Reservoirs obscured by complex heterogeneous salt bodies, where a constant salt velocity assumption fails, are extremely challenging to image. Properly modelling the heterogeneous salt velocity allows RTM to provide clearer images of the base salt and subsalt reservoirs, allowing more accurate interpretation.
Where the intervals and heterogeneities are sufficiently thick, intra-salt tomography can be used to derive a more accurate salt velocity. High resolution tomography uses a dense grid of picks, a fine ray-tracing grid and little smoothing.
RTM 3D angle gathers provide subsurface dipping angles, reflection/incident angles and azimuth information for ray tracing in tomography and are ideal for use in computing intra-salt velocity models with dirty salt tomography.
Constraining velocity updates to predefined dirty salt regions allows local updating. Ray tracing does not need to go through complexities all the way to the surface, making the tomographic inversion more stable.
Such velocity updates don't rely solely on amplitude so there is no confusion between inclusions with velocity variations and inclusions with only density contrasts. Furthermore, the velocity of the inclusions can be updated to be slower or faster than the surrounding salt velocity.
Salt bodies in the Santos Basin, offshore Brazil, comprise layered evaporites with strong reflectors. Intra-salt tomography derives a more accurate velocity model and better image, as illustrated.
For areas where gathers suffer poor illumination or intra-salt reflections are too sparse or erratic for manual picking or traveltime tomography to succeed, we have developed an iterative reflectivity inversion scheme to derive dirty salt velocities, which improves the imaging in these difficult areas.
RTM depth images from a WAZ dataset using a constant salt velocity (left) and a dirty salt velocity derived by reflectivity inversion (right). Both the base of salt and subsalt events are more coherent using the dirty salt velocity.
Retain the rich azimuth and reflection angle information carried in wide-azimuth data and bring the outstanding quality of RTM pre-stack.
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