Simultaneous estimation of residual static and crossdip corrections

Abstract

Seismic surveys on land are frequently conducted along nonlinear survey lines. Familiar examples include crooked lines controlled by existing road networks or by surface typography, lines that are otherwise linear but along which shotpoints occasionally must be offset laterally, and intentionally designed three‐dimensional (3-D) or broad‐line surveys. Departures from linear profiles introduce an element of complexity—crossdip—into the problem of estimating residual near‐surface reflection static time corrections (statics). Crossdip is the component of dip normal to the local profile direction. We have incorporated the effect of crossdip into the system of simultaneous equations that model residual static anomalies. The observed traveltimes of all reflections selected for analysis are represented as linear combinations of source and receiver static anomalies, structural shapes, residual normal moveouts, and crossdip terms. The static time components are taken to be surface‐consistent and independent of reflecting horizon, whereas the other solution parameters are subsurface‐consistent and pertain to specific horizons. Unfortunately, the inclusion of crossdip in the equations increases the degree of nonuniqueness of residual statics solutions. Its inclusion, however, is a necessity wherever horizons having differing crossdips are analyzed simultaneously. Such simultaneous analysis often is the best means for upgrading the reliability of the crosscorrelation estimates (i.e., the traveltime observations) upon which all statics are based. Synthetic‐data examples demonstrate the degree to which crossdip estimates and statics estimates can be separated from one another. Although estimates of crossdips are a useful by‐product, the accuracy of the static corrections is considered of prime importance. When critical crossdip terms are ignored in a statics solution, the quality of the common‐depthpoint (CDP) stacks suffer, as shown in comparison processings of field sections. Moreover, crossdip estimates from 3-D or broad‐line surveys are questionable if crossdip and static corrections are not considered in a unified solution.

Cross-profile acquisition and cross-dip analysis for extracting 3D information from 2D surveys, a case study from the western Skellefte District, northern Sweden

“…Given constant crossdip and medium velocity, the reflection times in the seismic traces within a CDP gather will vary according to the distance between the midpoint and the effective stacking line (Wu et al, 1995). These delays are neither surface consistent nor hyperbolic, hence, they cannot be corrected for by standard residual statics and reflection energy will not stack in phase after conventional NMO corrections (Larner et al, 1979;O'Dowd et al, 2004). Fig.…”

“…where ϕ i is the cross-dip angle at the ith common depth point, V i is the velocity above the shallowest dipping layer, Y ij is the transverse offset between the midpoint and the stacking line and j is the trace number within a CDP gather (Larner et al, 1979). By stacking data with different cross-dip corrections at a given stacking or NMO velocity, an estimate of the cross-dip component can be made.…”

“…Therefore, we applied a pseudo-3D data processing technique for cross-dip analysis on straight stacking lines (Wu et al, 1995) to determine the cross-dip component of some of the more clear reflections. Crossdip is defined as the component of reflector dip in the vertical plane perpendicular to the seismic profile (Larner et al, 1979;Wu et al, 1995;O'Dowd et al, 2004). Given constant crossdip and medium velocity, the reflection times in the seismic traces within a CDP gather will vary according to the distance between the midpoint and the effective stacking line (Wu et al, 1995).…”

MT and reflection seismics in northwestern Skellefte Ore District, Sweden

“…It is present when the survey line is not perpendicular to the strike of the reflectors (Larner et al, 1979;Wu et al, 1995;Nedimović and West, 2003; O'Dowd et al, Table 2. Processing steps of the seismic reflection survey (see Figure 2).…”

“…where ϕ i is the cross-dip angle at the ith CDP, V i is the velocity above the dipping layer, Y ij is the transverse offset between the jth midpoint and the stacking line (Larner et al, 1979). The estimation of the necessary cross-dip angle ϕ i , can be done by visually inspecting stacked sections cross-dip corrected with different ϕ i (e.g., Wu et al, 1995;O'Dowd et al, 2004;Rodriguez-Tablante et al, 2007;Dehghannejad et al, 2010;Lundberg and Juhlin, 2011).…”