Shear wave VSP in Amarume oil field

Abstract

Japan National Oil Corporation (JNOC) acquired the following shear wave seismic data around the TRC-AMR1 (JNOC research well) in the Amarume oil field which is located in the northern part of Japan: 1) 1989 Shear wave VSP by a single, coupled source 2) 1990 Shear wave surface seismic by a single, coupled source 3) 1992 Shear wave VSP by dual coupled sources Rotation analysis for VSP data of the first survey showed remarkable shear wave splitting. The surface seismic data of the second survey did not show this effect. In both surveys, the motion of the shear wave source was limited to a direction perpendicular to the source-receiver line. In general, rotation analysis requires data from dual coupled sources whose motions are perpendicular to each other. The last VSP data (3) were acquired in order to investigate the conflicting results of both surveys. Data from this survey confirmed the existence of an anisotropic layer shallower than 700 m. The direction of anisotropy was found to be almost east-west, that is, in line with the receiver direction of the second survey. It was concluded that this was the reason why the second survey did not show shear wave splitting. The time difference between S1-wave (the faster shear wave) and S2-wave (the slower one) was over 100 ms at 700 m depth. This survey also detected the location of a gas reservoir at around 870 m depth.