Obtaining velocity data for the model is the next step in the process. Let's begin with a simple marine data case, data which was collected over water. The velocity of water is approximately 1500 meters per second. Only the depth of the water is unknown and can be computed from the seismic data.
However, on land we are in for a terrible surprise. The weathered earth is just that, and the low velocity layer has truly variable properties. Just look across your favorite park and you will see stream beds with sand and gravel, hills with bedrock, fertile fields with soils, all places where the earth is really different. All these differences are seen in the near surface seismic data. Here both the velocity and depth are unknown and will be determined from the seismic data.
After the first layer either on land or at sea we still have mystery to unravel: what is the velocity of the rest of the subsurface? This we shall leave for another day. I will say that using the unstacked data and more powerful mathematics it is possible to do a very credible job of velocity analysis.