Geology Reference
In-Depth Information
Dispersion curves can also be extracted from multi-fold seismic reflection
data and can be inverted using laterally constrained inversion techniques to
obtain pseudo-2D models of the shear-wave velocity. This method gives
better results than the individual inversion of single dispersion curves.
14.2.2 Modelling
Forward modelling or linear least-squares inversion of dispersion curves
requires initial values for Poisson ratios and densities which can be based
on the known properties of local materials.
Modelled shear-wave velocity profiles are insensitive to reasonable vari-
ations in density, but differences in the P-wave velocity,
Vp
, in saturated and
unsaturated sediments (which cannot be obtained from the dispersion data)
can imply differences of up to 10-20% in surface wave velocities, because
of the dependence of the Rayleigh wave velocity on the Poisson ratio. The
ratio can be obtained by a combination of P-wave refraction and MASW (or
ReMi) surveys, at least to the maximum refractor depth.
Forward modelling is iterative and starts with an assumed shear-wave
velocity profile from which a theoretical dispersion curve can be derived.
The shear-wave model is adjusted until the measured dispersion curve and
the theoretical dispersion curve match. Linear inversion depends heavily on
the choice of initial model. If
apriori
information is unavailable, multiple
inversion solutions with equally good data fits are possible.
Calculation methods for fully automated 1D inversion, including sim-
ulated annealing, are beyond the scope of this topic. The starting points
are multi-layer models with predefined thicknesses, shear-wave velocities,
Poisson ratios and densities. More layers are used than are believed to be
present.
Modelling by 1D inversion or even pseudo-2D inversion (Figure 14.6) is
currently limited to flat-lying layers. The future development of full 2D and
3D modelling will take into account wavefield scattering and thus further
improve the accuracy and robustness of the method.
14.3 Limitations of the Method
Surface-wave methods are at an early stage of development, and many prob-
lems remain. Some limitations are fundamental, but improvements can be
expected in many areas. All methods currently suffer from near-field incom-
patibility effects. In some methods (e.g. SASW and CSWS) the Rayleigh
wave fundamental cannot be easily separated from higher harmonics and
body waves, and with these methods it has currently to be assumed that
the fundamental mode is dominant. This can cause errors in defining the
dispersion curves for modelling.
