Massachusetts Institute of Technology. Earth Resources Laboratory
Abstract
Zhang, Jie; Lavely, Eugene; Toksoz, M. NafiA useful approach to optimal seismic survey design is to simulate the seismic response
for a suite of a priori subsurface models and shot-receiver templates. The response can
be used to evaluate many criteria such as subsurface coverage, target resolution, noise
sensitivity, aquisition footprint, data redundancy, long-wavelength statics resolution,
and others. A key requirement for practical implementation is the use of an accurate
and rapid simulation method. For most cases survey optimization for a highly detailed
3D model would not be useful because (1) such information is often not available,
(2) some of the conclusions may not be robust to small changes in the model, and
(3) simulation of generally varying complex models would be prohibitively expensive. Instead, a more useful model class for survey design would be 3D models with constant
velocity layers separated by arbitrary (and possibly complex) interfaces. The models
may be from conjecture or previous seismic surveys.
We present a rapid 3D raytracing method optimized for the computation of reflection
and refraction wavefronts from a point source in this model class. We demonstrate
that the method simulates wave phenomena such as diffraction and head wave propagation.
The approach is extremely fast since it avoids traveltime expansion in the
volume between interfaces, and solves a simple 2D problem on each interface. Other
methods require local propagators (even in constant velocity regions), whereas our approach enables large jumps of wavefronts from interface to interface. The calculation
of 3D reflection or refraction traveltimes for a model with an arbitrary interface from
one source to any number of receivers requires less than 1 sec of CPU time on a DEC
3000/500 workstation.
We briefly review how our new method can be used to facilitate survey resolution
computations. We also develop a method for estimating an efficient source-receiver
distribution for resolving an assumed 3D structure. To design the receiver distribution,
we calculate continuous traveltime slices at the surface from a given source template
and plot the RMS curvatures of the wavefronts. The spatial density of the receiver
coverage should be in proportion to the locally-varying magnitude of the RMS curvature.
Similarly, to determine the optimal source distribution, we sum the RMS curvatures of
the wavefront traveltimes due to each source in the entire survey area. In the same way,
the magnitude of the curvatures suggests the most important areas for source locations.Gas Research Institute (Contract 5096-210-3781)Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
Consortiu
