Structural uncertainty of time-migrated seismic images

Introduction

The usual outcome of seismic data processing is an image of the subsurface (Yilmaz, 2001). In the conventional data analysis workflow, the image is passed to the seismic interpreter, who makes geological interpretation, often by extracting structural information, such as positions of horizons and faults in the image. Hidden in this process is the fact that structural information is fundamentally uncertain, mainly because of uncertainties in estimating seismic velocity parameters, which are required for imaging. Apart from the trivial case of perfectly flat seismic reflectors, which are positioned correctly in time even when incorrect stacking or migration velocities are used, seismic images can be and usually are structurally distorted because of inevitable errors in velocity estimation (Glogovsky et al., 2009).

Understanding and quantifying uncertainty in geophysical information can be crucially important for resource exploration (Caers, 2011). The issue of structural uncertainty in seismic images was analyzed previously by Thore et al. (2002) and Pon and Lines (2005). Tura and Hanitzsch (2001) studied the impact of velocity uncertainties on migrated images and AVO attributes. Bube et al. (2004b,a) studied the influence of velocity and anisotropy uncertainties on structural uncertainties.

In this paper, we propose a constructive procedure for estimating the degree of structural uncertainty in seismic images obtained by prestack time migration. The basis for our approach is the method of velocity continuation (Fomel, 2003b; Burnett and Fomel, 2011; Fomel, 2003a,1994; Hubral et al., 1996), which constructs seismic images by an explicit continuation in migration velocity. Velocity continuation generalizes the earlier ideas of residual and cascaded migrations (Rothman et al., 1985; Larner and Beasley, 1987; Rocca and Salvador, 1982). In addition to generating accurate time-migration images, it provides a direct access to measuring the structural dependence (sensitivity) of these images on migration velocities. We define structural uncertainty as a product of velocity picking uncertainty and structural sensitivity.

We use a simple data example to illustrate our approach and to show that structural uncertainty can be significant even when both structure and velocity variations are mild. Although the proposed approach is directly applicable only to prestack time migration, it can be extended in principle to prestack depth migration using velocity-ray approaches for extending the velocity continuation concept (Iversen, 2006; Duchkov and De Hoop, 2009; Adler, 2002).

Structural uncertainty of time-migrated seismic images