Can amplitudes resolve ambiguities in refraction inversion?

Abstract

The inversion of seismic refraction data with model-based methods is inherently ambiguous, and artefacts, which are geologically plausible and significant, can be introduced by the algorithms used to generate the starting model. In many cases, the minimum variance criterion of the generalized reciprocal method (GRM) can resolve whether lateral variations in the refractor wavespeeds are genuine, or whether they are artefacts of the inversion algorithm. As an additional constraint, this paper demonstrates that any genuine lateral changes in refractor wavespeed should also have an associated amplitude expression. Amplitudes are not commonly used in most seismic refraction studies, mainly because the very large geometric spreading component masks any variations related to geology. The amplitude decay is usually much more rapid than the commonly assumed inverse distance squared function, which only applies after the signal has travelled several wavelengths in the refractor. This study demonstrates that the refraction time section generated through the convolution of forward and reverse refraction traces, shows the same structure on the refracting interface, in units of time, as would be produced by the conventional reciprocal method (CRM) or the GRM. The traveltimes, which are contained within the phase spectra, are added with convolution. The amplitude spectra are multiplied, which is sufficient both to compensate for the large geometric effects and to facilitate convenient recognition of amplitude variations related to changes in refractor wavespeed. Convolution emphasises these amplitude variations through the squaring of the head coefficients and the convenient inclusion of transmission losses. In general, the higher the refractor wavespeed and/or density, the lower the amplitude. The results are applicable to the search for massive sulphide orebodies under electrically conductive regoliths, where other traditional exploration techniques, such as electrical and electromagnetic methods, may not be fully effective. These targets would be characterised by an increase in the thickness of the regolith and a decrease in amplitude caused by an increase in the density.