Reflectivity Dispersion for Gas Detection

Seismic wave energy attenuation and velocity dispersion happen when wave propagating viscoelastic rocks. A well known effect of attenuation is the change in frequency content and amplitude of a pulse propagating through an attenuating medium. Another is the presence of frequency dependent reflection coefficients. In viscoelastic media, frequency dependent reflection coefficients bring seismic energy dispersion of wave propagation, especially in seismic exploration where the attenuation is mainly due to the fluid content of the considered media. For the wave propagation in gas reservoirs, even no contrast in acoustic impedance, there is a reflection coefficient depending on the reflectivity dispersion. But most reflection coefficient equations cannot explain the above phenomena. Based on linear viscoelastic theory of velocity dispersion and the linearization of Zoeppritz equations, a new approximate reflection coefficient Rf is presented, which is pore fluid related and adds the effects of frequencies and quality factors. With Rf, the reflected energy change induced by reflectivity dispersion can be calculated and used to directly detect fluid. In our work, this method has been successfully applied to characterize the distribution of tight gas sands in a Triassic clastic sedimentary basin of southwest China. And the results can offer reliable foundations for well designing.