number of thick igneous rocks are encountered in the drilling process in the Permian. Their lithology and velocity are very strongly, which has a great influence on migration imaging of the “beaded” areas. It is very important to conduct the fine lithology identification and high-precision velocity modeling of the igneous rocks for the exploration and development of the reservoirs. A geostatistical inversion method to obtain the igneous-rock lithologic distribution pattern and velocity modeling in the FY area of the northern Tarim Basin is introduced in this paper. The results show that the application of the geostatistical inversion method greatly improves the resolution of lithology identification. This helps us further understand the Permian igneous rocks distribution in the FY area. Comparison between the seismic facies classification maps of the FY study area shows that the obtained velocity model can reflect the lateral distribution of igneous rocks well. At the same time, the velocity model can reflect the variation of igneous rocks velocity in detail and has a high precision. The average velocity error of the wells participating in the inversion is less than 2%, and the minimum average velocity error is 0.23%. Finally, the velocity model is applied to seismic data processing, and the processing results indicate that it can help to improve seismic migration imaging. The study demonstrates that the geostatistical inversion method can provide a high-precision velocity model for formation pressure prediction and seismic data processing and interpretation, ultimately guiding the exploration and development of oil.

Pre-stack Time Migration Processing for Block A of Putaohua Oilfield

According to the reservoir and structural characteristics of Heidimiao, this paper combines fine pre-stack pre-processing and small-panel pre-stack time migration, and optimizes targeted methods and parameters to obtain the seismic data with higher resolution and better amplitude retention. Compared with the previous processing results, the migration imaging of the new data has been improved, and the geological phenomenon of the whole area is clear. The fracture system is reasonable, the fracture points are crisp, and the imaging of small faults is clear. The wave group characteristics of the new treatment are obvious, and the transverse resolution, longitudinal resolution and signal-to-noise ratio are improved obviously. By using relative amplitude preserving and VTI pre-stack time migration, the signal to noise ratio and resolution of seismic processing results have been greatly improved, and the interlayer information is more abundant, which can meet the needs of target interpretation.

Research on the Improved Least Squares Reverse Time Migration Imaging Method

An improved Least Squares Reverse Time Migration (LSRTM) method is proposed in the paper, which can effectively improve convergence speed and imaging accuracy. Firstly, the key techniques in the implementation of LSRTM are discussed. Secondly, a condition factor is introduced in the iteration process of conjugate gradient method. Finally, the imaging effect and performance of the algorithm are analyzed. The experiment results indicate that it can speed up the convergence speed and improve the convergence accuracy, so as to improve the imaging effect. Compared with the conventional LSRTM, the data residual of improved LSRTM can be reduced by about 5%.