The effect of small random crustal reflectors on the complexity of Pg and PmP coda

Abstract

In the conventional method of incorporating heterogeneity into crustal models of the Earth, smoothed random small-scale velocity fluctuations are added to a uniform seismic velocity gradient field. Numerical experiments on such models show that the addition of the velocity fluctuations will tend to break up a smooth travel-time curve into segments with erratic amplitudes giving one the illusion that the Earth is layered with significant intra-crustal boundaries. Many experiments, which employ finite difference methods, produce synthetic seismic sections that closely match the nature of the observed seismic sections in appearance and many of the features that we have observed owe their origin to velocity fluctuations rather than to deterministic effects. In this paper, we investigated an alternative method of adding heterogeneity to an Earth model. Smoothed randomly-oriented, small-scale seismic reflectors are “embedded” in a uniform seismic velocity gradient field. The effect of the velocity gradient is to make the reflective field for downward waves much greater than upward waves. The on/off switching (“light bulb effect”) characteristics of the reflection coefficients of the intra-crustal reflectors, as a function of angle of incidence, plays a major role in generating the Pg coda in seismic wide-angle reflection experiments. Numerical experiments show that the complexity of the coda or the presence/absence of the PmP travel-time branch is related to random orientations and velocity contrasts of the small-scale reflectors. The shingling effect of Pg first arrivals is predicted by these experiments. The complexity of the record sections, which is predicted from the above analysis, was compared to actual data obtained from recent Canadian Lithoprobe seismic coincident near-vertical and wide-angle reflection experiments.