A study of the group velocity of PL for about fifty paths in Canada has been made. It is difficult to measure the dispersion of PL for long periods because two Airy phases arrive in the beginning of the wave train. It is also concluded that like Rayleigh waves PL waves cannot really give more than an S-velocity distribution because the partial derivatives in SV are too large compared to those in P for the period range where a reliable estimate of the dispersion can be obtained. The different dispersion curves are interpreted by looking for lateral variations of PL dispersion. As these curves have only one or two degrees of freedom, we label a curve with an index of dispersion. As in Santo's studies, this index is attributed to each region crossed by fitting the propagation times for a given period. Diagrams are then used giving the variation of the index with the average S velocity and the depth of the Moho. The structures found by this rather simple method are well correlated with tectonic regions and gravity measurements.

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