Horizon velocity analysis for depth conversion – A case study

Abstract

This case study compares three methods of deriving velocity information from seismic data for the purposes of depth conversion. Horizon stacking velocity analysis (HSVA) and Interval velocity analysis (IVA) have been used to derive average and interval velocity data over the Yolla structure in the Bass Basin, approximately 120km north of Tasmania. The resultant velocity maps are compared with those derived from (more commonly used) vertical function stacking velocities generated during processing. As HSVA and IVA are both horizon-based approaches to velocity picking, lateral velocity trends can be more readily identified and interpreted to be consistent with the geological structure, than regular stacking velocities. A velocity gradient between the 2 wells in the survey area, is known to exist from check-shot data. This gradient is shown to be more accurately determined from the horizon velocity analysis than from the stacking velocity vertical functions. This supports the conclusion that HSVA and IVA (in this case, and perhaps in general), are a more accurate source of velocity control for depth conversion than more routinely used regular stacking velocities. In this example, the relative accuracy of HSVA and IVA velocities is shown to be within 10m/s of check-shot data, compared to several tens of m/s for velocity derived from stacking velocity vertical functions.