P And S Wave Velocity Inverted From OBS Data Of South China Sea

In the past, geophysics character such as, BSR, blank, and polar inversion are used to identify gas-hydrate. But these characters don’t always coincide with BSR .For more finite describe distribute of gas hydrate, so it is needed to carry out more accurate method for the estimation of gas hydrate distribution and saturation. Compression wave is increasing sharply in Gas hydrate- rich formation. While there is little free gas in the formation wave velocity are decreasing and S wave velocity isn’t changing. Analyses of the seismic velocities and amplitude behavior at the BSR provide an efficient way of identifying and characterizing the distribution of gas hydrates within sediments.The method is based on P-wave velocities computed by reflection tomography applied to multi-component Ocean-Bottom Seismometer data. We interpret layer in character of compression wave from OBS vertical component. Then several horizontal models were building. There are two main inversion methods: first, according to the model, inversion was done from top to bottom; second, invert layer with strong reflect energy and obvious reflect character. Ray tracing is carry out according to receiver and shot point coordinates. Velocity- depths models are adapted basing on the difference between calculate travel time and actual measurement. When the differences are small than some given value, it is finished. Interpolation was done between main reflect layers, so other layers initial velocity and layer depth are known. In this method, we got the velocity-depth model which can better describe subsurface geology information.There aren’t any obvious features in conversion wave in horizontal component.For it is difficult to identify one layer reflect conversion wave, and tomography is easily affected by the picking travel time. As we have got P wave velocity, we adopt coherency inversion obtaining shear wave velocity. Because without obvious wave field character, we can’t identify P wave and PS wave which from same reflect layer.Coherency inversion is based on migrate section from which initial velocity-depth models were obtained, and P wave velocity is from tomography inversion. It is same as tomography inversion we carried out ray-tracing calculation,and coherence of reflection time-travel curve and actual conversion wave measurement was calculated. We choose the max coherence velocity as the finally velocity. The inverted result is consistent with blank tape of migrated section and quotient of compression velocity and shear velocity difference method. vp /vs and Poisson ration was calculate using the inversion result.