Estimation of Q from surface seismic reflection data

Reliable estimates of the anelastic attenuation factor, Q, are desirable fo r improved resolution through inverse Q deconvolution and to facilitate amp litude analysis. Q is a useful petrophysical parameter itself, yet Q is rar ely measured. Estimates must currently be made from borehole seismology. Th is paper presents a simple technique for determining Q from conventional su rface seismic common midpoint (CMP) gathers. It is essentially the classic spectral ratio method applied on a trace-by-trace basis to a designatured a nd NMO stretch-corrected CMP gather. The variation of apparent Q versus off set (QVO) is extrapolated to give a zero-offset Q estimate. Studies on synt hetics suggest that, for reasonable data quality (S/N ratios better than 3: 1, shallow (<5 degrees) dips, and stacking velocity accuracy <5%), source-t o-reflector average Q is recoverable to within some 3% and a for a specific interval (depending on its two-way time thickness and depth) is recoverabl e to 15-20%. Three case studies are reported. First, Q versus offset and vertical seismi c profiling (VSP) a estimates for a southern North Sea line were in close a greement, validating the method. For Chalk, Mushelkalk-Keuper, and Bunter-Z echstein, Q was estimated as 130 +/- 15, 47 +/- 8, and 156 +/- 18, respecti vely. Next, two alternative lithological interpretations of a structure see n in a frontier area were discriminated between when Q estimates of 680 to 820 were obtained (compared to some 130-170 in the overlying units), favori ng a metamorphic/crystalline lithology rather than (prospective) sediments. This was later confirmed by drilling. Third, a profile of Q estimates alon g a 200-ms-thick interval, known to include a gas reservoir, showed a clear and systematic reduction in Q to a low of 50 +/- 11, coincident with the m aximum reservoir thickness, from some 90-105 outside the reservoir. Q for t he reservoir interval itself was estimated at 17 +/- 7.