Using Long Offsets Information for Improved Pre-SDM Imaging -A Successful Application for the Sub-Salt Plays in Northern Germany

摘要

Salt intrusions are known as being the main cause of velocity contrasts in Northern Germany. These salt accumulations from the Zechstein formation often mask reflections from the under laying target zones at circa 4.5 km depth. In addition, significant velocity contrasts may occur in the Cretaceous series. For this reason PreSDM has gained popularity over PreSTM in this area, thanks to its ability of better dealing with significant velocity contrasts in the overburden. Nevertheless, even using advanced migration algorithms a reliable interpretation of target areas is not always possible. The usual offset range used for velocity model building (VMB) and migration is generally limited to 3 km for land acquisitions, which does not allow to fully explain ray path distortions and which makes anisotropy estimation difficult. Furthermore, the nominal fold is usually low and the data is therefore generally very noisy. Specific acquisition schemes have been designed to record longer offsets data up to 17 km in order to successfully penetrate the high velocity discontinuities or to undershoot them altogether. One challenge that arises with such long offset data is how to correctly process and integrate them into regular offset data before migration. Illumination studies are frequently used to assess the potential of long offset data and their possible contribution to the final image, but such studies are limited by the quality of the available initial depth model. In our paper we use two different long offset datasets from the northern part of Germany to discuss the challenges of integrating them into the standard PreSDM methodology, using both Kirchhoff and Wave Equation imaging techniques. We will demonstrate the benefit of including LO data in achieving a much improved image quality.