Raypath-dependent statics

The region of the earth which usually has the greatest effect on the quality of seismic reflection data is that between the surface and the first competent “bedrock” interface, which can occur at a depth of anywhere from zero to a few hundred metres. When the material in this region is much softer than the rocks beneath the bedrock boundary, seismic reflection energy travels along near-vertical paths to and from surface locations, and simple time shifts can be used as effective corrections for seismic traces. The application of these simple “static” time shifts to each trace is often sufficient to correct for velocity and/or thickness variations in the surface layer. When the near-surface material is harder than deeper material, however, as often occurs in arctic permafrost areas and in regions covered with volcanic surface flows or overthrust carbonate layers, the assumption of near-vertical raypaths is no longer valid. Each reflection raypath connecting one surface location to any other surface location has its own unique raypath segments through the near-surface material. This implies a separate static correction for each near-surface raypath segment, rather than a single average correction for all raypaths sharing a single surface location. We hypothesize how such statics would appear on field data, then seek evidence for their existence on a set of arctic field data. The Snell ray radial trace transform is used to map seismic trace gathers into the domain of common injection angles (or common reception angles). Common-angle Snell ray trace panels are analyzed for differential statics using static distribution functions. Arctic data analyzed in this way show consistent evidence of small statics that vary slowly with raypath angle. An early attempt to correct these data by applying match filters derived from the angle-dependent static functions have yielded interesting results that appear to verify the existence of raypath-dependent statics and to show that they can be corrected. INTRODUCTION Time picks vs static distribution functions Correcting seismic reflection traces for the effects of propagation through the nearsurface region of the earth is a significant ongoing problem for seismic data processors. Most often, data redundancy and favourable near-surface conditions allow the constraint of surface-consistency to be imposed in order to derive appropriate discrete “static” time shifts to apply to correct individual seismic traces. In a significant fraction of cases, however, especially in areas where the surface layer of the earth is significantly higher in velocity than underlying layers, surface-consistency is clearly violated, and some other constraint or condition must be used to assist in solving the “statics” problem. The violation of surface-consistency, the statistical uncertainty of discrete event arrival times, and the possibility of multi-path phenomena are among the near-surface seismic propagation issues which led to a proposal to pose the near-surface correction problem as