Velocity Analysis and Depth Conversion in the Offshore Northern Perth Basin

Brief description

The northern Perth Basin is an elongate sedimentary basin, located off the southwestern margin of Australia. The basin is prospective for petroleum resources, but is relatively under-explored, and the nature of the sediment-basement contact is relatively unknown due to a high degree of structuring and deep basement depth inhibiting seismic imagining. Accurate depth conversion of seismic interpretation is vital for use as constraints in gravity modelling and in other basin modelling tasks, but depth conversion requires good quality seismic velocity information. The number and distribution of wells with velocity information in the northern Perth Basin is poor, but there exists a large amount of seismic stacking velocities. Seismic stacking velocities are an outcome of seismic processing and are thus not a direct measurement of the speed of sound in rocks. To improve the quality of stacking velocities we propose a methodology to calibrate stacking velocities against well velocities, which is as follows: 1. Check each velocity dataset for errors 2. Modify the datum of each dataset to the sea floor 3. Convert all datasets to TWT and depth domain 4. Resample all velocity datasets to the same depth intervals 5. Cross plot stacking velocity depths near a well site with corresponding well depths 6. Fit a linear polynomial to this cross-plot (higher order polynomials were tried also), and determine calibration coefficient from the gradient of the polynomial. 7. Grid calibration coefficients 8. Multiply depths derived from stacking velocities by calibration coefficient grid An assessment of depth conversion errors relative to wells shows that this methodology improves depth conversion results to within ±50m; this depth uncertainty translates into a gravity anomaly error of about ±20 gu, which is acceptable for regional scale gravity modelling.

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