i_c0 - critical angle
V₀ - velocity of the first layer
V₁ - velocity of the second layer
h₀ - thickness of the first layer
T0₁ - intercept

${\displaystyle i_{c_{0}}=asin\left({V_{0} \over V_{1}}\right)}$

${\displaystyle T={2h_{0}cos(i_{c_{0}}) \over V_{0}}+{X \over V_{1}}=T0_{1}+{X \over V_{1}}}$

${\displaystyle h_{0}={T0_{1}V_{0} \over 2cos(i_{c})}}$

${\displaystyle h_{0}={X_{cross_{1}} \over 2}{\sqrt {V_{1}-V_{0} \over V_{1}+V_{0}}}}$

Forward:

${\displaystyle T_{n}=T0_{n}+{X \over V_{n}}}$

${\displaystyle T0_{n}=\sum _{j=0}^{n-1}{2h_{j}cos(i_{j}) \over V_{j}}}$

Inverse:

${\displaystyle h_{n}={V_{n} \over cos(i_{c_{n}})}\left({T0_{n+1} \over 2}-\sum _{j=0}^{n-1}{h_{j}{\sqrt {{1 \over V_{j}^{2}}-{1 \over V_{j+1}^{2}}}}}\right)}$

${\displaystyle T0_{n}=T_{cross_{n}}-{X_{cross_{n}} \over V_{n}}}$

${\displaystyle h_{n}={V_{n} \over cos(i_{c_{n}})}\left({\left(T_{cross_{n+1}}-{X_{cross_{n+1}} \over V_{n+1}}\right) \over 2}-\sum _{j=0}^{n-1}{h_{j}{\sqrt {{1 \over V_{j}^{2}}-{1 \over V_{j+1}^{2}}}}}\right)}$