Global Anisotropic Pre Stack Depth Migration (PSDM) Velocity Modelling to Mitigate Structural Heterogeneity and Eenhance Imaging

Abstract

Abstract Seismic imaging in time domain is extremely difficult because of the insufficient illumination at the crest of South Fuwaris Humma-ARQ structure with 32 (thirty two) 2D seismic lines which were acquired in two vintages 1978 with 24 fold and 1996 with 80 fold. Due to large near surface lateral velocity variations and severe ray distortion by low velocity layers in surface seismic data, first-arrival turning ray tomography in a 3D sense proved to be useful during the initial time domain processing for deriving statics corrections to minimize these distortions and mitigate different causes of statics problems. A key element of the reprocessing objectives was to generate an image in depth using prestack migration that takes near-surface velocity model derived from the turning ray tomography into account. For this purpose, the near-surface velocity model was used as the shallow layer of the depth initial velocity model, with the travel time effects calculated directly by ray tracing rather than by statics corrections. The deeper parts of the initial velocity model were derived from 3D smoothed velocity model that was coming from prestack time migration velocities for the re-processed 2D lines. Two iterations of global isotropic tomographic update for velocity model were computed prior to reach the final anisotropic velocity model in depth domain. Delta and epsilon (anisotropic parameters) were manually picked and then a tomographic update of the anisotropic velocity model and the anisotropic parameter (delta) took place. Special effort was taken to minimize the mistie at well locations at major seismic markers and at the intersection points of the dense intersecting 2D lines with interactive interpretation and processing technique. This paper presents an innovative Global Anisotropic Pre Stack Depth Migration (PSDM) velocity modelling technique to reduce structural uncertainties which enabled to drill exploratory wells in ARQ field.