Velocity from time and depth seismic images
Disclaimer: It a very crude way to do it.! Take it as the last resort.

It assumes that you know velocity for the shallower reflectors/layers. I will explain for one reflector and you can extend for deeper reflectors.

Step 1. You slice out the reflector event (in a trace) you want to find the velocity for. 

Step 2. Take a correlation with the respective trace in other time/depth domain part and find the max correlation time/depth. Here you may assume this time corresponds to the respective depth so you get total travel time.

Step 3. At this point you know the interval velocities for layers at the shallower depth, so you can find out the interval velocity for this layer.

You start from the top and continue to bottom side. You may treat it as layer stripping.