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

In the conventional method of incorporating heterogeneity into crustal mode ls of the Earth, smoothed random small-scale velocity fluctuations are adde d to a uniform seismic velocity gradient field. Numerical experiments on su ch models show that the addition of the velocity fluctuations will tend to break up a smooth travel-time curve into segments with erratic amplitudes g iving one the illusion that the Earth is layered with significant intra-cru stal boundaries. Many experiments, which employ finite difference methods, produce synthetic seismic sections that closely match the nature of the obs erved seismic sections in appearance and many of the features that we have observed owe their origin to velocity fluctuations rather than to determini stic effects. In this paper, we investigated an alternative method of adding heterogeneit y to an Earth model. Smoothed randomly-oriented, small-scale seismic reflec tors are "embedded" in a uniform seismic velocity gradient field. The effec t of the velocity gradient is to make the reflective field for downward wav es much greater than upward waves. The on/off switching ("light bulb effect ") characteristics of the reflection coefficients of the intra-crustal refl ectors, as a function of angle of incidence, plays a major role in generati ng the Pg coda in seismic wide-angle reflection experiments. Numerical expe riments show that the complexity of the coda or the presence/absence of the PmP travel-time branch is related to random orientations and velocity cont rasts of the small-scale reflectors, The shingling effect of Pg first arriv als is predicted by these experiments. The complexity of the record sections, which is predicted from the above an alysis, was compared to actual data obtained from recent Canadian Lithoprob e seismic coincident near-vertical and wide-angle reflection experiments. ( C) 2000 Elsevier Science B.V. All rights reserved.