# Wyllie time-average equation

${\displaystyle {\frac {1}{v}}={\frac {\phi }{v_{f}}}+{\frac {1-\phi }{v_{m}}}}$
${\displaystyle \Delta t=\phi \Delta t_{f}+(1-\phi )\Delta t_{m}}$
where, ${\displaystyle \phi }$ is the total porosity, ${\displaystyle v}$ is the wave's measured phase velocity, ${\displaystyle v_{m}}$ is the velocity of the rock matrix, and ${\displaystyle v_{f}}$ is the velocity of the fluid in the pores, ${\displaystyle \Delta t}$ is the measured interval travel-time, ${\displaystyle \Delta t_{m}}$ is the interval travel-time of the rock matrix, ${\displaystyle \Delta t_{f}}$ is the interval travel-time of the saturating fluid.