Seismic wavefields recorded at near-vertical incidence from a counterphase source

Zandong Sun, Michael J. Jones

To acquire direct shear-wave data without a shear-wave vibrator we have tested a different type of shear-wave source - the counterphase source (two P-wave vibrators running 180 degrees out of phase) in VSP experiments. Although the data were acquired in the deepest parts of wells (at near-vertical incidence), they allow us to compare counterphase shear waves, direct shear waves, and converted shear waves. It has been observed that a significant amount of shear-wave energy propagates in the vertical direction, which indicates a good potential for the counterphase source to be applied to VSP area. A comparison between counterphase shear waves and direct shear waves indicates the particle-displacement direction of counterphase shear is in, or close to, the direction of the line joining the two P-wave vibrators. Shear-wave velocities inverted from counterphase shear waves and converted shear waves are comparable.