Preliminary Studies of Velocity Variation on Weakly Anisotropic Medium
Velocity is the crucial attribute in reservoir characterization. In isotropic medium, velocity will propagate to all direction with the same value. However, in anisotropic medium velocity will be depending on the physical properties of the medium particularly the anisotropy parameters. The presence of the fractures in a medium will influence the physical properties of the medium and induce the anisotropy of the medium as well. Velocity is one of the properties that will be changed once the medium changing from isotropic to anisotropic medium. The physical modeling had been carried out to comprehend the behavior of the velocity in anisotropic medium especially the variation of velocity with offset and azimuth in medium with vertically fractures aligned (HTI). The model was made from epoxy resin which the dimension of the medium is 19 x 14 x 7 cm. The velocity was measured on five lines which has different direction each other by using Seismic Viewer SX. Strike direction of fractures is N0˚E and the directions of the lines are N0˚E, N30˚E, N45˚E, N60˚E, and N90˚E.Results of physical modelling confirmed that the velocity is varied with azimuth and incident angle in anisotropic medium with vertically fractures aligned. Moreover, the fractures affect the significant velocity changing and it is depend on the total of the fractures in a medium (fractures intensity). The higher velocity changing once the line is perpendicular with strike direction of fractures and the low velocity changing occurred once the direction of line closed and parallel to strike direction of fractures. In addition, the extended analysis namely Poisson’s ratio and Vp/Vs ratio also carried out study effect of fractures to both physical properties. The Poisson’s ratio value of model in isotropic and anisotropic condition are 0.383 and in between 0.25 – 0.36 respectively. Meanwhile, the value of Vp/Vs in isotropic condition closed to 2.5 and in anisotropic condition due the vertical fractures existence is in between 1.5 – 2.0.