Publications

The area of investigation covers 15,785 square km in Sarawak Malaysia. It is covered by 128 2D lines oriented in NW-SE and NE-SW direction. For depth conversion of two key horizons, seismic velocities and one exploration well are available. The seismic velocities were loaded as data-points, DIX converted and adjusted to the well velocities to address velocity anisotropy. Then the interval and average velocity points were derived for both seismic layers. These data points were visualized in a 3D display window that facilitated easy identification of true velocity and deletion of apparent outliers. The edited point sets were interpolated and smoothed in order to remove noise not addressed by the point editing. The optimum smoothing operator was determined thru maximizing the correlation coefficient between the time horizon and the filtered seismic velocities. The two time surfaces were depth converted based on the smoothed interval velocity surfaces (layer cake approach). Results demonstrate that this depth converted 2D interpretation is more accurate than depth converting the interpolated time horizons. In addition the lower horizon was depth converted using the average velocity set. Its depth surface showed differences of more than 100ft compared to the result from the layer cake approach. In order to estimate depth discrepancy the velocity differences at intersecting 2D lines were calculated. For each 2D time horizon the resultant residual velocity standard deviation was converted into depth, delivering a distribution of the time dependent depth error standard deviation. The analysis shows that the layer cake approach delivers smaller depth errors for the lower horizon than the average velocity approach. Apparently, smoothing * PETRONAS Carigali Sdn Bhd,Malaysia ** SCHLUMBERGER IS, Stavanger, Norway of the interval velocities can address the velocity noise more precisely than smoothing of the average velocities.