Long-offset Seismic Signal Detection And Exploration With Active Source

Seismic waves give us the light to illuminate the Earth's interior. So far, almost all the information about Earth's interior such as its structure, composition and status has been gained by seismic wave. With the development of Seismology, people have achieved great success in studying the Earth's interior in global scale by earthquake, such as Earth's layered structure and plate subduction. However, the limited spatial distribution and the unpredictable occurrence of earthquakes, as well as the errors in the location of their epicenters, all these unfavorable factors heavily restrict the application of earthquakes in understanding the subsurface structure and its temporal variation in regional scale. In contrast with earthquake, seismic exploration with active sources has gradually become the dominant means in studying such regional scale structure for its well-known source location and well-controlled excitation thus the possibility of reasonably distributed observation systems. However, the energy released by active sources is relatively weak compared with that from earthquakes, thus reaches shallower in depth and smaller in lateral distance while propagating in the earth. In order to improve the capability of exploration with active sources, one way people used to take is to use large volume dynamite, the usage of which is restricted more and more or even forbidden for the requirement of environmental protection nowadays. It is therefore urgent to find an alternative and effective way as well as related data process methods. This thesis mainly focuses on the characteristics of active sources, and probes into the methods in improving the capability of seismic signal detection and exploration in both the excitation of seismic source and the process of seismic data.Firstly, in order to study the feasibility and effectiveness of deep crustal structure exploration by reservoir based air gun source, we analyzed the characteristics of air gun excited in reservoir through field experiment. The results showed that the air gun source has advantages as follows: (1) High Efficiency. The energy released by air gun array of 6000 cubic inches in reservoir is equal to that generated by 1.6Kg dynamite, but the signal could be detected clearly even with offset bigger than 185Km. (2) Low dominant frequency. It is full of low frequency energy, and its dominant frequency is around 4 - 6Hz; (3) High repeatability. Seismic waves (99.7%) from air gun have big cross-correlation coefficients (> 0.94) with each other; (4) Green source. It has no damage to the reservoir dam and aquatic lives.Air gun source could be used for exploring the regional fine structure and monitoring the temporal variation. We analyzed the air gun signals in the 185Km-long seismic line. And the records showed that the air gun signal is full of seismic phase information (Pg, Pn, PmP, etc). Based on these phases, we developed the P-wave velocity model for this area and find an obvious low-velocity layer, which is coherent to what people obtained from other geophysical methods in this area. With consideration of air gun source's high repeatability, we proposed an idea to construct active and accurate doublet by combination of doublet method and active sources. This method could effectively minimize the uncertainty of the source locations, thus enhance the feasibility and reliability of the doublet method. It laid the foundation for continuous monitoring on the status of the underground substances through repeated source.Secondly, in order to explore the feasibility and potential of applying Code Principles to seismic signal excitation, which can be learned from radar detection, we developed Time Coded Impulsive Seismic Technique (TCIST). We studied the efficiency of this method through theoretical analysis, numerical simulation and field experiment. The principle of this method could be loosely described as follows, instead of using a high impact this method applies a series of moderate energy according to a time coding theme, and obtains the subterranean information by the correlation between the accurately recorded source function and long offset seismic record. Field experiment showed that this method could greatly improve seismic detection by moderate-energy source. In addition, we applied polarization filter to improve SNR of seismic record to contribute to its correlation coefficients with source function in TCIST. We tested the filter with three-component synthetic seismic data and actual record from field experiment. It is not only good for denoising, but also has good zero phase characteristics.Traditional seismic exploration with active sources is restricted by source energy, resolution, environmental requirement and etc., thereby it is of great significance to develop methods to conduct seismic exploration in large scale with small energy source. This thesis provides a new attempt and research for the development of the theories and methods with active source.