Whenever we are investigating multiple layer interfaces in a seismic reflection survey the root-mean-squared velocity is often employed to deduce travel time to a certain n-th layer and also to deduce velocity of any given interval. This is done because it is a way to account for refraction that is also occurring between layers which alters the reflection of a shot made through multiple layers.
What I don't understand is how do we know the travel times in any layer beforehand? How are we distinguishing between different travel times and assigning them to different layers we've distinguished? We need travel times of successive layers in order to compute the root-mean-square, but this is puzzling to me because how could you already know them? Who is to say it is multiple and not just 1? or 3, or 5 or some arbitrary number? What about the arrivals to the geophones tell us this?
I don't get how we know there are extra layers in the first place to which we must assign travel times, and when they do exist, from where do these travel times come? I have included a graphic to help show what I mean. The more I think about this, the more I start to think I don't understand multiple layer seismic reflection in general.