Near-surface scattering from high-velocity carbonates in West Texas

Seismic data acquired directly over near-surface limestone formations are c ommonly observed to be of inferior quality. For a dataset from West Texas, we found that data acquired over mesas are badly contaminated by wave-energ y scattered at shallow depths. Despite intensive data processing, we enhanc ed the data only marginally. One 'effective' method was a double dip-filter ing of common-source as well as common-receiver gathers (Larner et al., 198 3) which removed all events propagating at speeds less than the P-wave velo city at the surface. Another 'effective' procedure was to remove all cohere nt wave-energy propagating toward the zero-offset point by triple FK-filter ing common-source, common-receiver, and common-midpoint gathers. Possible mechanisms for this scattering in the near-subsurface are the weat hering layer, rough free-surface topography, or volume heterogeneities such as clusters of vugs, joints, or cavities. We applied different numerical s cattering schemes to study the effects of each of these scattering mechanis ms. For our West Texas dataset, we find that the weathering layer is the do minant cause of source-generated noise on records acquired in valleys. Howe ver on the mesas, we find that topography alone cannot cause the observed s cattering. We believe that near-subsurface volume heterogeneity is a major cause of scattering. Using energy-density and energy-flux vectors to study the frequency dependence of the different scattering models, we observe tha t near-surface heterogeneities form waveguides which efficiently trap seism ic energy near the surface. In addition, the wavefields penetrating the het erogeneous surface region are rendered incoherent masking reflections from deeper targets.