Study On Joint Tomographic Method For Active And Passive Seismic Data

As fine crustal structure characteristics play an important basic role in understanding spatial coupling relationship between geolo gical tectonic blocks with different scales, building the model of seismic dynamics, knowing seismogenic mechanism of destructive earthquake in mainland and the scientific fortification of potential destructive earthquake disaster, obtaining enough fine 3D images of crust structure has been one of the important goals for geophysicist, especially for seismologists. At present, the deep and shallow crustal structures and tectonic characteristics with high resolution mainly come from the research results of the seismological method, which has higher resolution and precision than other geophysical detection method s. This is mainly because the wavelength of seismic wave is the shortest among all kinds of waves, which are observed after passing through the interior structure of earth and being modulated. Meanwhile, seismic wave owns the smallest wavefront distortion and amplitude attenuation comparing with other geophysical observations, such as terrestrial heat flow, static displacement, strain, gravity and electromagnetic quantity. Newly developed fast marching tomography(FMtomo) method overcomes several shortages of traditional seismic imaging methods. Basing on the same regularization rule, the consistent optimization method and the same way of model building, FMtomo can get higher resolution of 3D fine structures and tectonic images of the research area with multiple seismic phases of the same or different data types and their respective distinguishing advantages of spatial region. Some examples applied abroad have proved its great potential for applications. In our country, with the operation of the digital earthquake observation network which has been constructed since “the Tenth Five-Year Plan”, the implementation of the C hina earthquake science array detectio n plan project(Chinarray), the successful development and bulk-production of the portable and low-power digital seismic instrument for active source in GEC, and the increase of artificial seism observation network in some local area s, active and passive source seismic observation data is accumulating rapidly, but the development of the processing method for these large data is obviously lagging behind. In addition, though GEC has owned hardware conditions for acquiring 3D deep seismic sounding data, it’s obvious that 3D observation system can not be evaluated and designed only by experience that is usually used in traditional 2D linear detection profiles,so it is very necessary and pressing to develop new technique to design and evaluate 3D observation system on the basis of modern seismology inversion algorithm.In this article, based on available special instruments of deep seismic sounding which GEC owns and achievements of deep detection, we builded numerical model and designed the 3D observation system. With the latest developed algorithm FMM(Fast Marching Method) which is unconditionally stable in any complex structure model and subspace inversion technique, basing on the same regularization rule, optimization method and way of model building, we analyzed joint wavefront imaging in different models such as the model with strong velocity contrast a nd Moho’s lateral undulations, using 3D active and passive source seismic refraction/reflection data that is obtained from the true model: to begin with, we did the numerical modeling of seismic imaging in order to test the ability of model reconstruction by single refractive seismic phase Pg, Moho wide-angle reflective seismic phase PmP and the combination of the two phases, and analyzed corresponding resolution capability. Then, to make up the lack of active source data, we added regional earthquake data in imaging simulation and obtained finer deep-shallow structure. Results show that the refraction phase Pg has higher resolution for fine upper crustal structures, and the middle-lower crustal structure and complex Moho geometry can be well recovered wit h wide angle reflection phase PmP, though it has poor resolution for the shallow structures. The joint inversion can effectively recover 3D velocity structure with strong velocity contrasts and the morphological characteristics of Moho interface in any depth. Resolution of active source can be improved by adding passive source seismic data and the joint inversion of active and passive source seismic data can effectively improve detecting resolution of target zone. These studies offer necessary basis for researching joint test of 3D reflection/refraction with active and passive sources, and are important for exploring deep fine structure of complex tectonic zone with 3D joint detection of active and passive seismic sources. Meanwhile, calculation method, debugged program module and related parameters used in the numerical simulation can also be the reference for designing 3D deep seismic sounding observation system.