For this article, please refer to Report 7 of
the consortium Seismic
waves in complex 3-D structures, Dept. of Geophysics, Charles
University, Prague, November 1998, pp. 349-374.
Stable finite-difference modelling of sedimentary basins
I. Oprsal, J. Zahradnik
Department of Geophysics, Faculty of Mathematics and Physics, Charles
University,
V Holesovickach 2, 180 00 Praha, Czech Republic;
FD modelling of the two-dimensional P-SV seismic
response is performed for the Volvi Lake sedimentary basin at
EURO-SEISTEST site. The basin is 6 km long and 200 m deep. Viscoelastic
linear rheology, with the attenuation described by Q proportional to
frequency is used. The FD synthetics are accurate up to 4Hz. The site is
characterized by complex structure, with high velocity contrasts
between individual blocks, (Vs=2600 m/s)/(Vs=425 m/s); also the Vp/Vs
velocity ratio is up to 5. Numeric instabilities have been found
at contacts between blocks with high and low Vp/Vs
ratios. A simple trick to stabilize the calculations for long FD
runs (e.g. 20 000 time levels, or more) has been suggested. It
consists of a special algorithmic treatment of the material
discontinuities, preventing mixing of very large and very low
values of the so-called effective material parameters within a grid
cell. Discussed are also various possible ways of numerical evaluating
the effective parameters in inhomogeneous media, some of which may yield
incorrect results. Appendix provides a comparison of the present
FD method with an independent method for a model with
non-planar topography and several Vp/Vs ratios.
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