Imaging refractors with the convolution section

Abstract

Seismic refraction data are characterized by large moveouts between adjacent traces and large amplitude variations across the refraction spread. The moveouts are the result of the predominantly horizontally traveling trajectories of refraction signals, whereas the amplitude variations are the result of the rapid geometric spreading factor, which is at least the reciprocal of the distance squared. The large range of refraction amplitudes produces considerable variation in signal‐to‐noise (S/N) ratios. Inversion methods which use traveltimes only, employ data with a wide range of accuracies, which are related to the variations in the S/N ratios. The time section, generated by convolving forward and reverse seismic traces, addresses both issues of large moveouts and large amplitude variations. The addition of the phase spectra with convolution effectively adds the forward and reverse traveltimes. The convolution section shows the structural features of the refractor, without the moveouts related to the source‐to‐detector distances. Unlike the application of a linear moveout correction or reduction, a measure of the refractor wavespeed is not required beforehand. The multiplication of the amplitude spectra with convolution, compensates for the effects of geometric spreading and dipping interfaces to a good first approximation, and it is sufficient to facilitate recognition of amplitude variations related to geologic causes. These amplitude effects are not as easily recognized in the shot records. The convolution section can be generated very rapidly from shot records without a detailed knowledge of the wavespeeds in either the refractor or the overburden.