A geological structure contains heterogeneities on all scales. A seismic wavefield is sensitive to heterogeneities of different wavelength in different ways. Smooth large-scale (long-wavelength, low-wavenumber) velocity heterogeneities affect forward wave propagation, whereas small-scale (short-wavelength, high-wavenumber) velocity or density heterogeneities cause wide-angle scattering. A seismic wavefield is nearly insensitive to large-scale density heterogeneities.
The forward propagation of seismic waves is not very sensitive to short-wavelength velocity or density heterogeneities. It is thus possible to approximately recover large-scale velocity heterogeneities without knowing small-scale heterogeneities. On the other hand, small-scale heterogeneities cannot be correctly recovered without large-scale velocity heterogeneities. It is thus natural and reasonable to recover the heterogeneities iteratively, gradually proceeding from large-scale heterogeneities to smaller scales (shorter wavelengths, higher wavenumbers).
A velocity macro model approximates, above all, the large-scale velocity heterogeneities. A vague smooth macro model is created at the early stage of the iterative inversion of seismic data and is gradually being improved, making large-scale heterogeneities more precise and adding heterogeneities of smaller scales. There are, however, reasons why to terminate incorporation of small heterogeneities into the macro model on a certain scale (wavelength, wavenumber) and leave the macro model considerably smooth.
Various aspects of macro models are discussed, with emphasis on their relation to geological structures and on their role in inversion of seismic data. Attention is also devoted to the construction and computer representation of macro models.
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