AVO inversion on depth migrated traces

Abstract

It is generally recognized that avo inversion is better conducted on depth migrated traces, although it is not often done because of computational costs and the fact that it may provide only marginal improvement in regions of low dip and small lateral velocity gradients. However, there are instances where it is necessary, in which case multiple common-offset migrations are undertaken. Aside from being computationally intensive, common offset migration is a trace mixing process where amplitudes can be distorted due to unattenuated migration noise and an inaccurate migration velocity. Careful attention must also be paid to the migration operator weights. Described in this paper is an alternate procedure where a reflection from a single time trace is mapped to its specular reflection point on a single unstacked migrated trace. This gives a one-to-one correspondence between a reflection on a raw input trace and its image. The shot and receiver locations are preserved and the true reflection point is known. The procedure begins with Kirchhoff prestack depth migration where the output from each trace is individually stored at selected image points. Most of the energy on these traces will be migration operator noise; however, the specular reflections (if any) can be recognized by a straightforward, but manual, process that will be elaborated upon later. Because manual intervention is required to pick the specular reflection, this process is not for reconnaissance. Presumably an efficient program for finding avo anomalies has already been run and some anomalies have been singled out for further analysis. Depth migration need only be run within the anomalous region so that the entire process is not as computationally intensive as one might expect. On the other hand, separating the specular reflections from the migration operator noise is manpower intensive and at first glance may seem somewhat tortuous. A good trace sorting program is required. The final result is a set of unstacked migrated traces, sorted by common image point and offset, and with known shot and receiver locations. Moreover the reflection of interest is known to be a specular reflection, not just the limb of the migration operator impulse response, which without the benefit of stacking would be unattenuated. Rays can then be traced to the reflection point, allowing both the incident angle and a geometric spreading factor to be calculated. After correcting for geometric spreading the trace amplitudes and incident angles can be used to compute the coefficients of any one of the common avo functions This procedure provides an opportunity to combine PS and PP avo inversion with a minimum amount of migration effort and no ambiguity with respect to their common reflection points. Misalignment of reflector depths on the common image traces can also be a quick way to evaluate the migration velocity.