# Translations:Migration in the case of constant velocity/7/en

What do we have? We have a set of traces in which each trace is associated with a given source point and a given receiver point. On each trace is a fixed number of amplitude values in which each value is associated with a discrete time index. Out of all these data, we just want to pick one amplitude value and then determine the set of depth points in the earth that could give rise to this value. Under the simplifying assumption of constant velocity, all such depth points lie on an ellipsoid. In other words, we know the location of all such depth points for the amplitude value in question. The next step is to assign a value to each point on the ellipsoid in question. We assign the same value as the amplitude value in question. For example, if the amplitude value is the number 3, then we assign the number 3 to each point on the ellipsoid in question. The terminology used is: Take the amplitude at each digital time instant *t* on the trace and *spread out the amplitude* onto the constructed ellipsoid. In other words, we have taken one amplitude value on a trace and, like butter on bread, we have *spread the lump out* onto an entire ellipsoidal surface. Then we repeat this operation for every amplitude value on every trace. In our mind’s eye, every amplitude value is spread out onto its own ellipsoid. Each lump of butter has its own piece of bread.