A modeling study of fault‐shadow, shallow‐gas, and low‐velocity chimney effects on seismic data

Abstract

Time distortions caused by shallow-gas anomalies, faults, and low-velocity chimneys are often serious exploration and development problems in the Gulf of Thailand. To investigate these problems, a synthetic 2D seismic modeling has been performed. The model included a rectangular shallow-gas pocket, a narrow low-velocity chimney, and a normal fault. The full two-way acoustic wave equation was solved using a finite-difference technique for the forward modeling. The time distortions and diffraction originating from the shallow-gas anomaly, low-velocity chimney, and fault were distinctively observed on the raw shot gathers, all the intermediate gathers, and final migrated section. It is reconfirmed that any dynamic time distortions (not static time shifts) can not be solved by time processes. They can only be corrected by accurate depth imaging techniques. The result of a full two-way wave equation depth migration showed all the layer boundaries, shallow-gas anomaly, low-velocity chimney, and fault at correct locations without distortion. However, in reality it is difficult to obtain an accurate velocity model with minute details for a real data set, particularly in a clastic sedimentary basin like the Gulf of Thailand.