oa A case study on using AVO walkaway VSP data - Barrow Sub-basin, Western Australia

In December 1999 the TL/1 JV partners acquired a 2D walkaway VSP survey in the Monty-2 well in the Barrow Sub-basin, Western Australia. The survey was designed to analyse AVO effects and possible later calibration of surface seismic data. The survey was recorded using CGG's 12 level array tool, the SST-500, and comprised a total of 4 walkaway lines, with 2 azimuths at each of two array positions. Each of the 4 walkaway lines had a length of 6000 meters. Walkaway VSP data was acquired by positioning the array tool just above the two target reflectors, the North Rankin and the Mungaroo "B" sandstone formations.

The VSP data was processed using an ‘isotropic’ sequence, which allowed for a perfect preservation of the amplitude and phase relationships between the three receiver components. AVO estimates were computed from the reflected P-wavefield after shaping deconvolution, derived from the down going P-wavefield. P-wave polarisation information was used to compute reflection angles at the target horizon and the local reflector dip. Finally, synthetic P-wave reflectivity curves were computed for the North Rankin and the Mungaroo "B" sandstone formation and compared with the borehole seismic measurements.

The AVO/AVA response at the top North Rankin reflection was found to be strongly affected by a thin overlaying carbonate layer. The best match between measured AVO curves and isotropic modeling therefore was found for a shale/carbonate model rather than a carbonate/sandstone model. In contrast, the AVO response of the Mungaroo "B" reflection was found to be in very good agreement with isotropic modeling results.