How Can We Use Cross-Spreads for Near-Surface Model Building?

In on-shore exploration, an adequate near-surface model is often crucial for a proper image at depth. Shallow velocities can be inverted from dispersion curves of surface waves and guided waves (`coherent noise'). This requires small arrays and dense sampling, as also short wavelengths of the coherent noise need to be sampled. However, properly sampling ground roll is often not cost effective. By staggering shots or receivers in a cross-spread acquisition geometry, and jointly transforming multiple gathers, we obtain adequate sampling in absolute offset. This allows the use of small arrays without introduction of aliasing. The increase in acquisition effort and the decrease in spatial resolution is only marginal, while a significantly larger bandwidth for the dispersion curve is obtained and hence shallower velocities can be resolved.