Automatic Velocity Analysis via Shot Profile Migration

Shot profile migration provides a convenient framework for implementation of a differential semblance algorithm for estimation of complex, strongly refracting velocity fields. The objective function minimized in this algorithm may measure either focussing of the image in offset or flatness of the image in (scattering) angle. Velocity estimation based on this measure of data-model consistency uses waveform data directly: it does not require any sort of traveltime picking. We show that the offset variant of differential semblance yields somewhat more reliable migration velocity estimates than does the scattering angle variant, and explain why this is so. We observe that inconsistency with the underlying model (Born scattering about a transparent background) may lead to degraded velocity estimates from differential semblance, and show how to augment the objective function with stack power to enhance ultimate accuracy. A 2D marine survey over a target obscured by the lensing effects of a gas chimney provides an opportunity for direct comparison of differential semblance with reflection tomography. The differential semblance estimate yields a more data-consistent model (flatter angle gathers) than does reflection tomography in this application, resulting in a more interpretable image below the gas cloud.