Changes

and  

and  

    <math> \rho = \frac{\rho_2+\rho_1}{2}, \Delta \rho = \rho_2 - \rho_1 </math>  

:<math> \rho = \frac{\rho_2+\rho_1}{2}, \Delta \rho = \rho_2 - \rho_1 </math>  

    <math> Vp = \frac{Vp_2+Vp_1}{2}, \Delta Vp = Vp_2 - Vp_1 </math>

:<math> Vp = \frac{Vp_2+Vp_1}{2}, \Delta Vp = Vp_2 - Vp_1 </math>

    <math> Vs = \frac{Vs_2+Vs_1}{2}, \Delta Vs = Vs_2 - Vs_1 </math>

:<math> Vs = \frac{Vs_2+Vs_1}{2}, \Delta Vs = Vs_2 - Vs_1 </math>

    <math> \theta = \frac{\theta_2+\theta_1}{2} </math>

:<math> \theta = \frac{\theta_2+\theta_1}{2} </math>

===Hilterman (1983)===

===Hilterman (1983)===