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

Seismic data acquired directly over near-surface limestone formations are commonly observed to be of inferior quality. For a dataset from West Texas, we found that data acquired over mesas are badly contaminated by wave-energy scattered at shallow depths. Despite intensive data processing, we enhanced the data only marginally. One 'effective' method was a double dip-filtering of common-source as well as common-receiver gathers (Larner et al., 1983) which removed all events propagating at speeds less than the P-wave velocity at the surface. Another 'effective' procedure was to remove all coherent wave-energy propagating toward the zero-offset point by triple FK-filtering common-source, common-receiver, and common-midpoint gathers. Possible mechanisms for this scattering in the near-subsurface are the weathering layer, rough free-surface topography, or volume heterogeneities such as clusters of vugs, joints, or cavities. We applied different numerical scattering schemes to study the effects of each of these scattering mechanisms. For our West Texas dataset, we find that the weathering layer is the dominant cause of source-generated noise on records acquired in valleys. However on the mesas, we find that topography alone cannot cause the observed scattering. 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 that near-surface heterogeneities form waveguides which efficiently trap seismic energy near the surface. In addition, the wavefields penetrating the heterogeneous surface region are rendered incoherent masking reflections from deeper targets.