Abstract: A Strategy for Near-Surface Model Construction and Subsurface Structural Discrimination

PECHOLCS, PETER I. and COSTAS G.

In Saudi Arabia, a wide range of near-surface seismic velocities caused by sand dunes, karst topography, buried channels, etc. overlie deeper structures of higher velocities. A reliable estimation of long and medium wavelength static components of the near-surface velocity-depth model is crucial for accurate mapping of these time structures which in some cases can have closures as small as 10 to 20 milliseconds (ms).

One of the difficulties in testing the reliability of time structures in stacked sections lies in separating the effects of the near-surface irregularities from structural changes in the deeper part of the section. We suppose a strategy for the estimation and verification of the velocity-depth models by using both midpoint and depthpoint oriented estimation techniques. First we construct an initial multilayer near-surface model and compute static corrections using uphole measurements, shallow refraction/reflection arrival times and optimum offset refraction stacks with geologic constraints. An initial interpretation of deeper reflection events is made from the resulting brute stack. This initial interpretation and near-surface model are used in a depthpoint oriented interval velocity estimation scheme operating from the surface to build a deeper velocity-depth macro model in a top-down approach. Pre-stack depth migration and forward modeling with iterative ray tracing are used to verify and update the near-surface and subsurface velocity-depth models. A real data example is shown to illustrate how this strategy of integrating geologic and geophysical data with midpoint and depthpoint oriented schemes can improve the interpreter's ability to reliably map subsurface structures.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria