Converting surface motions to subsurface wavefields

Dave Nichols

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ABSTRACT

The measured quantities in nine-component seismic data recorded at a free surface are displacements in the X, Y, and Z directions. The quantities we would like to use in seismic imaging are the amplitudes of the upcoming P, S1 and S2 waves. If the properties of the near-surface are known exactly it is possible to obtain the wave amplitudes from the measurements. However if the properties of the near-surface are unknown they must be estimated as the conversion is performed. It is not possible to estimate accurately all the elastic properties of the near-surface from reflection seismic data, so some approximate conversion scheme in which only a few parameters need to be estimated is required. The conversion scheme most often used is valid for waves propagating vertically or near vertically in a medium exhibiting orthorhombic symmetry. I present a scheme that uses approximations valid for waves arriving over a larger range of angles in a medium with the more general monoclinic symmetry. Three main classes of methods may be used to choose the parameters that ``best'' perform the conversion from surface motions to wave amplitudes. These are, searching the full parameter space, interactive selection of parameters, and automatic search of the parameter space.