  Efficient and accurate numerical simulation of 3D acoustic wave propagation
in heterogeneous media plays an important role in the success of seismic full
waveform inversion (FWI) problem. In this work, we employed the combined scheme
and developed a new explicit compact high-order finite difference scheme to
solve the 3D acoustic wave equation with spatially variable acoustic velocity.
The boundary conditions for the second derivatives of spatial variables have
been derived by using the equation itself and the boundary condition for $u$.
Theoretical analysis shows that the new scheme has an accuracy order of
$O(\tau^2) + O(h^4)$, where $\tau$ is the time step and $h$ is the grid size.
Combined with Richardson extrapolation or Runge-Kutta method, the new method
can be improved to 4th-order in time. Three numerical experiments are conducted
to validate the efficiency and accuracy of the new scheme. The stability of the
new scheme has been proved by an energy method, which shows that the new scheme
is conditionally stable with a Courant - Friedrichs - Lewy (CFL) number which
is slightly lower than that of the Pad\'{e} approximation based method.
