The Influence Of High Velocity Layer On Seismic Wave Propagation

Data from geology, geophysics and geochemistry all confirmed that large areas of the earth are covered by lavas. Because the velocity of lavas layers are always higher than of the surrounding sediments, conventional seismic reflection surveys can not pass through the types of high velocity layers, which highly lead to the unclear imaging of structure of the sub-basalt and low resolution data. Therefore it limits the high resolution exploration of oil and gas. Because high velocity layer can be considered as oil and gas reservoir or oil and gas reservoir and cap formation, it is important for oil and gas exploration to study the effects of high velocity layer on wave propagation. And it is also valuable to describe the reservoir and cap rock in lava.The objectives of this thesis are to study high velocity layer on seismic wave propagation and the consequent implications and theory for imaging sedimentary structures beneath them. To evaluate the effects of high velocity layers on seismic wave propagation, the thesis study the response of impedance contrast, reflections and transmission, refractions, multiplies, scatterings, attenuation and anisotropy, respectively.During the study, I consider two different types of high velocity layer structures. 1. high velocity layer by a single and homogeneous layer. 2. high velocity layers made by a multi-layered sequence. I examined the influence of structure and impedance on the effective reflection coefficients of high velocity layer reflections. I computed synthetic seismograms and amplitude calculations to determine the best possible wave types to image sediments beneath the high velocity layer. The calculation of effective reflection coefficients shows that it has the symmetrical at far offset only when the shear-wave velocity of the high velocity layer matches the P-wave velocity of the overlying and underlying sediments. The symmetrical PSP model will be the most powerful reflection for far distance between shots, and the PSS wave with the secondary. There will no amplitude for PSP reflection for real model which be structured by acoustic welling, it's very possible be caused by basalt listed thin bed and the unmatched velocity between P wave and S wave. In that case, the power reflected from the underlying basalt by PSS wave will be much greater than PSP, however, the local convert from the strongest amplitude will be transformed to an unsymmetrical PSSSSS. It's necessary to trim multiply of offing and interlays down, if the local convert will be used to image the under of basalt. However, if the data of the convert is depended for far distance between shots, the amplitude of P wave will be the strongest. The local convert can be detected for ideal model, such as single basalt layer, or singer bedrock.The model with multiply layers be modeled by finite difference method. We compare the result of the seismic data with different frequency. Furthermore, both the seismic synthetic and amplitude were calculated, the optimal wave were chosen to menstruate the aggradation under the high speed shielding layer. There is very strong heterogeneity both the lateral and the lengthwise direction in interior of the basalt layers and the place between basalt and wall rock, the heterogeneity will affect seismic field powerfully with scattering and shielding action. It is correlated between the heterogeneity of media and wave length, the signal from long wavelength with ability to penetrate the medial with strong heterogeneity; in that case, the action from strong heterogeneity basalt of scattering and shielding to seismic field will be depressed greatly by signal with low frequency. Furthermore, the synthetic seismic data shows that, the amplitude of reflection from underlying basalt will increase exponential with the decrease of frequency from the source, it is caused by basalt thin bed structure, which low velocity deposit be contained between layers, which like a low band filter, so, more information of the high velocity layer will be got if low frequency source and receiver be used. Therefore, it's necessary to detect the area of volcano with strong low frequency source. Furthermore, the action from multiply will be great, so, it's reasonable to study the high shield layer with convert, if the multiply can be depressed efficiently.