Improved Imaging and Resolution of Overburden Heterogeneity by Combining Amplitude Inversion with Tomography

In some areas of the North Sea there are irregular high-amplitude events that some call “gull-wing” reflections because of their characteristic appearance on seismic sections. The Oseberg area, the site of a recently acquired ocean-bottom cable (OBC) 3D survey, is severely affected by these anomalies, which are scattered over the area at a depth of about 1.5 km. Some gull-wing anomalies have been drilled and are seen in log data to have a thickness of up to about 50 m and a P-velocity of around 5500 m/s. The high velocity of these irregular features causes significant distortion in depth imaging of Oseberg seismic data, causing both vertical and lateral displacement of deeper events. Different approaches were explored to resolve the high-velocity anomalies during depth imaging of the OBC 3D data. One method that has proven successful is to use results of prestack AVO inversion to insert anomalies in the velocity model. Another method is high-resolution common-image-point (CIP) tomography using offset-vector-tile (OVT) input, which is also able to resolve a smooth representation of the gull-wing anomalies. Both of these methods reduce the distortion caused by the gull-wing anomalies and give improved depth-imaged results.