The AVO Lithology Identification Technology Of Deep Reflection Seismic Data In Middle And Lower Crust

With the deep seismic reflection techniques changing from solving structural problems to lithology problems, Amplitude versus offset (AVO) technique has become an important research direction in deep seismic sounding. In my research, based on Hudson and Thomsen's fracture media theory, I made fracture media rock physics model in middle and lower crust. Considering the anisotropic characterization of fracture media and the factor of cracks density, cracks directions, porosity, filled fluid, pressure and temperature, I used Ruger's AVO formula for HTI (include VTI) media, and applied it to the real WLR (Wollaston Lake Reflector) seismic reflection profile. I hope to establish the identification method and rule for discriminating lithology in middle or lower crust by using the AVO attributes . Furthermore, my study will improve our understanding of the petrophysics, form-mechanism and the dynamics process in middle or lower crust.My study shows that, the whole trend of AVO for WLR1 is increasing. The AVO gradient is small. We calculated four anisotropic models to fit WLR1 data, and found model one is the best fit model. Model three is the worst which was considered the best fittest model by using isotropic theory. So I think that in the present the four candidates of WLR reflection bright, the Model one is the best candidate. With the decreasing of Possion's ratio, the misfit error is gradually decreasing in Model one. Therefore, I can infer that the high amplitude of WLR may be caused by the lower Possion's ratio formation. By using the anisotropic AVO theory, I found that Possion's ratio and the P-wave velocity is the best factor to discriminate lithlogy for middle and lower crust metamorphic rock. Reflection coefficient crossplot, AVO gradient and intercept crossplot can also be used to discriminate lithology. AVO gradient and intercept crossplot of WLR reflection had prove this. It means that our anisotropic AVO theory are correctly to be used here as well.By analyzing the sensitivity of fracture-pore media, I found that with the increasing of effective pressure, the fracture in media is closed, the anisotropic phenomena is gradually disappeared, the velocity and reflection coefficient also increased. However, the increment of velocity and reflection coefficient is gradually decreasing. Comparsion of no-equant variation modle with the same elastic parameters shows that reflectivity or velocity difference is increasing and reach to 30MPa, and continue decreases and finally tend to constant with the increasing of effecive pressure. The reason is that variation of crack- porosity andpore-porosity is the largest from 0-30Mpa, and gradually tend to constant with the increasingof pressure.