Abstract

The dispersion imaging scheme for multichannel surface waves involves summation of a given frequency component over all traces in one record, always including wavefields in the full-offset range in the calculation. The scheme, therefore, does not have any way to take into account near- and far-field effects of surface waves, especially far-field effects, which can result in degraded imaging performance because of low amplitudes in the image space. Far-field effects are caused by noise wavefields dominating at far offsets where source-generated surface waves become relatively weak because of attenuation and geometrical spreading. The adverse influence of far-field effects can be minimized by considering an optimum far offset, which may not be the same as the farthest offset surveyed, but shorter depending on the wavelength being considered. Because the surface wave attenuation is proportional to distance in wavelength, the optimum far offset can be set to a specific number of wavelengths. On the other hand, the geometrical spreading affects all wavelengths equally and this alleviates the need for any other elaborate scheme. In this study, a selective-offset scheme is presented that limits the minimum and maximum offsets used for the imaging to 0.1–1.0 and 3–7 times, respectively, the wavelengths considered. It is shown that a selective-offset scheme can result in dispersion images with improved details, especially at those points dealing with relatively short wavelengths that are vulnerable to attenuation. The common full-offset scheme is explained in more intuitive ways than previously presented and, by extending it, the selective-offset scheme is explained.

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