Jin Zhang

Sensitivity analysis of P-waves and S-waves to gas hydrate in the Shenhu area using OBS

Compared to towed streamers, ocean-bottom seismometers (OBS) obtain both S-wave data and richer wavefield information. In this paper, the induced polarization method is used to conduct wavefield separation on OBS data obtained from the Shenhu area in the South China Sea. A comparison of the changes in P- and S-waves, and a comprehensive analysis of geological factors within the area, enable analysis and description of the occurrence of natural gas hydrate in the study area. Results show an increase in P-wave velocity when natural gas hydrate exists in the formation, whereas the S-wave velocity remains almost constant, as S-waves can only propagate through the rock skeleton. Therefore, the bottom-simulating reflection (BSR) response of the P-wave is better than that of the S-wave in the frequency analysis profile. In a wide-angle section, the refractive wave of the hydrate layer is evident when using P-wave components but identification is difficult with S-wave components. This velocity model illustrates the sensitivity of P- and S-wave components to gas hydrate. The use of this polarization method and results of analysis provide technical and theoretical support for research on hydrate deposits and other geological features in the Shenhu area.

Estimation of Elastic Parameters Using Two-Term Fatti Elastic Impedance Inversion

Elastic impedance (EI) inversion has been widely used in industry to estimate kinds of elastic parameters to distinguish lithology or even fluid. However, it is found that conventional three-term elastic impedance formula is unstable even with slight random noise in seismic data, due to the ill-conditioned coefficient matrix of elastic parameters. We presented two-term Fatti elastic impedance inversion method, which is more robust and accurate than conventional three-term elastic impedance inversion. In our method, density is ignored to increase the robustness of inversion matrix. Besides, P-impedance and S-impedance, which are less sensitive to random noise, are inverted instead of VP and VS in conventional three-term elastic impedance. To make the inversion more stable, we defined the range of K value as a constraint. Synthetic tests claim that this method can obtain promising results with low SNR (signal noise ratio) seismic data. With the application of the method in a 2D line data, we achieved λρ, μρ and VP/VS sections, which matched the drilled well perfectly, indicating the potential of the method in reservoir prediction.

Phase characteristic analysis of continuous depth air-gun source wavelet

Air guns are important sources for marine seismic exploration. Far-field wavelet of air gun arrays, as a necessary parameter for pre-stack processing and source models, plays an important role during marine seismic data processing and interpretation. When an air gun fires, it generates a series of air bubbles. Similar to onshore seismic exploration, the water forms a plastic fluid near the bubble; the farther the air gun is located from the measurement, the more steady and more accurately represented the wavelet will be. In practice, hydrophones should be placed more than 100 m from the air gun; however, traditional seismic cables cannot meet this requirement. On the other hand, vertical cables provide a viable solution to this problem. This study uses a vertical cable to receive wavelets from 38 air guns and data are collected offshore Southeast Qiong, where the water depth is over 1000 m. In this study, the wavelets measured using this technique coincide very well with the simulated wavelets and can therefore represent the real shape of the wavelets. This experiment fills a technology gap in China.

Research of SWD Data Processing Software Architecture

In view of the present domestic demand with data processing technology of seismic while drilling, a set of software system suitable for data processing of seismic while drilling was developed. The software was based on the hierarchical structure and the article described the architecture of the seismic while drilling data processing software system. It also defined the function at all levels in detail. The results showed that: the software system based on the hierarchical structure is stable and has good adaptability and extensibility.

Accuracy Comparison of Elastic Impedance Formula

Elastic impedance (EI) inversion is one of the prestack seismic inversion methods, which can obtain P-wave and S-wave velocity, density, Poisson ratio, Lame coefficients and other elastic parameters. But there have been many EI formulas nowadays, so which formula should be used in inversion is an urgent problem. This paper divides these formulas into two categories, and use several forward modeling to test the accuracy of these EI formulas. It shows that using the first kind of EI formulas in near offset seismic data can get high precision results.