Whereas t 2 - X 2 plots are straight lines for a reflection from a horizontal interface beneath an isotropic solid, the plots are curved for P-waves and SV-waves when the solid is transversely isotropic. Velocity and intercept time are normally found by fitting a least-squares straight line to the t 2 - X 2 data. However, we can define an instantaneous velocity and an instantaneous intercept time by fitting a tangent line to the t 2 - X 2 plot at each X. We obtain expressions for instantaneous velocity and intercept time for a transversely isotropic solid in terms of the plane-wave velocity v and plane-wave angle from the vertical theta . We relate the instantaneous quantities to measurable quantities for waves from a point source. A method is suggested for finding the minimum source-to-geophone separation at which the velocity becomes essentially the velocity for travel in the horizontal direction.

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