Anisotropic PSDM: A case study from Western Offshore, India

Abstract

Isotropic pre‐stack depth migration (PSDM) in the presence of anisotropy in the subsurface, suffers from positioning errors; vertical as well as lateral. It is well known that layering in shaly formations produces anisotropy which can be modeled as transverse Isotropy (TI). In the present paper we present a case study from shallow offshore India. The subsurface in the area is characterized by massive shale (800–1000 m) underlain by thick limestone — shale alternations. The isotropic PSDM showed a 50 m mismatch at this level with hockey‐stick effect visible at far offsets on PSDM gathers. We have estimated the heterogeneity and anisotropy parameters i.e. velocity gradient (G), and Thomson parameters (ε and δ) from seismic and well data. These parameters, estimated in a number of wells spread over 1000 sq. km., were found to be practically constant. These parameters were incorporated in depth velocity modeling and PSDM along 2D profiles. The study of PSDM depth image gathers has shown that in this area non‐hyperbolic effect due to heterogeneity dominates beyond offsets of 1140 m (offset‐depth ratio of 1.4). The similar effect due to anisotropy is visible only after offsets of 1440 m (offset depth ratio of 1.8). However after Anisotropic PSDM (APSDM) the gathers are flattened up‐to 1940 m (offsets depth ratio 2.5). APSDM images match well with well‐tops within seismic resolution with accompanying improvements in overall image quality at target horizons.