Model-based velocity analysis

Abstract

Semblance displays are commonly used to indicate the velocities required to flatten reflection hyperbolas during a velocity analysis of seismic data. "Model-based velocity analysis" is a scheme for interpreting semblance analyses that uses an elastic synthetic seismogram computed from a detailed depth model. The depth model is constructed from logs taken from an adjacent well. The semblance analysis of the synthetic and the P-wave RMS velocity function derived from the depth model are then used to guide the velocity analysis of nearby field records. Semblance analyses of additional synthetics, such as those computed without multiples, enable noise events to be more readily identified. Strong noise events remain identifiable on field data semblance analyses computed away from the well. A model-based approach thereby allows more effective extrapolation of the well-log information. A model-based velocity analysis revealed that interbed multiples had been mistaken for primaries in conventional velocity analyses from the Gippsland Basin. High amplitudes and lack of differential moveout of the interbed multiples relative to the primaries resulted in picks that were 5-10% lower than the stacking velocities of the primaries. Restacking using the model-based velocities provided far greater continuity across a 4 km seismic line from the Gippsland Basin that had been processed through to stack in the t-p domain. The model-based approach can be applied with greater accuracy by exploiting the higher resolution of semblance peaks computed in the t-p domain. The resolution improvement results from the use of wide-angle reflections and the theoretical advantages of the elliptical moveout correction. Semblance analyses of field records showed a 2´ resolution improvement in the target zone after the application of a point-source t-p transform. A 5´ resolution improvement was achieved at the shallow Miocene carbonates where the reflection ellipses were flatter and contained a supercritical segment that was muted in the t-x domain.