Characterisation And Mapping Of Stacking Velocity And Time Effects Of Calcite-Cemented Zones, Cooper And Eromanga Basins.
Jervis, Anthony James
Honours Degree, 2001
University of Adelaide
High velocity calcite-cemented zones (CCZs) in the Jurassic section of the Eromanga Basin are a common cause of depth conversion errors to underlying horizons in the Cooper Basin. The dense stacking velocity information provided by continuous velocity analysis (CVA) of 3D seismic survey data in the Moomba North Field may have a number of uses for dealing with such zones.
Raytracing results demonstrate that CCZs representative of the Moomba North CVA area show a slow stacking velocity at common midpoints (CMPs) above the CCZ and a fast stacking velocity either side of it. This bears little relationship to the true average velocity, which should increase coincident with the CCZ. Both 2D and 3D raytracing of four characteristic models reveal stacking velocity perturbations ranging between 31 m/s and 110 m/s in peak-to-trough amplitude and 900 m and 1575 m in length. A fifth model simulating the rapid variations in thickness of real CCZs demonstrates the distinctive stacking velocity signature despite its irregularity. In addition to these stacking velocity perturbations, the time section is distorted below the CCZs resulting in time pull-ups of up to 8 ms. Depth conversion using the raw stacking velocities and times results in an error of up to 37 m for a selected model.
The magnitude of the stacking velocity and time effects vary according to the character of the CCZ, the depth of the anomalies and the acquisition geometry. Comparison of real and model data for the Moomba 3D geometry demonstrates that regardless of these criteria the stacking velocity perturbations trend parallel with the shot line direction.
The small elliptical CCZs modelled do not cause significant mis-stacking on the stacked section due to insufficiently dense velocity control. The high density stacking velocity data obtained from CVA does not result in better event coherency on the stacked section. The main application of CVA therefore appears to be for identifying the effect of CCZs in the stacking velocity field, paving the way for more accurate depth conversion.