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